4 ;3 THE POPULAR SCIENCE MONTHLY. 



went out of their way.* Under similar circumstances, Matteucci ob- 

 served 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 pre- 

 vailing, 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 denned by the jjoints 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 aurorae boreales certainly appear to be connected with a par- 

 ticular condition of the atmosphere, and M. de La Rive finds in this a 

 confirmation of his theory. Nearly all the observers agree that cirro- 

 stratus clouds accompany or precede the phenomena, and are frequently 

 seen within the dark segment. Hardly less invariable is the simulta- 

 neous presence in the air of hosts of fine, transparent, microscopic 

 needles of ice, that favor the formation of lunar halos before the au- 

 rora 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 inces- 

 sant 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 re- 

 composition, 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 



* Nevertheless, if the observer is within the circle formed by the aurora, its action on 

 the needle is almost nothing. This fact has been noticed more than once. 



