Altimeter, Aviation Section US Signal Corps, 15,000FT, WWI
This is an Altimeter used in military aircraft of the Aviation Section, US Signal Corps, which existed from 1914-1918 and was the WWI-era precursor to today's US Air Force.
This altimeter crudely measured altitude from 0-15,000 ft. By loosening the small retaining lever, gripping the knurled rim of the faceplate and turning, the numerical scale rotates clockwise or counter-clockwise while the pointer remains stationary, to correct the altitude reading as required. This looks very similar to the altimeter depicted in the JN-4D Curtiss Jenny cockpit at the US Naval Aviation Museum website.
It is in very good condition given its age. Measures 6 inches diameter and ~2.5 inches deep.