DC -3 Airspeed Indicator. This is the kind in the CR Smith Museum Hangar.

Airspeed indicator connections

Along with the altimeter and vertical speed indicator, the airspeed indicator is a member of the pitot-static system of aviation instruments, so named because they operate by measuring pressure in the pitot and static circuits.
Airspeed indicators work by measuring the difference between static pressure, captured through one or more static ports; and stagnation pressure due to "ram air", captured through a pitot tube. This difference in pressure due to ram air is called impact pressure.^[1]