nOW COULD AN EXPLORER FIND THE POLE 91 



of the circle he was ; but he could not determine his position accurately. 

 If, some time after his first measure of the sun's altitude, he should 

 make a second similar measure, he would determine his position on a 

 second circle; and the intersection of those two circles would deter- 

 mine his position completely. The determination, of course, would 

 be more accurate if the circles cut each. other at a high angle, and this 

 could be insured by making the second set of observations on the sun 

 after it had changed its direction (measured on the horizontal plane) 

 by about 90°. Instead of making two sets of observations on the sun, 

 we might, in the evening, observe two stars properly located with 

 respect to each other, and we could then find two circles of position in 

 a few minutes and completely determine our position. 



If the sun is due north or south the part of the circle on which the 

 observer is will coincide with his parallel of latitude, which is thus 

 immediately determined; if the sun is due east or west, a part of the 

 circle will correspond with the meridian and the longitude will be 

 found. The old method of determining position at sea, and one still 

 in use, was to observe the sun at noon for latitude, and to accept as 

 local noon the poorly determined time when the sun reached its high- 

 est altitude; or to observe also in the morning or evening for time or 

 longitude, guessing at the latitude to work out the observations. But 

 the new method makes it possible to observe altitudes at any time and 

 to get satisfactory results even if the sun were hidden for several hours 

 during the middle of the day. And besides it makes clear just what 

 information regarding our position is yielded by a single observation 

 of the sun's altitude. This beautiful method was first used by Captain 

 Thomas H. Sumner, of Boston, Mass., in 1837; the short parts of the 

 circle which are drawn on the map in finding one's position are called 

 Sumner's lines. 



If an explorer were approaching the north pole, and had arrived, 

 let us say within a degree of it, it would be necessary for him to de- 

 termine his latitude in order to know his distance from the pole, and to 

 determine the direction of the pole in order to know his course. It 

 might be supposed that when approaching the pole he would, by means 

 of his compass, be able to follow his meridian; but the difficulty of 

 keeping a fixed direction when traveling over rough ice, and especially 

 the shifting of his position by the unknown drift of the ice, would 

 soon make a decided change in his longitude in a region where the 

 meridians converge so rapidly. 



In the neighborhood of the north pole the compass needle points 

 approximately in the direction of the meridian 155° west of Green- 

 wich, according to Neumayer, but the angle between the needle and the 

 meridian changes considerably for comparatively small variations of 

 position ; especially as the distance from the pole becomes smaller. 



