Page:South - the story of Shackleton's last expedition, 1914-1917.djvu/480

358 time owing to the pressure of the ice on the ship. An ionium collector, for which the radioactive material was kindly supplied by Mr. F. H. Glew, was used. The chief difficulty to contend with was the constant formation of thick deposits of rime, which either grew over the insulation and spoiled it, or covered up the collector so that it could no longer act. Nevertheless, a considerable number of good records were obtained, which have not yet been properly worked out.

Conditions during the Expedition were very favourable for observations on the physical properties and natural history of sea-ice, and a considerable number of results were obtained, which are, however, discussed elsewhere, mention of them being made here since they really come under the heading of physics.

In addition to these main lines of work, many observations of a miscellaneous character were made, including those on the occurrence and nature of parhelia or "mock suns," which were very common, and generally finely developed, and observations of the auroral displays, which were few and rather poor owing to the comparatively low magnetic latitude. Since most of the observations made are of little value without a knowledge of the place where they were made, and since a very complete set of soundings were also taken, the daily determination of the ship's position was a matter of some importance. The drift of the ship throws considerable light on at least one geographical problem, that of the existence of Morrell Land. The remainder of this appendix will therefore be devoted to a discussion of the methods used to determine the positions of the ship from day to day.

The latitude and longitude were determined astronomically every day when the sun or stars were visible, the position thus determined serving as the fixed points between which the position on days when the sky was overcast could be interpolated by the process known as "dead reckoning," that is to say, by estimating the speed and course of the ship, taking into account the various causes affecting it. The sky was often overcast for several days at a stretch, and it was worth while to take a certain amount of care in the matter. Captain Worsley constructed an apparatus which gave a good idea of the direction of drift at any time. This consisted of an iron rod, which passed through an iron tube, frozen vertically into the ice, into the water below. At the lower end of the rod, in the water, was a vane. The rod being free to turn, the vane took up the direction of the current, the direction being shown by an indicator attached to the top of the rod. The direction shown depended, of course, on the drift of the ice relative to the water, and did not take into account any actual current which may have been carrying the ice with it,