STEAM NAVIGATION.

[Pg441]
TOCINX

The manner in which the steam-engine is rendered an instrument for the propulsion of vessels must in its general features be so familiar to every one as to require but short explanation. A shaft is carried across the vessel, being continued on either side beyond the timbers: to the extremities of this shaft, on the outside of the vessel, are fixed a pair of wheels constructed like undershot water-wheels, having attached to their rims a number of flat boards called paddle-boards. As the wheels revolve, these paddle-boards strike the water, driving it in a direction contrary to that in which it is intended the vessel should be propelled. The moving force imparted to the water thus driven backwards is necessarily accompanied by a re-action upon the vessel through the medium of the paddle-shaft, by which the vessel is propelled forwards. On the paddle-shaft two cranks are constructed, similar to the cranks already described on the axle of the driving wheels of a locomotive engine. These cranks are placed at right angles to each other, so that when either is in its highest or lowest position the other shall be horizontal. They are driven by two steam-engines, which are placed in the hull of the vessel below the paddle-shaft. In the earlier steam-boats a single steam-engine was used, and in that case the unequal action of the engine on the crank was equalised by a fly-wheel. This, however, has been long [Pg443] since abandoned in European vessels, and the use of two engines is now almost universal. By the relative position of the cranks it will be seen, that when either crank is at its dead points, the other will be in the positions most favourable to its action, and in all intermediate positions the relative efficiency of the cranks will be such as to render their combined action very nearly uniform.

The steam-engines used to impel vessels may be either condensing engines, similar to those of Watt, and such as are used in manufactures generally, or they may be non-condensing and high-pressure engines, similar in principle to those used on railways. Low-pressure condensing engines are, however, universally used for marine purposes in Europe and to some extent in the United States. In the latter country, however, high-pressure engines are also in pretty general use, on rivers where lightness is a matter of importance.

The arrangement of the parts of a marine engine differs in some respects from that of a land engine. The limitation of space, which is unavoidable in a vessel, renders greater compactness necessary. The paddle-shaft on which the cranks to be driven by the engine are constructed being very little below the deck of the vessel, the beam and connecting rod could not be placed in the position in which they usually are in land engines, without carrying the machinery to a considerable elevation above the deck. This is done in the steam-boat engines used on the American rivers; but it would be inadmissible in steam-boats in general, and more especially in sea-going steamers. The connecting rods, therefore, instead of being presented downwards towards the cranks which they drive, must, in steam-vessels, be presented upwards, and the impelling force received from below. If, under these circumstances, the beam were in the usual position above the cylinder and piston-rod, it must necessarily be placed between the engine and the paddle-shaft. This would require a depth for the machinery which would be incompatible with the magnitude of the vessel. The beam, therefore, of marine engines, instead of being above the cylinder and piston, is placed below them. To the top of the [Pg445] piston-rods cross pieces are attached of greater length than the diameter of the cylinders, so that their extremities shall project beyond the cylinders. To the ends of these cross pieces are attached by joints the rods of a parallel motion: these rods are carried downwards, and are connected with the ends of two beams below the cylinder, and placed on either side of it. The opposite ends of these beams are connected by another cross piece, to which is attached a connecting rod, which is continued upwards to the crank-pin, to which it is attached, and which it drives. Thus the beam, parallel motion, and connecting rod of a marine engine, is similar to that of a land engine, only that it is turned upside down; and in consequence of the impossibility of placing the beam directly over the piston-rod, two beams and two systems of parallel motion are provided, one on each side of the engine, acted upon by, and acting on the piston-rod and crank by cross pieces.

The proportion of the cylinders differs from that usually observed in land engines, for like reasons. The length of the cylinder of land engines is generally greater than its diameter, in the proportion of about two to one. The cylinders of marine engines are, however, commonly constructed with a diameter very little less than their length. In proportion, therefore, to their power their stroke is shorter, which infers a corresponding shortness of crank and a greater limitation of play of all the moving parts in the vertical direction. The valves and the gearing by which they are worked, the air-pump, the condenser, and other parts of the marine engines, do not materially differ from those already described in land engines.

These arrangements of a marine engine will be more clearly understood by reference to fig. 119.[35], in which is represented a longitudinal section of a marine engine with its boiler as placed in a steam-vessel. The sleepers of oak, supporting the engine, are represented at X, the base of the engine being secured to these by bolts passing through them [Pg446] and the bottom timbers of the vessel; S is the steam-pipe leading from the steam-chest in the boiler to the slides c, by which it is admitted to the top and bottom of the cylinder. The condenser is represented at B, and the air-pump at E. The hot well is seen at F, from which the feed is taken for the boiler; L is the piston-rod connected by the parallel motion a with the beam H, working on a centre K, near the base of the engine. The other end of the beam I drives the connecting rod M, which extends upwards to the crank which it works upon the paddle-shaft O. Q R is the framing by which the engine is supported. The beam here exhibited is shown on dotted lines as being on the further side of the engine. A similar beam similarly placed, and moving on the same axis, must be understood to be at this side connected with the cross head of the piston in like manner by a parallel motion, and with a cross piece attached to the lower end of the connecting rod and to the opposite beam. The eccentric which works the slides is placed upon the paddle shaft O, and the connecting arm which drives the slides may be easily detached when the engine requires to be stopped. The section of the boiler, grate, and flues, is represented at W U. The safety-valve y is enclosed beneath a pipe carried up beside the chimney, and is inaccessible to the engine-man; h are the cocks for blowing the salted water from the boiler; and I I the feed-pipe.

The general arrangement of the engine-room of a steam-vessel is represented in fig. 120.

