1867.] 



Coimbra Montlily Magnetic Determinations. 



475 



graphic records were at that date on the pomt of commencing. The lati- 

 tude and longitude of the Observatory are 40° 12'-5 N., and 8° 25' W. — 

 E. S.] 



Observations of Deflection, Vi-bration, and Dip tahen at the Coimbra 

 Observatory, 1866. 



The horizontal, Tcrtical, and total forces are calculated to English 

 measure. 



The vertical and total forces are obtained from the absolute measures of 

 horizontal force and dip. 



The value of log K determined at the Kew Observatory is=l "64829 

 at temp. 62° Fahr. 



The value of log is =6-30487. 



The values of the coefficients q and q are respectively 0*000128, 

 0*00000030. The temperature correction was obtained from the formula 

 (^Q— 38) +2' (^0 — 38)^. The correction for error of graduation of the 

 deflection-bar at I'O foot is =-0*00006, at 1*3 is = — 0*00024. The 

 time of one vibration has been obtained from the mean of twenty-four de- 

 terminations of the time of 100 vibrations. 



The angles of deflection are each the mean of two determinations. The 

 difl^erence between the angles of every pair was found always less than 40". 



In deducing from these observations the ratio and product of the mag- 

 netic moment m of the magnet, and of the earth's horizontal magnetic in- 

 tensity X, no correction has been required for the rate of the chronometer, 

 or for the initial and terminal semiarcs of vibration, the former having been 

 always less than 2*'0, and the latter less than 70' at commencement, and 

 30' at end. 



But the induction and temperature corrections have ahvays been applied, 

 and the observed time of vibration has been corrected for the effect of tor- 

 sion of the suspending thread. 



The torsion for 90° was found no less than 5'* 18, and no greater than 

 8'*67. 



In the calculations of the ratio ~ , the third and subsequent terms of the 



X 



P o 



series 1 -V + + &c. have been omitted. The value of the constant P 

 r^ ^.1 



was found to be = —0*0022317, by the mean of thirty-one determinations 

 obtained each from two pairs of deflection observations at distances 1 '0 and 

 1*3 foot. 



The horizontal force for each day is the mean of those calculated for 

 each pair of deflections at distances 1*0 and 1*3 foot. 



