John Henry Pepper

James Watt.

Throughout the greater number of the preceding chapters it will be evident that the active properties of matter may be summed up under one general head, and may be considered as varieties of attraction—such as the attraction of gravitation, cohesive attraction, adhesive attraction, attraction of composition (or chemical attraction), electrical attraction, magnetical attraction.

The absolute or autocratic system does not, however, prevail in the works of nature; and she seems ever anxious, whilst imparting great and peculiar powers to certain agents, to create other forces which may control and balance them. Thus, for instance, the great force of cohesive attraction is an ever-present power discernible, as has been shown, in solids and liquids; but if this agent were allowed to run riot in its full strength and intensity, it would tyrannically hold in subjection all liquid matter, and every drop of water which is at present kept in the liquid state, would succumb to its iron rule, and retain the solid state of ice. Hence, therefore, the wise creation of an antagonistic force—viz., heat; which is not provided in any niggardly manner, but is liberally bestowed upon the globe from that all-sufficient and enormous source, the sun. And it is by the softening and liquifying influence of his rays that the greater proportion of the water on the surface of the globe is maintained in the fluid condition, and is enabled to resist the power of cohesion, that would otherwise turn it all, as it were, to stone.

Cohesion, electricity, and magnetism fully embody the notion of powers of attraction, or a drawing together; whilst heat stands almost alone in nature as the type of repulsion, or a driving back.

Mechanically, repulsion is demonstrated by the rebound of a ball from the ground; the parts which touch the earth are for the moment compressed, and it is the subsequent repulsion between the particles in those parts which causes them to expand again and throw off the ball.

The development of heat is produced from various causes, which may be regarded as at least four in number. Thus, it was shown by Sir Humphrey Davy, that even when two lumps of ice are rubbed together, sufficient heat is obtained to melt the two surfaces which are in contact with each other. Friction is therefore an important source of heat, and one of the most interesting machines at the Paris Exposition consisted of an apparatus by which many gallons of water were kept in the boiling state by means of the heat obtained from the friction of two copper discs against each other. The machine attracted a good deal of attention on its own merits, and especially because it supplied boiling water for the preparation of chocolate, which the public was duly informed was boiled by the heat rubbed out of the otherwise cold discs of copper. When cannon made on the old system are bored with a drill, it is necessary that the latter should be kept quite cool with a constant supply of water, or else the hard steel might become red-hot, and would then lose its temper, and be no longer capable of performing its duty.

Count Rumford endeavoured to ascertain how much heat was actually generated by friction. When a blunt steel bore, three inches and a half in diameter, was driven against the bottom of a brass cannon seven inches and a half in diameter, with a pressure which was equal to the weight of ten thousand pounds, and made to revolve thirty-two times in a minute, in forty-one minutes 837 grains of dust were produced, and the heat generated was sufficient to raise 113 pounds of the metal 70° Fahrenheit—a quantity of heat which is capable of melting six pounds and a half of ice, or of raising five pounds of water from the freezing to the boiling point. When the experiment was repeated under water, two gallons and a half of water, at 60° Fah., were made to boil in two hours and a half.

Chemical affinity has been so often alluded to in these pages, that it may be sufficient to mention only one good instance of its almost magical power in evoking heat. When a bit of the metal sodium is placed on the tip of a knife, and thrust into some warm quicksilver, or if a pellet of sodium and a few globules of mercury are placed on a hot plate just taken from the oven, and then gently squeezed together, a vivid production of heat and light is apparent; and when the mixture of the two metals is cold, it will be found that the quicksilver has lost its fluidity, and a solid amalgam of sodium and mercury is obtained, which gradually, by exposure to the air, returns to the liquid state, the mercury being set free, whilst the sodium is oxidized, and forms soda. Just as an ordinary alloy of copper and gold used by jewellers would lose its colour and brilliancy by the oxidation of the copper; and when the rusty, dirty film is removed by rubbing and polishing, the surface is again brilliant, and remains so until another film of the exposed copper is attacked: in like manner the sodium is attacked and changed by the oxygen of the air, whilst the mercury being unaffected retains its brilliancy, and at the same time regains its fluidity. The evolution of heat in the above case indicates that a chemical union has taken place between the two metals.

Examples of the production of heat by electricity and magnetism have been abundantly shown in the chapters on these subjects; and one of the best illustrations of this fact has been shown on the occasion of the opening of the telegraphic communication between France and England by means of the submarine cable, when cannon were fired alternately at both ends of the conducting cable by means of electricity, and the event thus inaugurated in both countries.

That heat is a product of living animal organization is shown, as it were, visibly by the marvellous phenomena that proceed in our own bodies. People do not very often trouble themselves to ask where the heat comes from, or even to think that this invisible power must be maintained in the body, and that slow combustion, or, as Liebig terms it, eremacausis, must continually go on inside our frail mortal tenements; and more than this, that we cannot afford to waste our heat. If the body is deprived of heat faster than it can be generated, death must inevitably occur; and a very melancholy instance of this remarkable mode of death has lately occurred in Switzerland to a Russian gentleman.

Such another instance of a man being slowly frozen to death within sight and sound of other beings, through whose veins the blood was flowing at its accustomed temperature (about 90º Fahr.), it would be difficult to find, and it stands forth, therefore, as a marked example and illustration of the statement already made, that living animal organisms are truly a source of heat, which is as essential to the well-being of the body as meat, drink, and air.

Heat is of two kinds, and may be either apparent to our senses, and therefore called sensible heat; or it may be entirely concealed, although present in solids, liquids, and gases, and is then termed insensible or latent heat.

Sensible Heat.

The first effect of this force is a demonstration of its repulsive agency, and the dilatation or expansion of the three forms of matter whilst under the influence of heat, admits of very simple illustrations. The expansion of a solid substance, as, for instance, a metal, on the application of heat, is apparent by fitting a solid brass cylinder into a proper metal gauge, which is accurately filed so as to admit the former when perfectly cold. If the brass rod is then heated, either by plunging it into boiling water or by the application of the flame of a spirit lamp, its particles are separated from each other; they now occupy a larger space, and expansion is the result, and this is clearly proved by the application of the gauge, which is no longer capable of receiving it. (Fig. 343.) When, however, the latter is cooled, the opposite result occurs, the particles of brass return to their old position, and contraction takes place; hence it is stated that "Bodies expand by heat and contract by cold;" and it is proper to state here that the term "cold" is of a negative character, and simply means the absence of heat.

Fig. 343. Fig. 343.

a b. Cylinder of brass. c d. Iron gauge, admitting a b longitudinally, and also in the hole e when cold, but excluding a b when the latter is heated and expanded.

Solid bodies do not expand equally on the application of the same amount of heat; thus, a bar of glass one inch square and one thousand inches long would only expand one inch whilst heated from the freezing to the boiling point of water. A bar of iron one inch square and eight hundred inches long would expand one inch in length, through the same degrees of heat; and a bar of lead one inch square and three hundred and fifty inches long would also dilate one inch in length. Hence,

Lead expands in volume	1/350th.
Iron	1/800th.
Glass	1/1000th.

The unequal expansion of the metals is well illustrated by an experiment devised by Dr. Tyndal, the respected Professor of Natural Philosophy in the Royal Institution of Great Britain, and is arranged as follows:—A long bar of brass and another of iron are supported on the edges of two pieces of wood placed at an angle, and resting against the sides of a mahogany framework. The metallic bars only touch one end of the frame, and are in metallic communication with a piece of brass inserted there, and forming part of a conducting chain connected with a voltaic battery; when heat is applied to both bars they expand unequally; the brass bar dilates first, and filling up the minute space left between the two ends of the frame, touches another brass plate and instantly completes the voltaic circuit, when a coil of platinum wire becomes ignited, showing the fact of expansion; and secondly, the difference in the power of dilatation possessed by each is clearly shown by removing the two angular supports of wood, when the iron falls away, whilst the brass remains and still completes the voltaic circuit. (Fig. 344.)

Fig. 344. Fig. 344.

a a. The brass bar which has expanded by the heat from the gas jet b, and making the contact between the brass plates in connexion with the binding screws c c, the voltaic circuit is completed, and a coil of platinum wire in the glass tube d, is immediately ignited. The iron bar at e e has not expanded sufficiently, which is shown afterwards by removing the angular wooden supports k k, when the iron falls off, and the brass remains on the two ledges of the mahogany framework l l l.

The force exerted by the expansion of solids is enormous, and reminds us again of the amazing power of all the imponderable agents; and it is truly wonderful to notice how the entry of a certain amount of heat into and between the particles of metals, or other solids, endues them with a mechanical force which is almost irresistible, and is capable of working much harm. KussnÉ made an experiment with an iron sphere, which he heated from a temperature of 32° Fahr. to 212° Fahr., and he found that the expansion of the ball exerted a force equal to 4000 atmospheres—i.e. 4000 × 15—on every square inch of surface, or a pressure equal to thirty millions of pounds; the entry of only 180° of heat into the iron sphere produced this remarkable result, just as Faraday has calculated that a single drop of water contains a sufficient quantity of electricity to produce a result equal to the most powerful flash of lightning, provided the electricity of quantity in the drop of water is converted into electricity of high tension or intensity.

The practical applications of this well-known property of solids with respect to heat are very numerous; thus, the iron bullet-moulds are always made a little larger than the requisite size, in order to allow for the expansion of the hot liquid lead, and the contraction of the cold metal. The tires of wheels and the hoops of casks are usually placed on whilst hot, in order that the subsequent contraction may bind the spokes and fellies, or the staves, closely together. If an allowance was not made for the expansion and contraction of the iron rails on the permanent ways of railroads, the regularity of the level would be constantly destroyed, and the position of the rails, chairs, and sleepers would be most seriously deranged; indeed it is calculated that the railway bars between London and Manchester are five hundred feet longer in the summer than in the winter.

