12 AKSEL S. STEEN. TERRESTRIAL MAGNETISM. [NORW. POL. EXP. 



then approximately, provided the intervals are of about equal length, 



* + a > - - 8 ft + ft - - a *+ a > .. s etc 

 2 ~ Pi > 2 ~ ' ' 2 rt ' IC ' 



and the circle readings 



L^L and bt t &LJJ_A_ and a3 , *L+_*L an d b , , etc. 



may be considered to be simultaneous for both positions of the needle. For 

 each such combination, a value is obtained for the error of collimation, 

 2(n 1) values in all. If these values be indicated by c t , c 2 , c s , etc. 

 we have 



c, --*- 



C i(H _ l)== 



2 

 and the mean error of collimation 



_ c, + c, + c, + ...... + c, a.-, 



By this formula I have calculated the error of collimation from the series 

 of observations (noted when the disturbance was not very great) for cases 

 in which the difference in time 



a m + 1 fl ft + i 



Pm ) Or - ~ a m + j 



was less than 1 minute. The results obtained are given in the following 

 table. 



