ELECTRICITY. 



137 



pare numerically its depth on different bodies, or on different parts of the same 

 body. 



With this instrument he measured the proportion in which electricity was 

 shared between insulated conductors when brought into contact, and also the 

 law according to which its depth varied on different parts of the same insulated 

 conductor. These results acquired, at a later period, still greater importance, 

 supplying, as they did, tests by which the mathematical analysis of the science 

 could be tried. 



The same apparatus supplied the means of investigating the law according to 

 which an insulated electrified conductor had its charge gradually diminished by 

 dissipation in the surrounding air, and by the escape of the fluid by the imper- 

 fect insulation of the supports. 



The results of the observations of Coulomb on the distribution of the elec- 

 tric fluid on the surfaces of conductors illustrated satisfactorily the doctrine of 

 points, which formed so prominent a part of Franklin's researches. The the- 

 oretical solution of this problem was not, however, effected till a later period. 



The demonstration of the identity of lightning and electricity naturally di- 

 rected the attention of philosophers to the solution of other meteorological phe- 

 nomena by means of the same agency. The explanation of the aurora borealis 

 had long exercised the sagacity and baffled the attempts of those devoted to 

 physical researches. Some ascribed this appearance to solar light refracted 

 in the higher regions of the air, others assigned it to the agency of the mag- 

 netic fluid. Euler imagined it to proceed from the same ether which formed 

 the tails of comets ; Mairan conceived it to arise from the mixture of the at- 

 mosphere of the sun with that of the earth ; but when the properties of elec- 

 tric light became known, and when its appearance in rarefied air had been ob- 

 served, all these hypotheses were by common consent abandoned, and no 

 doubt was entertained that, whatever might be the details of the natural process 

 by which it was produced, the aurora borealis was an effect of atmospheric 

 electricity. Eberhart, professor at Halle, and Paul Frisi at Pisa, were the first 

 who proposed an explanation of it, founded on the following facts : "1. Elec- 

 tricity transmitted through rarefied air exhibits a luminous appearance, precise- 

 ly similar to that of the aurora borealis." " 2. The strata of atmospheric air 

 become rarefied as their altitude above the surface of the earth is increased." 

 Hence they argued that the aurora is nothing more than electrical discharges 

 transmitted through parts of the upper regions of the atmosphere, so rarefied 

 as to produce that peculiar luminous appearance which they exhibit. This 

 theory, which was embraced and improved in its details by Canton, Beccaria, 

 Wilke, Franklin, and other contemporary electricians, has received further 

 countenance from more recent researches. 



Attempts were also made to explain on electrical principles other meteorolo- 

 gical effects ; such as waterspouts, whirlwinds, rain, fogs, hail, &c., but no 

 satisfactory conclusions resulted from these investigations, and the discussion 

 of such phenomena forms a part of the meteorological inquiry of the present 

 time. 



While the series of experimental researches which have just been related 

 were in progress, many attempts were made to trace electricity in the phenom- 

 ena of vegetable and animal life, and more especially to apply it as a medical 

 agent in cases of organic disease in the animal system. None of these at- 

 tempts, however, led to any consequences sufficiently important to entitle them 

 to attention in this brief sketch. 



After electroscopes had been much improved, and in their application to at- 

 mospheric electricity had derived great power from the addition of a long 

 pointed conductor, extending from the diverging balls to a height of several 



