Galvanism. 119 



quently the degree of danger, is not, however, 

 difficult to be estimated. As light travels at the 

 rate of seventy-two thousand four hundred and 

 twenty leagues in a second of time, its effects 

 may be considered as instantaneous within any 

 moderate distance ; but sound, on the contrary, 

 is transmitted only at the rate of three hundred 

 and eighty yards in a second. By accurately ob- 

 serving the time, therefore, which intervenes be- 

 tween the flash, and the noise of thunder which 

 succeeds it, a very near calculation may be made 

 of its distance. Or, the distance may be very well 

 estimated by means of the pulsations in the wrist, 

 allowing five and a half to a mile ; and in the same 

 proportion for any other number of pulsations in 

 the interval between the flash and the thunder. 



The discovery of Dr. Franklin, which ascer- 

 tained the identity of lightning and the electric 

 fluid, suggested to the same philosopher the 

 means of preserving buildings from lightning, by 

 means of metallic conductors attached to the out- 

 side of high buildings. As these are now com- 

 mon, it is unnecessary to describe them. The 

 principle on which they are constructed rests on 

 the well-known fact of metallic bodies being better 

 conductors of the electrical fluid than any others. 

 The conducting rod is pointed at the top, in order 

 the more gradually to attract the electricity from 

 the clouds and the atmosphere ; and the upper 

 part should be made of copper, to prevent its 

 rusting, and the remainder painted. The con- 