The walls of the Cathedral of Armagh, as also those of the Conservatoire des Art et MÉtiers, were brought back to a nearly perpendicular position, by the insertion (through the opposite walls) of great bars of iron, which being alternately heated, expanded, and screwed up tight, then cooled and contracted, gradually corrected the bulging out of the walls or main supports of these buildings. The principle of these famous practical experiments is neatly illustrated by means of an iron framework with a bar of iron placed through both its uprights, and screwed tight when hot; on cooling, contraction occurs, which is shown by a simple index. (Fig. 345.)

Fig. 345. Fig. 345.

The iron frame, with c c, wrought-iron bar heated by putting on the semicircular piece of iron e e, which is first made red-hot, and as the heat is communicated to the wrought iron rod c c, it is screwed up tight by the nut k. g g. The index attached to the iron frame screwed up when hot; the arms come together at p, and separate further to h h as the contraction takes place by cooling the bar c d.

It has often been remarked that there is no rule without an exception, and this applies in a particular instance to the law that "bodies expand by heat and contract by cold"—viz., in the case of Rose's fusible metal, which consists of

Two parts	by weight of	bismuth,
One part	"	lead,
One part	"	tin.

To make the alloy properly, the lead is first melted in an iron ladle, and to this are added first the tin, and secondly the bismuth; the whole is then well stirred with a wooden rod, and cast into the shape of a bar.

When placed in the pyrometer and heated, the bar expands progressively till it reaches a temperature of 111° Fahr.; it then begins to contract, and is rapidly shortened, until it arrives at 156° Fahr., when it attains a maximum density, and occupies no more space than it would do at the freezing-point of water. The bar, after passing 156°, again expands, and finally melts at about 201°, which is 11° below the boiling-point of water. Fusible metal is sometimes made into teaspoons, which soften and melt down when stirred in a cup of hot tea or basin of soup, to the great surprise and bewilderment of the victim of the practical joke.

Unequal expansion is familiarly demonstrated with a bit of toasted bread, which curls up in consequence of the surface exposed to the fire contracting more rapidly than the other; and the same fact is illustrated with compound flat and thin bars of iron and brass, which are fixed and rivetted together; when heated, the compound bar curves, because the iron does not expand so rapidly as the brass, and of course forms the interior of the curve, whilst the brass is on the exterior.

The experiment with the compound bar is made more conclusive and interesting by arranging it with a voltaic battery and platinum lamp. One of the wires from the battery is connected with the extremity of the compound bar, and as long as it remains cold, no curve or arch is produced, but when heat is applied, the bar curves upwards, and touching the other wire of the battery, the circuit is completed, and the platinum lamp is immediately ignited. (Fig. 346.)

Fig. 346. Fig. 346.

a b. Compound bar resting on two blocks of wood. The end a is connected with one of the wires from the battery. The circuit is completed and the platinum lamp d ignited directly the bar curves upwards by the heat of the spirit lamp, and touches the wire c C connected with the opposite pole of the battery.

The expansion and contraction of liquids by heat and cold is also another elementary truth which admits of ample illustration, and indeed introduces us to that most useful instrument called the thermometer.

If a flask is fitted with a cork through which a long glass tube, open at both ends, is passed, and then carefully filled with water coloured with a little solution of indigo, so that when the cork and tube are placed in the neck, all the air is excluded, a rough thermometer is thus constructed, which, if placed in boiling water, quickly indicates the increased temperature by the rising or expansion of the coloured water inside the flask. (Fig. 347.)

Expansion of liquids shown at a by the coloured water rising in the tube from the flask, which is quite full of liquid, and heated by boiling water. b. The expansion of the water heated by the spirit-lamp is shown by the rising of the piston and rod c c. d represents a retort filled up like a to show the expansion of a liquid by heat.

The thermometer embraces precisely the same principle as that already described in Fig. 347, with this difference only, that the tube is of a much finer bore, and the liquid employed, whether alcohol or mercury, is boiled and hermetically sealed in the tube, so that the air is entirely excluded. To make a thermometer, a tube with a capillary bore is selected of the proper length; it is then dipped into a glass containing mercury, so that the tube is filled to the length of half an inch with that metal. The half-inch is carefully measured on a scale, and the place the mercury fills in the tube marked with a scratching diamond; the mercury is then shaken half an inch higher, and again marked, and this proceeding is continued until the whole tube is divided into half inches. The object of doing this is to correct any inequalities in the diameter of the bore of the glass tube, because if wider at one part than another, the spaces filled with the mercury are not equal; as the bore is usually conical, the careful measurement of the tube with the half inch of mercury in the first place gives the operator at once a view of the interior of his tube, and enables him to graduate it correctly afterwards. (Fig. 348.)

The next step is to heat one extremity by the lamp and blowpipe, and whilst hot, to blow out a ball upon it; if this operation were performed with the mouth, moisture from the breath would deposit inside the fine bore of the glass tube, and injure the perfection of the thermometer afterwards. In order to prevent any deposit of water, the bulb is blown out, whilst red-hot, with the air from a small caoutchouc bag fitted on to the other extremity of the tube. The operator now marks off the intended length of his thermometer, and above that point the tube is again softened with the flame and blowpipe, and a second bulb blown out. (Fig. 349 a.)

The open end of the tube is now placed under the surface of some pure, clean, dry quicksilver, and heat being applied to the upper bulb, the air expands and escapes through the mercury, and as the tube cools a vacuum is produced, into which the mercury passes. By this simple method, the mercury is easily forced into the tube, as otherwise it would be impossible to pour the quicksilver into the capillary bore of the intended thermometer. (Fig. 349 b.)

The tube is now taken from the glass containing the mercury, and simply inverted; but in consequence of the very narrow diameter of the bore the air will not pass out of the first bulb until heat is applied, when the air expands, and the mercury, first stationary in the second bulb, will now displace the air, and fall into the first bulb when the tube is again cool.

The ball, No. 1 (Fig. 349 a), is now full of mercury, and there is also some left in No. 2; in the next place, the tube is supported by a wire, and held over a charcoal fire, when it is heated throughout its entire length, and the mercury being boiled expels the whole of the air, so that there is nothing inside the bulbs and capillary bore but mercury and its vapour. (No. 1, Fig. 350.) The open end of the intended thermometer is now temporarily closed with sealing-wax, and the whole allowed again to cool with the sealed end uppermost, so that the ball No. 2, Fig. 350, and the tube above it, are quite filled with quicksilver.

After cooling, the tube is placed at an angle with the sealed end uppermost, and, guided by experience, the operator heats the lower bulb so as to expand enough mercury into the upper one to leave space for the future expansion and contraction of the mercury in the tube, which has now to be hermetically sealed. This is done by dexterously heating the tube at the cross whilst the mercury in the first bulb is still expanded; and by drawing it out rapidly with the help of the heat obtained from the lamp and blowpipe, the second bulb is separated from the first at the little cross (b, No. 3, Fig. 350), and the thermometer tube at last properly filled with quicksilver, and hermetically closed. (No. 4, Fig. 350.)

In order to procure a fixed starting-point, the thermometer tube is placed in ice, with a scale attached; the temperature of ice never varies, it is always at 32 degrees. When, therefore, the mercury has sunk to the lowest point it can do by exposure to this degree of cold, the place is marked off in the scale, and represents that position in the graduated scale where the freezing point of water is indicated.

The tube is placed in the next place in a vessel of boiling water, care being taken that the whole tube is subject to the heat of the water and the steam issuing from it, and when the mercury has risen to the highest position attainable by the heat of boiling water, another graduation is made which indicates 212 degrees—viz., the boiling point of water. This graduation should be made when the barometer stands at 30 inches, because the boiling point of water varies according to the weight of the superincumbent air pressing upon it.

Between the graduation of the freezing and the boiling point of water the space is divided into 180 parts, which added to 32 make up the boiling point of water to 212 degrees, being the graduation of Fahrenheit, who was an instrument-maker of Hamburg. Why he divided the space between the freezing and boiling point of water nobody appears to know, unless he took a half circle of 180 degrees as the best division of space. If the thermometer contains air the mercury divides itself frequently into two or three slender threads, each separated from the other in the capillary bore, and thus the instrument is rendered useless until the threads again coalesce. If the thermometer has been well made, and is quite free from air, it may be tied to a string and swung violently round, when the centrifugal force drives the slender threads of mercury to their common source—viz., the bulb containing the quicksilver, and the whole is again united. The string must be attached, of course, to the top of the thermometer scale.

When travelling on the Continent it is sometimes desirable to be able to read the thermometers which are graduated in a different manner to that of Fahrenheit. In France the Centigrade scale is preferred, and in many parts of Germany Reaumur's graduation. The difference of the graduation is seen at a glance.

The number of degrees, therefore, between boiling and freezing is 100 in the Centigrade, 80 in Reaumur, and (212-32, that is) 180 in Fahrenheit.

If, then, the letters C, R, F, be taken to denote the number of degrees from the freezing point at which the mercury stands in the Centigrade, Reaumur, and Fahrenheit thermometers, we have the following proportions:—

The following examples will show how to apply these formulÆ:—

(1).—Suppose the Reaumur stands at 28°, at what height does the Centigrade stand? We have C = 5/4 of R (in this case), 5/4 of 28 = 35: that is, the Centigrade stands at 35°.

