298 On Lightning Conductors to Powder Magazines, [No. 99. 



16. The cause of a lightning discharge selecting the con- 

 ductor is to be traced in the law of electrical induction, which 

 I hope to be pardoned for briefly exposing. 



A cloud highly charged with electricity of either kind 

 (let us say, positive) approaches the earth, and by the approxi- 

 mation causes the natural electricities of the earth to sepa- 

 rate, and that of the negative kind to accumulate at the sur- 

 face opposite to the cloud. The intervening particles of air 

 are thrown into a polar state. The cloud is attracted by the 

 earth, the electricity of which becomes most accumulated on 

 the buildings and objects on its surface, in proportion to their 

 degree of conducting power. At length the resistance to the 

 rushing together of the two opposite electricities is overcome, 

 and a discharge by explosion takes place, the best conductor on 

 the earth receiving all the electric discharge it is capable of 

 conducting in the time the discharge occupies. Of all such 

 bodies a pointed metallic rod is the most likely to receive a 

 discharge, and will lead off the greatest quantity thereof to 

 the earth. 



17. Mr. Daniell has indeed stated, that a pointed bar must 

 cause a silent discharge without explosion.* I am unable to 

 comprehend how Mr. Daniell could have fallen into such a mis- 

 conception. The whole history of lightning accidents, teems 

 with instances of well constructed pointed rods having been 

 struck, and the points melted. Look at the accident to Mr. 

 Trower's house for example. f The conductor is faultless in its 

 construction, and the flash was seen to strike it by Dr. Goodeve. 



* "A pointed conductor will indeed draw off silently and safely, a con- 

 siderable portion of electricity from a charged cloud, but it can possess no 

 power of determining a disruptive, and destructive discharge, at a point 

 where it would not otherwise occur." Mr. Daniell 's paper. See para. 9. 



t Described in my first report.— W. B. O'S. 



