Page:Popular Science Monthly Volume 18.djvu/532

516 These two conclusions, applicable to condensation, may be applied to the frequently observed fact that a vivid flash of lightning is often quickly followed by a sudden and heavy down-pouring of rain. It is clearly impossible to tell which is antecedent to the other, the discharge or the sudden condensation; for, while the flash reaches the observer first, it is clear that light travels from the place of action with a vastly greater velocity than that of the falling rain, and the discharge may therefore have been subsequent to the sudden condensation. If the discharge occurs first, then the lowering of the electric potential permits approach of aqueous spherules and consequent coalescence after collision, in accordance with Lord Rayleigh's experiments. On the other hand, if the condensation is antecedent, it follows from the result reached above that it must be accompanied by a sudden rise of potential in the enlarged drops, leading to an electric discharge.

It will be observed that neither of these principles accounts for the original electrification of aqueous vapor in the air. It has been the custom to regard thunder-clouds as primarily charged by some unexplained process; and these, acting inductively, as producing a corresponding charge of opposite sign in the earth underneath. This view appears to have no conclusive evidence in its favor, but corresponds rather to appearances merely—a very unreliable guide.

In the theory here proposed, the earth is the charged body, acting inductively on the air, aqueous vapor, and clouds about it. Whenever moisture condenses to cloud, a better conductor is thereby formed, and increased inductive action takes place, causing an accumulation of electricity both in the cloud above and the earth beneath. If, then, the lower part of the cloud, under this inductive influence, condenses to rain and falls away from the upper part, a separation of the two electricities is effected, and an increase in potential results from the enlargement of the drops, as explained above. Consequently, a discharge may then take place either between the upper and lower cloudy strata, or between the lower portion and the earth, according as one path or the other offers the least resistance.

Further, when evaporation takes place from an electrified locality, the rising vapor must carry away a charge of electricity by convection, as air and dust carry away electricity from a charged conductor. The condensation of this vapor increases its potential, and, if sufficiently rapid, gives rise to electrical displays. It is a fact of recent establishment that "northern lights" occur in various high latitudes only with southerly winds, which come laden with moisture and probably with electricity.

The following considerations in favor of this view of the origin of atmospheric electricity may be briefly enumerated:

1. Continuous observations of the electrical state of the atmosphere at Kew Observatory and elsewhere for several years show that the air is always more or less electrified. The average potential of