The nature of the effect required to be produced by marine engines does not render either necessary or possible that great regularity of action which is indispensable in a steam-engine applied to the purposes of manufacture. The agitation of the surface of the sea will cause the immersion of the paddle-wheels to be subject to great variation, and the resistance produced by the water to the engine will undergo a corresponding change. The governor, therefore, and other parts of the apparatus, contrived for giving to the engine that great regularity required in manufactures, are omitted in nautical engines, and nothing is introduced save what is [Pg447] necessary to maintain the machine in its full working efficiency.

To save space, marine boilers are constructed so as to produce the necessary quantity of steam within the smallest possible dimensions. With this view a more extensive surface in proportion to the capacity of the boiler is exposed to the action of the fire. The flues, by which the flame and heated air are conducted to the chimney, are so constructed that the heat may act upon the water on every side in thin oblong shells or plates. This is accomplished by constructing the flues so as to traverse the boiler backwards and forwards several times before they terminate [Pg448] in the chimney. Such an arrangement renders the expense of the boilers greater, but their steam-producing power is proportionally augmented, and experiments made by Mr. Watt, at Birmingham, have proved that such boilers with the same consumption of fuel will produce, as compared with common land boilers, an increased evaporation in the proportion of about three to two.

The form and arrangement of the water-spaces and flues in marine boilers may be collected from the sections of the boilers used in some of the government steamers, exhibited in figs. 121, 122, 123. A section made by a horizontal plane passing through the flues is exhibited in fig. 121. The furnaces F communicate in pairs with the flues E, the air following the course through the flues represented by the arrows. The flue E passes to the back of the boiler, then returns to the front, then to the back again, and is finally carried back to the front, where it communicates at C with the curved flue B, represented in the transverse vertical section, fig. 122. This curved flue B finally terminates in the chimney A. There are in this case three independent boilers, each worked by two furnaces communicating with the same system of flues; and in the curved flues B, fig. 122., by which the air is finally conducted through the chimney, are placed three independent [Pg449] dampers, by means of which the furnace of each boiler can be regulated independently of the other, and by which each boiler may be separately detached from communication with the chimney. The letters of reference in the horizontal section, fig. 121., correspond with those in the transverse vertical section, fig. 122., E representing the commencement of the flues, and C their termination.

A longitudinal section of the boiler made by a vertical plane extending from the front to the back is given in fig. 123., where F, as before, is the furnace, G the grate-bars sloping downwards from the front to the back, H the fire-bridge, C the commencement of the flues, and A the chimney. An elevation of the front of the boiler is represented in fig. 124., showing two of the fire-doors closed, and the other two removed, displaying the position of the grate-bars in front. Small openings are also provided, closed by proper doors, by which access can be had to the under side of the flues between the foundation timbers of the engine for the purpose of cleaning them.

Each of these boilers can be worked independently of the others. By this means, when at sea, the engine may be worked by any two of the three boilers, while the third is being cleaned and put in order. In all sea-going steamers multiple boilers are at present provided for this purpose.

In the boilers here represented the flues are all upon the same level, winding backwards and forwards without passing one above the other. In other boilers, however, the flues, [Pg450] after passing backwards and forwards near the bottom of the boiler, turn upwards and pass backwards and forwards through a level of the water nearer its surface, finally terminating in the chimney. More heating surface is thus obtained with the same capacity of boiler.

The most formidable difficulty which has been encountered in the application of the steam-engine to sea-voyages has arisen from the necessity of supplying the boiler with sea-water instead of pure fresh water. The sea-water is injected into the condenser for the purpose of condensing the steam, and it is thence, mixed with the condensed steam, conducted as feeding water into the boiler.

But besides the deposition of salt sediment in a loose form, some of the constituents of sea-water having an attraction for the iron of the boiler, collect upon it in a scale or crust in the same manner as earthy matters held in solution by spring-water are observed to form and become incrusted on the inner surface of land-boilers and of common culinary vessels.

The coating of the inner surface of a boiler by incrustation and the collection of salt sediment in its lower parts, are attended with effects highly injurious to the materials of the boiler. The crust and sediment thus formed within the boiler are almost non-conductors of heat, and placed, as they are, between the water contained in the boiler and the metallic plates which form it, they obstruct the passage of heat from the outer surface of the plates in contact with the fire to the water. The heat, therefore, accumulating in the boiler-plates so as to give them a much higher temperature than the water within the boiler, has the effect of softening them, and by the unequal temperature which will thus be imparted to the lower plates which are incrusted, compared with the higher parts which may not be so, an unequal expansion is produced, by which the joints and seams of the boiler are loosened and opened, and leaks produced.

These injurious effects can only be prevented by either of two methods; first, by so regulating the feed of the boiler that the water it contains shall not be suffered to reach the point of saturation, but shall be so limited in its degree of saltness that no injurious incrustation or deposit shall be formed; secondly, by the adoption of some method by which the boiler may be worked with fresh water. This end can only be attained by condensing the steam by a jet of fresh water, and working the boiler continually by the same water, since a supply of fresh water sufficient for a boiler worked in the ordinary way could never be commanded at sea.

This process of blowing out, on the due and regular observance of which the preservation and efficiency of the boiler mainly depend, is too often left at the discretion of the engineer, who is, in most cases, not even supplied with the proper means of ascertaining the extent to which the process should be carried. It is commonly required that the engineer should blow out a certain portion of the water in the boiler every two hours, restoring the level by a feed of equivalent amount; but it is evident that the sufficiency of the process founded on such a rule must mainly depend on the supposition that the evaporation proceeds always at the same rate, which is far from being the case with marine boilers. An indicator, by which the saltness of the water in the boiler would always be exhibited, ought to be provided, and the process of blowing out should be regulated by the indications of that instrument. To blow out more frequently than is necessary is attended with a waste of fuel; for hot water is thus discharged into the sea while cold water is introduced in its place, and consequently all the heat necessary to produce the difference of the temperatures of the water blown out and the feed introduced is lost. If, on the other hand, the process of blowing out be observed less frequently than is necessary, then more or less incrustation and deposit [Pg453] may be produced, and the injurious effects already described ensue.

