396 WELLS'S NATURAL PHILOSOPHY. 



position of surrounding bodies must therefore be attended to, whether a poT- 

 Bon be insulated or not. 



The apprehension and solicitude respecting lightning are proportionate to 

 the magnitude of the evils it produces, rather than the frequency of its occur- 

 rence. The chances of an individual being killed by Ughtning are infinitely 

 less than those which he encounters in his daily walks, in his occupation, or 

 eyen during his sleep from the destruction of the house in which he lodges by 

 fire. 



B^w are the ^^^- ^^^ mcchanical power exerted by light- 

 ni.^chanicai ef- jjjjjgr is cnormous and difficult to account for. 



lects of lignt- o 



ning accounted ^j-agQ supposcd that the heat of the light- 

 ning in passing through any substance, in- 

 stantly converted all the moisture contained in it into 

 steam of a highly exi)losive character, and that the great 

 mechanical effects observed are due to this agent rather 

 than to the direct effect of the electric current. A tem- 

 perature that can instantly render iron red hot, is known 

 to be sufficient to generate steam of such an clastic force 

 that it would overcome all obstacles, and if the water con- 

 tained in the pores of bodies is at once converted into 

 steam of this character, its force would be capable of pro- 

 ducing any of the mechanical effects witnessed in lightning 

 discharges. 



Another theory supposes that the natural electricities 

 of non-conducting bodies are forcibly decomposed by the 

 presence of the electric fluid which forms the lightning, 

 and that their violent separation forces every thing asun- 

 der which tends to confine them. 



What is the ^'^^' The phcnomenou of the aurora borcalis 

 Mroroboreaiis? ^^ supposcd to bc duc to the passage of electric 

 currents throu'jrh the higher rcij-ions of the 

 atmosphere — the different colors manifested being pro- 

 duced by the passage of the electricity through air of dif- 

 ferent densities. 



Where doesthe I^ ^hc northcm hemisphere the aurora al- 

 •urora appear? ^^^g appcars iu the north, but in the south- 

 em hemisphere it appears in the south ; it seems to origin- 

 ate at or near the poles of the earth, and is consequently 



