140 



Dr. T. E. Thorpe. 



for which, it was originally calculated, no longer accords with the ob- 

 servations of the present epoch. On June 15, 1878, Mr. Schott found 

 the magnetic dip, at his observatory, to be 70 o, 80: my own observa- 

 tions at the same place, three months later, made it 70"' 78 ; the value 

 calculated from the above formula is 70°* 5 7, or 14' less than the mean 

 of our observations, an amount far in excess of the error of the ob- 

 servation. It is not at all improbable, as indeed Mr. Schott has already 

 remarked, that although on the whole the clip is decreasing at 

 Washington there are subordinate periods of comparatively short 

 duration during which an increase occurs, the former rate of diminu- 

 tion being afterwards re-established. Inequalities of the same kind, 

 although by no means of the same extent, have also been traced in the 

 observations of inclination made at Toronto and Kew by Sir Edward 

 Sabine and Dr. Balfour Stewart (Sabine, " Phil. Trans., 1863 ; " 

 Stewart, " Proc. Roy. Soc," vol. xviii, 1870). 



Intensity. 



The horizontal component of the earth's magnetic force was deter- 

 mined by the method of vibrations and deflections in the manner 

 described in the " Admiralty Manual of Scientific Inquiry," 3rd edi- 



tion, 1869. By the aid of the deflections we obtain ^jF, or the ratio 



between the magnetic moment of the magnet employed and the hori- 

 zontal component ; from the vibrations we find the product mX, 

 whence either m or X may be obtained. The total force I is obtained 

 by dividing X by the cosine of the observed dip ; and from this we 

 readily find the vertical component V from the formula : — 



Y2 = I2-X 3 . 



The final results are stated in British and metrical units ; one foot, 

 one second of mean solar time, and one grain being the units of space, 

 time, and mass in the one case ; and one millimetre, one second of time 

 and one milligramme being the units in the other. By dividing the 

 latter values by 10, the corresponding values in C. G. S. are also ob- 

 tained. The factor for converting the British to metric values is 

 0-46108. 



The value of log 7r 2 K at 60° is 1*62832, as determined by Mr. 

 Whipple, at Kew. 



The induction coefficient /a is 0*0001615. 



The correction for temperature t° to the adopted standard tempera- 

 ture 35°P., is 



0*0001383 (T-35°) +0*000000607 (T-35°). 



The correction for error of graduation of the brass deflection bar at 

 1 foot is -0*000135 foot and at 1-5 foot —0 00087 foot. 



