514 THILOSOPHICAL TRANSACTIONS. [aNNO 1774. 



coating, on one side a knob, and on the other side a sharp-pointed wire, both 

 being insulated, and standing 5 inches from each other, fig. 6, he placed a large 

 copper ball, c, 8 inches in diameter (insulated also) so as to stand exactly at half 

 an inch distance both from the knob and the point. The jar being fully charged, 

 he delivered it on the copper ball by the discharging rod, whence it leaped to 

 the knob a, which was ^ inch in diameter, and the jar was discharged by a loud 

 and full explosion, and the chain was very luminous. He could perceive no 

 light on the chain, which connected the pointed wire b with the coating of 

 the jar. ' '■' 



Exp. 6. Mr. H. insulated his 3 largest jars, containing together about l6 

 square feet of coated surface; fig. 7. From the bottom of these jars projected a 

 wire, terminated by a ball, 4 inch in diameter; and at the distance of l^- inch 

 from it, he placed the insulated ball c ; on which he brought down the charge 

 of the 3 jars, by the discharging rod; which leaped from it to the ball in contact 

 with the jars, and discharged them by a loud and full explosion ; but the same 

 thing did not happen if he removed the insulated ball only -f of an inch farther 

 from the other. He then removed the wire, which was terminated by the ball, 

 from the bottom of the jars; and placed another in its stead, of the same length 

 and diameter, but very nicely tapered to a point, as usiial. Then placing the 

 insulated ball c one inch from the point, he brought down the charge of the 3 

 jars, as before, which flew upon the point, and melted it a little. The jars 

 were discharged with a loud and full explosion. But having removed the ball c 

 to the distance of l-f inch from the point, the charge could not strike it ; though 

 much of it was presently drawn oflT silently by the point, as appeared by the fall- 

 ing of the index of the electrometer. 



From this experiment, he thinks it seems somewhat more than probable, that 

 a conductor terminated by a ball, of 4 inch in diameter, would be in danger of 

 a stroke from a highly electrified cloud, at a much greater distance than another 

 with a sharp termination. Intleed he cannot help remarking, how very impro- 

 bable it appears, that a sharp-pointed conductor should at any time invite or 

 solicit a stroke of lightning. Imagine, if you please, that a large cloud is, by 

 the force of the wind, driven violently towards such a point, and actually strikes 

 on it: yet as the point would act as such, at somewhat more than the striking 

 distance, it seems probable that part of the electricity of the cloud would be 

 drawn off silently, before the actual stroke could be made; and the stroke itself 

 might thereby perhaps be a little lessened. 



Mr, H. here inserts what seems to afford a sufficient proof of the truth of this 

 reasoning; viz. 



Extract of a Letter from Capt. Richard Nairne, of the Generous Friends, dated 

 Montreal, June 24, 1773, to Mr. Thos. Marsham, in the Borough. 



• I shall make every observation I can for the good of electricity, and the sa- 



