Page:Popular Science Monthly Volume 18.djvu/534

518 farther from the surface of the earth, the potential of points in the air differs more and more from that of the earth, the difference being approximately simply proportional to the distance. Also the electrical density is greater on projecting parts of the earth's surface than on those which are plane or concave. These facts are precisely what we should find to be the case in the vicinity of an irregular, charged conductor; and they are sufficiently explained if we regard the earth itself as charged with electricity varying in density. In fact, observations of the potential of so-called atmospheric electricity are simply "determinations of the quantity of electricity residing on the earth's surface at the place of observation." (Professor Everett, in Deschanel's "Natural Philosophy.")

In the Rocky Mountains electrical storms are of frequent occurrence. They consist of electrical displays without precipitation of rain, hail, or snow. Usually, though not always, the sky is overcast. In February, 1880, a remarkable electrical excitation was manifest at Boulder, Colorado. The miners were unable to kindle fire in the stove till eleven o'clock in the morning, every attempt to touch the metal about the stove resulting in a severe electrical shock. With every strong gust of wind the manifestations were more marked. Similar reliable evidence comes to the writer from other parts of Colorado. Long's Peak, an isolated mountain, 14,271 feet high, is noted for these peculiar electrical manifestations. The density of electricity on the peaks, projecting so far above the general level of the earth's surface, is greater than elsewhere; hence they possess a power to discharge like points on an electrified conductor. The air and aqueous vapor become surcharged with electricity; and only a slight condensation, sufficient merely to form clouds without rain, serves to produce discharges of lightning. With heavy gusts of wind the charged air is removed, and a fresh supply is provided into which the peak again pours its electricity.

In view of these facts, the theory is submitted as worthy of consideration that the earth itself is the seat of those disturbances that manifest themselves in atmospheric electricity. Fluctuating currents ebb and flow through the confining walls of this immense reservoir of cosmic energy. These follow naturally from the great changes in temperature to which the earth's crust is subjected; from those seismic disturbances occasioned by vast internal convulsions; from immeasurable local strain and compression, the result of upheaval and contraction. The earth, unlike the moon, contains still a vast store of unexpended energy; and in the ebb and flux of its mighty internal, contending forces, and the bending and swaying of its magnetic lines of force, in obedience to the magic wand of the sun, there is ample room for the generation of those comparatively feeble forms of energy that manifest themselves in the electrical disturbances of the air.