William Gilbert, a physician of Colchester, first showed in 1600 that the earth as a whole has the properties of a magnet, and consequently that the directive action exerted by it upon a compass needle represents only a special case of the mutual action of two magnets. In 1845, Faraday established the fact that susceptibility to magnetic force is not, as was generally believed, confined to iron, nickel, and a few other substances, but is a property of all substances. According to Balfour Stewart, aurorÆ and earth currents may be regarded as secondary currents resulting from changes in the earth’s magnetism. Magnetic phenomena are included under the general term terrestrial magnetic elements, and consist of magnetic declination, inclination, and intensity.

These are for convenience determined separately; the first by an instrument called a Declinometer, and the second by an Inclinometer or Dipping Needle. The Declinometer is also made to serve the additional purpose of measuring the intensity of the earth’s magnetic force, which it effects on a principle similar to that by which the force of gravity is determined by the oscillations of a pendulum of known length on any given portion of the earth’s surface. The declinometer needle is made to oscillate, and the number of oscillations in a given time counted; due allowance being made for the strength of the needle, it is obvious that the force which restores the needle to rest can be estimated. To ascertain the angle of declination, the zero line of the compass card is made to coincide with the geographical north and south line; and the angle which the direction of the needle makes with this line is then read off on a graduated circle over which the needle turns. The magnetic inclination or dip of the needle is estimated by observing the inclination to a horizontal plane of a needle turning on the vertical plane which passes through the magnetic north and south points.

Fig. 62 shows a simple form of magnetic needle suspended on a fine steel point, which is supported by a brass stand; the addition of a graduated circle would constitute such an arrangement a Declinometer.

Fig. 63 gives the appearance of the dipping needle, or Inclinometer, and Fig. 64 an arrangement by which both kinds of terrestrial as well as local attraction may be shown.

These components of the earth’s magnetism undergo not only an annual but a daily and even hourly variation, apparently connected in some occult manner with the frequency of the sun’s spots. The needle sometimes suffers such exceptional perturbations as to suggest the idea of a magnetic storm. These disturbances are usually accompanied (in polar regions) by luminous phenomena called aurorÆ. Continuous automatic records of them, therefore, is of great value, as facilitating inductive research which may lead to valuable practical results.

Accordingly the Royal Society have adopted for the Kew and other observatories the form of Magnetograph, or Self-recording Magnetometer, shown at Fig. 61, by means of which the variations just referred to are registered by the oscillations of three magnets on photographically prepared paper, stretched on a drum revolved by clockwork.

One magnet is suspended in the magnetic meridian by a silk thread, and, by the aid of a mirror attached, it describes on the cylinder, moved by clockwork in the centre pier, all the variations in the magnetic declination.

The other two components of the magnetic force of the earth are given by the other magnets. That recording the vertical variations rests on two agate edges under a glass shade, while the horizontal component magnet is suspended by a double silk thread, under the shade to the right of the picture, being retained by the tension of the thread in a position nearly at right angles to the magnetic meridian.

The clock box in the centre covers the three revolving cylinders bearing the sensitive photographic paper, and to each magnet is attached a semicircular mirror, which reflects the rays from a gas jet to one of the cylinders, and thus describes by a curved line the oscillations of the magnet. A second semicircular mirror is fixed to the pier on which the instrument stands, and consequently describes a straight line, or zero, from whence the curves are measured.

To avoid errors attending sudden changes of temperature, underground vaults are always chosen for magnetic observations, and also on account of light being more easily and perfectly excluded.

Since the performance of Franklin’s famous kite experiment, by which he determined the identity of lightning with the electrical discharge from a machine, much attention has been devoted, not only to that form of atmospheric electricity which displays itself in the thunder-cloud, but to the electric condition of the air in all states of the weather. These researches have established the fact that the air is always in an electrical condition, even when the sky is clear and free from thunder-clouds. The instruments employed for ascertaining the kind and intensity of atmospheric electricity are called Electroscopes. Fig. 65 shows a modification of Saussure’s Electroscope, the basis of which is a narrow-mouthed flint glass bottle with a divided scale to indicate the degree of divergence of the gold leaves or straws. To protect the lower part from rain, it is covered by a metallic shield about five inches in diameter. Bohnenberger’s Electroscope indicates the presence and quality of feeble electric currents. Peltier’s Electrometer yields the same result by the deflection of a magnetic needle. This latter has been in use at Brussels for thirty years, and at Utrecht for twenty years, and is highly recommended.

