CHAP, iv.] ELECTRICITY AND MAGNETISM. 257 



partial vacuum thus produced. If the spark be straight 

 and short, the observer will hear but one short sharp clap. 

 If its path be a long one and not straight, he will hear 

 the successive sounds one after the other, with a charac- 

 teristic rattle^ and the echoes from other clouds will 

 come rolling in long afterwards. The lightning -flash 

 itself never lasts more than ^-jftnnr f a second. 



The damage done by a lightning-flash when it strikes 

 an imperfect conductor appears sometimes as a disrup- 

 tive mechanical disintegration, as when the masonry 

 of a chimney-stack or church-spire is overthrown, and 

 sometimes as an effect of heat, as when bell-wires and 

 objects of metal in the path of the lightning-current are 

 fused. The physiological effects of sudden discharges 

 are discussed in Art. 226. The remedy against disaster 

 by lightning is to provide an efficient conductor com- 

 municating with a conducting stratum in the earth. 



The " return-stroke " experienced by persons in the 

 neighbourhood of a flash is explained in Art. 26. 



3O5. Lightning Conductors. The first suggest- 

 ion to protect property from destruction by lightning 

 was made by Franklin in 1749, in the following words : 



" May not the knowledge of this power of points be of use 

 to mankind, in preserving houses, churches, ships, etc., from 

 the stroke of lightning, by directing us to fix on the highest 

 parts of those edifices upright rods of iron made sharp as a 

 needle, and gilt to prevent rusting, and from the foot of those 

 rods a wire down the outside of the building; into the ground, 

 or round one of the shrouds of a ship, and dpwn her side till 

 it reaches the water ? Would not these pointed rods probably 

 draw the electrical fire silently out of a clouij before it came 

 nigh enough to strike, and thereby secure us : |from that most 

 sudden and terrible mischief." 



The four essential points of a good lightnjng-conductor 



are ( I ) that its apex be a fine point elevaied above the 



highest point of the building ; (2) that its lower end passes 



either into a stream or into wet stratum of ground ; (3) 



s 



