C. GENERAL PHYSICS. 1V3 



dition of the earth at the two ends of the line, but even earth- 

 quakes are also frequently accompanied by similar phenomena. 

 This latter interesting fact, which has been vaguely announced 

 several times during the past thirty years, is set forth in a very 

 clear light in a remarkable series of observations made bv 

 James Graves at Valencia, Ireland, on the spontaneous currents 

 observed in the Atlantic cables in 18V 1. Indefinite ideas also 

 prevail among telegraphers to the effect that the approach of 

 storms is frequently heralded by special earth currents. The 

 importance of having regular observations, at different places, 

 of natural earth currents is more and more frequently in- 

 sisted upon, and to a certain extent an attempt has been 

 made to do this in India. Every one of the regular tests of 

 the government telegraph lines in that country is made with 

 positive and negative currents, and from the different values 

 is obtained the electro-motive force of the natural line cur- 

 rent in terms of that of a single Daniell's cell. These deter- 

 ruinations are as regularly made as are the tests for conduction 

 and insulation of the lines. The powerful earth currents that 

 occur during heavy thunder-storms cause trouble similar to 

 that produced by the earth currents of the aurora. On the 

 approach of thunder-storms, the French Atlantic cable often 

 gave indications several days in advance; coming first in- a 

 series of slight galvanic shocks, increasing as the storm came 

 onward to the coast, when they were very strong. As to 

 the origin of these currents, Mr. Varley advanced the idea 

 that they are due to the rotation of the earth, which, accord- 

 ing to well-known principles, should produce an electric cur- 

 rent passing through the solid earth from west to east. The 

 variations of intensity of the horizontal magnetic force are 

 due to the variations of the currents of electricity passing 

 through the earth. Jour. Soc. Tel Mig.\ II., 80-120. 



THE FUNCTION OF THE AUMATURES IN MAGNETS. 



Jamin has recently established the following propositions: 



1. The number of elementary magnetic threads, and, con- 

 sequently, the quantity of magnetism which can be contained 

 in a magnet, docs not depend upon the average section. 



2. The expansion (?) of the poles of these magnetic threads, 

 or the distribution of their intensities, is regulated by the 

 form and extent of the exterior surfaces of the magnet. 



