THE CONQUEST OF TIME AND SPACE 



of the explorer himself, the fact of his arrival at the 

 pole. 



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 con- 

 stitute the essential equipment of the arctic explorer no 

 less than of the navigators of the seas of more accessi- 

 ble latitudes. There is one important matter of de- 

 tail, 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 hori- 

 zon 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 sur- 

 face, 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. Un- 

 fortunately mercury freezes at about 39 degrees below 

 zero; it is therefore often necessary for the arctic ex- 

 plorer 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 adjust- 

 ing the sextant that the direct image coincides with the 



[50] 



