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.