102 ATMOSPHERIC ELECTRICITY. 



normal fall of potential should, according to the views of the author, 

 have a different sign in the polar and equatorial regions, which is con- 

 trary to the observed fact. This theory does not, however, exhaust 

 the possibility of explaining atmospheric electricity as a phenomenon 

 of electromagnetic induction, and it is not disproved that in some form 

 or other the rotation of the earth's magnetic field may play a part in 

 the origin of the electric field. The theories which take solar radiation 

 as the source of the energy divide themselves into several groups. 

 We may think of a direct thermoelectric or actinic action, but there 

 is, so far, no experimental support to such views. One of the earliest 

 and most natural suppositions is the belief in evaporation as a source 

 of electrification. This was Volta's theory, and experiments have at 

 various times been produced in its support; but so far no one has been 

 able to invalidate Faraday's conclusion that whenever electrification 

 seemed to appear as a consequence of evaporation, it was really due 

 to secondary causes, such as the friction of the liquid spray against 

 the sides of the containing vessel. Rejecting Yolta's theory, there is 

 nothing left but the belief in some form of contact or frictional elec- 

 tricity either between drops of water and air or water and ice, or any 

 two of the various bodies present in the atmosphere. The possibility 

 of contact electricity between a solid or liquid and a gas is not quite 

 easy to submit to the test of experiment. If we rub two solid bodies 

 together, we may, by separating them, investigate the electric field 

 produced; but, supposing we have a drop of water surrounded on all 

 sides by air, the water niay be covered with an electric layer of, say, 

 positive electricity, the air in contact with the water with the opposite 

 kind, and it is not at all clear how we could experimentally demonstrate 

 the difference of potential between the air and the drop which is thus 

 produced. A current of air flowing past the drop might carry away 

 some of the negative layer, and in this way an electric field may be 

 established while clouds are forming, but the conditions necessary for 

 an experimental demonstration would be very difficult to realize. Two 

 methods have been devised which practically demonstrate some form 

 of contact electricity between gases and water. 



Leuard, wishing to imitate the electric field observed in the neigh- 

 borhood of waterfalls, has established by careful experiment a number 

 of important facts, which are all consistent with the following explana- 

 tion. If we imagine two oppositely electrified layers at the surface of 

 a drop of water such as has been referred to, and if the drop falls on to 

 a layer of the same liquid, or if similar drops impinge on each other, 

 the difference of potential produced by the fusion of the surface layers 

 becomes greater than is consistent with equilibrium. For, taking the 

 case of drops falling into a mass of water contained in a cylindrical 

 vessel, the extent of surface between air and water is not increased 

 by the falling drops, and we must imagine that surface to be already 

 covered with a sufficient electrical sheet to establish the required dif- 



