METEOROLOGY. 3GI 



Meteorological Society, gives his views on the source of the elec- 

 tricity of the air about as follows: The new kinetic theory of gases 

 teaches that in a cubic inch of saturated aqueous vapor under atuios- 

 ])henc pressure there are contained some three hundred trillion of par- 

 ticles. This corresponds approximately to y,4;, y of a cubic? inch of water, 

 or to one rain drop of ordinary size. When eveiy i)article of \a]»or be- 

 comes electrified for any reason, and all are at the same potential, then, 

 after the union of all into one rain drop, the potential of the latter will 

 be fifty billion times greater. From this it is evident that if from any 

 reason every pajticle of vapor received so slight a potential that it is im- 

 perceptible to our most delicate electrometers, yet the formation of drops 

 of water from these particles would explain the most terrible strokes of 

 lightning. Some years ago it occurred to me that the simi)le contact of 

 vai)or particles with those of the air by the process of difl'usion going on 

 betweeu them must be sufficient to produce this extremely small poten- 

 tial. Thus the source of atmospheric electricity is the contact of two 

 substances, as in Yolta's apparatus, where it is the contact of two dry 

 metals. Experiments upon a small scale have not as yet confirmed this 

 view, nor can it be thoroughly investigated without making them on so 

 large a scale that private means are inadequate to cover the expense. 

 yZ. 0. G. if., XIX, p. 301.) 



333. Prof. P. G. Tait communicates a critical summary of all suggested 

 theories as to the source of atmosi)heric electricity. His own contact 

 theory is given above ; the other theories may be briefly summarized as 

 follows: (1) Aerial friction; (2) Fouillet, combustion and evaporation; 

 (3) Saussure, vapor condensation ; (4) Peltier; and von Lamont, the per- 

 manent negative electrical charge of the earth by conduction and in- 

 duction electrifies the air and vapor; (5) Sir William Thomson, air is 

 electrified by contact with the ground, and remains so after ascending 

 in the ordinary convection currents; (G) Becquerel, eleclified corpuscles 

 travel from the sun to the earth's atmosphere ; (7) Miihry, a direct effect 

 of solar radiation ; (8) Liiddens, friction of vapor against the dry air; 

 (9) The capillary surface tension of water drops; (10) The production 

 of hail-storms; (11) Friction of air against the ground or against cur- 

 rents of air. [Nature, xix, p. 517). 



334. Prof. Edlund, of Stockholm, has endeavored to determine luimer- 

 ically the amount of the unipolar induction due to the relative move- 

 ments of the atmosphere and the earth considered as a magnet. The 

 measures which he has made in Stockholm give for a layer 1 meter thick 

 at the equator an electromotive force of 0.0321 Daniell's cell. Allowing 

 that the moist air is a good conductor, and that the clouds have an 

 average height of 1,000 meters, he reckons that between the earth and 

 the clouds there must exist an electro-motive force of at least 23 Dani- 

 ell's, which abundantly explains the electric tension ordinarily observed 

 in the air. The extraordinary electric tension necessary to produce a 

 lightning flash originates according to him in the increase of tension 



