282 ELECTRIC CLOUDS. . SECT. XXVIII. 



any one part of a cloud is extremely small. The inten- 

 sity of the flash arises from the very great extent of 

 surface occupied by the electricity; so that clouds may 

 be compared to enormous Leyden jars thinly coated 

 with the electric fluid, which only acquires its intensity 

 by its instantaneous condensation. The rapid and irreg- 

 ular motions of thunder clouds are, in all probability, 

 more owing to strong electrical attractions and repul- 

 sions among themselves than to currents of air, though 

 both are no doubt concerned in these hostile move- 

 ments. 



Since the air is a non-conductor, it does not convey 

 the electricity from the clouds to the earth, but it ac- 

 quires from them an opposite electricity, and when the 

 tension is very great the force of the electricity becomes 

 irresistible, and an interchange takes place between the 

 clouds and the earth ; but so rapid is the motion of light- 

 ning, that it is difficult to ascertain when it goes from the 

 clouds to the earth, or shoots upward from the earth 

 to the clouds, though there can be no doubt that it does 

 both. In a storm which occurred at Manchester, in the 

 month of June, 1835, the electric fluid was observed to 

 issue from various points of a road, attended by explo- 

 sions as if pistols had been fired out of the ground. A 

 man appears to have been killed by one of these explo- 

 sions taking place under his right foot. M. Gay-Lussac 

 has ascertained that a flash of lightning sometimes darts 

 more than three miles at once in a straight line. 



A person may be killed by lightning, although the 

 explosion takes place at the distance of twenty miles, 

 by what is called the back stroke. Suppose that the 

 two extremities of a cloud highly charged with electri- 

 city hang down toward the earth : they will repel the 

 electricity from the earth's surface, if it be of the same 

 kind with their own, and will attract the other kind ; 

 and if a discharge should suddenly take place at one 

 end of the cloud, the equilibrium will instantly be re- 

 stored by a flash at that point of the earth which is un- 

 der the other. Though the back stroke is often suffi- 

 ciently powerful to destroy life, it is never so terrible in 

 its effects as the direct shock, which is frequently of 

 inconceivable intensity- Instances have occurred in 



