416 



PROCEEDINGS OF THE AMERICAN ACADEMY 



way that only one of the two components of the earth's magnetism acts 

 upon it at one time. I first placed a coil vertically in the meridian 

 and turned it 90° around a vertical axis, having connected the ends 

 of the wire with a Thomson reflecting galvanometer. The horizontal 

 lines of magnetic force are the only ones cut by the coil, and hence the 

 scale reading is proportional to H. 



In the next place, I turned the coil, again placed vertically on the 

 meridian, 90*^ around an horizontal axis. In this case, the vertical 

 component is cut, and a deflection proportional to it produced. 



Therefore, we have found the ratio of the two components ; and, if 



i = the inclination, we have 



V 

 ta.ni == — ; (C.) 



The results of my measurements are given below. 



Preliminary Measurements, Dec. 19 and 20, 1877. 



(«) Determination of MH. 



To find the time of oscillation t, two measurements were made aa 

 follows : — 



O^was found equal to .01387. 

 Substituting these values in equation (A.), we get 

 MH=i 2954000. 



(^) Determination of 



In this case, a ring-magnet was used, carrying a plane mirror, and 



suspended by a piece of silk thread, the torsion of which was measured, 



and gave 



© = .01719 



