EXPLORATION BY AIRCRAFT 



391 



The difficulty can be met by using a sun compass (Fig. 3), provided 

 the explorer starts out with the sun showing. He can compare the sun 

 compass with the magnetic compass and procure at once the error of 

 the compass for the particular course he is on. This error is the sum 

 of the variation and the deviation of the compass. But since the 

 deviation changes with changes of headings of the plane, the air navi- 

 gator cannot apply this error to his return course (the opposite course 

 to the one he is flying away 

 from his base). 



If the air navigator 

 wants to get back to his 

 base, he should circle at in- 

 tervals, head the plane on 

 the return course, and take 

 the error of the compass — 

 the combined variation and 

 deviation. If he is cutting 

 across magnetic meridians, 

 the error will change by 

 the amount of the change 

 in variation, which, in some 

 localities, is a disconcert- 

 ingly rapid one. 



If the air navigator, 

 then, is conservative and 

 turns back in case the sun 



goes behind a cloud, he can get back to his base unless he gets over 

 fog. Then he will be taken off his course by the amount of the drift 

 of the wind, for no instrument has yet been developed that will give 

 the drift of a plane when flying above fog. 



It is sometimes possible, and certainly it is highly desirable, to 

 select a spot in the plane where the compass has no deviation — ^where 

 there is no local magnetism. 



This was done in the case of the transatlantic flight of the N. C. 

 flying boats and the polar flight of the Josephine Ford. This is easy 

 of accomplishment where an earth-induction compass is used. The 

 coil of the compass that cuts the magnetic line of force can be placed 

 in the stern of the fuselage, or out at the end of a wing, where there is 

 little metal. 



Where there is no deviation, the application of the variation alone 

 to the "return-to-base course" will give accurate results, and time 

 need not be lost circling for the return compass error. It is a good rule 

 to use as large a compass as possible. Cutting down on weight in an air- 

 plane by taking a smaller compass seems to be poor economy where 

 long-distance navigation flights are to be made. Though it is ad- 



FiG. 3 — A sun compass (Bumstead model) of the type 

 used by the writer on the north polar flight. Note the 

 shadow of the shadow pin on the pear-shaped hand of the 

 24-hour clock. 



