392 POLAR PROBLEMS 



mitted that the magnetic compass is comparatively sluggish in parts 

 of the Arctic, it is generally possible, with a good pilot and moderately 

 smooth air, to steer by it. 



The most difficult dead-reckoning navigation occurs in flying 

 at right angles to the lines of variation when using the magnetic 

 compass, or at right angles to the meridians of longitude when using 

 a sun compass. 



In the former case, the change in variation must be allowed for 

 at regular periods, depending upon the accuracy desired by the navi- 

 gator; in the latter (when using the sun compass), it is accurate enough 

 for practical purposes to set the compass to the latitude and time of 

 the position of the mid-point of the line to be flown. 



For example, in flying from Point Barrow direct to Etah, the 

 lines of variation would be cut almost at right angles throughout 

 the flight, and the variation of the compass would range from about 

 95° at Etah to 330° at Point Barrow. Frequent changes of steering 

 course would have to be made to make good a straight course. When 

 200 miles from Axel Heiberg Island the variation would change ap- 

 proximately a degree for a minute of flying time, and, on top of this, 

 the value of the variation is only theoretical. If the navigator hap- 

 pens to be in error as to the distance he has traveled, then of course 

 he applies the wrong variation and gets into trouble. Therefore, 

 in using the magnetic compass alone, a flight of this kind would 

 be difficult. 



If practicable, two sun compasses should be carried so that the 

 shadow from the wings at certain angles of the sun may not render 

 the compass useless. It is advantageous to have good-sized windows 

 in the navigating cabin and to have the passageway open forward to 

 the pilots' seats — forward' of the leading edge of the wing — and aft 

 to a trap door, just abaft the trailing edge of the wing. 



DETERMINATION OF DRIFT, SPEED, AND DIRECTION 



The air navigator in the Arctic, as elsewhere, must allow frequently 

 and carefully for the drift caused by the wind. The drift indicator 

 used by the United States Navy is a simple and accurate instrument. 

 A wire is stretched taut in a frame, and the object below — an ice 

 hummock, a blotch on the snow, an irregular place in a lead, or a smoke 

 bomb which can be dropped — is simply made to follow the wire by 

 screwing the wire to the proper angle to the course. Not only the 

 angle of drift but the approximate speed over the ground can be 

 procured very quickly, by using a stop watch and a table. 



The speed of the craft through the air can be obtained from 

 air-speed meter. Then, knowing the angle of drift of the craft and 

 its speed over the ground, it is a simple matter to calculate the speed 



