ATMOSPHERIC ELECTRICITY. 335 



tlie divergence "of tlie gold leaves or straws of an electrometer is owing 

 to a superior positive induction, and not to the electricity which the 

 surrounding air has communicated to this instrument. Finally, if 

 we recollect that the presence of an ignited body is sufiicient to render 

 this divergence greater, we may perhaps conclude that the daily vari- 

 ations of atmospheric electricity, such as have been observed by 

 De Saussure and Schiibler are only in a great degree the effect of 

 changes which, during the course of the day, take place in the electric 

 radiation of the electrometer. In fact, these changes depend directly 

 on the greater or less facility with which the negative electricity 

 can escape, and consequently on the conductibility of the air. 

 Everything which tends to increase this conductibility will favor the 

 electric radiation, and consequently the divergence of the gold leaves 

 produced by the repelling electricity ; on the contrary, everything that 

 diminishes the conductibility also weakens the radiating power and the 

 divergence of the instrument. But as the conductibility of the air is 

 in its turn dependent on the quantity of vapors it contains, it appears 

 to us that an electrometer with a metallic rod ending in a point, 

 would give in the same circumstances, that is to say, under a clear sky, 

 indications in direct accordance with the hygrometric state of the air. 

 Accordingly, the electric radiation, and consequently the diver- 

 gence of the gold leaves, will increase at sunrise, on account of the 

 vapor which rises in the air, and render this medium a better con- 

 ductor of electricity; it will be the same in the evening towards sun- 

 set, v/hen the decrease in the temperature will produce the condensation 

 of the vapor which is in the lower strata of the air. Toward the 

 middle of the day, on the contrary, the warmer air is at the same 

 time dryer and consequently a poorer conductor of electricity; 

 hence the radiation of the electrometer as well as the divergence of its 

 leaves will diminish. Finally, this divergence ought likewise to 

 decrease some hours after sunset, when, by the fall of the dew_, the air 

 becomes relatively less a conductor. 



Thus the daily variations which the electrometer exhibits in its 

 indications appear to us to explain in part the changes which the 

 electric radiation of this instrument undergoes in the course of the 

 day. In the same manner we account for many observations made by 

 De Saussure and other scientists. It is known that De Saussure 

 found the intensity of positive electricity under a clear sky stronger 

 onquays, and particularly on bridges, than in the open country. 

 This ought evidently to be the case, not because the vapors which rise 

 constantly from the surface of the water convey to the air a greater 

 quantity of positive electricity, as has been supposed, but because the 

 air, being in these places more saturated with moisture than elsewhere, 

 ought to possess greater conductibility, and consequently render easier 

 the electric radiation of the electrometer. In the same manner may 

 be explained why the electric intensity is more feeble in summer than 

 in winter, since, in summer the air being less moist than in winter, 

 would also be less a conductor of electricity. We see, also, why in the 

 times of prolonged drought the intensity of the electric periods decrease 

 gradually, as has been observed by Schiibler.* 



* Journal de Sch»veigger, torn. VIII, p. 28. 