As the specific gravity of water holding salt in solution is increased with every increase of the strength of the solution, any form of hydrometer capable of exhibiting a visible indication of the specific gravity of the water contained in the boiler would serve the purpose of an indicator, to show when the process of blowing out is necessary, and when it has been carried to a sufficient extent. The application of such instruments, however, would be attended with some practical difficulties in the case of sea-boilers.

The temperature at which a solution of salt boils under a given pressure varies considerably with the strength of the solution; the more concentrated the solution is, the higher will be its boiling temperature under the same pressure. A comparison, therefore, of a steam-gauge attached to the boiler, and a thermometer immersed in it, showing the pressure and the temperature, would always indicate the saltness of the water; and it would not be difficult so to graduate these instruments as to make them at once show the degree of saltness.

If the application of the thermometer be considered to be attended with practical difficulty, the difference of pressures under which the salt water of the boiler and fresh water of the same temperature boil, might be taken as an indication of the saltness of the water in the boiler, and it would not be difficult to construct upon this principle a self-registering instrument, which would not only indicate but record from hour to hour the degree of saltness of the water. A small vessel of distilled water being immersed in the water of the boiler would always have the temperature of that water, and the steam produced from it communicating with a steam-gauge, the pressure of such steam would be indicated by that gauge, while the pressure of the steam in the boiler under which pressure the salted water boils might be indicated by another gauge. The difference of the pressures indicated by the two gauges would thus become a test by which the saltness of the water in the boiler would be measured. The two pressures might be made to act on opposite ends of the same column of [Pg454] mercury contained in a siphon tube, and the difference of the levels of the two surfaces of the mercury would thus become a measure of the saltness of the water in the boiler. A self-registering instrument founded on this principle formed part of the self-registering steam-log which I proposed to introduce into steam-vessels some time since.

The ordinary method of blowing out the salted water from a boiler is by a pipe having a cock in it leading from the boiler through the bottom of the ship, or at a point low down at its side. Whenever the engineer considers that the water in the boiler has become so salted that the process of blowing out should commence, he opens the cock communicating by this pipe with the sea, and suffers an indefinite and uncertain quantity of water to escape. In this way he discharges, according to the magnitude of the boiler, from two to six tons [Pg455] of water, and repeats this at intervals of from two to four hours, as he may consider to be sufficient. If, by observing this process, he prevents the boiler from getting incrusted during the voyage, he considers his duty to be effectually discharged, forgetting that he may have blown out many times more water than is necessary for the preservation of the boiler, and thereby produced a corresponding and unnecessary waste of fuel. In order to limit the quantity of water discharged, Messrs. Seaward have adopted the following method. In fig. 125. is represented a transverse section of a part of a steam-vessel; W is the water-line of the boiler, B is the mouth of a blow-off pipe, placed near the bottom of the boiler. This pipe rises to A, and turning in the horizontal direction, A C is conducted to a tank T, which contains exactly a ton of water. This pipe communicates with the tank by a cock D, governed by a lever H. When this lever is moved to D', the cock D is open, and when it is moved to K, the cock D is closed. From the same tank there proceeds another pipe E, which issues from the side of the [Pg456] vessel into the sea governed by a cock F, which is likewise put in connection with the lever H, so that it shall be opened when the lever H is drawn to the position F', the cock D' being closed in all positions of the lever between K and F'. Thus, whenever the cock F communicating with the sea is open, the cock D communicating with the boiler is closed, and vice versÂ, both cocks being closed when the lever is in the intermediate position K. By this arrangement the boiler cannot, by any neglect in blowing off, be left in communication with the sea, nor can more than a ton of water be discharged except by the immediate act of the engineer. The injurious consequences are thus prevented which sometimes ensue when the blow-off cocks are left open by any neglect on the part of the engineer. When it is necessary to blow off, the engineer moves the lever H, to the position D'. The pressure of the steam in the boiler on the surface of the water W forces the salted water or brine up the pipe B A, and through the open cock C into the tank, and this continues until the tank is filled: when that takes place, the lever is moved from the position D' to the position F', by which the cock D is closed, and the cock F opened. The water in the tank flows through the pipe E into the sea, air being admitted through the valve V, placed at the top of the tank, opening inwards. A second ton of brine is discharged by moving the lever back to the position D', and subsequently returning it to the position F'; and in this way the brine is discharged ton by ton, until the supply of water from the feed which replaces it has caused both the balls in the indicator to sink to the bottom.

To save the heat of the brine, a method has been adopted in the marine engines constructed by Messrs. Maudslay and Field similar to one which has been long practised in steam-boilers, and in various apparatus for the warming of buildings. The current of heated brine is conducted from the boiler through a tube which is contained in another, through which the feed is introduced. The warm current of brine, therefore, as it passes out, imparts a considerable portion of its heat to the cold feed which comes in; and it is found that by this expedient the brine discharged into the sea may be reduced to a temperature of about 100°.

This expedient is so effectual that when the apparatus is properly constructed, and kept in a state of efficiency, it may be regarded as nearly a perfect preventive against the incrustation, and the deposition of salt in the boilers, and is not attended with any considerable waste of fuel.

