LIGHTNING CONDTJCTOKS. 



37 



it is extremely dangerous to be near ; but when the thunder 

 is not heard till some time after the flash is seen, there can 

 be no danger, as it is then far away. Sound travels at about 

 eleven hundred feet per second, therefore (in round numbers) 

 it may be known that the lightning is one mile distant for 

 every five seconds that elapse between the flash and the 

 thunder. The notion which prevails that iron and steel 



attract lightning is entirely erroneous, they do not even 

 conduct it so well as copper and many other metals. 



Lightning conductors (fig. 8) are wires of iron or copper, 

 made to project above the highest parts of buildings and 

 carried down to the earth; they have the power of con- 

 ducting the lightning down without injury to the building, 

 for electricity (which lightning is) travels through metallic 

 wires for any distance without disturbance or noise (as in 

 the electric telegraph), but bad conductors are apt to be burst 

 and rent by it if it be strong enough, creating flashes of light 

 and reports. Lightning strikes most readily any projecting 

 substance or point, the highest point is therefore selected 

 for the protrusion of the upper part of conductors. In a 

 plain or open space, trees and animals are often struck, as 