(2).—Suppose Fahrenheit to stand at 41°, what will Reaumur stand at? R = 4/9 of (41-32) (that is, the number above freezing in Fahr.) = 4/9 of 9 = 4. Reaumur stands at 4.

(3).—Suppose Fahrenheit stands at 23°, what will the Centigrade stand at? C = 5/9 of F = 5/9 of (32-23) = 5/9 of 9 = 5 below freezing (or-5).

(4).—If Fahrenheit stands at 4 below 0, what will Reaumur indicate? R = 4/9 of F = 4/9 of (32 + 4) = 4/9 of 36 = 16 below 0 (or-16).

The only liquid which has the exceptional property of expanding by cold is water, and it will be seen presently that this curious anomaly is of the greatest importance in the economy of nature.

If a box containing a mixture of ice and salt is placed round the top of a long cylindrical glass containing water at a temperature of 60° Fahr., the intense cold of the freezing mixture, which is zero—that is to say, 32° below the freezing point of water—very soon reduces the temperature of the water contained in the glass, and as it becomes colder it contracts, is rendered heavier, and sinks to the bottom of the vessel, and its place is taken by other and warmer water. This circulation commencing downwards, proceeds till the water has attained a temperature of about 40° Fahr., when the maximum density is obtained and the circulation stops, because after sinking below 40° the cold water becomes lighter, and continues to be so until it freezes, and of course, being of a less specific gravity than the warmer water, it floats (like oil on water) upon its surface; so that a small thermometer placed at the bottom of the jar indicates only 40° Fahr., whilst the solid ice enveloping the other or second thermometer placed at the top may be as low as 29°, or even lower, according to the quantity of ice and salt used in the box surrounding the top of the glass. (Fig. 351.)

The importance of this curious anomaly cannot be overrated. If water did not possess this rare property, all the seas, rivers, canals, lakes, &c., would gradually become impassable from the presence of enormous blocks of ice formed during the winter. The whole bulk of water contained in them would have to sink below 32° before it could solidify provided water increased in density or continued to contract by cold. Having once solidified, the warmth of the rays from a summer's sun would certainly melt a great deal of the ice, but not the whole, and winter would come again before the solid masses had disappeared. The ocean could not be navigated in safety even near our own shores, in consequence of the vast icebergs that would be formed, and float about and jostle each other even in the British Channel.

The earth has been wonderfully prepared for God's highest work—Man, and in nothing is this supreme wisdom more apparent than in the fact that water offers the only known exception to the law "that bodies expand by heat and contract by cold."

The expansion of gases by heat and contraction by cold take place in obedience to a law to which there is no exception, except in degree. It was discovered in 1801 by M. Gay Lussac, of Paris, and also about the same period by the famous English philosopher who established the atomic theory—viz., by Dr. Dalton. Since these experiments and calculations Rudberg, Magnus, and Regnault have made other researches, and their successive experiments give the following results:—

As a natural result, air at 32° Fahr, expands 1/491 part of its volume for every degree of heat on the scale of Fahrenheit; and a volume of air which measures 491 cubic inches at 32° will measure 492 at 33°, 493 at 34°, and so on. The exception is only in degree, and Magnus and Regnault discovered by their searching experiments that the gases easily liquified are more expansible by heat than air and those gases (such as oxygen, hydrogen, and nitrogen) which have never been liquified.

The expansion of air is easily shown by placing the open end of a tube with a large bulb blown at the other extremity, under the surface of a little coloured water; on the application of heat the air expands and escapes, and its place is taken, when cool, by the coloured liquid. Such an arrangement represents the first thermometer constructed by Sanctorio about a.d. 1600, which might certainly answer for rough purposes, but as the ascent and descent of the fluid depend on the bulk of air contained in the bulb, and as this is affected by every change of the height of the barometer, no satisfactory indication of an increase or decrease of temperature could be obtained with it, although the instrument itself is interesting in an historical point of view, and in a modified form as an air thermometer has been employed by Sir John Leslie, under the name of the "Differential Thermometer," in his refined and delicate experiments with heat.

Fire balloons are a good example of the expansion of gases, and the levity of the air thus increases in bulk was taken advantage of by Montgolfier in the construction of his famous balloon, which, with a cage containing various animals, ascended, in the presence of the King and royal family of France, at Versailles; and in spite of huge rents in two places, it rose to a height of 1440 feet, and after remaining in the air for eight minutes, fell to the ground at the distance of 10,200 feet from the place whence it started, without injury to the animals. When it is considered that a volume of air heated from 32° to 491° is doubled, and tripled when heated to 982°, it will at once be understood how great must be the ascending power of such balloons, provided the air within them is kept sufficiently hot.

That gallant aËronaut, Pilate de Rozier, offered himself to be the first aËrial navigator; and having joined Montgolfier, they made three successful ascents and descents with a large oval-shaped balloon, forty-eight feet in diameter, and seventy-four feet high. On the fourth occasion he ascended to a height of 262 feet, but in the descent a gust of wind having blown the machine over some large trees of an adjoining garden, the situation of the brave aËronaut was extremely dangerous, and if he had not possessed the strongest presence of mind, and at once given the balloon a greater ascending power, by rapidly supplying his stove with some straw and chipped wood, he might on this occasion have met with that untimely end which subsequently, in another rash aËronautic adventure, befell this brave but foolhardy Frenchman.

On descending again, he once more, and without the slightest fear, raised himself to a considerable height by feeding his fire with chopped straw. Some time after he ascended, in company with M. Giroud de Vilette, to the height of 330 feet, hovering over Paris at least nine minutes, in sight of all the inhabitants, and the machine keeping all the while perfectly steady.

The danger in using this method of inflating the balloon arises from the possibility of generating gas, which escaping unburnt into the body of the balloon, may accumulate and blow up, or burn afterwards.

Fire balloons, as usually made, are very dangerous toys, and may sometimes prove rather costly to the person who may send them off, in consequence of their being blown by the wind on a hay or corn rick, or other combustible substances. The safest mode of using fire balloons is to fill them with hot air from a lighted gas stove (Wessel's, for instance); the balloons may then be used in large rooms, or out in the air, without fear of doing any harm to neighbouring property, as of course the stove and the fire remain behind, and will fill any number of air balloons. (Fig. 353.)

After all the fuss made about the novelty of the American hot-air engine, it is somewhat amusing to look back to the records of civil engineering, and in the "Transactions of the Institution of Civil Engineers," to read Mr. James Stirling's account of his improved air engine, in which the great expansion of air mentioned at p. 365 has been successfully applied. The engine was constructed about the year 1843, and the principle, discovered thirty years before by Mr. R. Stirling, will be comprehended by reference to the cut. (Fig. 354.)

Stirling's air engine.

Two strong air-tight vessels are connected with the opposite ends of a cylinder, in which a piston works in the usual manner. About four-fifths of the interior space in these vessels is occupied by two similar air-tight vessels or plungers, which are suspended to the opposite extremities of a beam, and capable of being alternately moved up and down to the extent of the remaining fifth. By the motion of these interior vessels, which are filled with non-conducting substances, the air to be operated upon is moved from one end of the exterior vessel to the other, and as one end is kept at a high temperature, and the other as cold as possible, when the air is brought to the hot end it becomes heated, and has its pressure increased; and when it is brought to the cold end, its heat and pressure are diminished. Now, as the interior vessels necessarily move in opposite directions, it follows that the pressure of the enclosed air in the one vessel is increased, while that of the other is diminished. A difference of pressure is thus produced upon the opposite sides of the piston, which is thereby made to move from the one end of the cylinder to the other, and by continually reversing the motion of the suspended bodies or plungers, the greater pressure is successively thrown upon a different side, and a reciprocating motion of the piston is kept up. The piston is connected with a fly-wheel in any of the usual modes; and the plungers, by whose motion the air is heated and cooled, are moved in the same manner, and nearly at the same relative time, with the valves of a steam engine.

The pressure is greatly increased and made more economical by using somewhat highly-compressed air, which is at first introduced, and is afterwards maintained, by the continued action of an air-pump. The pump is also employed in filling a separate magazine with compressed air, from which the engine can be at once charged to the working pressure. Mr. Stirling's chief improvement consists in saving all or nearly all the heat of the expanded air after it has done its work, by passing it from the hot to the cold end of the air vessel through a multitude of narrow passages, whose temperature is at the beginning of the tubes nearly as great as that of the hot air, but gradually declines till it becomes nearly as low as the coldest part of the air vessel. The heat is therefore retained by these passages, so that when the mechanism is reversed, the cold air returns again through these hot pipes, and is thus made nearly hot enough by the time it reaches the heating vessel to do its work. Thus, instead of being obliged to supply at every stroke of the engine as much heat as would be sufficient to raise the air from its lowest to its highest temperature, it is necessary to furnish only as much as will heat it the same number of degrees by which the hottest part of the air vessel exceeds the hottest part of the intermediate passages. This portion of the engine may be called the economical process, and represents the foundation of all the success to which it has attained in producing power with a small expenditure of fuel. No boiler being required, of course the danger of explosions is much lessened. The higher the pressure under which the engine was worked the greater was the effect produced. A small engine on this principle was worked to a pressure of 360 pounds on the square inch; and perhaps the best popular notion of the novelty in the arrangement is that suggested by Mr. George Lowe, who compared the economical part of the machine to a "Jeffrey's Respirator" used by consumptive patients. The heat from the air expired being retained by the laminÆ, and again used when cold air is inspired or drawn into the lungs. Mr. Stirling states that the consumption of fuel as compared to the steam engine which the air engine had replaced was as 6 to 26; the same amount of work being now performed by about six cwt. of coals which had formerly required about twenty-six cwt., though he ought to have stated that the steam engine removed was not of the best construction, nor had the boiler any close covering. (Fig. 354.)

