-1859] WRITINGS OF JOSEPH HENRY. 385 



tinct metallic masses completely separated from each other, 

 it will be difficult to tell which of them will be struck in 

 preference. The safest practice is to unite all these masses 

 by rods or bands of iron, copper, or other metal, so that each 

 of them may be in metallic communication with a rod 

 which may transmit the lightning to the damp earth. 



" We thus deduce from facts established by observation 

 alone without borrowing anything from theory," says Arago, 

 "a simple, uniform, and rational means of protecting build- 

 ings from the effects of lightning. But when we refer, in 

 addition to these facts, to the precise principles or laws of 

 electrical action, as deduced from cautious and refined ex- 

 periments in the laboratory, we are enabled to give rules for 

 the protection of buildings which, when properly observed 

 reduce almost to insignificance the danger to be apprehended 

 from the ordinary occurrences connected with the terrific 

 exhibitions of thunder-storms." 



From what has been said on the principles of induction, 

 and also on the fact of the negative condition of the earth, it 

 will be readily perceived that the upper end of an elevated 

 conductor must become highly negative under the repulsive 

 energy of a positive cloud, and though it may not be suffi- 

 cient in itself to cause a rupture of the thick stratum of air 

 intervening between the cloud and the earth, yet if a dis- 

 charge does take place in the vicinity of this body, it will 

 be drawn toward it, and if the conductor extends to the 

 earth, and is in intimate connection with the damp ground, 

 the discharge will pass innoxiously into this great reservoir. 

 We further know from theory as well as experiment and ob- 

 servation, that the intensity of attraction is increased when 

 the conductor is terminated above in a single sharp point. 

 Although the attraction at a distance may be greater on a 

 metallic globe of a few feet in diameter than on a metallic 

 point, (since the former is able to receive a greater induced 

 charge, which by the well-known law of attraction will act 

 as if the whole were concentrated at the centre of the sphere,) 

 3'et the intensity of action of the point and its tendency to 

 open a passage through the air is so great that it is preferred 

 in protecting a given circumscribed space from lightning. 

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