VOL. LXVIII.] PHILOSOPHICAL TRANSACTIONS. 315 



when he placed balls of 4 ot" an inch diameter, instead of the small ones, the 

 cloud, every time it passed over them, made explosions, and fired the magazines 

 a, b, c ; and yet the index of the electrometer does not descend above 20°, and 

 starts up again as suddenly as it fell. 



If balls were safer at the upper ends of conductors than points, it should 

 follow, that the larger the balls are, the greater the security ; but from all these 

 experiments he never found a shock with a point, and not always with a very 

 small ball : but the electrical matter passes off silently with the points, and so 

 entirely, that the electrometer falls to 5°. With balls J- of an inch diameter in- 

 deed it passes off with a little hissing noise, but this seldom amounts to a shock : 

 but with balls f of an inch diameter an explosion constantly happens, and the 

 magazines are fired. To put this matter still more out of doubt, he placed a 

 ball of 9 inches diameter on one of the conductors, and the explosion was very 

 violent, always more certain ; and yet the machine does not discharge itself, for 

 the electrometer falls not more than 20°. 



The next experiment he makes with the water-conductor is, placing the 

 houses A, B, c, in a negative state, by connecting them with the cushion of 

 the machine, or with the outside of a battery : when the cloud is charged 

 and passes over the houses, with points at the upper end of their conductors, 

 there is no explosion ; the points seem to draw off all the electrical matter during 

 the passage of the clouds of 44- feet long : but when, in this position of the 

 houses, balls of -f of an inch diameter are placed instead of points, there is a 

 small explosion, and a considerable residuum of the matter is left in the battery. 

 He then changes tlie insulated water for wire to complete the circle : on the 

 passage of the clouds over the houses there is a considerable explosion, whether 

 points or balls are the upper terminations of the conductors of the houses ; but 

 no residuum is left in the battery. Hence appears the difference of effect, 

 whether the houses stand in a state of nature, or in a negative state ; and whe- 

 ther the conductors be made complete with wire, or water insulated. 



I have, says Mr. Swift, by 16 years practice been convinced how difficult it is 

 to draw general conclusions from any electrical experiments, and therefore it 

 becomes me to propose my conjectures with the greatest diffidence ; but I appre- 

 hend the result of many experiments show, that points at the upper termination 

 of conductors gradually diminish or draw off the electrical matter, so as to pre- 

 vent any damage to the buildings on which they are placed, by preventing a vio- 

 lent explosion ; and that, on the contrary, balls, though perhaps they will repel 

 the electrical matter in some degree, yet from that very circumstance probably 

 the explosion, when it happens^ is violent and attended with danger. 



X. An Account of the Island of Sumatra, &c. By Mr. Charles Miller, p. 160. 



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