ATMOSPHERIC ELECTRICITY. 



157 



friction of the arrow against the air, the experiment was repeated, projecting 

 the arrow honzontally, at the height of three feet from the ground. In this 

 case no effect was produced on the electrometer. 



Becquerel made experiments with a like object in clear weather, on the 

 summit of the rock called Sanadoire, near the Mont d'Or. This summit, sep- 

 arated from the surrounding mountains, is terminated by a platform of the ex- 

 tent of several square yards, at the height of about 4,600 feet above the level 

 of the sea. The electrometer of Saussure was surmounted by a pointed con- 

 ductor, about twenty inches long. A divergence of the straws, amounting to 

 an eighth of an inch, was produced, when the apparatus was raised about three 

 feet above the head. The divergence was doubled, when a wire, attached to 

 the electrometer, was projected upward by means of a 'stone, to the height of 

 about thirteen feet, and when projected to greater heights, the divergence con- 

 tinued to augment. 



When the apparatus, elevated to a certain height above the head, and show- 

 ing a certain divergence, was carried down the side of the hill, the divergence 

 gradually diminished, and disappeared altogether, before attaining the foot of 

 the hill. 



In the ascent made in a balloon by MM. Gay-Lussac and Biot, the increase 

 of positive electricity in the ascending strata of air was also rendered mani- 

 fest. These philosophers attached a metallic ball to a wire, about 170 feet 

 long, and suspended it from the car of the balloon, the upper end of the wire 

 being attached to an electrometer. The weather being perfectly clear, the in- 

 strument diverged with negative electricity. This result, which was in appa- 

 rent discordance with the results of observations in general, was, however, 

 easily shown to be consistent with them. The wire, in this case, supplied a 

 conducting communication between two strata of air, one 170 feet above the 

 other. If they were equally charged with the same species of electricity, the 

 electrometer would not have been affected ; for the natural electricities of the 

 wire being placed between two equal and contrary decomposing influences, no 

 decomposition would take place, and the wire would remain in its natural state. 

 If, however, the two strata at the ends of the wire were electrified positively, 

 in different degrees, a decomposition of the electricities of the wire would en- 

 sue, the positive fluid being repelled toward that stratum having the weaker 

 positive charge, and the negative fluid being attracted toward that stratum hav- 

 ing the stronger charge. Since, then, the electrometer at the upper extremity 

 of the wire showed negative electricity, it follows that the higher stratum was 

 more intensely positive than the lower. 



In a similar experiment made by Saussure, the electrometer was placed at 

 the lower end of the wire, and, in accordance with what has been just ex- 

 plained, the instrument diverged with positive electricity. 



The method of explaining the apparently inconsistent results of the experi- 

 ments of Biot and Saussure, proposed by the former, is imperfect, unless it be 

 admitted that the two strata of air are both electrified positively ; for if they 

 were both electrified negatively, the lower stratum having the stronger charge, the 

 same effects would ensue ; or even if they were differently electrified, the 

 upper stratum being positive and the lower negative, the effects would be the 

 same. 



Strictly speaking, therefore, the consequence which legitimately follows, 

 from all observations made on the electricity of the air at different heights, by 

 means of a vertical conducting-rod or wire extending from the electroscope to 

 the stratum of which it is desired to ascertain the electric state, is, not that the 

 electricity with which the instrument diverges, is that of the air in which the 

 remote extremity of the conductor is placed, but that if E' be the electricity of 



