390 WRITINGS OP JOSEPH HENRY. [1855- 



which the experiment was tried took from the clouds as much 

 lightning as would have been sufficient in the short space of 

 an hour to kill upwards of three thousand men. From the 

 foregoing facts and conclusions we may infer that the light- 

 ning-rods of a city have considerable effect in silently dis- 

 charging the clouds, and in preventing explosions which 

 would otherwise take place; but we must recollect that on 

 account of the upward rushing of the moist air, the elec- 

 tricity of the cloud is constantly renewed. 



We cannot suppose that the sparks observed by Beccaria 

 in his experiment, and the ringing o*f bells by Franklin, were 

 due entirely to the electricit}' immediately received from the 

 cloud. By the powerful induction of the redundant elec- 

 tricity of the latter, and the negative action of the earth be- 

 neath, the natural electricity of the top of the rod would be 

 forced down into the earth, the point would become intensely 

 negative, and in this condition would draw from the air 

 around streams of electricity, and in this way a large volume 

 of air around the top of the rod would become negatively 

 electrified ; and in case a discharge of lightning took place 

 its first effect would be to neutralize or fill up, as it were, 

 this void of electricity in the large mass of air surround- 

 ing and above the top of the rod, before the remainder of the 

 discharge could pass to the earth. The peculiar sound which 

 is heard when a discharge from a thunder-cloud is transmit- 

 ted through a lightning-rod may possibly be attributed to 

 this cause. 



The Smithsonian building, with its high towers, situated 

 in the middle of a plain, at a distance from all other edifices, 

 is particularly exposed to discharges of lightning, and we 

 have reason to believe that in as many as four instances 

 within the last ten years the lightning has fallen upon the 

 rods and been transmitted innoxiously to the ground. 



In two of the instances the lightning was seen to strike 

 the rod on one of the towers; in a third, a bright spark due 

 to induction and attended with an explosion as loud as that 

 of a pistol was perceived ; and in the fourth instance, although 

 the platinum top of the rod, which was one hundred and fifty 



