Wikipedia
Drift meter
A drift meter, also drift indicator and drift sight, is an optical device used to improve dead reckoning for aircraft navigation. It consists of a small telescope extended vertically through the bottom of the aircraft with the eyepiece inside the fuselage at the navigator's station. A reticle, normally consisting of spaced parallel lines, is rotated until objects on the ground are seen to be moving parallel to the lines. The angle of the reticle then indicates the aircraft's drift angle due to winds aloft, and can be used to calculate the ground speed.
Usage examples of "drift meter".
There must, Franklin decided, be powerful currents scouring this area clear-but there was certainly no current now, as his drift meter assured him.