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Rh color. In case we had to come down, a little bright spot bobbing about on the water would have stood a better chance of attracting attention than one of neutral tints.

In what space the tanks left in the cabin, a small table was set up for navigating instruments. Our rolled-up flying suits and a five-gallon can of water constituted the available seats. In the cabin floor was a hatch which had to be opened for each cal­culation to show drift or actual velocity over the ground. For, of course, speed over the ground may not be the same as air speed.

Airplanes are equipped with air speed indicators which tell the pilot how fast a stream of air is passing the wing of the ship. If there is little or no wind, it may read approximately true for ground speed. It reads the same whether the plane is fly­ing with the wind or against it. A plane which travels 100 miles in still air would be going only eighty miles an hour over the ground if a twenty-mile wind blew against it, head on. But the air speed indicator doesn’t know the difference and gives its hundred m.p.h. reading just the same. Conversely, if the twenty-mile wind were blowing in the same direction as the airplane was flying, the speed of the plane would be increased to 120 miles per hour. So there must be other means of deter­mining actual ground speed. Over a mapped ter­ritory the pilot without much trouble can clock his speed with the landmarks he can recognize. Where landmarks aren’t available, different types