The question has, indeed, been largely answered in the earlier pages of this chapter, in our discussion of the sextant and the _Nautical Almanac_; for these constitute the essential equipment of the arctic explorer no less than of the navigators of the seas of more accessible latitudes. There is one important matter of detail, however, that remains to be noted. This relates to the manner of using the sextant. On the ocean, as we have seen, the navigator levels the instrument at the visible horizon; but it is obvious that on land or on the irregular ice fields of the arctic seas no visible horizon can be depended upon as a basis for measuring the altitude of sun or stars. So an artificial horizon must be supplied. The problem is solved by the use of a reflecting surface, which may consist of an ordinary mirror or a dish of mercury. The glass reflector must be adjusted in the horizontal plane with the aid of spirit levels; mercury, on the other hand, being liquid, presents a horizontal surface under the action of gravitation. Unfortunately mercury freezes at about 39 degrees below zero; it is therefore often necessary for the arctic explorer to melt it with a spirit lamp before he can make use of it. These, however, are details aside from which the principles of use of glass and mercury horizon are identical. The method consists simply in viewing the reflected image of the celestial body--which in practice in the arctic regions is usually the sun--and so adjusting the sextant that the direct image coincides with the reflected one. The angle thus measured will represent twice the angular elevation of the body in question above the horizon,--this being, as we have seen, the information which the user of the sextant desires.