On the Connexion of the Physical Sciences

The earth possesses a powerful electrical tension, and the atmosphere when clear is almost always positively electric. Its electricity is stronger in winter than in summer, during the day than in the night. The intensity increases for two or three hours from the time of sunrise, comes to a maximum between seven and eight, then decreases towards the middle of the day, arrives at its minimum between one and two, and again augments as the sun declines till about the time of sunset, after which it diminishes and continues feeble during the night. The mere condensation of vapour is a source of atmospheric electricity; but although it is also produced by the vapour that rises from the surface of the earth, it is not under all circumstances. M. Pouillet found that electricity is only developed when accompanied by chemical action: for example, when the water whence the vapour proceeds contains lime, chalk, or any solid alkali, negative electricity is produced; and when it holds in solution either gas, acid, or some of the salts, the vapour is positively electric.

The actual quantity of electricity in any part of a cloud is extremely small. The intensity of the flash arises from the great extent of surface over which it is spread, so that clouds may be compared to enormous Leyden jars thinly coated with electricity, which only acquires its intensity by its instantaneous condensation. The rapid and irregular motions of thunder clouds are probably more owing to strong electrical attractions and repulsions among themselves than to currents of air, though both are no doubt concerned in these hostile movements. The atmosphere becomes intensely electric on the approach of rain, hail, snow, sleet, and wind; but it varies afterwards, and the transitions are very rapid on the approach of a thunderstorm.

When the air is rarefied by heat, its coercive power is diminished, so that the electricity escapes from the clouds in those lambent diffuse flashes without thunder so frequent in warm summer evenings; and when the atmosphere is highly charged with electricity, it not unfrequently happens that electric light, in the form of a star, is seen on the topmasts and yard-arms of ships. In 1831 the French officers at Algiers were surprised to see brushes of light on the heads of their comrades, and at the points of their fingers when they held up their hands. This phenomenon was well known to the ancients, who reckoned it a lucky omen.

A battery charged with water alone, instead of acid, is constant in its action, but the quantity of electricity it develops is comparatively very small. Mr. Cross, of Broomfield in Somersetshire, kept a battery of this kind in full force during twelve months. M. Becquerel had invented an instrument for comparing the intensities of the different kinds of electricity by means of weights; but, as it is impossible to make the comparison with Voltaic electricity produced by the ordinary batteries, on account of the perpetual variation to which the intensity of the current is liable, he has constructed a battery which affords a continued stream of electricity of uniform power, but it is also of very feeble force. The current is produced by the chemical combination of an acid with an alkali. Metallic contact is not necessary for the production of Voltaic electricity, which is entirely due to chemical action. The intensity of the Voltaic electricity is in proportion to the intensity of the affinities concerned in its production, and the quantity produced is in proportion to the quantity of matter which has been chemically active during its evolution. Dr. Faraday considers this definite production to be one of the strongest proofs that electricity is of chemical origin.

A magneto-electric machine has been recently constructed by Mr. Henley, of enormous power. It consists of two permanent magnets, from which the induction is obtained; each of these is formed of thirty horseshoe steel magnets, two feet and a half long, and from four to five inches broad, and each is surrounded by a coil of wire six miles long, coated with silk to insulate the coils. A shock from these wires would be instantaneous death. This apparatus will ultimately be employed to send a stream of electricity through long submarine and subterraneous wires; but a Volta-electric machine has hitherto been used, in which the electricity is generated by a galvanic battery instead of magnets. Induction, or the effect of the spiral wires in augmenting the power of Voltaic electricity, is admirably illustrated in the Atlantic telegraph.

Ruhmkorff’s electro-inductive apparatus has either been improved, or new machines constructed, by Messrs. Grove, Gassiot, and Joule, of intense energy. Indeed, so great is the energy of electro-induction, that hopes were entertained of its superseding steam as a motive power. For the current of electricity from an electro-magnet can be made to flow in opposite directions, so as to produce alternate attractions and repulsions, and consequently a continued motion, which might be applied as a motive force to machinery. However, Mr. Joule has proved that the power developed by one pound of coal in combustion is to that produced by one pound of zinc consumed in Mr. Grove’s powerful electro-magnetic apparatus as nine to one, so that, even if zinc were as cheap as coal, and a Voltaic battery as easily kept in order as an engine-furnace, electricity will not supersede steam as a motive power. A current of electricity traversing a conductor gives out a quantity of heat determined by fixed laws, the amount of which is invariable as long as the machine to which it is applied remains at rest; but the instant the machine is set in motion a reaction takes place in the intensity of the current, causing a diminution in the quantity of heat, because the heat that disappears is converted into the mechanical force exerted by the engine.

