VOL. LXVIIl.] PHILOSOPHICAL TRANSACTIONS. 445 



as preservatives from lightning. And here it is necessary to observe, that build- 

 ii^s may be exposed to a stroke of lightning in several different ways. The 

 lightning which, to avoid prolixity, I shall only speak of as positive electricity : 

 the lightning, I say, may accumulate directly over the building ; or it may be 

 brought towards the building by a small cloud fetching it in several successive 

 trips from a large -cloud at some distance ; or a large electrified cloud may be car- 

 ried rapidly towards it by the wind : a circumstance this by no means rare, there 

 being no less than 4 instances of it upon record in the Phil. Trans. In the first 

 of these supposed cases, a sharp-pointed conductor might possibly drain the 

 cloud of its lightning as fast as it began to accumulate, and so prevent any ex- 

 plosion whatever. In the 2d, as the cloud, by supposition, not being driven in 

 one direction by the wind, could not move with any remarkable velocity, it is 

 reasonable to imagine, that in this case also there might be no explosion ; and 

 that the electricity of the larger cloud might be gradually exhausted. But if, 

 according to the 3d supposition, a cloud of great extent, and highly electrified, 

 should be driven with great velocity in such a direction, so as to pass directly 

 over the sharp-pointed conductor, there can be no doubt but that such a point, 

 from its superior readiness to admit electricity, would take the explosion at a 

 much greater distance than a rounded end, and in proportion to the difference 

 of that striking distance would do mischief, instead of good. 



But perhaps it will be said, that every stroke of lightning falling on a sharp 

 point is previously diminished by that point, and therefore may more easily be 

 transmitted through the conductor, than when it falls undiminished on a rounded 

 end. On this supposition I must observe, that it not only contradicts Mr. 

 Wilson's experiments at the Pantheon, but also Mr. Henly's experiment already 

 referred to in this paper, where the fire flew to a very taper point, and melted 

 the end with a strong and loud explosion. So also the sharp-pointed conductors 

 affixed in America to the houses of Mr. West, Mr. Raven, and Mr. Mayne, do 

 not seem to have diminished the force of the explosion, if we may judge from 

 the violence of its effects as related at large in Dr. Franklin's works. It should 

 seem therefore, that the power of diminishing a stroke, like that of preventing 

 it, is only contingent, and depends, as before said, on the degree of velocity 

 with which the lightning moves. 



The sum of the whole is, that conductors, terminated by sharp points, are 

 sometimes advantageous, and at other times prejudicial. Now as the purpose 

 for which conductors are fixed on buildings is, not to protect them from one 

 particular sort of clouds only, but, if possible, from all, it cannot surely be ad- 

 viseable to use that kind of conductors, which, if they diminish danger on one 

 hand, will increase it on the other. It is the duty of a pilot to keep out of the 



