SECT, xxvin.] ELECTRIC CLOUDS. 321 



may be accumulated in this manner, which may be either 

 positive or negative. When two clouds, charged with oppo- 

 site kinds, approach within a certain distance, the thickness 

 of the coating of electricity increases on the two sides of the 

 clouds that are nearest to one another; and, when the accu- 

 mulation becomes so great as to overcome the coercive pres- 

 sure of the atmosphere, a discharge takes place, which occa- 

 sions a flash of lightning. The actual quantity of electricity 

 in any one part of a cloud is extremely small. The intensity 

 of the flash arises from the very great extent of surface occu- 

 pied 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 conden- 

 sation. The rapid and irregular motions of thunder clouds 

 are, in all probability, more owing to strong electrical attrac- 

 tions and repulsions among themselves than to currents of 

 air, though both are no doubt concerned in these hostile 

 movements. 



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

 electricity from the clouds to the earth, but it acquires 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 lightning, that it is difficult to 

 ascertain when it goes from the clouds to the earth, or shoots 

 upwards from the earth to the clouds, though there can be no 

 doubt that it does both. In a storm which occurred at Man- 

 chester, in the month of June, 1835, the electric fluid was 

 observed to issue from various points of a road, attended by 

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

 man appears to have been killed by one of these explosions 

 taking place under his right foot. M. Gay-Lussac has ascer- 

 tained 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 explo- 

 sion takes place at the distance of twenty miles, by what is 



Y 



