SOURCES OF AERIAL ELECTRICITY.

211

a discharge from an apparatus, similar to that of M. Dalibard, with which he was experimenting.

Cavallo, in 1777, raised an electric kite repeatedly in the neighbourhood of London, and obtained an enormous quantity of electricity; he found that the electricity frequently changed its character, as the kite passed through different aerial layers or strata.

309. Perhaps the readiest mode of investigating the electric state of the upper regions, is by means of the apparatus used by MM. Becquerel and Breschet, on the great St. Bernard.* These gentlemen fixed one end of a cord covered with tinsel, about ninety yards in length, to the cap of an electrometer, and tying the other to an arrow, they projected it, with the aid of a bow into the air, and they found that the gold-leaves diverged in proportion as the arrow ascended into the atmosphere.

310. The probable cause of the free electricity in the air has been referred to various sources; the phenomena of animal and vegetable life, as well as chemical action, have been called in to explain its origin. Among others, the evaporation of water, and other fluids, constantly taking place on the earth's surface, may certainly be regarded as one of the sources of atmospheric electricity. The evolution of electricity by evaporation, may be readily proved by placing on the cap of a gold-leaf electrometer (239) a small metallic cup containing water, in which some common salt has been dissolved. On dropping into it a piece of hot cinder, the vapour will arise copiously and carry off positive electricity, leaving the cup negatively electrified, with which electricity the gold-leaves will diverge. If water, containing a minute portion of an acid, be substituted for the weak brine, the reverse will occur, the gold-leaves diverging with positive. electricity, the vapour being negatively electrified.

311. The clouds, consisting of immense masses of aqueous

* Traité de l'Electricité et du Magnétisme. t. iv. p. 110.

vapour, are tolerably good conductors of electricity, and consequently, contain a considerable quantity of the latter in a free state. Two clouds, being in different electric states, act upon each other through the particles of the intervening dielectric, or air; like the inducing surfaces or metallic coatings of a charged jar (280), and when sufficiently near to each other, discharge occurs, producing the vivid flash well known as lightning, generally accompanied by the loud reverberated sound of thunder. When, on the other hand, induction, and consequent charge takes place through the air, between an electrified cloud and the earth, an explosion or discharge ensues, when the intervening particles of the dielectric are so arranged as to admit of its occurring; producing a second, and much dreaded form of lightning. This mode of establishing an equilibrium between two oppositely electrified bodies, often ensues through the medium of the nearest most prominent conductor, which, if a tree, is often riven in sunder; if a building, as a church, is frequently dashed in pieces; and if an animal, too often severely injured or even killed.

312. Several instances have occurred of the fatal effects of a tempest, having been exerted on animals at a considerable distance from the spot where the most serious effects have taken place, and where the violence of the lightning appeared to have been chiefly exerted. This will readily admit of explanation, on the supposition of a lateral explosion or returning shock (299) having occurred. Thus, if AB be

a large cloud, positively electrified, approaching at its end, a, within striking distance of the church-steeple c, the extremity

B, will, by its inductive action, decompose the electricities present in any object at D, as a traveller for example, repelling the positive to the earth, and leaving him in a negative state. When A has approached sufficiently near to c, an explosion will occur, and electric equilibrium will ensue. B being thus left unelectrified, no longer exerts a coercing force on the negative electricity in D, which, attracting the positive electricity previously repelled by B, causes it to rush with violence into D, producing discharge, and a restoration of electric equilibrium, with such mechanical force, however, as too often to kill the unfortunate individual situated at D.

*

313. Science, and mankind generally, must ever remain debtors to the ingenuity of Dr. Franklin, for proposing, at least, a partial protection against these dreaded effects of the tempest, in the invention of the paratonnerre, or lightning rod. These consist of metallic conductors, of sufficient thickness, usually fixed against the sides of the building they are destined to protect, their upper extremity extending some feet above it, and terminates in a point, which is best constructed of some metal not liable to oxydation. The lower end is buried in the earth, to the depth of a few feet. For ships, flexible paratonnerres, composed of copper chain, or slips of that metal, are fixed to the masts, and reaching from their highest points to the outside of the keel of the vessel, so as to conduct the electricity harmlessly to the water in which the vessel floats. Whatever form is used, one general precaution is necessary, that all and every portion of the paratonnerre should be as perfectly continuous as possible, for wherever a break or interruption occurs, the electric fluid, in rushing from one portion to another, is liable to produce the very danger which these instruments are intended to avert.

314. To illustrate some of these positions, the thunderhouse, as it is termed, was invented by Dr. Franklin: A B

• See Traité Elémentaire de Physique, par M. L'Abbé Hauy, p. 434. Paris, 1806.

is a piece of hard dry wood, cut into the shape of the gable end of a house, with a brass rod, terminating in a ball at c,

At E

fixed against its side, and terminating at D in a hook. this conductor is interrupted by a block of wood, fitting loosely into a cavity made to receive it, having a wire fixed across it; so that when E is fitted in its place, as in the figure, the conductor CD is perfect; but when placed in the opposite direction, as shown by the dotted line, the paratonnerre CD is interrupted in its centre.

EXP. (A.) Charge the jar F; connect its outside with the hook at the end of D, and its knob with the pointed wire supported on its insulating stand н, and bearing on its apex the brass rod K, terminating in balls, and moving on it in any direction, as on a pivot. Place the window E in its place, so that the brass conductor may be continuous, and cause к to revolve, so that one of the balls terminating it may pass within half an inch of c. The jar will be discharged, and the window E remain unmoved.

(B.) Repeat the last experiment, with the window E, placed so that its wire may be at right angles to the axis of the wire CD. On discharging the jar as before (A), the effects of the explosion will be exerted on E, and will project it with violence, from the cavity into which it fits.

(C.) Let things be arranged as in Exp. (B), but remove the knob on c, and leave the paratonnerre pointed. On allowing K to revolve, the jar will be silently discharged.

The electric current, during this gradual discharge by the point, never acquiring sufficient tension to act energetically on E, although it was displaced with violence when D terminated in a knob.

(D.) The protecting influence of pointed conductors is more strikingly shown by the electrical toy, called the powder magazine, in which the interrupted portion of the conductor reposes in a mass of gunpowder, placed in a wooden model of a house. If the jar be discharged whilst the paratonnerre terminates in a point, the powder is unaffected; but if a knob be screwed on, the discharge explodes the powder, and blows the model to pieces. In repeating this experiment, a piece of wet string should be used to connect the jar with the base of the paratonnerre, for reasons already mentioned (293 d).

315. The well-known meteoric appearances so frequent on the pointed masts of shipping, known as Castor and Pollux, the feu de St. Elm of the French, and Elmsfeuer of the Germans, appear to depend upon the slow discharge of atmospheric electricity by the pointed masts of the vessel.

316. The beautiful aurora borealis, so frequent in the north of Europe, and of late years not of unfrequent occurrence in the neighbourhood of the metropolis, depends, in all probability, on the passage of electricity through an highly rarefied medium. From the calculations of Mr. Cavendish, it is probable that the aurora usually appears at an elevation of about seventy-one English miles above the earth's surface; at which elevation the atmosphere must be of 148,567 times the density of that at the earth's surface, a degree of rarefaction far above that afforded by our best air-pumps. As electricity is diffused in a quantity nearly proportionate to the elevation above the earth's surface, it appears very probable, that under favorable circumstances, it would appear luminous to us, in the vast regions of rarefied air terminating our atmosphere, in a manner analogous to that in which it appears on an infinitely smaller

but