348 PHILOSOPHICAL TRANSACTIONS. [aNNO 1/78. 



great cylinder being charged, the model was suffered to pass : and though the 

 velocity was less than f of a mile in an hour, the point was struck. 



Exp. 32. When the great ball of the less substitute was placed at the greatest 

 striking distance from the ball at the end of the great cylinder, I fixed a needle 

 into the under side of the remote end of the less substitute, with the point 

 downwards : opposite to this point, and on the same stand described in the 22d 

 experiment, was fixed another needle; so that the 2 points were opposed to each 

 other. The space between them was varied from time to time, in order to find 

 the greatest distance at which the lower one could be struck. On charging the 

 great cylinder, it appeared that the greatest distance in this case was 5^- inches. 

 — Exp. 33. Upon repeating this experiment, whilst every circumstance remained 

 the same, excepting that, instead of the point below, a rounded end was put in 

 its place, and after charging the cylinder again, it appeared, that the greatest 

 distance, at which the lightning struck the rounded end, was not more than 2f 

 inches. And the largeness of the spark and the loudness of the explosion ap- 

 peared to be less considerable than in the 32d experiment. 



During the course of this inquiry, having occasion to try some experiments in 

 the dark, I observed a curious circumstance, which seeuied to show, that a 

 point had a far greater influence on the charged substitute, in certain circum- 

 stances, than a rounded end had when it was placed in the same situation. 



Exp. 34. The circumstance alluded to was an appearance of light on the 

 brass ball that was fixed at the end of the great cylinder, when tlie copper ball of 

 the less substitute was opposed to it at the greatest striking distance, as in the 

 1 8th experiment, every other circumstance remaining the same; and while 

 the model, with its pointed conductor, stood on the table directly under the tin 

 ball, fixed at the remote end of the less substitute : for soon after 7 or 8 turns 

 of the wheel, a light began to appear on the brass ball, and continued to increase 

 in brightness till the moment it burst forth in an explosion towards the copper 

 ball. The part of the brass on which the light appeared, was that next to the 

 copper ball : and the general appearance of it was round, and sometimes more 

 than half an inch in diameter. — Exp. 35. On repeating this experiment with a 

 rounded end, instead of a point, and at the same distance from the tin ball, 

 though every other circumstance continued the same, there was no such appear- 

 ance. — Exp. 36. But when the rounded end was moved considerably nearer, that 

 is, within -^ of an inch, a light was visible ; but then it was faint, and not 

 more than -yV of an inch in diameter, even at the instant before the explosion 

 happened. 



\Zth Observation. — By the 1st of these experiments it appears, that the in- 

 fluence which the point had on the whole of the fiuid contained in the great 



