A conjecture has recently been started that electricity may be generated by the mere impact of minute particles of water vapor against minute particles of air.[13] If this conjecture could be established as a fact, it would be amply sufficient to account for all the electricity of the atmosphere. From the very nature of a gas, the molecules of which it is composed are forever flying about with incredible velocity; and therefore the particles of water vapor and the particles of air, which exist together in the atmosphere, must be incessantly coming into collision. Hence, however small may be the charge of electricity developed at each individual impact, the total amount generated over any considerable area, in a single day, must be very great indeed. It is evident, however, that this method of explaining the origin of atmospheric electricity can only be regarded as, at best, a probable hypothesis, until the assumption on which it rests is supported by the evidence of observation or experiment.
Length of a Flash of Lightning.—It would seem, then, that we are not yet in a position to indicate with certainty the sources from which the electricity of the atmosphere is derived. But whatever these sources may be, there can be little doubt that the electricity of the atmosphere is intimately associated with the minute particles of water vapor of which the thundercloud is eventually built up. This consideration is of great importance when we come to consider the special properties of lightning, as compared with other forms of electricity. The most striking characteristic of lightning is the wonderful power it possesses of forcing its way through the resisting medium of the air. In this respect it incomparably surpasses all forms of electricity that have hitherto been produced by artificial means. The spark of an ordinary electric machine can leap across a space of three or four inches; the machine we have employed in our experiments to-day can give, under favorable circumstances, a spark of nine or ten inches; the longest electric spark ever yet produced artificially is probably the spark of Mr. Spottiswoode’s gigantic induction coil; and it does not exceed three feet six inches. But the length of a flash of lightning is not to be measured in inches, or in feet or in yards; it varies from one or two miles, for ordinary flashes, to eight or ten miles in exceptional cases.
This power of discharging itself violently through a resisting medium, in which the thundercloud so far transcends the conductor of an electric machine, is due to the property commonly known among scientific men as electrical potential. The greater the distance to which an electrified body can shoot its flashes through the air, the higher must be its potential. Hence the potential of a thundercloud must be exceedingly high, since its flashes can pierce the air to a distance of several miles. And what I want to point out is, that we are able to account for this exceedingly high potential, if we may only assume that the minute particles of water vapor in the atmosphere have, from any cause, received ever so small a charge of electricity. The number of such particles that go to make up an ordinary drop of rain are to be counted by millions of millions; and it is capable of scientific proof that, as each new particle is added, in the building up of the drop, a rise of potential is necessarily produced. It is clear, therefore, that there is practically no limit to the potential that may be developed by the simple agglomeration of very small cloud particles, each carrying a very small charge of electricity.[14]
This explanation, which traces the exceedingly high potential of lightning to the building up of rain drops in the thundercloud, suggests a reason why it so often happens that immediately after a flash of lightning “the big rain comes dancing to the earth.” The potential has been steadily rising as the drops have been getting larger and larger, until at length the potential has become so high that the thundercloud is able to discharge itself, and almost at the same moment the drops have become so large that they can no longer be held aloft against the attracting force of gravity.
Physical Cause of Thunder.—Let us now proceed to consider the phenomenon of thunder, which is so intimately associated with lightning, and which, though perfectly harmless in itself, and though never heard until the real danger is past, often excites more terror in the mind than the lightning flash itself. The sound of thunder, like that of the electric spark, is due to a disturbance caused in the air by the electric discharge. The air is first expanded by the intense heat that is developed along the line of discharge, and then it rushes back again to fill up the partial vacuum which its expansion has produced. This sudden movement gives rise to a series of sound waves, which reach the ear in the form of thunder. But there are certain peculiar characteristics of thunder which are deserving of special consideration.
Rolling of Thunder.—They may be classified, I think, under two heads. First, the sound of thunder is not an instantaneous report like the sound of the electric spark—it is a prolonged peal lasting, sometimes, for several seconds. Secondly, each flash of lightning gives rise, not to one peal only, but to a succession of peals following one another at irregular intervals. These two phenomena, taken together, produce that peculiar effect on the ear which is commonly described as the rolling of thunder; and both of them, I think, may be sufficiently accounted for in accordance with the well-established properties of sound.
To understand why the sound of thunder reaches the ear as a prolonged peal, we have only to remember that sound takes time to travel. Since a flash of lightning is practically instantaneous, we may assume that the sound is produced at the same moment all along the line of discharge. But the sound waves, setting out at the same moment from all points along the line of discharge, must reach the ear in successive instants of time, arriving first from that point which is nearest to the observer, and last from that point which is most distant. Suppose, for example, that the nearest point of the flash is a mile distant from the observer, and the farthest point two miles—the sound will take about five seconds to come from the nearest point, and about ten seconds to come from the farthest point; and moreover, in each successive instant from the time the first sound reaches the ear, sound will continue to arrive from the successive points between. Therefore the thunder, though instantaneous in its origin, will reach the ear as a prolonged peal extending over a period of five seconds.
Succession of Peals.—The succession of peals produced by a single flash of lightning is due to several causes, each one of which may contribute more or less, according to circumstances, toward the general effect. First, if we accept the results arrived at by Professor Ogden Rood, of Columbia College, what appears to the eye as a single flash of lightning, consists, in fact, as a general rule, of a succession of flashes, each one of which must naturally produce its own peal of thunder; and although the several flashes, if they follow one another at intervals of the tenth of a second, will make one continuous impression on the eye, the several peals of thunder, under the same conditions, will impress the ear as so many distinct peals.
The next cause that I would mention is the zigzag path of the lightning discharge. To make clear to you the influence of this circumstance, I must ask your attention for a moment to the [diagram] on next page. Let the broken line represent the path of a flash of lightning, and let O represent the position of an observer. The sound will reach him first from the point A, which is nearest to him, and then it will continue to arrive in successive instants from the successive points along the line A N and along the line A M, thus producing the effect of a continuous peal. Meanwhile the sound waves have been traveling from the point B, and in due time will reach the observer at O. Coming as they do in a different direction from the former, they will strike the ear as the beginning of a new peal which, in its turn, will be prolonged by the sound waves arriving, in successive instants, from the successive points along the line B M and B H. A little later, the sound will arrive from the more distant point C, and a third peal will begin. And so there will be several distinct peals proceeding, so to speak, from several distinct points in the path of the lightning flash.