Conduction of Heat.

This property of heat with reference to matter, and the consideration of the curious manner in which it creeps, as it were, through solid substances, brings the thoughtful mind at once to the bold question of What is heat? Is it to be regarded as something real or material? or must it be considered only as a property or state of matter? These questions are not to be solved easily, and they demand a considerable amount of experiment and reasoning even to appreciate their meaning.

If a red-hot ball is placed in the focus of a concave metallic speculum, it gives out certain emanations that are quite invisible, but which are reflected from the surface of the mirror in the same manner as visible rays of light, and may be collected in the focus of another and second concave speculum, when they can be concentrated on to a bit of phosphorus, and will cause the combustion of that substance. If the air from a pair of bellows is blown forcibly across the rays of heat as they are being concentrated upon the phosphorus, the rays are not moved from their course, they are no more blown away than a sunbeam darting through an aperture in a cloud on a stormy, windy day. The heat has, therefore nothing to do with the air, and is wholly independent of that medium in its passage from one mirror to the other. Such an experiment as that described would at once suggest the idea that heat is a matter sui generis, a component part of all bodies, and given off from incandescent matter, the sun, &c., and that it may be propagated through space much in the same manner as light. (Fig. 355.) The mechanism may be very much like the corpuscular movement of light as defined by Sir Isaac Newton, and already explained in another portion of this book. Hence it has been supposed that heat is propagated through the air, water, and solid substances by a direct emission of material particles from the heat-giving agent, and that these molecules of heat force their way into, or along, or through them, according to circumstances.

Heat reflected by mirror, but not blown away by air from bellows.

Certain bodies are almost transparent to heat rays, such as air, whilst others take an intermedial position, and only stop a certain quantity of the heat molecules, such as rock crystals, mirror glass, and alum. A third class of bodies absorbs the heat plentifully, such as charcoal, black cloth, &c.; and a fourth, when polished and placed at the proper angle, reflects or throws off the heat, as in the case of polished mirrors. The transparency or opacity of substances (so far as light is concerned) does not affect the transmission of heat. Light of every colour and from all sources is equally transmitted by all transparent bodies in the liquid or solid form, but this is not the case with heat.

The rays of heat emitted by the sun and other luminous bodies have properties quite different to the rays of light with which they are accompanied. From these statements it will be evident that the material theory of heat is surrounded with difficulties and anomalies that cannot be reconciled the one with the other, or neatly adapted, fitted in, and dovetailed with all the puzzling phenomena that arise. Our knowledge of the theory of heat has been greatly assisted by the researches of Melloni, who has demonstrated that different species of rays of heat are given off by the same body at different temperatures, which may be distinctly sifted and separated from each other. Long before the experiments of Melloni philosophers had endeavoured to weigh heat; trains of the most delicate levers were exposed, without effect, to the action of heat rays; and all attempts, experimental as well as theoretical, to define heat by the material theory, are imperfect, crude, and unsatisfactory. We are perforce obliged to adopt another theory, and the one that obtains the greatest favour, as offering the best definition of heat, is the dynamical theory, which is more or less analogous to the undulatory theory of light. At pages 262, 328, 335, this theory has been partly explained, and in speaking of it again, great care must be taken not to confuse the undulations of heat with those of light. The sun and the stars swim in a molecular medium, and 39,180 vibrations or waves must occur in one inch to produce the sensation of red light, and 57,490 undulations in the space of one inch to produce a violet light. As vibrations of the ethereal molecules affect the eye, so there may be other nerves in our bodies which are peculiarly sensitive to the waves of heat. It requires eight vibrations of the air to occur in a second to produce an audible sound; whilst if the vibrations of the air amount to 25,000 per second they cannot be appreciated by the human ear, although it is possible to conceive that the ears of certain animals may be so susceptible of rapid vibrations that they may be able, for certain wise purposes of the Creator, to appreciate sounds which are inaudible to human ears.

Melloni exhibited a spectrum to a number of persons, and found that there was more light apparent to some eyes than to others. Lubeck put a scarlet cloth on a donkey, and found that the two were frequently confounded together by the eyes of many spectators. These facts indicate that there may be vibrations of molecules that produce the sensation of heat, but which do not affect the nerves that are sensitive to the action of light waves, and vice versÂ; and it is also probable that all these different undulations, some affording heat and some light, may be generated and propagated through space, as from the sun; or through shorter distances, as from burning lamps and fires, without in any way interfering with or impeding each other's progress.

The dynamical theory seems to offer the best idea of the transmission of heat which is carried, conducted, or propagated through solids with variable rapidity, either by the vibration of the constituent molecules of the body itself, or by the undulation of a rare subtle fluid which pervades them. If a copper and iron wire of the same length and diameter are bound together and heated at the point of union, the waves of heat travel faster through the copper than the iron, and the former is said to be the best conductor of heat; and the fact itself is demonstrated by placing a bit of phosphorus at the end of each metallic wire, and it will be found by experiment that the combustible substance melts first and takes fire on the copper, and that a considerable interval of time elapses before the phosphorus ignites on the iron.

The same fact is exhibited in a most striking manner by inserting a series of rods of equal lengths and thicknesses in the side of a rectangular box, allowing them to pass across the interior to the opposite side. The rods are composed of wood, porcelain, glass, lead, iron, zinc, copper, and silver, and have attached to each of their extremities, by wax or tallow, a clay marble. When the water placed in the box is made to boil, the heat passes along the different rods, and melting the wax or tallow, allows the marble to drop off. Consequently the first marble would drop from the silver rod, the next from the copper, the third from the iron, the fourth from the zinc, the fifth from the lead, whilst the porcelain, glass, and wooden rods would hardly conduct (in several hours) sufficient heat to melt the wax or tallow, and discharge the marbles.

Conduction of Metals.

The experiment is made more striking if the marbles are allowed to fall on a lever connected with the detent of a clock alarum, which rings every time a marble falls from one of the rods. (Fig. 357.)

During a cold frosty day, if the hand is placed in contact with various substances, some appear to be colder than others, although all may be precisely the same temperature; this circumstance is due to their conducting power: and a piece of slate seems colder than a bit of chalk, because the former is a much better conductor than the latter, and carries away the heat from the body with greater rapidity, and diffuses it through its own substance.

The gradual passage of heat along a bar of iron as compared with one of copper, is well illustrated by supporting the ends of the two bars on the top of the chimney of an argand lamp, whilst the other extremities are held in a horizontal position by little blocks of wood. If marbles are attached by wax to the under side, they fall off as the heat travels along the metallic bars, and more rapidly from the copper than the iron, because the former is a better conductor of heat than the latter. (Fig. 358.)

From the experiments of Mayer, of Erlangen ("Ann. de Ch.," xxx.), it would appear that the conducting powers of different woods are to a certain extent to be regarded as in the inverse proportion to their specific gravities—i.e., the greater the density of the wood the less conducting power, and the contrary.

If a cylindrical bar or thick tube of brass, six inches long, and about two inches in diameter, is attached to a wooden cylinder of the same size, the conducting powers of the two substances are well displayed by first straining a sheet of white paper over the brass, and then holding it in the flame of a spirit lamp. The heat being conducted rapidly away by the metal will not scorch the paper, until the whole arrives at a uniform high temperature; whereas the paper is rapidly burnt when strained over the wooden cylinder, because the heat of the flame of the lamp is concentrated upon one point, and is not diffused through the mass of the wood. (Fig. 359.)

In the course of the highly philosophical experiments of Sir H. Davy, which led him gradually to the discovery of the construction of the safety lamp, he connected together, by a copper tube of a small bore, two vessels, each containing an explosive mixture composed of fire damp and air. When the mixture was fired in one vessel he found that the flame did not appear to be able to travel, as it were, across the bridge—viz., the copper tube—and communicate with the other magazine, because it was deprived of its heat whilst passing through the tube, and was no longer flame, but simply gaseous matter at too low a temperature to effect the inflammation of the mixture in the second box.

A mass of cold metal may be suddenly applied to a small flame, such as that of a night light, and depriving it rapidly of heat (like the case of the unfortunate Russian described at page 354), it is almost immediately extinguished (fig. 360), not by the mere exclusion of the oxygen of the air, but on account of the withdrawal of the heat necessary for the maintenance of the combustion.

Sir H. Davy first thought of making his safety lamp with small tubes, which would supply fresh air, and carry off the burnt or foul air, at the same time they were to be so narrow that no flame could pass out of his lamp to communicate with an outer explosive atmosphere; and in speaking of his lamp with tubes he says:—"I soon discovered that a few apertures, even of very small diameter, were not safe unless their sides were very deep; that a single tube of one-twenty-eighth of an inch in diameter, and two inches long, suffered the explosion to pass through it; and that a great number of small tubes, or of apertures, stopped explosion, even when the depths of their sides was only equal to their diameters. And at last I arrived at the conclusion that a metallic tissue, however thin and fine, of which the apertures filled more space than the cooling surface, so as to be permeable to air and light, offered a perfect barrier to explosion, from the force being divided between, and the heat communicated to an immense number of surfaces. I made several attempts to construct safety lamps which should give light in all explosive mixtures of fire damp, and after complicated combinations, I at length arrived at one evidently the most simple, that of surrounding the light entirely by wire gauze, and making the same tissue feed the flame with air and emit light."

