Note 1 — Chapter XXXII
Reading edition
The instruments used in taking observations for latitude may be either a sextant and an artificial horizon, or a small theodolite. Both these instruments were taken on the sledge journey; but the theodolite was not used, owing to the low altitude of the sun. Had the expedition been delayed on the return until May or June, the theodolite would then have been of value in determining position and variation of the compass.
The method of taking meridian observations with a sextant and an artificial horizon on a polar sledge journey is as follows: if there is any wind, a semi-circular wind-guard of snow blocks, two tiers high, is put up, opening to the south. If there is no wind, this is not necessary.
The instrument box is firmly bedded in the snow, which is packed down to a firm bearing and snow is packed around the box. Then something, usually a skin, is thrown over the snow, partly to prevent any possible warmth from the sun melting the snow and shifting the bearing of the box; partly to protect the eyes of the observer from the intense reflected glare of light from the snow.
The mercury trough of the artificial horizon is placed on top of the level box, and the mercury, which has been thoroughly warmed in the igloo, is poured into the trough until it is full. In the case of the special wooden trough devised and used on the last expedition, it was possible to bring the surface of the mercury level with the edges of the trough, thus enabling us to read angles very close to the horizon.
The mercury trough is covered with what is called the roof—a metal framework carrying two pieces of very accurately ground glass, set inclined, like the opposite sides of the roof of a house. The object of this roof is to prevent any slightest breath of wind disturbing the surface of the mercury and so distorting the sun's image in it, and also to keep out any fine snow or frost crystals that may be in the atmosphere. In placing the trough and the roof on the top of the instrument box, the trough is placed so that its longer diameter will be directed toward the sun.
A skin is then thrown down on the snow close to the box and north of it, and the observer lies down flat on his stomach on this, with his head to the south, and head and sextant close to the artificial horizon. He rests both elbows on the snow, holding the sextant firmly in both hands, and moving his head and the instrument until the image or part of the image of the sun is seen reflected on the surface of the mercury.
The principle on which the latitude of the observer is obtained from the altitude of the sun at noon is very simple. It is this: that the latitude of the observer is equal to the distance of the center of the sun from the zenith, plus the declination of the sun for that day and hour.
The declination of the sun for any place at any hour may be obtained from tables prepared for that purpose, which give the declination for noon of every day on the Greenwich meridian, and the hourly change in the declination.
Such tables for the months of February, March, April, May, June, and July, together with the ordinary tables for refraction to minus 10° Fahrenheit, I had with me on pages torn from the "Nautical Almanac and Navigator."