Singer’s Atmospheric Electroscope is an efficient form of the instrument in which an ordinary gold-leaf electrometer has attached to its circular brass plate a brass rod two feet in length, with a clip at its upper extremity to receive a lighted paper or cigar fusee. The electricity of the air in immediate contact with the flame, causes, by induction, electricity of the opposite nature to accumulate at the upper extremity, where it is constantly carried off by the convection currents in the flame, leaving the conductor charged with the same kind and power of electricity as that contained in the air at the time of the experiment. The principle of this method was initiated by Volta, and has been extended and applied by Sir William Thomson in his Water-dropping Collector, which consists of an insulated cistern from which water escapes through a jet so fine that it breaks into drops immediately after leaving the nozzle of the tube. The result of this is that in half a minute from the starting of the stream the can is found to be electrified to the same extent as the air at the point of the tube. The scale value of each instrument has to be separately determined by repeated comparative experiments, and involves much delicacy of manipulation.

It is chiefly important for the ordinary observer to know that the occurrence of thunder and lightning should be always noted in the column headed “Remarks.”

The destructive effects of lightning are too well known to need description here; the means, however, by which these may be averted demand a brief notice. Lightning when discharged from a cloud will always choose the better of any two conductors which may present themselves. The stone of a church steeple and the wood of a ship’s mast are bad conductors, but a galvanized iron wire rope is the best possible conductor, and accordingly this material is now generally employed for the purpose. A lightning conductor consists of three parts: 1, the rod, which extends beyond the summit of the building, 2, the conductor, which connects the rod with the underground portion, and 3, the part underground. The connection between each of these must be absolutely perfect, or the conductor will be faulty. The top is usually of solid copper tipped with platinum (Fig. 66), the body of galvanized iron rope, so as to adapt itself to the inequalities of the building and yet have no sharp turns in it, while the part underground is of solid iron rod. This latter portion should extend straight underground for two feet, and being bent at right angles away from the wall, should rest in a horizontal drain 10 to 15 feet long filled with charcoal, and be again bent downwards into a well of water. Should water not be available, it should rest in the centre of a hole 15 feet deep and 10 inches in diameter, tightly packed with charcoal, which, while conducting the electricity from the rod into the earth, serves also to preserve the iron from rusting.

OZONE.

The atmosphere, besides holding the vapour of water diffused throughout its mass, contains also minute traces of carbonic acid and ammonia, and a very remarkable substance called Ozone. Oxygen, one of the component gases of the atmosphere, is capable of existing in two conditions; one in which it is comparatively passive, and another in which it possesses exceptional chemical activity, dependent apparently upon its electrical condition, and in which state it possesses a peculiar smell which has caused it to be named ozone.^[16] The characteristic odour is always observable near a powerful electric machine when it is being worked, near a battery used for the decomposition of water, and in the air after the passage of a flash of lightning. Its presence is most marked near the sea-coast, and in localities remarkable for their salubrity; and on account of its influence on health, it has been proposed by Schonbein and others to include ozonometrical observations with the ordinary meteorological observations.

Although in minute quantities it is favourable to health, when existing in undue proportion it irritates the mucous membrane of the nose and throat, producing painful sores. It attacks india-rubber, bleaches indigo, and oxidizes silver and mercury, differing in all these points from ordinary atmospheric oxygen.

The chemical energy it possesses (which exceeds that of ordinary oxygen as much as the latter exceeds atmospheric air as an oxidizing agent) affords the means of ascertaining its presence and quantity. It liberates iodine from its combination with potassium, and free iodine colours starch a deep blue.

Schonbein, the discoverer of ozone, found that when strips of paper previously saturated with starch and iodide of potassium and dried were exposed freely to the air but protected from rain and the direct action of the sun, they underwent a peculiar discoloration (when immersed in water) after an exposure of 24 hours. A scale of tints numbered from one to ten afforded the means of comparative observation, and thus the Ozonometer was constructed, and a means established of registering the amount of ozone in the air of various localities from day to day.

Schonbein also observed that the proportion of ozone was largely augmented after heavy falls of snow. For the exposure of the ozone papers, an ozone cage is employed, as shown at Fig. 67.

Ozone may be prepared artificially as a disinfectant by cautiously mixing without friction or concussion equal parts of peroxide of manganese, permanganate of potash, and oxalic acid. For a room containing 1,000 cubic feet, two teaspoonfuls of the powder, placed in a dish and moistened with water occasionally, will develop the ozone and disinfect the surrounding air without producing cough.