If a number of square metallic tubes of a fine bore are placed upright side by side, and a section cut off horizontally, it would represent the wire gauze which possesses such marvellous powers of sifting away the heat from a flame, so that it is destroyed in its attempted passage through the metallic meshes; and of this fact a number of proofs may be adduced.

A gas jet delivering coal gas may be placed under a sheet of wire gauze, the gas permeates the gauze, and may be set on fire at the upper side, but the flame is cut off from the mouth of the jet by the cooling action of the wire gauze. The same experiment reversed, by holding the gauze over the gas burning from the jet, shows still more decidedly that flame will not pass through the metallic tissue. (Fig. 361.)

Sir H. Davy again says: "Though all the specimens of fire damp which I had examined consisted of carburetted hydrogen mixed with different small proportions of carbonic acid and common air, yet some phenomena I observed in the combustion of a blower induced me to believe that small quantities of olefiant gas may be sometimes evolved in coal mines with the carburetted hydrogen. I therefore resolved to make all lamps safe to the test of the gas produced by the distillation of coal, which, when it has not been exposed to water, always contains olefiant gas. I placed my lighted lamps in a large glass receiver through which there was a current of atmospherical air, and by means of a gasometer filled with coal gas, I made the current of air which passed into the lamp more or less explosive, and caused it to change rapidly or slowly at pleasure, so as to produce all possible varieties of inflammable and explosive mixtures, and I found that iron gauze wire composed of wires from one-fortieth to one-sixtieth of an inch in diameter, and containing twenty-eight wires or seven hundred and eighty-four apertures to the inch, was safe under all circumstances in atmospheres of this kind; and I consequently adopted this material in guarding lamps for the coal mines, when in January, 1816, they were immediately adopted, and have long been in general use."

The remarkable conducting power of wire gauze is further shown by placing some lumps of camphor on a piece of this material, and when the heat of a spirit-lamp is applied on the under side of the gauze, the camphor volatilizes, and as the vapour is remarkably heavy, it falls through the meshes of the gauze, and takes fire; but the most curious and further illustration of the conducting power of the wire meshes is shown in the fact that the fire does not communicate through the thin film of gauze to the lumps of camphor placed upon it.

The camphor may be ignited by applying flame to the upper side of the gauze, showing that, although this substance is so exceedingly combustible, it will not take fire even if placed at no greater distance from flame than the thickness of the wire gauze, provided the latter material is interposed between it and the flame.

A square box made of wire gauze, with a hole at the bottom to admit a candle or spirit-lamp, may have a considerable jet of coal gas forced upon it from the outside, or a large jug of ether vapour poured upon it; and although the box may be full of flame, arising from the combustion of the gas or ether, the fire does not come out of the wire box or communicate with the jet or the ether vapour as it is poured from the jug. (Fig. 362.)

Sir Humphrey Davy's safety lamp consists of a common oil-lamp, f, with a wire through the cistern for the purpose of raising or depressing the cotton wick without unscrewing the wire gauze; b is the male screw fitting the screw attached to the cylinder of wire gauze, which is made double at the top. The entire lamp is shown at a, whilst the platinum coil which Sir H. Davy recommends should be wound round the wick is shown at h. The small cage of platinum consists of wire of one-seventieth to one-eightieth of an inch in thickness, fastened to the wire for raising or depressing the cotton wick, and should the lamp be extinguished in an explosive mixture, the little coil of platinum begins to glow, and will afford sufficient light to guide the miner to a safe part of the mine. With respect to this platinum coil, Sir H. Davy gives a careful charge, and says:—"The greatest care must be taken that no filament or wire of platinum protrudes on the exterior of the lamp, for this would fire externally an explosive mixture."

Sir Humphrey Davy's safety lamp.

Since the invention of the Davy lamp, a great number of modifications have been brought forward, some of which for a short time have occupied the public attention, but whether from increased cost or a sort of inertia that arrests improvement, it is certain that the lamp originally devised by Sir Humphrey Davy is still the favourite. It was perhaps unfortunate that the lamp was called the safety lamp, because it is not so under every circumstance that may arise, unless it happens to be in the hands of persons who have taken the trouble to study it and understand how to correct the faults. The lamp might have escaped the incessant attacks that have been made upon its just merits, if the name had simply been that of its illustrious inventor—"a Davy lamp." No one could carp at that, whilst "safety" was held to mean perfect immunity from every possible and probable danger that might arise in the coal-pits. The lamps are now usually placed under the charge of one man, who trims them and ascertains that the wire gauze is in perfect order; this latter is usually locked upon the lamp, and as it is a penal offence, and punishable by a heavy fine and imprisonment, to remove the wire gauze from safety lamps in dangerous parts of the mine, of course the miners are being gradually brought to a sense of the obligations they owe themselves and their brother-miners, and the rash, ignorant, and foolhardy offences of breaking open safety lamps for more illumination, or to light pipes, are becoming much less frequent than formerly. One of the most ingenious "detector lamps" is that of Mr. Symons, of Birmingham. (Fig. 364.) It consisted of the old-fashioned Davy, but inside the rim of the wire gauze is placed a small extinguisher and spring, which does not move so long as the gauze is screwed on to the lamp, but directly the gauze is unscrewed, the reversed movement releases the detent, and the extinguisher falls upon the light. In spite of the manifest ingenuity of this lamp, it is not adopted, because it costs a trifle more than the ordinary "Davy." To show the remarkable perfection of the wire gauze principle, some turpentine may be poured upon a lighted safety lamp, when a great smoke is produced by the evaporation of the spirit, but no flame passes through to the outside, although the turpentine burns inside the lamp. If some coarse gunpowder is laid upon two thicknesses of fine wire gauze, it may be heated from below with the flame of the spirit lamp, and the sulphur will gradually volatilize without setting fire to the mass of powder. To show the security of the Davy lamp, it may be lighted and hung in a large box with glass sides, open at the top, and a jet of coal gas supplied at the bottom; as this rises and diffuses in the air, the mixture becomes explosive, and the fact is at once evident by the alteration in the appearance of the flame of the lamp, which enlarges, flickers, and frequently goes out, in consequence of the suddenness with which the explosion of the mixture takes place inside the lamp, producing a concussion that extinguishes the flame. In this case the utility of the platinum coil is very apparent, and it continues to glow with a red heat until the explosive character of the air in the box is changed.

Symons' self-extinguishing Davy lamp.

If a large washhand-basin is first warmed by some boiling water, which is then poured away, and a drachm of ether thrown in, a highly-combustible atmosphere is obtained, and when a lighted Davy lamp is placed into the basin so prepared, the flame inside the lamp immediately enlarges and flickers, but is not extinguished, and does not communicate to the combustible vapour outside. The contrast between the safety lamp and an unprotected flame is very striking; if a lighted taper is thrust into the basin, the ether catches fire, and burns with a very large flame. The solid conductors of heat, which are said to enjoy this property in the highest degree, are the metals, marble, stone, slate, and other dense and compact solid substances; whilst the opposite quality of being non-conductors, or nearly so, is possessed by fur, wood, silk, cotton, wool, eider and swansdown, paper, sand, charcoal, and every substance which is of a light or porous nature. The practical application of this knowledge is very apparent in the affairs of every-day life. Thus we rise in the morning, and immediately after the necessary ablutions, if it is winter time, proceed to encase the body in non-conductors, such as flannel and wool. When we sit down to the breakfast table to make tea, we may notice the contrivances for preventing the handle of the top of the urn, or that of the teapot, from becoming too hot for the fingers, by the interposition of ivory or wood. If asked to place water in the teapot from the kettle, we instinctively seek for the well-worn kettle-holder made of Berlin wool, and therefore a bad conductor. As we cut our meat or fish at the same meal, we may shiver with cold, but our fingers are not quite frozen by contact with the steel knives, as we hold them by ivory handles; and we are agreeably reminded that some metals are good conductors of heat, by the pleasant warmth of the silver teaspoons, as we stir our tea or coffee.

Even the polish of the well-rubbed mahogany is protected from the heat of the dishes by non-conducting mats, and plates are handed about, if "nice and hot," with a carefully-wrapped non-conducting linen napkin. Supposing we prefer a bit of fresh-made toast, the fork is provided with a non-conducting handle; and should we peep out of window some wintry morn whilst the baker delivers his early work in the shape of hot rolls, we notice they come out of nicely-wrapped flannel or baize, which being a bad conductor is employed to retain their heat. We read, occasionally, in the military intelligence, statements respecting some newly-constructed shells which are to burst and scatter melted iron (!!); and of course the idea of the interposition of a good non-conductor of heat between the bursting charge and the molten metal must be realized in their construction.

The central heat of our globe is a reality that cannot be disputed, and after digging beyond a depth of twenty feet the thermometer gradually rises at the rate of one degree of Fahrenheit's scale for every fifteen yards. The bad conducting power of the crust of the earth must, therefore, be apparent, as it is easy, knowing the diameter of our globe, to calculate that the increase of heat downwards amounts to 116° for each mile, consequently at a depth of thirty and a half miles below the surface, there will be a temperature most likely equal to 3500°, or a heat that might easily melt cast-iron, and would help to account for the earthquakes and eruptions of volcanoes, which still remind us by their terrible warnings, that we live only on the bad conducting upper crust of a globe, the inside of which is still, perhaps, in a liquid and molten state. Monsieur Fourier has demonstrated the non-conducting power of this shell by calculating that, supposing the globe was wholly composed of cast-iron, the central heat would require myriads of years to be transmitted to the surface from a depth of 150 miles; and by inverting the process of reasoning, we may come to the conclusion that the internal heat must be excessive, because it is confined and shut out from those influences that would carry off and weaken the intensity.

