534 THE APPLICATIONS OF PHYSICAL FORCES. [BOOKV. 



the same kind cannot be repelled far through this ground, because 

 it is but a very bad conductor for large charges of electricity. 



If the tree, on the contrary, is on very wet ground of great 

 extent, it will be much influenced, because the electricity of the same 

 kind can spread a long way in this good conductor, and the whole 

 amount of possible induction will take place, if this good conductor, 

 at its limits, is also in communication with other sheets of water of 

 indefinite size. 



When we are dealing with the electricity of our machines, the 

 surface of the earth, whatever it may be, is what we call the ground 

 or the common reservoir. We can call it so, because its conductibility 

 is sufficient to disperse and neutralize all our little electrical charges. 



When we are dealing with lightning, the vegetable soil, in its usual 

 state, is not what we can call the common reservoir, it becomes rela- 

 tively a bad conductor, according to the geological formations of 

 various kinds on which it reposes. It must reach the first water- 

 bearing stratum, that is the stratum supplying wells which never dry 

 up (we will call it here the subterranean stratum), to find abed whose 

 conductibility is sufficient. This, on account of its extent and 

 numerous ramifications, cannot be insulated from the neighbouring 

 water-courses; and with them, the streams and rivers, and the sea 

 itself, it constitutes what we may call the common reservoir for 

 thunder-clouds, and consequently the common reservoir for the light- 

 ning-conductors. 



" In fact, while the storm-cloud exercises its induction everywhere 

 below it, attracting the contrary, and repelling the like electricity, 

 the subterranean stratum is affected by the induction to an incom- 

 parable degree. Then all the upper surface becomes charged with the 

 opposite electricity which the cloud accumulates there by its attrac- 

 tion, while the electricity of the same kind is repelled and dispersed at 

 a distance in the common reservoir. So when the lightning falls the 

 two poles are one on the cloud and the other on the subterranean 

 stratum, which acts as a second cloud necessary for the explosion of 

 the lightning. 



" It is in this way that the globe, always on the whole in a natural 

 state, is eventually electrified at certain points by the presence of the 

 storm-clouds. 



" Buildings, trees, and living animals, which are struck by lightning 



