ATMOSPHERIC ELECTRICITY. 297 



Great St. Bernard, they stretclied on the earth a piece of taffeta pre- 

 j^ared with gum, on which they coiled a silk tliread, covered Avith 

 tinsel, 2fi2 feet long. They placed one of the ends in communication 

 with the stem of a straw electrometer by means of a knot I'astened 

 loosely to it; and after having attached the otlierend to tlie iron point 

 of an arrow, they shot it upwards by means of a strong bent bow. The 

 arrow as it rose bore away the conducting thread, as in the experi- 

 ments of De Saussure, and separated it from the stem of the elec- 

 trometer. Messrs. Bccquerel and Breschet observed that the straws 

 of their instrument diverged gradually as the arrow rose, so as at last 

 to strike strongly against the sides of the bell. They also ascertained 

 that the electricity transmitted to the electrometer by the arrow did 

 not proceed from its friction against the air ; for they observed that the 

 experiment was unsuccessful when the arrow was shot horizontally at 

 three feet above the ground. 



In 1785 De Saussure* added an important improvement to his at- 

 mospheric electrometer. Availing himself of the action which points 

 exercise on the electric fluid, he surmounted it with a conductor, ter- 

 minating in a pointy two feet long and composed of three pieces which 

 could be adjusted to each other. The length of two feet appeared to 

 him the most proper, because a longer conductor would render the 

 instrument much more difficult to manage without increasing its deli- 

 cacy. To the conductor was attached a little umbrella of very thin brass 

 plate, of a conical form, and four inches and a half in diameter, de- 

 signed to preserve the electrometer from the rain or snow. By this 

 addition the instrument acquired such a delicacy that the two small 

 balls would diverge when a stick of sealing wax that had been elec- 

 trified was passed rapidly over it, at a distance of fifteen or eighteen 

 inches. 



To make his observations De Saussuref chose an open place, free 

 from trees and houses, and there put the plate and conductor of his 

 instrument into communication with the ground ; he then elevated the 

 point so as to bring it to the height of the eye. After having no- 

 ticed to how many lines the divergence of the little balls corresponded, 

 he slowly lowered the electrometer towards the earth, and marked 

 the height of the point of the conductor at the moment when the di- 

 vergence had entirely ceased, as the distance from the ground at which 

 the atmospheric electricity began to be perceptible. He regarded the 

 electricity as exhibiting itself at the surface of the earth, when the 

 electrometer without a conductor diverged when it was simply placed 

 on the ground. Finally, if on being placed at the height of the eye, 

 and consequently its point two I'eet higher — that is to say, seven 

 ieet — the electrometer exhibited no divergence, he raised it one or two 

 feet higher ; but as then he could no longer observe the balls, he 

 touched the bottom of the conductor with the other hand, and then 

 brought down the instrument near the earth to see if it was electrified. 

 If it was so, he concluded that the atmospheric electricity was sensible 

 at eight or nine feet. If it was not so, he marked in his journal 0, 



'' Voyage dans les Alpes, torn. II, sec. 791, page 203, and sec. 793, page 211. 

 ■j- Voyage dans les Alpes, torn. II, sec. 798, page 216. 