There are no two words, says Tyndal, with which we are more familiar than matter and force. The system of the universe embraces two things, an object acted upon, and an agent by which it is acted upon; the object we call matter and the agent we call force. Matter, in certain respects, may be regarded as the vehicle of force; thus, the luminiferous ether is the vehicle or medium by which the pulsations of the sun are transmitted to our organs of vision. Or, to take a plainer case, if we set a number of billiard balls in a row, and impart a shock to one end of the series in the direction of its length, we know what will take place; the last ball will fly away, the intervening balls having served for the transmission of the shock from one end of the series to the other. Or we might refer to the conduction of heat. If, for example, it be required to transmit heat from the fire to a point at some distance from the fire, this may be effected by means of a conducting body—by a poker, for instance; thrusting one end of a poker into the fire, it becomes heated, the heat makes its way through the mass, and finally manifests itself at the other end. Let us endeavour to get a distinct idea of what we here call heat; let us first picture it to ourselves as an agent apart from the mass of the conductor, making its way among the particles of the latter, jumping from atom to atom, and thus converting them into a kind of stepping stones to assist its progress. It is a probable conclusion, even had we not a single experiment to support it, that the mode of transmission must, in some measure, depend upon the manner in which those little molecular stepping stones are arranged. But we must not confine ourselves to the molecular theory of heat. Assuming the hypothesis, which is now gaining ground, that heat, instead of being an agent apart from ordinary matter, consists in a motion of the material particles; the conclusion is equally probable that the transmission of the motion must be influenced by the manner in which the particles are arranged. Does experimental science furnish us with any corroboration of this inference? It does. More than twenty years ago MM. De la Rive and De Candolle proved that heat is transmitted through wood with a velocity almost twice as great along the fibre as across it. This result has been recently expanded, and it has been proved that this substance possesses three axes of calorific conduction; the first and greatest axis being parallel to the fibre; the second axis perpendicular to the fibre and to the ligneous layers; while the third axis, which marks the direction in which the greatest resistance is offered to the passage of the heat, is perpendicular to the fibre and parallel to the layers.

If many solids are bad conductors of heat, they are at all events greatly surpassed by fluids, and especially by water. The conduction of heat by that fluid is almost imperceptible, so much so, that it has even been questioned whether liquids do really conduct heat downwards at all. It has, however, been found that liquid mercury will conduct heat downwards, and therefore by analogy it may be assumed that other liquids must possess a conducting power, although it may be exceedingly limited.

In order to prove that water is an exceeding bad conductor of heat, a tube with a large glass bulb blown at one end is partly filled with tincture of litmus, until it will just sink below the surface of water placed in a tall cylindrical or open jar. If a copper basin, containing burning ether, is now floated on the top of the water, so as to leave about a quarter of an inch between the top of the air thermometer—viz., the bulb containing the coloured liquid—and the bottom of the copper pan, it will be noticed that whilst the water surrounding the latter almost boils, not the slightest effect arising from the conduction of heat can be perceived in a downward direction. After the ether has burnt out of the copper vessel, it may be removed, and the boiling water stirred down and around the air thermometer, when the air within it expands, drives out the colouring liquid, and the bulb becoming specifically lighter, rises to the top of the containing glass. (Fig. 365.)

Again, if the tube of an air thermometer is placed through a cork in the neck of a gas jar, inverted and standing on a ring stand, and the jar is then filled with water, and boiled at the top with a red-hot iron heater, the heat does not pass downwards and affect the thermometer. By introducing a syphon the water surrounding the thermometer at the bottom of the jar may be drawn off, until the hot water is within a fraction of an inch of the air thermometer, and still no heat is conducted, and the liquid in the latter remains stationary. (Fig. 366.)

The diffusion of heat through water does not take place like that of solids, but is effected by the motion of the particles of the water. When heat is applied to the bottom of a vessel containing water, such as an inverted glass shade, the first effect is to expand the layer of water which is first affected by the heat; this expanded layer being specifically lighter than the cold water above, it rises to the upper part of the glass shade, and its place is immediately taken by other, colder and heavier, water, which in like manner moves upwards, and is again succeeded by a fresh portion. Now, the first and succeeding strata of water all carry off so much heat, and thus by the convective or carrying power of the water the heat is diffused finally in the most perfect manner through the whole bulk of fluid; and indeed, the movement itself of the particles of water may easily be watched by putting a little paper pulp at the bottom of the inverted glass shade containing the water. (Fig. 367.)

This bad conducting power is not merely confined to water, but is likewise apparent with oil and other fluids, and if some water is frozen at the bottom of a long test-tube by means of a freezing mixture, oil may then be poured upon it, and some alcohol above the latter. If the flame of a spirit-lamp is now applied to the alcohol at the top of the tube it may be entirely boiled away, and no heat will travel down the oil and communicate with the ice, and even after the alcohol has been evaporated away the tube can be filled up with water; this may also be boiled, and whilst demonstrating the bad conducting power of the oil, the curious anomaly is observed of a vessel or tube containing ice at the bottom and boiling water at the top, and further showing the wisdom of the Supreme Creator in preventing the freezing of the water of lakes, rivers, and seas, by the exceptional law of the expansion of water by cold. It is evident from what has been stated that liquids acquire and lose their heat by means of those currents and movements of the particles of water which have already been partly explained. Whatever interferes with this movement must prevent the passage of heat, and consequently thick viscous liquids are always difficult to boil, and in consequence of their motion being impeded they rise to too high a temperature and are burnt. This fact is remarkably apparent in the manufacture of nice white lump sugar; as the syrup is evaporated it becomes very thick, and if boiled over a fire might frequently be burnt, but it is boiled by the heat of steam, and under a vacuum produced by an air-pump, and thus the sugar-boiler is enabled to avert all danger from burning.

It is, then, by a continual and perpetual motion, involving circulation of the particles, that heat travels through water; and the fact already described is still further elucidated by one of Professor Griffith's simple but telling experiments. A glass tube, about three feet in length and half an inch in diameter, is bent as at A (Fig. 368), and then being filled with water, is suspended by a string attached to any convenient support inside a copper dish containing water, so that the straight end is at the top of the water, and the curved end at the bottom. Just before it is used some ink or other colouring matter is poured into the copper pan of water; and it should not be added till the moment the experiment is to begin, as any rise of temperature in the room promotes circulation, and interferes with the colourlessness of the water in the tube, which is compared with the inky fluid in the basin. Directly heat is applied the hot water rises to the top of the copper vessel, and thence gradually up the tube; and this movement is rendered visible by the hot coloured liquid matter creeping slowly up the tube, and displacing the colourless water, which falls gradually into the copper pan. (Fig. 368.)

The principle of the circulation of the particles of water being once understood, it is easy to comprehend how it is applied to the heating of buildings by what is called the "Hot Water Apparatus." A coil of pipe is enclosed in a proper furnace, and the bottom end communicates with a pipe coming from a second tube or set of coils, placed above it in another apartment, whilst the top of the latter coil communicates with the top pipe of the first coil. When the fire is lighted, the circulation through the first coil of pipe commences, and is communicated to the second, and from that back again to the first; so that the "hot water system" involves an endless chain of pipes of water, provided with proper safety valves to allow for the escape of any expanded air or steam; and serious accidents have occurred in consequence of persons neglecting to look after the perfection of this safety valve. The fearful accident which occurred to the hot water casing around one of the funnels of the Great Eastern offers a painful but memorable example of the heating of water, and of the dangers that must arise if the pipe, casing, or other vessel which contains it, is not provided with an escape or safety valve, which must always be in good working order.

Mr. Jacob Perkins, in 1824, made his name remarkable for experiments with the circulation of water through tubes, and his account of the invention and improvement of the "Steam Gun," in which the improvement consists chiefly in the circulation of water through coils of pipe, is so important that we give it verbatim, with a drawing of the steam gun; and the author is enabled to vouch for the accuracy of the statements made in the description of the apparatus, as he purchased one of the improved steam guns, and exhibited it at the Polytechnic Institution, where it discharged three hundred bullets per minute.

The charging tube and gun-barrel of steam gun.

"The expansive power of steam has often been proposed as a substitute for gunpowder, for discharging balls and other projectiles; the great danger, however, which was formerly thought to be inseparably connected with the generation and use of steam, at so extraordinary a pressure as appeared necessary to produce an effect approximating to that of gunpowder, prevented scientific men from testing the power of this new agent by experiment. It was also apparent that the apparatus which was ordinarily used for generating steam for steam-engines was wholly inadequate to sustain the necessary pressure, and that one of a totally different character must be contrived before steam could be sufficiently confined to come into competition with its powerful rival.

"In the year 1824, Mr. Jacob Perkins succeeded in constructing a generator of such form and strength, as allowed him to carry on his experiments with highly elastic steam without danger, although subjected to a pressure of 100 atmospheres. The principle of its safety consisted in subdividing the vessel containing the water and steam into chambers or compartments, so small, that the bursting of one of them was perfectly harmless in its effects, and only served as an outlet, or safety valve, to relieve the rest.

