Polytechnic Association. (599 



the others being far more potent; there is the heat of the sun, 

 already spoken of, with its direct rays beating upon the opposite 

 masses of cloud ; the heated and ascending cloud coming in contact 

 with the cold strata of the higher atmosphere; the numberless 

 cavities of these irregular cumuli y seething cauldrons beneath the 

 sun's rays ; the motion of the clouds, agitated by the winds driving 

 the masses into collision — all conduce to the production and accumula- 

 tion of electricity and its unequal distribution. The tendency of 

 electricity is to prominences and points ; and when a surcharge 

 occurs by an accumulation at any place beyond its capacity to con- 

 tain it, it breaks forth in the form of lightning in the direction requi- 

 site to restore the equilibrium. This may be upward, downward, 

 horizontally or obliquely, according to the situation of the locality 

 negatively electrified, whether it be the earth, the adjacent clouds, 

 the atmosphere beyond the borders of the clouds in the same stratum, 

 or the region of the atmosphere above. Thus we often behold in 

 thunder clouds some miles remote the streaks of lightning flying in 

 every direction in lines variously curved, direct or zigzag. When 

 we reflect that the most powerful spark that can be artificially pro- 

 duced is but a little more than a foot long, we may form some con- 

 ception of the enormous quantity that is eliminated in a stroke of 

 lightning, which reaches the earth in a continuous stream from a 

 cloud more than a mile in height. These explosions are often 

 repeated every few minutes for hours together. Hence it is evident 

 that the accumulated quantity of electricity in these thunder storms 

 is vast beyond description. 



The theory of Franklin is, that the conductor draws electricity from 

 the atmosphere and clouds, transmitting it to the earth and con- 

 tributing to equalize the positive and negative electricity in regard to 

 the earth and clouds ; and that it thereby prevents the lightning 

 stroke where it is placed. According to the theory of electric induc- 

 tion, accepted by the French s'avans, when a thunder cloud passes, 

 positively electrified, it repels the positive and attracts the negative 

 fluid accumulated in bodies on the surface of the earth, and which 

 are, therefore, exposed to the danger of an electric discharge. By 

 means of the lightning rod, the negative electricity withdrawn from 

 the surface by the cloud flows off from the points of the conductor 

 into the atmosphere, and, neutralizing the positive fluid of the cloud, 

 prevents an explosion. By either theory the utility of the conductor 

 is asserted. 



The difference of opinion in regard to so notable an incident as the 



