Additions to the Kew Magnetometer. 



125 



The following observations wore made in a lofty room on 

 an object the elevation of which was 40° : — 



Scale Reading of 



Image of Cross 



Wires [x). 



Reading on 

 Circle (p). 



x-x . 



P~Po- 



-060 

 -3-60 



-0 20 

 +2-S0 

 +5*45 



284 42-1 

 204 39-5 

 264 35-7 



2G4 326 

 264 29-0 



— 5-43 

 -3-40 



3-00 

 5-65 



6-4 



3-8 



-31 

 -67 



All the readings are the mean of two which differed but 

 little, and between which the axle of the mirror was inverted 

 in its bearings. Hence if a be the angular value of a scale- 

 division, i. e. the error of collimation of the mirror which 

 corresponds to a displacement of the image through one 

 division, 



2(>- t i' n )«sin 2 20° 

 P-P°= cos 40° 



= 0-306 O-.r )a. 



We thus get four values of a, viz. 3'-8, 3'*7, 3'-4, and 3'-9, the 

 mean of which, viz. 3'* 7, may be taken as correct. 



To check this value, observations were made at South 

 Kensington on the sun, and on a pinnacle of the Natural 

 History Museum, both when the mirror was in good and 

 when it was in bad adjustment. It will be seen that the 

 corrected values are in close accord. 



Thus on May 9th, at 12 h 46 m 36 s , when the altitude of the 

 sun was 54° 39**8, the N point on the scale was found to be 

 58° ll'*2. The mirror was then thrown out of adjustment, 

 the mean reading for the image of the cross wires being 

 + 5*7 div. Another observation on the sun was made at 

 l h 13 ra 12 s , when the altitude was 52° 51'- 5, which gave for 

 the reading of the north point 57° 56' # 9. 



The correction is 



2x5-7x3'7sin 2 26°26' 



13'-8. 



Hence 



cos 52° 51' 



57°56'-9 + 13'-8 = 58°10'-7, 