But as soon as the plate is inclined to that plane, electricity begins to be developed by its motion across the lines of magnetic force; it becomes more powerful as the inclination increases, and arrives at a maximum when the plate revolves at right angles to the line of dip. When the revolution is in the same direction with that of the hands of a watch, the current of electricity flows from its centre to the circumference; and when the rotation is in a contrary direction, the current sets the opposite way. Thus a copper plate, revolving at right angles to the line of the dip, becomes a new electrical machine, differing from the common plate-glass machine by the copper being the most perfect conductor, whereas glass is the most perfect non-conductor; besides insulation, which is essential to the glass machine, is fatal to the copper one. The quantity of electricity evolved by the metal does not appear to be inferior to that devolved by the glass, though very different in intensity. Even a ship crossing the lines of force must have electric currents running through her. Dr. Faraday observes that such is the facility with which electricity is generated by the magnetic lines of force, that scarcely any piece of metal can be moved without a development of it; consequently, among the arrangements of steam-engines and metallic machinery, curious electro-magnetic combinations probably exist which have never yet been noticed. Thus magnetic lines of force certainly issue from the surface of the globe.

Travail is a word used in mechanics, to express that _work done_ is equal to the labouring force employed. The work done may be resistance overcome or any other effect produced, while the labouring force may be a horse, a steam-engine, wind, falling water, &c. NOTE 223, p. 313. When a stream of positive electricity descends from P to n, fig. 72, in a vertical wire at right angles to the plane of the horizontal circle A B, the negative electricity ascends from n to P, and the force exerted by the current makes the north pole of a magnet revolve about the wire in the direction of the arrow-heads in the circumference, and it makes the south pole revolve in the opposite direction. When the current of positive electricity flows upwards from n to P, these effects are reversed. [Illustration: _Fig. 72._] [Illustration: _Fig. 73._]

James, Colonel, measurements of, in the General Survey of Great Britain, 47; density of the earth determined by, 58. Jamin, M., remarks of, on substances producing elliptical polarization, 193. January, epoch of its beginning the year, 85. Jews, denominations of time in their calendars, 85. Josephstadt, discovery of a comet from, 367. Joule, Mr., heat considered a mechanical force by, 275; his view of elastic force, 276; amount of latent force in a pound of coal, computed by, 278; furnishing data to Professor Thomson, 279; quantity of heat generated in a unit of time by electricity computed by, 302; powerful magnet obtained by electricity, 315; electric machines constructed by, 328; experiments proving heat and mechanical power convertible, 329. Jovial system, mass of the whole, 55. Julian Calendar, year of, the first of our era, 86. June, 1833, reappearance of Saturn’s rings, 67; coincidence of times in, 84. Juno, the diameter of, 56; astronomical tables of, 63.

Magneto-electricity, principle suggesting, 322; machine constructed on the principle of, 325; relation of heat to, 329. Magnets, influence of, on electric light, 307; fish possessing the power of making, 311; effect of an electric stream on, 312-314; obtained by electricity, 315; power of electro, measured, 315; cylinders acting as, 316, 317; producing electrical effects, 322, 323; evolving electricity by rotation, 330; classification of substances in relation to, 332; polarity a property of, 336; effect on themselves of imparting paramagnetism, 337; experiment showing the lines of force of, 338; properties of, indestructible by subdivision, 338, 339; the earth reckoned among, 342; planets reckoned among, 346; action of an electro, on copper, 351. Maguire, Captain, his observations on magnetic storms, 345, 346. Malo, St., rising of the tide at, 98. Malus, M., discovery of polarization of light by, 195; attempts of, to polarize heat, 264. Malta, observations on Saturn’s rings made at, 66. Manchester, thunderstorm near, in 1835, 292. Mankind, distinct tribes of, 255; limited perceptions of, 267. Marcet, M., rate of increase in temperature below the earth’s surface calculated by, 230. Marco Polo, atmospheric effects observed by, in ascending mountains, 118.

Waterspouts, origin and cause of, 128. Waterstone, Mr., magnetic property of the ethereal medium maintained by, 357. Waves neutralized by interference, 99. ——, atmospheric, over local districts, periods, dimensions of, 121, 122. —— of sound, 131; furnishing an illustration of reflections of sound and light, 137; interference of, producing calm, 139. Wedgwood, Dr., attempts of, to trace objects by means of light, 203, 204. Week, the, of seven days, the most ancient and universal division of time, 85. Wells, increase of temperature in, 230, 231. Welsh, Mr., observations made by, in a balloon ascent, 119. West Indies, the, cause of hurricanes in, 126. Wheels invented to test intensity of sound, 132, 133. Wheat, range of its cultivation, 250. Wheatstone, Professor, experiments in acoustics of, 132; musical instruments invented by, 143; paper on musical vibrations read by, 145; experiments on sounding boards of, 150; experiments on sound reinforced by resonance, 151; instrument measuring velocities of electricity and light invented by, 202; spectrum of an electric spark observed, 289; speed of electricity measured, 289, 290; experiments on the spectrum of Voltaic flame, 303. Willis, Mr., articulating machine invented by, 151; investigations of, into the mechanism of the larynx, 152.