This contrivance has been of late years revived by Mr. Samuel Hall of Basford, near Nottingham, with a view to supersede in marine engines the necessity of using sea-water in the boilers. Mr. Hall proposes to make marine boilers with fresh water to condense the steam without injection, by a tubulated condenser, and to provide by the distillation of sea-water the small quantity of fresh water which would be necessary to make good the waste. These condensers have been introduced into several steam-vessels: in some they have been continued, and in others abandoned, and various opinions are entertained of their efficacy. I have not been able to obtain the results of any satisfactory experiments on them, and cannot therefore form a judgment of their usefulness. Mr. Watt abandoned these condensers from finding that the condensation of the steam was not sufficiently sudden, and that consequently at the commencement of the stroke the piston was subject to a resistance which [Pg459] injuriously diminished the amount of the moving power, whereas condensation by jet was almost instantaneous, and the efficiency of the piston throughout the entire stroke was more uniform.

Mr. Watt also found that a fur collected around the tubes of the condenser, so as to obstruct the free passage of heat from the steam to the water of the cold cistern; and that, consequently, the efficiency of the condenser was gradually impaired, and could only be restored by frequent cleansing.

It is stated by Mr. Hall that a vacuum is preserved in his condensers as perfect as that which is maintained in the ordinary condensers by injection. It is objected, on the other hand, that without the injection water and the air which accompanies it being introduced into his condensers, Mr. Hall uses as large and powerful an air-pump as those which are used in engines of equal power condensing by injection; that, consequently, the vacuum which is maintained is produced, not as it ought to be altogether by the condensation of steam, but by the air-pump drawing off the uncondensed steam. To whatever extent this may be true, the efficacy of the machine, as indicated by the barometer-gauge, is only apparent; since as much power is necessary to pump away any portion of uncondensed vapour as is obtained by the vacuum produced by the absence of that vapour.

A tubular condenser of the form proposed by Mr. Hall is represented in fig. 126.; a is the upper part of the condenser to which steam is admitted from the slide after having worked the piston; k is the section of a thin plate, forming the top of the condenser, perforated with small holes, in which the tubes are inserted so as to be steam-tight and water-tight. Water is admitted to flow around these tubes between the top k and the bottom d of the condenser, so as to keep them constantly at a low temperature. The steam passes from a through the tubes to the lower chamber f of the condenser, where it is reduced to water by the cold to which it has been exposed. A supply of cold water is constantly pumped through the condenser, so as to keep the tubes at a low temperature. The air-pump g is of the usual construction, having valves in the piston opening upwards, and [Pg460] similar valves in the cover of the pump also opening upwards. The water formed by the condensed steam in f is drawn through the foot-valve, and after passing through the piston-valves, is discharged by the up-stroke of the piston into the hot well. Any air, or other permanent gas, which may be admitted by leakage through the tubes of the condenser, or by any other means, is likewise drawn out by this pump, and when drawn into the hot well is carried from thence to the feeding apparatus of the boiler, to which it is transferred by the feed-pump.

A provision is likewise made by which the steam escaping at the safety-valve is condensed and carried away to the feeding cistern.

Besides the injury arising from the deposition of salt and the incrustation on the inner surface of boilers, an evil of a formidable kind attends the accumulation of soot mixed with salt in the flues, which proceeds from the leaks. In the seams of the boiler there are numerous apertures, of dimensions so small as to be incapable of being rendered stanch by any practicable means, through which the water within the boiler filters, and the salt which it carries with it mixes with the soot, forming a compound which rapidly corrodes the boilers. This process of corrosion in the flues takes place not less in copper than in iron boilers. In cleansing the flues of a copper boiler, the salt and soot which was thrown out upon the iron-plates which formed the flooring of the engine-room, having remained there for some time, left behind it a permanent appearance of copper on the iron flooring, arising from the precipitation of the copper which had combined with the soot and salt in the flues.[36] In this case the leaks from whence the salt proceeded were found, on careful examination, so unimportant, that the usual means to stanch them could not be resorted to without the risk of increasing the evil.

Much of the efficiency of fuel must depend on the management of the fires, and therefore on the skill and care of the stokers. Formerly the efficiency of firemen was determined by the abundant production of steam, and so long as the steam was evolved in superabundance, however it might have blown off to waste, the duty of the stoker was considered as well performed. The regulation of the fires according to the demands of the engine were not thought of, and whether much or little steam was wanted, the duty of the stoker was to urge the fires to their extreme limit.

Since the resistance opposed by the action of the paddle-wheels of a steam-vessel varies with the state of the weather, the consumption of steam in the cylinders must undergo a corresponding variation; and if the production of steam in the boilers be not proportioned to this, the engines will either work with less efficiency than they might do under the actual circumstances of the weather, or more steam will be produced in the boilers than the cylinders can consume, and the surplus will be discharged to waste through the safety-valves. The stokers of a marine engine, therefore, to perform their duty with efficiency, and obtain from the fuel the greatest possible effect, must discharge the functions of a self-regulating furnace, such as has been already described: they must regulate the force of the fires by the amount of steam which the cylinders are capable of consuming, and they must take care that no unconsumed fuel is allowed to be carried away from the ash-pit.

The economy of fuel depends in a considerable degree on the arrangement of the furnaces, and the method of feeding them. In general each boiler is worked by two or more furnaces communicating with the same system of flues. While the furnace is fed, the door being open, a stream of cold air rushes in, passing over the burning fuel and lowering the temperature of the flues: this is an evil to be avoided. But, on the other hand, if the furnaces be fed at distant intervals, then each furnace will be unduly heaped with fuel, a great quantity of smoke will be evolved, and the combustion of the fuel will be proportionally imperfect. The process of coking in front of the grate, which would insure a complete combustion of the fuel, has been already described (147.). A frequent supply of coals, however, laid carefully on the front part of the grate, and gradually pushed backwards as each fresh feed is introduced, would require the fire-door to be frequently opened, and cold air to be admitted. It would also require greater vigilance on the part of the stokers than can generally be obtained in the circumstances in which they work. In steam-vessels the furnaces are therefore fed less frequently, fuel introduced in greater quantities, and a less perfect combustion produced.

