Drift Sight Air Corps US Army Type A-3

This is a 1930's pre-WWII Drift Sight Type A-3 for aircraft of the Air Corps US Army. A drift sight is used to determine the degree of drift  from the intended heading due to cross winds left or right, to permit correction of the course by flight controls. 

This is the first and only Type A-3 that we've come across. The serial number indicates a year of manufacture of 1935.

The instrument would either be installed pointed directly to the ground, or onto an angled periscope installed in the side of the fuselage pointed 90deg to the ground.  As the aircraft travels over the earth's surface, terrain as viewed through the drift sight should be traveling parallel with the direction of the aircraft if there are no crosswinds.  With crosswinds, the terrain would appear to travel at an angle to the heading of the aircraft, which is measured by comparing the linear scale nearest the eye to the scale at the bottom of the sight. By turning the knob, the scale aligns and drift angle can be measured, and course correction determined.  Note the markings on the face that advise to "add to" or "subtract from" the the compass heading.

This instrument is in very good condition given its age. All of the glass is intact. Note that images only appear in the viewfinder from a distance. Measures 6 x 8 inches.