Cooke airship sight, used for measuring both the Drift and Ground Speed of an airship.

This instrument, produced in 1917, was used for measuring both the Drift and Ground Speed of an airship.

In the optical system are two parallel wires and a cross wire; the separation between the parallel wires is adjusted for the height of the airship by means of the milled ring below the eyepiece.

The Bearing Plate is first rotated so that the course steered is shown against the lubber’s line. To measure Drift, the optical system is rotated by means of the lugs on the central ring so that a prominent object on the ground appears to move along the drift wire, the track made good over the ground is then read off at the arrow against the bearing plate.

To measure the Ground Speed, the height of the airship is set against the height index by rotating the milled ring above the height scale. A prominent object is again followed and using a special stop-watch, calibrated directly in m.p.h., the time is taken for the object to pass from the forward to the second wire in the eyepiece. This movement corresponds to a ground distance of 169 yards.

Reference: Air Publication 803 (1921)