"Although Mr. Perkins' generator was originally intended for working steam engines (it having long been evident to him that highly elastic steam used expansively would be attended with considerable economy), the idea occurred to him, in the course of his experiments, that he had already solved the problem of safely generating steam of sufficient power for the purposes of steam gunnery; and that the steam which daily worked his engine possessed an elastic force quite adequate to the projection of musket balls. He therefore caused a gun to be immediately constructed, and connected by a pipe to the generator, the first trial of which fully realized his most sanguine anticipations. Its performance, indeed, was so extraordinary and unexpected, that it gave rise to a paradox, which was difficult of explanation—viz., that steam, at a pressure of only forty atmospheres, produced an effect equal to gunpowder; whereas it was known that the combustion of gunpowder was attended with a pressure of from 500 to 1000 atmospheres.

"Mr. Perkins gives the following explanation of this apparent discrepancy, by referring to the small effect produced by fulminating powder, compared to gunpowder, although many times more powerful; he supposes that the action of fulminating powder, however intense, does not continue sufficiently long to impart to the ball its full power. The explosion of gunpowder, although not so powerful at the instant of ignition, is nevertheless, in the aggregate, productive of greater effect than that of fulminating powder, because the subsequent expansion continues in action upon the ball (but with decreasing effect), until it has left the barrel. The action of steam differs from either of these agents, inasmuch as it continues in full force until the ball has left the barrel; and to this is assigned the cause of its superiority.

"In the year 1826, Mr. Perkins had so perfected the mechanism of the gun and generator that, at an exhibition and trial of its power, in the presence of the Duke of Wellington and other distinguished officers of the Ordnance Department, balls of an ounce weight were propelled, at the distance of thirty-five yards, through an iron plate one-fourth of an inch in thickness; also, through eleven hard planks, one inch in thickness, placed at distances of an inch from each other. Continuous showers of balls were also projected with such rapidity, that when the barrel of the gun was slowly swept round in a horizontal direction, a plank, twelve feet in length, was so completely perforated, that the line of holes nearly resembled a groove cut from one of its ends to the other.

Perkins's steam gun.

"a is an iron furnace, containing a continuous coil of iron tubing, 80 feet in length, 1 inch of external and 5/8th inch of internal diameter, within which the fire is made; the upper end of this tube, b, called the flow-pipe, is extended any required distance to the top of the generator.

"The furnace is provided with a very ingenious heat governor or regulator, by which the intensity of the fire is always proportionate to the temperature which it may be requisite to maintain in the tubes.

"h is an iron box, containing a series of levers, b b b; c, a nut screwed upon the flow-pipe, and in contact with the short arm of the lowest of the levers. e. A lever, from one end of which is suspended the damper f, and from the other end the rod g, which rests upon the long arm of the highest of the levers, b b b. When the apparatus has arrived at the required temperature, the nut c is screwed down until it bears upon the lever. Any farther increase of temperature will expand or lengthen the flow-pipe, and depress the short arm of the lever, which is in contact with the nut. The combined and multiplied action of the levers will then elevate the rod g, and the damper f will descend to check the draught. When the fire slackens, and the apparatus cools, the action of the levers will be reversed, and the damper will open. The space through which the damper moves, compared with the nut c, is as 200 to 1.

"c is the generator, composed of a strong iron tube, 3 inches diameter and 6 feet in length, within which are eight smaller tubes, having their ends welded to the ends of the larger tube. These small tubes communicate at the top with the flow-pipe b, and at the bottom with the return-pipe d, which is continued to the bottom of the furnace-coil of tubing. The circulation in the tubes is occasioned by the difference in the specific gravities of the water composing the ascending and descending currents; the portion contained in the flow-pipe and fire coil becoming expanded by the heat, ascends by its superior levity; while that contained in the small tubes of the generator, having given off its heat, acquires increased density, and descends through the return-pipe d to the bottom of the furnace-coil, to take the place of the ascending current. When the hot-water current has arrived at a temperature of 212° and upwards, cold water is injected into the generator, and becomes converted into steam by its contact with the small tubes; the rapidity of evaporation and the pressure of the steam depending, of course, upon the temperature of the hot-water current, which at 500° will cause a pressure within the tubes of 50 atmospheres, or 750 lbs. upon the square inch. The whole apparatus is proved to be capable of sustaining a pressure of 200 atmospheres, or 3000 lbs. upon the square inch.

"g. A force pump for injecting water into the generator.

"i. The indicator for exhibiting the pressure of the steam in the generator, and of the water in the boiler; it may be connected with either by means of the valves attached to the levers.

"j. Valve to regulate the pressure of water.

"j l. Valve to regulate the pressure of steam.

"k. The steam pipe.

"l. The gun.

"m. The discharging lever acting upon the valve n.

"o. The discharging cock, by a simple adjustment in which balls are transferred from the charging tube p to the gun barrel, singly or in a continuous shower.]

"As the perfection and introduction of the steam gun was not a field for private enterprise, and the British Government having declined to institute experiments at its own expense, Mr. Perkins was reluctantly compelled to leave the project, and to engage in others of a more lucrative, although, perhaps, of a less important nature. He did not suspend his operations, however, until he had constructed for the French Government a piece of artillery which discharged balls weighing five pounds at the rate of sixty per minute.

"The gun and generator exhibited at the Polytechnic Institution during the time that Mr. Pepper was the Resident Director were the production of Mr. A. M. Perkins, of London, who has invented an entirely new method of generating steam, which has been successfully applied to steam engines, and is at once so simple, safe, and economical, as to leave little doubt that, with its aid, the steam gun will ere long rank amongst the first instruments of warfare.

"The gun, except in a few minor mechanical details, does not differ from that originally constructed by Mr. Jacob Perkins.

"The novelty which distinguishes the generator from all others, consists in the manner of conveying the heat from the fire to the water, without exposing the generator to the action of the fire. This is accomplished by means of the circulation, in iron tubes, of a current of hot water, which is entirely separate from, and independent of, that to be evaporated in the generator.

"The following are the principal advantages which this generator possesses over all others: Freedom from all wear or deterioration consequent upon exposure to the fire, an important quality in a generator that is to be subjected to great pressure, inasmuch as its original strength remains unimpaired; no accident can arise from want of water in the generator, and the precautions indispensably requisite when a generator is in contact with the fire are quite unnecessary, as the water may be drawn off with impunity without producing the least injurious effect, and the grossest neglect is followed by no worse consequences than an inefficient supply of steam; an explosion of the generator is impossible, as the temperature of the furnace-coil always exceeds that of any other part of the apparatus, and consequently, being the weakest part, is invariably the first to yield when the pressure is carried beyond the strength of the pipes; economy of fuel is also obtained, with a small amount of fire surface. The circulation of the water has likewise the effect of preserving the fire-coil from the decay to which boilers are liable; many such coils, which have been in constant use for eight years, being apparently as good as when first erected.

"The whole apparatus is exceedingly simple, and will be readily understood by reference to the accompanying diagram. (Fig. 370.)

"The steam has often been raised to a pressure of 700 lbs. on the square inch, but one-third of that pressure is sufficient to completely flatten the balls when discharged against an iron target one hundred feet distant from the gun; and a pressure of 400 lbs. per square inch, at the same distance, shivers the ball to atoms, with the production in a dark room of a visible flash of light. Steam guns are generally mounted upon a ball and socket joint, which allows the barrel to move freely in every direction."

The conduction of heat through gases is also very slow when heat is applied to the upper part of any stratum of air. Heat appears to be diffused through air only by the circulation and rising of the heated and lighter strata, and the sinking of the colder currents which take their places; hence the danger of sitting in a room under an open skylight. A current of cold air may descend upon the head of the individual, whilst the warmer air takes some other opening to escape from. No doubt the movement of heated volumes of air is subject to definite laws, which apply themselves under every case, but are rather difficult to grasp when the subject of ventilation is concerned. The philosophical ventilator is often dreadfully teased by the inversion of all that he had planned, or the total failure of his apparatus. No specific mode of ventilation can be found to suit all rooms and buildings; they are like the patients of a physician who cannot be cured by one medicine only, but must have a treatment adapted properly to each case. If the fires, candles, gas, or oil-lamps, doors, windows, and chimneys, were always under the control of the scientific ventilator, his task would be very simple, but it is well understood that a ventilating system which answers well if certain doors communicating with lobbies are closed, fails directly they are accidentally opened. The watchful care of the ventilator must begin with the lowest area door, and in his calculations he must study the effect of every other door or window that may be opened, so that if a scientific man undertakes to ventilate a house, he must have a well-drawn plan hung up in the hall, and it must be clearly understood by the inmates that any interference with that plan will prejudice the whole.

There are a few common principles which will guide in ventilation, and these are, first, the rise of hot and the fall of cold air; second, that if an aperture is provided at the top of a room for the escape of hot air, an equally large aperture must be left for the entry of cold air; third, the aperture for the escape of hot air must be adapted in size to the number of persons likely to enter the room, and the number of gas or other lights burning in it. During the daytime, moderate apertures for the exit and entrance of air may suffice, but these must be largely increased at night, when the room is filled with people and lighted up. Expanding and contracting openings are therefore desirable, and they are to be regulated by rules stated on the plan of the ventilating system (already alluded to as being hung up in the hall) of the house which has submitted itself to a perfect system of ventilation, and no hall-keeper, footman, or butler should be allowed to remain in his post unless he undertakes to comprehend the system and work it properly by the written rules.

