NO. 7.] DECLINATION. 15 



readings with this needle, as increasing numbers on the horizontal circle cor- 

 respond to increasing east declination. There has been no subsequent veri- 

 fication of this determination, and I have therefore been obliged to take 

 8'6' all through as the constant value of the small needle's error of collimation. 

 In these pages, the small needle will be indicated by L. 



THE MARK. 



All through the time that the vessel was drifting with the ice, the mark 

 used was the objective of the astronomical altazimuth instrument, which was 

 set up on the ice at some distance, or, after the 26th Sept. 1895, when the 

 ice observatory was taken into use, its pillar. As already mentioned, the 

 setting at the mark was always the first step to be taken after the instrument 

 was levelled, whereupon followed a longer or shorter series of settings of the 

 magnetic needle, and finally a sight was once more taken of the mark. In 

 most cases, the first -and last mark-readings agree within less than 1'; but 

 sometimes it happened that the relative position of the two instruments changed 

 somewhat during the observation, either because of the movement in the ice, 

 or when the sun had acted upon the ice under the legs of the stand, so that 

 they had been a little displaced. Captain SCOTT-HANSEN was always observant 

 of this, however, and in such circumstances often made supplementary test- 

 settings at the mark. 



THE AZIMUTH. 



The azimuth determinations necessary for the calculation of the absolute 

 declination, were generally made with the astronomical altazimuth, the mag- 

 netic apparatus serving as mark. In the accompanying figure (Fig. 1), the 

 relative position of the two instruments is shown. T is the centre of the 

 magnetic apparatus, and U the centre of the astronomical altazimuth. The 

 arrow N gives the direction of the astronomical meridian, and the arrow n 

 the direction of the magnetic meridian. If the east declination be indicated 

 by D, then, as will be seen from the figure, we have 



D = x . 