When several furnaces are constructed under the same boiler, communicating with the same system of flues, the process of feeding, and consequently opening one of them, obstructs the due operation of the others, for the current of cold air which is thus admitted into the flues checks the draft and diminishes the efficiency of the furnaces in operation. It was formerly the practice in vessels exceeding one hundred horse-power, to place four furnaces under each boiler, communicating with the same system of flues. Such an arrangement [Pg464] was found to be attended with a bad draft in the furnaces, and therefore to require a greater quantity of heating surface to produce the necessary evaporation. This entailed upon the machinery the occupation of more space in the vessel in proportion to its power; it has therefore been more recently the practice to give a separate system of flues to each pair of furnaces, or, at most, to every three furnaces. When three furnaces communicate with a common flue, two will always be in operation, while the third is being cleared out; but if the same quantity of fire were divided among two furnaces, then the clearing out of one would throw out of operation half the entire quantity of fire, and during the process the evaporation would be injuriously diminished. It is found by experience, that the side plates of furnaces are liable to more rapid destruction than their roofs, owing, probably, to a greater liability to deposit. Furnaces, therefore, should not be made narrower than a certain limit. Great depth from front to back is also attended with practical inconvenience, as it renders firing tools of considerable length, and a corresponding extent of stoking room necessary. It is recommended, by those who have had much practical experience in steam-vessels, that furnaces six feet in depth from front to back should not be less than three feet in width, to afford means of firing with as little injury to the side plates as possible, and of keeping the fires in the condition necessary for the production of the greatest effect. The tops of the furnaces almost never decay, and seldom are subject to an alteration of figure, unless the level of the water be allowed to fall below them.[37]

A quantity of mercury is placed in a shallow wrought-iron vessel over a coke fire, by which it is maintained at a [Pg465] temperature varying from 400° to 500°. The surface exposed to the fire was computed at three fourths of a square foot for each horse-power. The upper surface of the mercury was covered by a very thin plate of iron in contact with it, and so contrived as to present about four times as much surface as that exposed beneath the fire. Adjacent to this a vessel of water was placed, maintained nearly at the boiling point, and communicating by a nozzle and valve with the chamber immediately above the mercury. At intervals corresponding to the motion of the piston a small quantity of water was injected from this vessel, and thrown upon the plate of iron resting upon the hot mercury. From this it received not only the heat necessary to convert it into common steam, but to give it the qualities of highly superheated steam. In fact, the steam thus produced had a temperature considerably above that which corresponded to its pressure, and was, therefore, capable of being deprived of more or less of its heat without being condensed. (94.) The quantity of water injected into the steam-chamber was regulated by the power at which the engine was intended to be worked. The fire was supplied with air by a blower subject to exact regulation. The steam thus produced was conducted to a chamber surrounding the working cylinder, and this chamber itself was enclosed by another space through which the air from the furnace passed before it reached the flue. By this contrivance the air imparted its redundant heat to the steam, as the latter passed to the cylinder, and raised its temperature to about 400°, the pressure, however, not exceeding 25 lbs. per square inch. The valves, governing the admission of steam to the piston, were adapted for expansive action.

The vacuum on the opposite side was maintained by condensation in the following manner:—The condenser was a copper vessel placed in a cistern of cold water, and the steam was admitted to it from the cylinder by an eduction pipe in the usual way. A jet was introduced from an adjacent vessel filled with distilled water, and the condensing water and condensed steam were pumped from the condenser as in common engines. The warm water thus pumped out of the [Pg466] condenser was drawn through a copper worm, carried with many coils through a cistern of cold water, so that when it arrived at the end of this pipe it was reduced nearly to the temperature of the atmosphere. The pipe was thus brought to the vessel of distilled water already mentioned, and the water supplied by it replaced. The water admitted to the condenser through the condensing jet being purged of air, a small air-pump was sufficient, since it had only to exhaust the condenser and tubes at starting, and to remove the air which might be admitted by leakage. Mr. Howard stated that the condensation took place as rapidly and perfectly as in the best engines of the common kind.

An engine of this construction was in the spring of 1835 placed in the government steamer called the Comet. It was stated, that though the machinery was not advantageously constructed, a part of the engine being old, and not made expressly for a boiler of this kind, the vessel performed a voyage from Falmouth to Lisbon, in which the consumption of fuel did not exceed a third of her former consumption when worked by Boulton and Watt's engines, the former consumption of coals being about eight hundred pounds per hour, and the consumption of Mr. Howard's engine being less than two hundred and fifty pounds of coke per hour.

The advantages claimed for this contrivance were the following: first, the small space and weight occupied by the machinery, arising from the absence of a boiler; second, the diminished consumption of fuel; third, the reduced size of the flues; fourth, the removal of the injurious effects arising from deposit and incrustation; fifth, the absence of smoke.

A section of such an engine, made by a plane passing through the two piston-rods P P' and cylinders, is represented in fig. 127. The piston-rods are attached to a cross-head C, [Pg468] which ascends and descends with them. This cross-head drives upwards and downwards an axle D, to which the lower end of the connecting rod E is attached. The other end of the connecting rod drives the crank-pin F, and imparts revolution to the paddle-shaft G. A rod H conveys motion by means of a beam I to the rod K of the air-pump E.

By this arrangement the force by which the piston is driven in its ascent and descent is communicated to the connecting rod, not, as usual, through the intervention of a piston-rod, but directly from the piston itself by the cross-pin I, and from thence to the crank C, which it drives without the intervention of beams, cross-heads, or any similar appendage.