The most important and interesting series of facts, however, connected with ozone are those established by the researches of M. Houzeau, who states:—

1. That country air contains an odorous oxidizing substance, with the power of bleaching blue litmus, without previously reddening it, of destroying bad smells, and of bluing iodized red litmus.

2. That this substance is ozone.

3. That the amount of ozone in the air at different times and places is variable, but this is at most 1/700,000 of its volume, or 1 volume of ozone in 700,000 of air.

4. That ozone is found much more frequently in the country than in towns.

5. That ozone is in greatest quantity in spring, less in summer, diminishes in autumn, and is least in winter.

6. It is most frequently detected on rainy days, and during great atmospheric disturbances.

7. That atmospheric electricity is apparently the great generator of ozone.

The subject is one of great interest in its bearings on health, and opens a wide field of scientific research, as may be inferred from the opinion expressed by the Vienna Congress, which is that “the existing methods of determining the amount of ozone in the atmosphere are insufficient, and the Congress therefore recommends investigations for the discovery of better methods.”

Mr. Lowe has published the valuable weather warnings tabulated on page 94, which are interesting as showing from a given number of observations the value of each phenomenon:—

Among the animals whose movements give weather warnings few are more trustworthy than the leech. The reader may verify this by placing one in a broad glass bottle, tied over with perforated leather, or bladder. If placed in a northern aspect, the leech will be found to behave in the following manner:—

1. On the approach of fine or frosty weather, according to the season, it will be found curled up at the bottom. 2. On the approach of rain, snow, or wind, it will rise excitedly to the surface. 3. Thunder will cause it to be much agitated, and to leave the water entirely.

Periods.—M. KÖppen states, as the result of his examination into the chances of a change of weather, that the weather has a decided tendency to preserve its character. Thus, at Brussels, if it has rained for nine or ten days successively, the next day will be wet also in four cases out of five; and the chance of a change decreases with the length of time for which the weather from which the change is to take place has lasted.

In the case of temperature for five-day periods, the same principle holds good;^[17] for if a cold five-day period sets in after warm weather, we can bet two to one that the next such period will be cold too; but if the cold has lasted for two months, we can bet nearly eight to one that the first five days of the next month will be cold too. The chance of change is, however, greater for the five-day periods than for single days. Similar results follow for the months, but here again the chance of change shows an increase.

“If we revert to the instance first cited, that of rain, the result is, not that if it once begins to rain the chances are in favour of its never ceasing; all that is implied is, that the chances are against its ceasing on a definite day, and that they increase with the length of time the rain has lasted. The problem is similar to that of human life: the chance of a baby one year old living another year is less than that of a man of thirty.

“The practical meaning of all this is, that although we know that a compensating anomaly for all extraordinary weather exists somewhere on the earth’s surface, e.g., the very common case of intense cold in America, while we have a mild winter in Britain, there is no reason as yet ascertained to anticipate that this compensation will occur at any given place during the year. In other words, when definite conditions of weather have thoroughly established themselves, it is only with great difficulty that the courses of the atmospheric currents are changed.”

To bring within the limits of a popular pamphlet a notice of the various phenomena classed under the head of Meteorology, it has been necessary to exercise the utmost brevity. Brief, however, as the treatment has been, reference has been made to the sciences of Heat, Light, Electricity, Magnetism, Gravitation, Astronomy, Chemistry, Geography, and Geology, thus corroborating the testimony of Sir John Herschel, who states that “it can hardly be impressed forcibly enough on the attention of the student of nature that there is scarcely any natural phenomenon which can be fully and completely explained in all its circumstances without a union of several—perhaps of all—the sciences; and it cannot be doubted that whatever walk of science he may determine to pursue, impossible as it is for a finite capacity to explore all with any chance of success, he will find it illuminated in proportion to the light which he is enabled to throw upon it from surrounding regions. But, independently of this advantage, the glimpse which may thus be obtained of the harmony of Creation, of the unity of its plan, of the theory of the material universe, is one of the most exalted objects of contemplation which can be presented to the faculties of a rational being. In such a general survey he perceives that science is a whole whose source is lost in infinity, and which nothing but the imperfection of our nature obliges us to divide. He feels his nothingness in his attempts to grasp it, and he bows with humility and adoration before that Supreme Intelligence who alone can comprehend it, and who ‘in the beginning saw everything that He had made, and behold it was very good.’”