1840.] On Lightning Conductors to Powder Magazines, 305 



and then not to badly conducting matter, as wood or stone, but 

 only to neighbouring masses of good conducting matter, as the 

 metals, which either ought not to be there, or if there, be in 

 metallic connexion with the conductor. It is on this point that 

 an apparent difference exists between Mr. Faraday's opinions 

 and mine. 



32. It is however always easy to obtain this spark to the 

 knuckle, and to many other imperfect conductors. Further, the 

 spark now alluded to, whatever be its cause or nature, may be ex- 

 pected to increase in power in direct proportion to the quantity 

 of electricity in the original flash. If with a quart Leyden jar 

 we can procure, as I have repeatedly done, a secondary or lateral 

 spark half an inch long, capable of inflaming gases and gun- 

 powder, I think I am not straining the inference too far, when I 

 believe that the discharge of 10,000 acres of excited cloud may 

 cause a secondary spark or flash capable of passing through the 

 wall of a magazine and exploding its contents. Mr. Harris has 

 indeed recently asserted that increasing the primary spark does 

 not increase the secondary one ; but I must state, with every 

 respect to this gentleman, that I have repeatedly exhibited 

 to my classes, long before his paper was published, the experi- 

 ment described at para. 30 — and that I have often shewn, that 

 while success is uncertain with a small jar, it is infallible with a 

 large one. I had not the means of measuring the spark, but 

 its increase was plainly visible, and palpable, as we increased 

 the battery and its charge. The magazine, moreover, contains 

 powder barrels lined with copper, and even though no flash or 

 spark pass through the wall, the barrels themselves may give 

 sparks to each other under the influence of the electricity 

 passing outside. Mr. Faraday has shewn in one of the most 

 perfect of all his matchless researches, that without the direct 

 conveyance of electricity, the walls of an apartment in which 

 a common electrifying machine is worked, are in a state of 

 active electrical excitement. 



S3. Mr. Harris, who is doubtless a highly accomplished elec- 



