HOW COULD AN EXPLORER FIND THE POLE 95 



the pole, to remain twenty-four or even eighteen hours at one camp 

 for the purpose of exactly determining his position. By making daily 

 observations on the sun, at different hours, so as not merely to fix his 

 successive positions on a series of parallel lines, but on lines having 

 different directions; by keeping his direction with the compass and 

 estimating the drift of the ice and his rate of travel, he could always 

 know where he was without too large an error. But when he was in 

 the immediate neighborhood of the pole he should make as many ob- 

 servations, with the sun in different directions, as circumstances would 

 permit. 



Fatigue, severe cold, the condition of his commissariat, and the 

 anxiety to return after having succeeded in his bold undertaking, 

 might prevent him from making as many observations as would be 

 desirable; but nevertheless they might be sufficient to be convincing 

 that he had been within a few miles of the pole; it would surely be 

 a quibble to dispute with an explorer the honor of having reached the 

 pole if his observations showed, without reasonable doubt, that he had 

 been within ten or fifteen miles of it. 



There are two kinds of instruments used for measuring altitudes; 

 the transit-theodolite and the sextant. The former consists of a tele- 

 scope so mounted that it can turn in a vertical and in a horizontal 

 plane; it is provided with vertical and horizontal graduated circles, to 

 measure the angle turned through, and with leveling screws and spirit 

 levels to adjust it in position. It is supported by a tripod, and after 

 being properly leveled, the reading of the vertical circle gives the alti- 

 tude of the object sighted through the telescope. It is by far the best 

 instrument for an explorer on land, because it is very easy to use, and 

 its adaptation to measure horizontal angles enables the explorer to 

 carry on an ordinary survey. 



The sextant was originally invented for use at sea, where a steady 

 support can not be found. It consists of a telescope mounted on a 

 frame, which is held in the hand. To measure the angle between two 

 objects, one of them is sighted directly through the telescope, and the 

 image of the second is reflected into the telescope by means of two 

 mirrors, one fixed rigidly to the frame in front of the telescope, and 

 covering half its field, and the other movable around an axis fastened 

 to the frame. The movable mirror is turned by an arm, which moves 

 along a graduated arc on the frame, and its reading, when the two 

 objects appear in the telescope superposed upon each other, gives the 

 angle beteween the objects. In determining the altitude of the sun at 

 sea the edge of the sun is made to touch the horizon; a movement of 

 the ship moves the sun and the horizon together and the contact is not 

 destroyed. On land, when the sea horizon is not available, a so-called 

 artificial horizon must be used. The ordinary mercurial artificial 



