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



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



a *+ a > . . 8 ft + ft - g + a = * etc 



~ Pi > ' ft ( lc - 



and the circle readings 



^+_^ a nd 6l , ^L+A_ and , , j- and 6 8 , 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 8 , c s , etc. 

 we have 



C, _-t 



2 



C, = 



S _J4* 



a > __ 6, 



2 

 and the mean error of collimation 



_ 



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 +a m + 1 fl, -f fl 



- p m , or - 



a m 



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

 table. 



