4»68 Mr. Snow Harris on Lishim?iQ Conductors 



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covering a portion of the earth's surface N. Let cche a dis- 

 charging rod, and o some near body. Then by Lord Stan- 

 hope's experiment the charged cloud M will displace from the 

 surface N, and all the bodies on it as c c, o, &c., a portion of 

 their natural electricity, which will again return when the dis- 

 charge has been effected. The conditions of Lord Mahon's 

 experiment cannot obtain between the conductor c c and the 

 body o, since they are both in the same forced state *. It is 

 very easy to perceive, that the electrical relations of two 

 bodies o and c between the boards, is different from that 

 between a conductor J, fig. 1, charged with electricity, and a 

 body B in its natural state ; or that of a conductor C, fig. 6, 

 carrying off the displaced electricity of the lower plate N, and 

 a body B neutral. Besides, in Lord Mahon's experiment, 

 fig. 7, the electricity of the return spark is different from that 

 of the primitive charge in M; whereas, in Mr. Sturgeon's ex- 

 periment, the spark is of the same kind. So little did His 

 Lordship anticipate any objection to the use of lightning rods 

 in consequence of his experiments, that he declares his con- 

 viction of their passive operation, and reproves those who 

 " ignorantly conclude " that they are of a dangerous nature. 



14. We have been here discussing what the author calls a 

 tJiird Wnd of lateral discharge; but he mentions o. Jirst and 

 second kind also. The first kind, he says, "takes place at 

 every interruption of a metallic circuit;" "it displaces loose 

 bodies," &c. This is evidently the effect of mechanical ex- 

 pansion, and is the very effect we avoid by means of a lightning 

 rod. He alludes to Dr. Priestley as authority on this point; 

 how unfortunate for his whole doctrine ! Let us consider for 

 a moment what Dr. Priestley says : " That the cause of this 

 dispersion of bodies in the neighbourhood of electrical ex- 

 plosions is not their being suddenly charged xmth electric matter.^ 

 is, I think, evident. I never observed the least attraction of 

 these bodies toward the brass rods, through which the explosion 

 passed, although I used several methods which could not fail 

 to show it. I even found that the explosion of a battery made 

 ever so near a brass rod, did not so much as disturb its elec- 

 tric fluid ; for when I had insulated the rod, and hung a pair 

 of pith balls on the end opposite to that near which the ex- 

 plosion passed, I found the balls were not in the least 

 movedf." 



* This applies to Mr, Sturgeon's Exp. (9.) — If B fig. c, 3. were on the same 

 insulation with the jar J and rod c, no spark could occur at o, except by a 

 division of the charge, whatever quantity passed through c. 'i'his fact alone 

 is conclusive of the point in question, proving clearly that the spark is not 

 a lateral explosion. 



f The reader will distinguish here between this experiment and Lord 



