156 



ATMOSPHERIC ELECTRICITY. 



electric fluid, accumulated in the higher regions, more abundantly to the earth. 

 The electric intensity would, therefore, increase with the humidity and the 

 dew until two or three hours after sunset. Finally, when the air begins to be 

 exhausted, the electricity again diminishes, and continues to decrease till the 

 next morning. According to the same principles, the annual variation of the 

 electricity is explained. In clear weather, the mean intensity of the electrici- 

 ty of the air will be much less in summer than in winter ; for the air in sum- 

 mer, being warm and dry, resists more strongly the transmission of the elec- 

 tric fluid accumulated in the higher regions of the atmosphere, while in winter 

 the air, beiug more humid, produces a contrary effect. 



DISTRIBUTION OF ELECTRICITY OF THE AIR. 



Although the negative electricity of the surface of the globe be a conse- 

 quence of the ascertained fact, that positive electricity is supplied by it to the 

 air, it is necessary, nevertheless, that it be ascertained by immediate observa- 

 tion. This has, accordingly, been done by different observers, at different 

 times, and in different places. Among the more recent observations of this 

 kind, are those of M. Peltier. To ascertain the electricity of the ground, this 

 philosopher used a multiplier, placing one extremity of the platinum wire in a 

 humid part of the soil, and attaching the other end to a pointed metallic con- 

 ductor, raised in the air. When the air was sufficiently humid to give it a 

 conducting power, a current was established through the wire, by which the 

 needle was sensibly affected, and the deflection of the needle proved that the 

 negative current came from the ground, and the positive from the air. 



The negative electricity of the ground, and the positive electricity of the 

 stratum of air contiguous to it, have a continual tendency to re-combine and 

 neutralize each other. From this cause, the lowest stratum of air in clear 

 weather, apart from disturbing causes, is found to be in its natural state. This 

 effect extends to the height of three or four feet from the ground, above which 

 height the positive electricity begins to be perceivable, and increases in its in- 

 tensity in ascending, according to some definite law, which observation has 

 not yet disclosed. 



To ascertain the increase of electricity in the ascending strata of air, Bec- 

 querel and Breschet made some experiments on the Great Saint Bernard, ac- 

 cording to a method suggested by Saussure. These electricians selected a 

 convenient platform of ground near the monastery, extended upon it a piece of 

 gummed sarcenet, about ten feet long and seven feet wide, upon which they 

 unrolled a silk cord, interlaced with metallic wire, measuring about 250 feet in 

 length. They attached one end of this cord to the hook or rod, which commu- 

 nicated with the straws of an electrometer, by means of a loose knot, in such a 

 manner that when drawn upward, it would be detached from the electrometer 

 without disturbing the instrument. The other extremity of the cord was tied 

 to the tail of an iron arrow, which was projected upward by means of a bow 

 with such force that, attaining a height of more than 250 feet, it detached the 

 lower end of the cord from the electrometer. As the arrow ascended, the 

 electrometer showed a gradually increasing divergence, which soon became so 

 considerable that the straws struck the sides of the case enclosing them. 

 When the cord was detached, the instrument retained the electricity it had re- 

 ceived, which, on examination, proved to be positive. 



Hence, it appears that from three feet above the ground, to the height of 250 

 feet, the air is charged with positive electricity, constantly increasing in inten- 

 sity, at least, in localities like that in which this experiment was made. 



Lest it might be supposed that the electricity obtained, was produced by the 