Dr. Angus Smith, in a very able paper "On the Air of Towns," says—"One of the conditions of health, and a most important, if not the most important of all, is to be found in the state of the atmosphere. As to the effect on the inhabitants, the question becomes exceedingly complicated; but the Registrar-General's returns are an unanswerable reply as to the results of the lethal influences of the district. Few people seem clearly to picture to themselves the meaning of a decimal plan in the percentage of death, and few clearly see that there are districts of England where the deaths at least in some years, and when no recognised epidemic occurs, are three times greater than in others. When we hear of the annual deaths in some districts being 3.4 per cent., and in the whole of England 2.2, it is simply that 34 die instead of 22, whilst even that is too slightly stated, as the whole of England would show a lower death-rate if the towns were not used to swell it."

This quotation is given here to remind our readers of the important question of a supply of pure air as well as pure water and pure food; and if the agricultural labourer, with all his exposure to variable weather, can take the first place in the scale of mortality, and outlive the members of all other trades and professions, it is evident that the importance of pure air is not overrated.

Every effort ought, therefore, to be made in large schools, hospitals, and barracks, to enforce a rigid system of supply of fresh air, and a sewage or removal of the impure; and in the use of a certain test employed by Dr. Smith for the detection of organic matter in the air a number of approximations were obtained, which clearly demonstrated that 1 grain of organic matter was detected in 72,000 cubic inches of air in a room, and the same quantity in 8000 cubic inches taken from a crowded railway carriage.

To show the rising of heated air, a long glass tube, about three-quarters of an inch in diameter, may be provided and held over the flame of a spirit lamp at an angle of sixty degrees. As the tube warms, the heated air rushes past the flame with great rapidity, and pulls it out or elongates it so much, that the sharp point of the spirit-flame will frequently be seen at the end of a tube ten feet six inches in length. The flame is, as it were, the sign-post that indicates the path or direction of the air. (Fig. 371.)

Upon the like principle, heated air may be dragged down the short arm of a syphon, provided the other arm is sufficiently long to impart a strong directive tendency to the upward current, and this mode of setting air in motion has been frequently proposed in numerous schemes for ventilation. In order to prove the fact that an inverted syphon will act in this manner, an iron pipe of three inches diameter and six feet long may be bent round during the construction into the form of a syphon, so that the short length is about one foot long, and the long length the remaining four feet, allowing one foot for the bend. If the interior of the long arm is first warmed by burning in it a little spirits of wine from a piece of cotton or tow wetted with the latter (which can be easily done by dropping in such a wetted piece into the bend of tube, so that it is just under the opening of the long part of the tube), the air is soon set in motion up the long pipe, and as it must be supplied with fresh volumes of air to take the place of that which rises, and as the only entrance for the fresh air can be down the short arm of the syphon, the circulation soon commences, and it proceeds as long as the upper arm is kept sufficiently warm. If a flame is held over the mouth of the short arm, it is immediately dragged downward, whilst, if held at the mouth of the long pipe, the motion of the air is seen by the assistance of the flame to be in the contrary direction. (Fig. 372.)

This plan of ventilation was proposed to be used in rooms in connexion with the chimney and chimney-piece, and in order to give it an ornamental appearance, the chimney-piece was supplied with two ornamental hollow columns, the ends of which were open at the mantel-shelf, and the tubes or columns were continued under the hearthstone, proceeding up the back of the grate and entering the chimney, in which there would be a constant current of heated air, and it was expected that the syphon arrangement would keep a current of air always in motion, and thus help to ventilate the room. (Fig. 373.) This plan, however, does not appear to have been adopted, and wisely so, because half the time the syphon arrangement might invert itself, and vomit smoky air out of the chimney into the room; indeed it is surprising what odd and contradictory freaks are performed by currents of air. The author remembers a case where two rooms on the same floor, the one a dining-room and the other a drawing-room, were always exhibiting the most absurd phenomena of smoke. If the fire in one room was lit, then the other, in a few moments, began to smell exactly like the inside of a gas manufactory, and was, of course, more or less filled with smoke, whilst the room in which the fire was actually burning remained quite free from this annoyance. The smoke appeared to issue from the wainscot or moulding which runs round at the bottom of the wall, and was at first thought to be an escape from the chimney of the kitchen beneath, the inside of which was duly examined and thoroughly stopped with cement in every place likely to afford a channel to the smoke, and the crevice whence the smoke issued was also filled in neatly with cement. But it was all in vain; the smoke then made its way out from another part of the cornice, and at last the rooms exhibited a beautiful reciprocating action. If the drawing-room fire was lighted the dining-room was full of smoke, and if the latter was lighted the former had the agreeable visitation. At last the backs of the two grates were examined, and in each was discovered a hole about one inch in diameter; and it was also found that the spaces at the back of the stoves had not been filled in properly, and, indeed, communicated with the hollow space behind the cornice. When, therefore, the fire was lighted, and coals heaped on just above the hole, the gas and smoke distilled through the orifice and travelled on, where it found the most convenient exit; and the fact is sadly at variance (apparently) with theory, because it might be considered that cold air would rush towards a fire, and that the draught ought to have been from the cornice to the chimney instead of vice versÂ. The fact seems to be that the coal in all grates is, in the act of burning, distilling and giving off inflammable gas; when the coal was, therefore, heaped above the orifice, and was, possibly, caked hard at the top, the gas distilling from it escaped more easily from the little orifice than elsewhere, and chance determined that the channel or delivery pipe should be in the direction of the drawing-room when the fire was burning in the dining-room, and in the contrary direction when the fire was lighted in the latter chamber. The nuisance was stopped by plugging the holes at the back of the grate with clay, and putting a sheet of iron over the orifice.

Before Dr. Faraday was appointed as a scientific counsellor to assist the deliberations of the Trinity Board in connexion with lighthouses, all the lamps were burnt in the lanterns with the smallest and most imperfect arrangement for carrying off the heated air and products of combustion; as a natural consequence, and particularly on cold nights, the windows of the lantern of the lighthouse were covered with ice derived from the condensation of the water produced by the combustion of the hydrogen of the oil, whilst the carbon generated such quantities of carbonic acid that the light-keepers were unable to stay in the lantern, and if obliged to visit the latter (whilst looking to improving the light of any single lamp that might be burning dimly), they were almost overpowered with the excess of carbonic acid, and stated, in their evidence, that it produced headache and sickness, and a tendency to insensibility. Faraday immediately established a system of ventilation; and by attaching a copper tube to the top of each lamp-chimney, and centering them all in one large funnel passing to the top of the lighthouse, the whole of the water which previously condensed on the glass windows and impeded the light, besides injuring the brass and copper fittings, was carried off, as also the poisonous carbonic acid gas; and thus, as Dr. Faraday expressed himself, a complete system of sewage was applied to the lamps of the lighthouses.

If any one of the numerous stories of ships saved by the Eddystone Lighthouse could demonstrate more than another the value of this beacon in mid ocean, it must be the graphic account in the Times of the gallant conduct of the British Admiral with his fleet whilst breasting the frightful storm of October, 1859, and endeavouring to reach Plymouth Sound:—

"It was on Saturday, the 22nd October, that the Hero, the Trafalgar, the Algiers, and the Aboukir, accompanied by the Mersey, the Emerald, and the Melpomene, put to sea from Queenstown. Up to the afternoon of Monday the squadron met with no remarkable adventure, but about that time, just after the crews had been exercised at gunnery practice, heavy storms of hail and sleet began to set in. Still there was no immediate indication of the tempest at hand, and at sunset topsails were double-reefed and courses reefed for the night, with no particular character about the wind, except that of extreme variability. As the morning broke on Tuesday—the day of the storm—the Land's-end was sighted, and the rain and the wind continued to increase. About nine a.m. the advent of the gale was no longer doubtful; topgallantyards were sent on deck and topgallantmasts struck, and the signal was given from the flagship, 'Form two columns; form line of battle; Admiral will endeavour to go to Plymouth.' To Plymouth, accordingly, the course of the fleet was shaped, but so terrifically had the wind increased that it became very questionable whether the sternmost ships of the line could possibly succeed in entering the Sound. Upon this the Admiral determined to wear the fleet together, stand off, and face the storm, a manoeuvre which, under circumstances of great difficulty, was most gallantly executed. The ships were close upon the Eddystone Lighthouse, round which they 'darted like dolphins' under the tremendous pressure of the gale, the Trafalgar stopping in the midst of the storm to pick up a man who had fallen overboard. The whole squadron now stood off the land, the Mersey and Melpomene furling their sails, and the former vessel steaming along 'like an ocean giant.' Still the gale increased till about three p.m., when there occurred that remarkable phenomenon by which these rotatory tempests are characterized. The fleet had got into the very centre of the storm, the 'eye' of the tornado, and, though the sea towered up and broke in tremendous billows all around, the wind suddenly ceased and the sun shone. When, however, the signal had been given and obeyed for setting sail again, the ships soon encountered the gale once more—not, as before, from the S.E., but the N.W.—and in greater force than ever. It was now a perfect hurricane; and for three hours the whole fury of the tempest was poured upon the squadron. When it began, at length, to abate a little, the four line-of-battle ships and one of the frigates were still in company, and all doing well. The Mersey and the Emerald had steamed into Plymouth, but the five remaining vessels kept in open order throughout that terrible night, wore in succession by night signal at about one a.m., made the land at daylight, formed line of battle, came grandly up Channel under sail at the rate of eleven knots an hour, steamed into Portland, and 'took up their anchorage without the loss of a sail, a spar, or a ropeyarn.'"

After making the important improvement in the ventilation of lighthouses, many letters were addressed to the learned philosopher by numerous light-keepers, one of which in plain but striking language related that "the enemy (alluding to the water and carbonic acid) was now driven out."

The British fleet rounding Eddystone Lighthouse during the great storm of October, 1859.