C. GENERAL PHYSICS. 163 



two movable soft iron masses suspended by a vertical thread. 

 Many of the laws of electro-dynamics may be readily illus- 

 trated by this instrument, and not only may different cur- 

 rents be compared with the greatest accuracy, but the ab- 

 solute mechanical value of the current may be at once ar- 

 rived at. 7 A, XL VIII., 297. 



EARTH CURRENTS IN TELEGRAPH LINES. 



Mr. Schwendler, who in 1868 was intrusted with the in- 

 troduction of a system of testing telegraph lines in India, 

 took that opportunity to do his work so thoroughly as to 

 secure all the data necessary for the quantitative determi- 

 nation of the electro-motive force on the line. Over 10,000 

 determinations have been made during the past six years, 

 and he deduces from these the conclusion that all the lines 

 in India are affected by natural currents of electricity. These 

 currents are, as it were, a negative or copper current, flowing 

 from the east to the west. The strength of the natural cur- 

 rent is very variable, even on the same line. The direction 

 is also variable, but far more constant than the strength. 

 The variations in strength and direction, on parallel lines 

 of telegraph, are very uniform. The prevailing direction 

 of the current is generally also the direction of the maxi- 

 mum current. He considers himself now fully justified in 

 establishing further improvements for the purpose of minute- 

 ly investigating these currents, and his propositions having 

 been strongly urged upon the attention of the Indian gov- 

 ernment, have been favorably received by it. Proc. Asiatic 

 Soc. Bengal, June, 1874, 145. 



VARIATIONS OF SHIPS' COMPASSES. 



Sir William Thomson communicates to the British As- 

 sociation for the Advancement of Science an investigation 

 of the perturbations of the compass produced by the rolling 

 of the ship the so-called "heeling error" which has been 

 studied by Airy and Smith. This heeling error may be 

 defined as the angle between the directions for the ship up- 

 right and the ship inclined, the resultant of the horizontal 

 magnetic forces of the earth and the ship at the position of 

 the compass a definition that would be rigorous for a com- 

 pass supported on a point in the ordinary manner, if this bear- 



