Tachometer

Part of speech: noun (n.)

1. An instrument for measuring the velocity, or indicating changes in the velocity, of a moving body or substance. Specifically: --

2. An instrument for measuring the velocity of running water in a river or canal, consisting of a wheel with inclined vanes, which is turned by the current. The rotations of the wheel are recorded by clockwork.

3. An instrument for showing at any moment the speed of a revolving shaft, consisting of a delicate revolving conical pendulum which is driven by the shaft, and the action of which by change of speed moves a pointer which indicates the speed on a graduated dial.