horizon consists of a flat dish about three inches wide and five or six long, filled to a small depth with mercury, the surface of which becomes perfectly horizontal. The image of the sun seen in the mercury will be as much below the horizontal plane as the actual sun is above it; and the angle between the sun and its image is twice the altitude of the sun. Except in very quiet air, the surface of the mercury must be protected from the wind by an accurately made glass cover.
The glass artificial horizon is a piece of perfectly flat dark glass, which will absorb the light which enters it and only reflect from its upper surface. It is provided with leveling screws and spirit levels so that it can be made perfectly horizontal. It is used in exactly the same way as a mercurial horizon.
Each form has its advantages; the glass horizon is easily transported, and can be used at temperatures below the freezing point of mercury (about 39° below zero Fahrenheit). On the other hand, it requires very careful leveling, and is liable to be broken. The mercury of a mercurial horizon is usually carried in an iron bottle; in pouring it back and forth it might be spilled and lost; and at very low temperatures it would be necessary to heat it to keep it liquid; but then it immediately takes a level surface and requires no leveling.
Lieutenant Shackleton, traveling over the Antarctic continent, determined his position by means of a small transit. Commander Peary and Dr. Cook, traveling over the floating ice of the Arctics, used sextants. The former used a mercurial and the latter a glass horizon.
It is interesting to note that if a man were taking an observation standing, with the sun about 6 degrees above the horizon and the artificial horizon on the level of his feet, it would have to be about 45 feet from him, and as he would look at it from an angle of about 6 degrees, it would only appear about half an inch long. If the altitude of the sun were 12 degrees, the artificial horizon would be 25 feet away and appear about an inch long. This can be easily imitated by putting a sheet of paper on the ground and looking at it from distances of 25 and 45 feet. Under such conditions the difficulties of making a good observation would be much increased. If, however, the artificial horizon were raised on a support, the observer would stand much closer to it, and the observation could be more easily made.
Another important instrument is the chronometer keeping mean Greenwich time; for, as has already been shown, the determination of position in general requires a knowledge of Greenwich time, though at the pole itself this is not necessary. Whenever an explorer remained as long as a week in one place he should determine, as well as he could, how much his chronometers were gaining or losing per day; and he should be most particular to determine the changes in their errors, between the times of leaving and returning to his base station.