The slide-valves regulating the admission and eduction of steam are represented at a; the rod of the air-pump is shown at d, being worked by a crank placed on the centre of the great crank shaft.[38]

The ordinary paddle-wheel, as has been already stated, is a wheel revolving upon a shaft driven by the engine, and carrying upon its circumference a number of flat boards, called paddle-boards, which are secured by nuts and braces in a fixed position; and that position is such that the planes [Pg473] of the paddle-boards diverge nearly from the centre of the shaft on which the wheel turns. The consequence of this arrangement is that each paddle-board can only act in that direction which is most advantageous for the propulsion of the vessel when it arrives near the lowest point of the wheel. In fig. 132. let O be the shaft on which the common paddle-wheel revolves; the position of the paddle-boards are represented at A, B, C, &c.; X, Y represents the water line, the course of the vessel being supposed to be from X to Y; the arrows represent the direction in which the paddle-wheel revolves. The wheel is immersed to the depth of the lowest paddle-board, since a less degree of immersion would render a portion of the surface of each paddle-board mechanically useless. In the position A the whole force of the paddle-board is efficient for propelling the vessel; but as the paddle enters the water in the position H, its action upon the water, not being horizontal, is only partially effective for propulsion: a part of the force which drives the paddle is expended in depressing the water, and the remainder in driving it contrary to the course of the vessel, and, therefore, by its re-action producing a certain propelling effect. The tendency, however, of the paddle entering the water at H, is to form a hollow or trough, which the water, by its ordinary property, has a continual tendency to fill up. After passing the lowest point A, as the paddle approaches the position B, where it [Pg474] emerges from the water, its action again becomes oblique, a part only having a propelling effect, and the remainder having a tendency to raise the water, and throw up a wave and spray behind the paddle-wheel. It is evident that the more deeply the paddle-wheel becomes immersed, the greater will be the proportion of the propelling power thus wasted in elevating and depressing the water; and if the wheel were immersed to its axis, the whole force of the paddle-boards, on entering and leaving the water, would be lost, no part of it having a tendency to propel. If a still deeper immersion take place, the paddle-boards above the axis would have a tendency to retard the course of the vessel. When the vessel is, therefore, in proper trim, the immersion should not exceed nor fall short of the depth of the lowest paddle; but for various reasons it is impossible in practice to maintain this fixed immersion: the agitation of the surface of the sea, causing the vessel to roll, will necessarily produce a great variation in the immersion of the paddle-wheels, one becoming frequently immersed to its axle, while the other is raised altogether out of the water. Also the draught of water of the vessel is liable to change, by the variation in her cargo; this will necessarily happen in steamers which take long voyages. At starting they are heavily laden with fuel, which as they proceed is gradually consumed, whereby the vessel is lightened.

Feathering paddle-boards must necessarily have a motion independently of the motion of the wheel, since any fixed position which could be given to them, though it might be most favourable to their action in one position would not be so in their whole course through the water. Thus the paddle-board when at the lowest point should be in a vertical position, or so placed that its plane, if continued upwards, would pass through the axis of the wheel. In other positions, however, as it passes through the water, it should present its upper edge, not towards the axle of the wheel, but towards a point above the highest point of the wheel. The precise point to which the edge of the paddle-board should be directed is capable of mathematical determination. But it will vary according to circumstances, which depend on the motion of the vessel. The progressive motion of the vessel, independently of the wind or current, must obviously be slower than the motion of the paddle-boards round the axle of the wheel; since it is by the difference of these velocities that the re-action of the water is produced by which the vessel is propelled. The proportion, however, between the progressive speed of the vessel and the rotative speed of the paddle-boards is not fixed: it will vary with the shape and structure of the vessel, and with its depth of immersion; nevertheless it is upon this proportion that the manner in which the paddle-boards should shift their position must be determined. If the progressive speed of the vessel were nearly equal to the rotative speed of the paddle-boards, the latter should so shift their position that their upper edges should be presented to a point very little above the highest point of the wheel. This is a state of things which could only take place in the case of a steamer of a small draught of water, shallop-shaped, and so constructed as to suffer little resistance from the fluid. On the other hand, the greater the depth of immersion, and the less fine the lines of the [Pg476] vessel, the greater will be the resistance in passing through the water, and the greater will be the proportion which the rotative speed of the paddle-boards will bear to the progressive speed of the vessel. In this latter case the independent motion of the paddle-boards should be such that their edges, while in the water, shall be presented towards a point considerably above the highest point of the paddle-wheel.

A vast number of ingenious mechanical contrivances have been invented and patented for accomplishing the object just explained. Some of these have failed from the circumstance of their inventors not clearly understanding what precise motion it was necessary to impart to the paddle-board: others have failed from the complexity of the mechanism by which the desired effect was produced.

