an instrument for measuring the velocity, or indicating changes in the velocity, of a moving body or substance.-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.-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.-an instrument for measuring the velocity of the blood; a haematachometer.