94 THE POPULAR SCIENCE MONTHLY 



this is about 22 minutes, or about 25 English miles, from the pole, and 

 nearly on the 90th meridian. He now knows his position, and by the 

 Greenwich time and the position of the sun he knows the direction 

 of the 90th meridian, and therefore of the pole. 



He then travels in the direction of the pole, keeping this direction 

 by means of his compass or by the sun and his chronometer. Knowing 

 about how fast he travels, he knows when he is in the immediate neigh- 

 borhood of the pole, and he checks his position again by another pair 

 of observations similar to the last. 



Suppose, however, the drift of the ice has been quite strong; it 

 may have carried him several miles from the line AA' during the six 

 hours between his observations; at the time of the second observation 

 he would, indeed, be on the line BB f , but he would no longer be on the 

 line AA'. If he should wait another six hours and observe the sun 

 when on the 210th meridian, he would then find himself, let us say, on 

 the line CC; and, assuming a uniform drift of the ice, his position at 

 the time of the second observation would have been on the line BB' 

 half way between the lines AA' and CC — that is, at B" ; but he has 

 drifted away from the line BB' during the six hours since he deter- 

 mined his position on that line, and he does not know exactly where 

 he is on the line CC. If he waits six hours longer, and observes the 

 sun when on the 300th meridian, and then finds himself on the line 

 DD', his true position at that time will be at T>, and the drift of the 

 ice during the twelve hours between his second and fourth observations 

 will have been in the direction B"D, and it will have drifted a distance 

 equal to the length of the line B"D on the scale of the figure. 



An explorer may find his position by pure calculation, and ma"y not 

 use the graphic method described, but the principle in the two methods 

 is exactly the same, and the graphic method shows more clearly what 

 the observations mean. 



An important source of error enters all these observations, namely, 

 atmospheric refraction, or the bending down of the light rays as they 

 pass through the atmosphere. The amount of this bending increases 

 rapidly as the sun is nearer the horizon; it also varies with the baro- 

 metric pressure, and with the temperature. On April 21, 1908, the sun 

 was only about llf degrees, and on April 6, 1909, only about 6^ degrees 

 above the horizon at the north pole; on both dates the refraction was 

 considerable, and it is hardly well enough known to prevent errors of 

 several minutes of arc in the determinations of position. If, however, 

 the observer should wait for twenty-four hours after his first observa- 

 tion and should measure a fifth altitude of the sun, he could find a fair 

 correction for the refraction and greatly improve the determination of 

 his position. 



It would be unreasonable to expect an explorer, making a dash for 