This paddle-wheel is represented in fig. 133. The contrivance may be shortly stated to consist in causing the wheel which bears the paddles to revolve on one centre, and the radial arms which move the paddles to revolve on another centre. Let A B C D E F G H I K L be the polygonal circumference of the paddle-wheel, formed of straight bars, securely connected together at the extremities of the spokes or radii of the wheel which turns on the shaft which is worked by the engine; the centre of this wheel being at O. So far this wheel is similar to the common paddle-wheel; but the paddle-boards are not, as in the common wheel, fixed at A B C, &c., so as to be always directed to the centre O, but are so placed that they are capable of turning on axles which are always horizontal, so that they can take any angle with respect to the water which may be given to them. From the centres, or the line joining the pivots on which these paddle-boards turn, there proceed short arms K, firmly fixed to the paddle-boards at an angle of about 120°. On a motion given to this arm K, it will therefore give a corresponding angular motion to the paddle-board, so as to make it turn on its pivots. At [Pg477] the extremities of the several arms marked K is a pin or pivot, to which the extremities of the radial arms L are severally attached, so that the angle between each radial arm L and the short paddle-arm K is capable of being changed by any motion imparted to L; the radial arms are connected at the other end with a centre, round which they are capable of revolving. Now, since the points A B C, &c., which are the pivots on which the paddle-boards turn, are moved in the circumference of a circle, of which the centre is O, they are always at the same distance from that point; consequently they will continually vary their distance from the other centre P. Thus, when a paddle-board arrives at that point of its revolution at which the centre round which it revolves lies precisely between it and the centre O, its distance from the former centre is less than in any other position. As it departs from that point, its distance from that centre gradually increases until it arrives at the opposite point of its revolution, where the centre O is exactly between it and the former centre; then the distance of the paddle-board from the former centre is greatest. [Pg478] This constant change of distance between each paddle-board and the centre P is accommodated by the variation of the angle between the radial arm L and the short paddle-board arm K; as the paddle-board approaches the centre P this gradually diminishes; and as the distance of the paddle-board increases, the angle is likewise augmented. This change in the magnitude of the angle, which thus accommodates the varying position of the paddle-board with respect to the centre P, will be observed in the figure. The paddle-board D is nearest to P; and it will be observed that the angle contained between L and K is there very acute; at E the angle between L and K increases, but is still acute; at G it increases to a right angle; at H it becomes obtuse; and at K, where it is most distant from the centre P, it becomes most obtuse. It again diminishes at K, and becomes a right angle between A and B. Now this continual shifting of the direction of the short arm K is necessarily accompanied by an equivalent change of position in the paddle-board to which it is attached; and the position of the second centre P is, or may be, so adjusted that this paddle-board, as it enters the water and emerges from it, shall be such as shall be most advantageous for propelling the vessel, and therefore attended with less of that vibration which arises chiefly from the alternate depression and elevation of the water, owing to the oblique action of the paddle-boards.

[Pg479] The theoretical effect of this wheel is the same as that of the common wheel, and experience alone, the result of which has not yet been obtained, can prove its efficiency. The number of bars, or separate parts into which each paddle-board is divided, has been very various. When first introduced by Mr. Galloway each board was divided into six or seven parts: this was subsequently reduced, and in the more recent wheels of this form constructed for the government vessels the paddle-boards consist only of two parts, coming as near to the common wheel as is possible, without altogether abandoning the principle of the split paddle.

It will be apparent that every improvement which takes place in the application of the steam-engine to navigation will modify all these data on which such an investigation must depend. Every increased efficiency of fuel, from whatever cause it may be derived, will either increase the useful tonnage of the vessel, or increase the length of the voyage of which it is capable. Various improvements have been and are still in progress, by which this efficiency has undergone continual augmentation, and voyages may now be accomplished with moderate economy and profit, to which a few years since marine engines could not be applied with permanent advantage. The average speed of steam-vessels has also undergone a gradual increase by such improvements. During the four years ending June, 1834, it was found that the average rate of steaming obtained from fifty-one voyages made by the Admiralty steamers between Falmouth and Corfu, exclusive of stoppages, was seven miles and a quarter an hour direct distance between port and port. The vessels which performed this voyage varied from 350 to 700 tons measured burden, and were provided with engines varying from 100 to 200 horse-power, with stowage for coals varying from 80 to 240 tons. The proportion of the power to the [Pg481] tonnage varied from one horse to three tons to one horse to four tons. Thus the Messenger had a power of 200 horses and measured 730 tons; the Flamer had a power of 120 horses, and measured 500 tons; the Columbia had a power of 120 horses, and measured 360 tons. In general it may be assumed that for the shortest class of trips, such as those of the Channel steamers, the proportion of the power to the tonnage should be about one horse for every two tons; but for the longer class of voyages, the proportion of power to tonnage should be about one horse-power to from three to four tons measured tonnage. These data, however, must be received as very rough approximations, subject to considerable modifications in their application to particular vessels. We have already stated that the nominal horse-power is itself extremely indefinite; and if, as is now customary in the longer class of voyages, the steam be worked expansively, then the nominal power almost ceases to have any definite relation to the actual performance of the vessel. It is usual to calculate the horse-power by assuming a uniform pressure of steam upon the piston, and, consequently, by excluding the consideration of the effect of expansion. The most certain test of the amount of mechanical power exerted by the machinery would be obtained from the quantity of water actually transmitted in the form of steam from the boiler to the cylinder. But the effect of this would also be influenced by the extent to which the expansive principle has been brought into operation.

From the reported performances of the larger class of steam-ships within the last few years, it would appear that the average speed has been increased since the estimate above mentioned, which was obtained in 1834; and on comparing the consumption of fuel with the actual performance, it would appear that the efficiency of fuel has also been considerably augmented. No extensive course of accurate experiments or observations have, however, been obtained from which correct inferences may be drawn of the probable limits to which steam-navigation, in its present state, is capable of being extended. The jealousy of rival companies has obstructed the inquiries of those who, solicitous more [Pg482] for the general advancement of the art than for the success of individual enterprises, have directed their attention to this question; and it is hardly to be expected that sufficiently correct and extensive data can be obtained for this purpose.

Iron steam-vessels on a very large scale are now in preparation in the ports of Liverpool and Bristol, intended for long sea-voyages. The largest vessel of this description which has yet been projected is stated to be in preparation for the voyage between Bristol and New York, by the company who have established the steam-ship called the Great Western, plying between these places.

