Drift Sight, Mk II-B Pelorus, WWII US Navy Aircraft

Here is an early WWII-era US Navy aircraft Pelorus Drift Sight. A Pelorus Drift Sight is designed to function either as a Pelorus, for taking bearings on objects, or for improving dead reckoning aircraft navigation by measuring the angle between the heading and the track of an airplane. It consists of a small 'telescope' viewing vertically towards the ground, angled toward the direction of the aircraft. When viewing the ground through the scope while in flight, the navigator observes the travel of fixed objects across the lens. By rotating the scope on its base to align the path of these objects, the drift angle is determined by reading the location of the pointer on the base's scale. Once done, the course correction is calculated and communicated to the pilot. The instructions for use can be seen starting on page 32 of the the US Navy's Aircraft Navigational Manual dated 1941, which is available to download here. 

Mark II-B consists of the sighting scope, the upright post, and the base mount. This example is in good condition given its age.  Measures ~12 inches tall, base is ~5 inches diameter, and the scope is ~8 inches long.