Page:South - the story of Shackleton's last expedition, 1914-1917.djvu/481

Rh but the true current seems never to have been large, and the direction of the vane probably gave fairly accurately the direction of the drift of the ice. No exact idea of the rate of drift could be obtained from the apparatus, although one could get an estimate of it by displacing the vane from its position of rest and noticing how quickly it returned to it, the 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 prevailing 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 drift 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. This is no doubt due to the ice to windward being set in motion, the resulting disturbance travelling through the ice more rapidly than the approaching wind.

For the astronomical observations either the sextant or a theodolite was used. The theodolite employed was a light 3´´ Vernier instrument 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 lightness. 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 theodolite proved to be a more generally useful instrument than the sextant. The ice-floes are quite steady in really thick pack-ice, and the theodolite can be set up and levelled as well as on dry land. The observations, both for latitude and longitude, consist in measuring altitude of the sun or of a star. The chief uncertainty in this measurement is that introduced by the refraction of light by the air. At very low temperatures, the correction to be applied on this account is