ATMOSPHERIC ELECTRICITY. 517 



fair weather being + 4, it was found rarely to fall as low as + 1 ; 

 often during sudden showers it equals 20 or 30 ; during snow- 

 storms with high wind it sometimes reaches + 100 ; and during thun- 

 der-showers, 100 and even 200. A predominance of negative 

 electricity is characteristic of thunderstorms. Hence some origin of 

 atmospheric electricity, always operative, but in different degrees of 

 intensity, must be sought. Clearly, any form of energy connected 

 with mere cloud-formation will not answer the requirements. 



2. The observations show also that the potential of the air is ex- 

 ceedingly fluctuating, no natural phenomenon being comparable with 

 it in changeableness except wind-pressure. A change in the electrical 

 state of the air indicates a corresponding change in the earth's surface. 

 It is more reasonable to suppose that the earth, the great reservoir of 

 electricity, should control the air and clouds electrically than that the 

 clouds should control the earth. 



3. The surface of the earth is perhaps never in electrical equi- 

 librium ; in other words, it is not an equipotential surface. Some 

 time about 1865 Matteucci made prolonged experiments on earth-cur- 

 rents, and reached several interesting results. He found, for instance, 

 that a tolerably steady current of electricity flowed through a line 

 established along a meridian, uniformly from south to north ; that 

 fluctuating currents of low electromotive force flowed through an east 

 and west line, sometimes in one direction, sometimes in the other ; 

 that when one terminal of a long line was in a valley and the other 

 at a considerably higher elevation, a current flowed uniformly up the 

 wire toward the more elevated end. A flash of lightning, in this case, 

 was always accompanied by a sudden increase in the deflection of the 

 galvanometer needle. (" Smithsonian Reports," 1867.) 



4. Irregular and spontaneous earth-currents are the usual accom- 

 paniment of great terrestrial disturbances. James Graves showed in 

 1871 that spontaneous currents in the Atlantic cables frequently occur 

 during earthquake-shocks. ("Journal Soc. Tel. Eng.," ii, pp. 80-120.) 

 That spontaneous currents flow through land lines during auroral dis- 

 plays is a well-known fact. It is also asserted that any great meteor- 

 ological change, as the motion of a heavy storm with considerable 

 barometric fluctuation, is announced at a distance by irregular galvanic 

 shocks through submarine cables. 



5. Marked electrical disturbances in the atmosphere not infrequent- 

 ly accompany earthquakes and volcanic eruptions. A vivid flash of 

 lightning was seen during an earthquake-shock in West Cumberland, 

 England, on October 25, 1879. So frequently are these two phe- 

 nomena conjoined that some writers have attributed South American 

 earthquakes to electrical action. Lightning is often seen playing 

 about the boundary between the condensing vapor from a volcano 

 and the adjacent cool air. 



6. Measurements of the potential of the air show that, as we pro- 



