104 



SCIENCE 



[N. S. Vol. LI. No. 1309 



the section of this conductor being equal to 

 the entire surface of the globe is enormous, 

 compared with the distance to be bridged; 

 and thus the minute specific conductivity of 

 the air multiplied by the section and divided 

 by the length of the path is still an appre- 

 ciable quantity even locally, and a very large 

 one taking the earth as a vfhole. Moist air 

 conducts better than dry, and the electrifica- 

 tion at a given altitude is on the average 

 several times as great in winter as in summer, 

 because the drier air of winter is a better 

 insulator. 



The following examples are from U. S. 

 Weather Bureau observations at Drexel, 

 Nebraska, in 191Y (e^mean pressure of 

 aqueous vapor in the air up to the given 

 height, measured in millibars ; v = positive 

 electric potential of the upper layer in volts). 



arily negatively charged by contact with the 

 ground, these conditions of electric distribu- 

 tion in the atmosphere are fairly persistent. 

 The two electricities are continually com- 

 bining, but are as constantly replenished. 



The incoming electrons from the sun may 

 be absorbed by the upper air, but they serve 

 to increase the absolute potential of the earth 

 by a process which is independent of the 

 radiant ionization; and as I have shown that 

 there is conduction between the upper and 

 lower layers of the atmosphere and adjust- 

 ment of its ever varying charges, the in- 

 creased absolute potential of the upper air is 

 eventually, and probably pretty rapidly, trans- 

 ferred to the ground. Thus the ground re- 

 ceives its permanent negative charge from 

 the sun; and in spite of all sorts of irregular 

 electric variations in the intervening atmos- 



On the given dates in January, which are 

 fairly typical, the average positive electric 

 potential was 20,332 volts at 3,000 meters for 

 e^l.92 m.b., and in June a potential of 

 1,375 volts was found at the same height for 

 e=:Y.39 m.b. So far as ionization of water 

 vapor is concerned, there should be more of 

 it in June per unit volume of air; but in 

 •spite of this, the greater atmospheric con- 

 ductivity at that time cuts down the potential 

 to a much lower value than the winter one. 

 Evidently there is continual conduction from 

 the air to the ground. This does not neutral- 

 ize the negative charge of the ground, partly 

 because of the large electric capacity of the 

 latter, but mainly because the prevalent nega- 

 tive charge of the earth as a whole is con- 

 tinually being restored. Except for convec- 

 tive uplifting of local bodies of air tempor- 



phere, the permanent negative charge of the 

 ground is maintained with only such minor 

 fluctuations as occur in magnetic storms. In 

 these, the showers of electrons received by 

 the earth from the sun at times of great solar 

 activity certainly penetrate into the earth's 

 solid substance almost immediately, in spite 

 of atmospheric obstruction, and produce elec- 

 tric " earth currents " of considerable magni- 

 tude. We must conclude that the absolute 

 potential of the earth is continually varying. 

 Newcomb's investigations of the inequali- 

 ties of the moon's motion* indicate the exist- 

 ence of unexplained fluctuations in the moon's 

 mean motion — a great fluctuation possibly 

 with a period of between 250 and 300 years, 



+ Monthly Notices of the Eoyal Astronomical So- 

 ciety, Vol. LXIX., p. 164, January, 1909. 



