Page:Popular Science Monthly Volume 24.djvu/494

478 went out of their way. Under similar circumstances, Matteucci observed the iron of the Tuscan telegraphic apparatus to be so strongly magnetized that the entire service between Florence and Pisa was interrupted. In the United States, when the like conditions are prevailing, the telegraphers work their instruments without the batteries.

The beautiful arcs of light which are observed in the polar regions have their culminating point on the magnetic meridian, as the vertical plane defined by the points of a horizontal magnetic needle is called. Bravais thought these arcs, or the circles of which they form part, were concentric with the magnetic axis of the globe, or with the straight line uniting the two magnetic poles and passing through the center of the earth. The arcs, then, do not coincide "with the geographical parallels, a fact which the earlier observers had already perceived. The magnetic pole is, moreover, not immovable, but its position may vary during a century several degrees in longitude or latitude.

The auroræ boreales certainly appear to be connected with a particular condition of the atmosphere, and M. de La Rive finds in this a confirmation of his theory. Nearly all the observers agree that cirrostratus clouds accompany or precede the phenomena, and are frequently seen within the dark segment. Hardly less invariable is the simultaneous presence in the air of hosts of fine, transparent, microscopic needles of ice, that favor the formation of lunar halos before the aurora itself breaks out. The essential points of M. de La Rive's theory are that the earth is charged with negative fluid, and the same is the case with the strata of air very near the soil. The upper regions of the atmosphere are, on the other hand, positively electrified. This double fact, the result of certain experiments, is not denied by any one. The two electricities of opposite polarity, accumulated near the tropics in enormous masses, are combined at the poles, where the air, less moist, is a better conductor. The polar discharges produce incessant calls of fluid, if we may use such an expression, and currents of electricity are constantly departing from the equator toward the poles, one kind traveling through the rarefied gases of the upper strata, and the other kind through the ground. It is from the phenomenon of recomposition, favored by the presence of infinitesimal vesicles of air, of imperceptible snow-crystals, and of little icy needles, that proceeds the meteor of which we are trying to present the history.

M. de La Rive satisfied himself of the sufficiency of his theory by an experiment. Tubes were inserted opposite to each other into the sides of a glass bottle. The air within the bottle was exhausted by means of one of the tubes, while in the other one was fixed a rod of iron projecting on the outside, and having its other end prolonged to