NO. 7.] DECLINATION. 11 



its error of collimation. Its two positions, which, during the observations, 

 were noted by the expressions n Skr. op" (Heads of the screws up) and n Skr. 

 ned" (Heads of the screws down) will be indicated in the following pages by 

 P, and P 2 respectively. 



By the determination of the constants of the instrument in Hamburg, 

 in 1893, the needle's total error of collimation (mirror and magnetic axis) was 

 found to equal 30.90', so that the correction 30.90' would be employed 

 for readings in the position P, , wich gave too great an east declination, 

 and -f- 30.90' for readings in position P l , which gave too small an east 

 declination. 



After the return of the expedition, a value of 37.74' for the error of 

 collimation was deduced by the observations in Wilhelmshaven, in 1897. It 

 appears from this that the position of the mirror, and the direction of the 

 magnetic axis of the needle, have remained relatively almost unchanged; and 

 thus the following constant correction of the error of collimation may be em- 

 ployed for all the observations made with this needle during the expedition: 



C = + 87 ' 7 ' - 34-3' 



There is also, however, a considerable amount of material for the veri- 

 fication of the error of collimation by the aid of these very series of observations 

 made during the expedition; and I have tried to utilise this material in the 

 following manner. The observations of the declination were generally made by 

 taking a longer or shorter series of readings with the needle alternately in 

 position P, , and in position P a . The hour was noted by the watch to tenths 

 of a minute; and after each reading, if the needle was not specially restless, 

 there was an interval of only a few minutes for reversing and resetting. 



If the readings in the two positions of the needle, P t and P s are called 

 respectively 



a, n . a . ..... On 



and b, b a 6, ..... b. 

 and the corresponding times by the watch, 



a, a, a, .... 



and ft ft ft , . . . ft n , 



