438 - PHILOSOPHICAL TBANSACTIONS. [aNNO 17 78. 



ment is much more agreeable to nature than the 23d, for clouds are not fixed 

 but floating bodies. 



In order to see the effect of rods terminating with balls of different sizes, or 

 terminating with a point, moving swiftly under the artificial cloud, Mr. N. made 

 use of the following apparatus, viz. A hollow tube uf wood covered with tin- 

 foil, with a heavy weight fastened to one end of this tube; and at about 3 inches 

 above the weight was an axis, it was then suspended by this axis between two 

 wooden pillars: in this wooden tube was a brass rod, v\hich was moveable, so 

 that a ball or point fixed on it could be raised to the height required. 

 " Exp. 25. A ball of 1-^ inch diameter was fixed to the under part of the arti- 

 ficial cloud, and then this apparatus was placed under it with a point, the swing-* 

 ing rod was held down to the floor, and the point covered: then the artificial 

 cloud was charged by a certain number of turns of the glass cylinder; the swing- 

 ing rod with the point was then let go, and passed swiftly and very near to the 

 ball under the artificial cloud. This was repeated several times, removing the 

 point lower each time, till the greatest striking distance to the point was found, 

 which was generally 1-^ inch. 



Exp. lQ. The point being removed, a ball of -^ diameter was placed in its 

 stead, and tried as the point in the preceding experiment; the striking distance 

 was generally found to be 2-;-'^ inches. 



Exp. 17. The -rVball being removed, another of l-rV "as tried as in the last 

 2 experiments, and the striking distance was generally 15 inches. 



Obiervation. — In the 25th experiment it appears, that the point is struck by 

 means of a swift motion; and from the 26th experiment, that the ball of-jSj-was 

 struck farther than the point; and the ball of l^V 'r> the 27th experiment, at a 

 much greater distance than either, even with the swift motion. From these ex- 

 periments Mr. N. is induced, first, to prefer elevated pointed conductors; next 

 to them those that are pointed, though they project but a little distance above 

 the highest part of the building; and after them those terminating in a ball, and 

 placed even with the highest part of a building, though it appears from these 

 experiments, that they are more liable to be struck, and have not the property of 

 guarding the distant parts of a building as elevated points have; but if they have 

 a good metallic communication with the earth, the building might not be hurt, 

 though the lightning should strike on the conductor. Those conductors which 

 are recommended to be within the inside of a building, and a foot or 2 below the 

 highest part,* are certainly very dangerou?, especially for all that part of the 

 building above the conductor. 

 - Mr. N. was a witness of the dreadful effects of a stroke of lightning on a 



• Mr. Wilson's letter to ibt- Marquis of Rockingliam, Philos. Trans, vol. 54, p. C47. — Orig. 