Several projects for the extension of steam-navigation to voyages of considerable length have lately been entertained both by the public and by the legislature, and have imparted to every attempt to improve steam-navigation increased interest. A committee of the House of Commons collected evidence and made a report in the last session in favour of an experiment to establish a line of steam-communication between Great Britain and India. Two routes have been suggested by the committee, each being a continuation of the line of Admiralty steam-packets already established to Malta and the Ionian Isles. One of the routes proposed is through Egypt, the Red Sea, and across the Indian Ocean to Bombay, or some of the other presidencies; the other across the north part of Syria to the banks of the Euphrates, by that river to the Persian Gulf, and from thence to Bombay. Each of these routes will be attended with peculiar difficulties, and in both a long sea-voyage will be encountered.

In the route by the Red Sea it is proposed to establish steamers between Malta and Alexandria (eight hundred and sixty miles). A steamer of four hundred tons' burden and one hundred horse-power would perform this voyage, upon an average of all weathers incident to the situation, in from five to six days, consuming ten tons of coal per day. But it is probable that it might be found more advantageous to establish a higher ratio between the power and the tonnage. From Alexandria the transit might be effected by land across the isthmus to Suez—a journey of from four to five days—by caravan and camels; or the transit might be made either [Pg484] by land or water from Alexandria to Cairo, a distance of one hundred and seventy-three miles; and from Cairo to Suez, ninety-three miles, across the desert, in about five days. At Suez would be a station for steamers, and the Red Sea would be traversed in three runs or more. If necessary, stations for coals might be established at Cosseir, Judda, Mocha, and finally at Aden or at Socatra—an island immediately beyond the mouth of the Red Sea, in the Indian Ocean; the run from Suez to Cosseir would be three hundred miles—somewhat more than twice the distance from Liverpool to Dublin. From Cosseir to Judda, four hundred and fifty miles; from Judda to Mocha, five hundred and seventeen miles; and from Mocha to Socatra, six hundred and thirty-two miles. It is evident that all this would, without difficulty, in the most unfavourable weather, fall within the present powers of steam-navigation. If the terminus of the passage be Bombay, the run from Socatra to Bombay will be twelve hundred miles, which would be from six to eight days' steaming. The whole passage from Alexandria to Bombay, allowing three days for delay between Suez and Bombay, would be twenty-six days: the time from Bombay to Malta would therefore be about thirty-three days; and adding fourteen days to this for the transit from Malta to England, we should have a total of forty-seven days from London to Bombay, or about seven weeks.

If the terminus proposed were Calcutta, the course from Socatra would be one thousand two hundred and fifty miles south-east to the Maldives, where a station for coals would be established. This distance would be equal to that from Socatra to Bombay. From the Maldives, a run of four hundred miles would reach the southern point of Ceylon, called the Point de Galle, which is the best harbour (Bombay excepted) in British India: from the Point de Galle, a run of six hundred miles will reach Madras, and from Madras to Calcutta would be a run of about six hundred miles. The voyage from London to Calcutta would be performed in about sixty days.

At a certain season of the year there exists a powerful physical opponent to the transit from India to Suez: from [Pg485] the middle of June until the end of September, the south-west monsoon blows with unabated force across the Indian Ocean, and more particularly between Socatra and Bombay. This wind is so violent as to leave it barely possible for the most powerful steam-packet to make head against it, and the voyage could not be accomplished without serious wear and tear upon the vessels during these months.

The attention of parliament has therefore been directed to another line of communication, not liable to this difficulty: it is proposed to establish a line of steamers from Bombay through the Persian Gulf to the Euphrates.

The run from Bombay to a place called Muscat, on the southern shore of the gulf, would be eight hundred and forty miles in a north-west direction, and therefore not opposed to the south-west monsoon. From Muscat to Bassidore, a point upon the northern coast of the strait at the mouth of the Persian Gulf, would be a run of two hundred and fifty-five miles; from Bassidore to Bushire, another point on the eastern coast of the Persian Gulf, would be a run of three hundred miles; and from Bushire to the mouth of the Euphrates, would be one hundred and twenty miles. It is evident that the longest of these runs would offer no more difficulty than the passage from Malta to Alexandria. From Bussora, near the mouth of the Euphrates, to Bir, a town upon its left bank near Aleppo, would be one thousand one hundred and forty-three miles, throughout which there are no physical obstacles to the river-navigation which may not be overcome. Some difficulties arise from the wild and savage character of the tribes who occupy its banks. It is, however, thought that by proper measures, and securing the co-operation of the pacha of Egypt, any serious obstruction from this cause may be removed. From Bir, by Aleppo, to Scanderoon, a port upon the Mediterranean, opposite Cyprus, is a land-journey, said to be attended with some difficulty, but not of great length; and from Scanderoon to Malta is about the same distance as between the latter place and Alexandria. It is calculated that the time from London to Bombay by the Euphrates—supposing the passage to be successfully [Pg486] established—would be a few days shorter than by Egypt and the Red Sea.

Whichever of these courses may be adopted, it is clear that the difficulties, so far as the powers of the steam engine are concerned, lie in the one case between Socatra and Bombay, or between Socatra and the Maldives, and in the other case between Bombay and Muscat. This, however, has already been encountered and overcome on four several voyages by the Hugh Lindsay steamer from Bombay to Suez: that vessel encountered a still longer run on these several trips, by going, not to Socatra, but to Aden, a point on the coast of Arabia, near the Straits of Babel Mandeb, being a run of one thousand six hundred and forty-one miles, which she performed in ten days and nineteen hours. The same trip has since been repeatedly made by other steamers; and, in the present improved state of steam navigation, no insurmountable obstacles are opposed to their passage.

[35] This cut is taken from the plate of the engine of the Red Rover, manufactured by Boulton and Watt, given in the last edition of Tredgold on the Steam Engine.