EXPLORATION BY AIRCRAFT 393 



and direction of the wind at the altitude flown, which may be quite 

 different, of course, from the direction of the wind at the surface. 



There is an instrument called the course-and-distance indicator 

 that will quickly and automatically solve the triangles of velocity and 

 direction. The parts of this instrument consist of a disk upon which 

 a movable disk, squared with parallel lines, is superimposed. The 

 length of the side of each square on the disk represents, let us say, 5 

 nautical miles. Two movable arms, graduated to scale, radiate from 

 the common center of the two disks. The two arms are graduated 

 to the same scale as the disks and are marked every 5', from 5' to 100'. 

 On each arm is placed a rider, enabling the user to indicate a particular 

 graduation. The solution of problems on the instrument is similar 

 in every respect to the graphical method, except that no lines need 

 be drawn. 



The Arctic air navigator should equip his plane with a turn indi- 

 cator for use in fog to enable the pilot to tell instantly when he is 

 turning. He can then steer a straighter compass course. 



POSITION FINDING 



For long flights, dead-reckoning navigation is not sufficient. The 

 navigator must obtain lines of position from some celestial body or 

 bodies. The usual method of forming an artificial horizon by means 

 of mercury seems too cumbersome to use in an airplane, therefore 

 some form of artificial-horizon sextant should be used. The sun does 

 not get very high in the Arctic, and it is generally possible so to ma- 

 neuver the aircraft that the sight of the celestial body can be taken 

 through the open window of the cabin, with the observer comfort- 

 ably seated, since he does not need to see the horizon. It is necessary 

 for him to get his position line quickly on account of the great speed 

 of travel. 



Suppose an altitude of the sun is taken. The sun can be bisected 

 by the horizon line in the sextant, and if this sextant has no error (as 

 it should not have when properly regulated) the true altitude can be 

 obtained by applying only the parallax and refraction. This makes 

 for rapidity of work. 



Some short method of obtaining the position line must be used. 

 There are a number of different methods which are equally satis- 

 factory. 



The air over the Arctic Sea seems generally to be without "bumps " 

 and so affords a steady platform for the observer. This is not the 

 case in flying over the rugged land of the Arctic. The air there is 

 sometimes very rough. But in that case the navigator may be aided 

 by landmarks in determining his position, unless the land is entirely 

 unknown. 



