APPENDIX I 
speed of return being greater the more rapid the drift. Another 
means of estimating the speed and direction of the drift was from 
the trend of the wire when a sounding was being taken. The rate 
and direction of drift appeared to depend almost entirely on the wind- 
velocity and direction at the time. If any true current-effect existed, 
it is not obvious from a rough comparison of the drift with the pre- 
vaiHng wind, but a closer investigation of the figures may show some 
outstanding effect due to current.* The drift was always to the left 
of the actual wind-direction. This effect is due to the rotation 
of the earth, a corresponding deviation to the right of the wind- 
direction being noted by Nansen during the diaft of the Fram. 
A change in the direction of the wind was often preceded by some 
hours by a change in the reading of the drift vane. Tliis is no doubt 
due to the ice to windward being set in motion, the resulting dis- 
turbance travelling through the ice more rapidly than the approach- 
ing wind. 
For the astronomical observations either the sextant or a theodolite 
was used. The theodolite employed was a light 3* Vernier instru- 
ment by Carey Porter, intended for sledging work. This instrument 
was fairly satisfactory, although possibly rigidity had been sacrificed 
to lightness to rather too great an extent. Another point which 
appears worth mentioning is the following : The foot-screws were of 
brass, the tribrach, into which they fitted, was made of aluminium 
for the sake of Kghtness. The two metals have a different coefficient 
of expansion, and while the feet fitted the tribrach at ordinary 
temperatures, they were quite loose at temperatures in the region of 
20** Fahr. below zero. In any instrument designed for use at low 
temperatures, care should be taken that parts which have to fit 
together are made of the same material. 
For determining the position in drifting pack-ice, the theodoUte 
proved to be a more generally useful instrument than the sextant. 
The ice-floes are quite steady in really thick pack-ice, and the theo- 
dolite can be set up and levelled as well as on dry land. The obser- 
vations, both for latitude and longitude, consist in measiu-ing the 
altitude of the sun or of a star. The chief imcertainty in this measOTC- 
ment is that introduced by the refraction of hght by the air. At 
very low temperatures the correction to be applied on this account 
is uncertain, and, if passible, observations should always be made in 
pairs with a north star and a south star for a latitude, and an east 
star and a west star for a longitude. The refraction error WiU then 
usually mean out. This error affects observations both with the 
theodolite and the sextant, but in the case of the sextant another 
♦ Cf. "Scientific Results of Norwegian North Polar Expedition, 1893-96," 
vol. iii, p. 367. 
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