C. GENERAL PHYSICS. 151 



of these earth currents need not be of an expensive charac- 

 ter; and it is greatly to be desired that such observations 

 should be made in various portions of the world. Mr. Win- 

 ter believes the absolute magnetism of the earth to be much 

 more constant than is generally imagined, and that the sup- 

 posed variation in its elements is largely due to the great 

 action of the earth stream upon the magnet and magnetom- 

 eters. From his observations in December, 1871, he finds 

 the general direction of the earth current to be from the 

 south, being most easterly at 5 P.M., and most westerly at 

 9 A.M. ; strongest at 5 P.M., and weakest at 1 A.M. Mr. 

 James Graves has presented to the same society a very valu- 

 able paper on the same subject. Mr. Graves was stationed, in 

 1871, at Valencia, during the period of nearly three months 

 when both the cables of the Anglo-American Telegraph Com- 

 pany were disabled. During this interval of seventy-eight 

 days he embraced the opportunity of recording, every half- 

 hour, the nature and strength of the earth currents that were 

 manifested upon these wires; one cable being about 1850 and 

 the other 1820 nautical miles in length. The record for the 

 two cables was practically the same. The recorded deflec- 

 tions of the galvanometer needles have been reduced to rel- 

 ative values in terms of one battery cell as a unit of force. 

 lie shows that there are two maxima of force positive, and 

 two negative; the first maximum positive at 3 or 4 A.M.; 

 the first negative at 7 or 8 A.M. ; the second and larger maxi- 

 mum positive about 12 or 1 P.M.; and the second maximum 

 negative at 6 or 7 P.M. The maximum observed at any 

 time was equal to 112 cells during an aurora, at 10 P.M., 

 April 9. Great meteorological changes, as well as sudden 

 changes of wind, are, he finds, generally accompanied by an 

 earth current of unusual force. The currents repeatedly re- 

 verse from plus to minus, and vice versa, during the display 

 of auroras, precisely as when a short cable or land-wire is 

 used. Whenever an earthquake is experienced in any part 

 of the globe, the telegraph lines are more than usually influ- 

 enced by earth currents, and Mr. Graves has observed the 

 same with reference to tire ocean cables, showing that the 

 great internal convulsions of the earth give rise to electrical 

 currents. Jour. Soc. Tel. Engineers, II., 89. 



