292 ATMOSPHERIC ELECTRICITY. SECT. XXVIII. 



sation be sufficient to produce drops of rain, they carry the 

 electricity to the ground, so that in general a shower is a con- 

 ductor between the clouds and the earth. When two clouds 

 charged with opposite kinds, but of equal tension, approach within 

 a certain distance, the intensity increases on the sides of the clouds 

 that are nearest to one another ; and when the tension is great 

 enough to overcome the coercive pressure of the atmosphere, a 

 discharge takes place which causes a flash of lightning, the stroke 

 being given either by the cloud or the rain. The actual quantity 

 of electricity in any part of a cloud is extremely small. The 

 intensity of the flash arises from the great extent of surface 

 over which it is spread, so that clouds may be compared to 

 enormous Leyden jars thinly coated with electricity, which only 

 acquires its intensity by its instantaneous condensation. The 

 rapid and irregular motions of thunder clouds are probably more 

 owing to strong electrical attractions and repulsions among them- 

 selves than to currents of air, though both are no doubt con- 

 cerned in these hostile movements. The atmosphere becomes 

 intensely electric on the approach of rain, hail, snow, slefct, and 

 wind ; but it varies afterwards, and the transitions are very rapid 

 on the approach of a thunderstorm. 



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

 from the clouds to the earth, but it acquires from them an oppo- 

 site kind, 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 whether 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 that occurred at Manchester in June 1835, the lightning 

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

 explosions as if pistols had been fired out of the ground, and a 

 man seems to have been killed by one of these explosions taking 

 place under his foot. M. Gay Lussac ascertained that a flash of 

 lightning sometimes darts more than three miles in a straight 

 line. A person may be killed by lightning, although the explo- 

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

 the back stroke. Suppose that the two extremities of a highly 

 .charged cloud hang down towards the earth, they will repel the 



