ATMOSPHERIC ELECTRICITY. 357 



of the opposite electricity, will, by virtue of its excess, act directly 

 on the instrument placed at the surface of the earth, producing there a 

 negative divergence. If, aiterwards, these two clouds happen to sej^arate 

 from each other, the electrometer must exhibit the positive divergence 

 which it at first had. We may conceive, says Volta, that these clouds, 

 driven by the wind and the different currents of air which appear 

 everywhere to exist in stormy weatlier, undergo a sort of fluctuating 

 movement sufficient to cause the various electric states which are 

 observed in the electrometer. For the purpose of reproducing, by ex- 

 l^eriment, an imitation of this phenomenon, Volta placed, horizontally, 

 one below the other, two disks of metal,, insulated and movable, 

 charged with opposite electricities. As the distance that se})arated 

 them was very great, the lower disk, which was negative, produced^ 

 in an electrometer placed below it, a negative divergence ; but in 

 proportion as the same disk was raised or the other was lowered the 

 negative signs gradually decreased till they sunk to zero, after which 

 the opposite electricity was manifested. 



It is very probable that a great number of changes which are 

 observed in electricity during storms are only effects similar to those 

 under discussion. If we unite to them the numerous discharges 

 which, in disturbing the electrical equilibrium at every moment, modify 

 the action, of induction, to which the instruments are subjected, we 

 shall, without difficulty, coD:>prehend that the numerous oscillations 

 ought to be exhibited in the intensity as well as in the nature of their 

 electricity. M. Peltier* thoui^ht that these oscillations might also 

 result from the partial electrical changes which take place from the 

 interior towards the exterior of a thunder cloud. In fact, we have sc.>n 

 that, in the way shown by this philosopher, the partial conductability 

 of the clouds gives to them distinct atmospheres — the one exterior 

 and a great number of others interior. When the external atmo- 

 sphere has escaped by an instantaneous discharge, all the internal 

 masses, which have their own atmospheres, restore gradually to the 

 surface the electricity which it had lost. These partial changes which 

 follow the external discharge of a cloud must be manifest to the in- 

 struments placed at the surface of the earth, if the latter have an 

 adequate sensibility; for example, in an electrometer we see the leaves 

 spring out, open, close suddenly, or strike against the armatures 

 without any explosion being heard. 



It is after a certain number of these partial discharges that we see 

 the flash of lightning which indicates the discharge from the surface, 

 after which the internal exchanges recommence, and with them the 

 oscillations of the electrometer. 



We design to state in the second part of this memoir all the results 

 to which the observations hitherto made on atmospheric electricity 

 lead, in order to present in the same outline the state of our knowledge 

 of this branch of meteorology. We may remark that among these 

 results some of them are derived from observations too limited in 

 number to definitively take rank in science, and that as to others, 

 they were obtained by means of too imperfect instruments, or those of 



^ Ann. dc Chira. ct de rhj'sifiue, 3d series, tome IV, page 423. 



