1840.] On Lightning Conductors to Powder Magazines. 283 



19. With respect to the materials and dimensions of conduc- 

 ting bars, I think it is altogether unnecessary either to construct 

 them of copper, or to make them one inch in diameter.* Iron can 

 be preserved bright for an indefinite period by attaching to it small 

 pieces of zinc, on the principle of the galvanic preservation of 

 copper. If its point be gilt or platinized, the rod will on the con- 

 trary corrode much more rapidly than if entirely unprotected. 

 As to size, I have known very many instances of violent dis- 

 charges of lightning through window rods, through ill-con- 

 structed conductors, over picture frames, railings, through the 

 metallic head of a spear with a wooden shaft, &c. &c. and in no 

 case was complete fusion, or an approach to it, effected, except 

 at the ends where the discharge entered, and from whence it 

 proceeded. The drawing a shews the extremities of one of 

 the window rods from Dr. Goodeve's house, and b of the spear 

 of the Britannia from Government House, Calcutta. 



20. As for the 



silent passage of at- ^__ 



mospheric electrici- 

 ty causing the heat- 

 ing of conductors, I scarcely think it possible — certainly no 

 instance of it has been recorded, and even were it to occur, it 

 could not occasion any mischief. 



21. To apply the preceding facts to the question before me, I 

 think it inexpedient to attach ordinary conductors, or such as 

 those described in your letter, to powder magazines. 



i. Because, being of slight elevation, of rounded surface, and 

 of non-conducting materials, these buildings are scarcely more 

 exposed to lightning than an equal area of ordinary ground. 



ii. Because a discharge may occur too great for the capacity 

 of a single conductor, in which case the electricity will divide 

 itself to all adjacent objects. 



iii. Because though the discharge may pass to the ground, the 

 lateral electric disturbance may occasion an explosion within the 

 magazine. 



* That is where several conductors are employed as subsequently 

 proposed.— W. B. O'S. 



2o 



