Anemom´eter

(Gr. anĕmos , wind, metron , measure), an instrument for measuring the force and velocity of the wind. This force is usually measured by the pre ure of the wind upon a square plate attached to one end of a spiral spring (with its axis horizontal), which yields more or le according to the force of the wind, and transmits its motion to a pencil which leaves a trace upon paper moved by clockwork. Various instruments have been devised for this purpose, but the one most commonly adopted by 163 meteorological stations is after the type invented by Dr. Robinson of Armagh. It consists of four hemispherical cups A attached to the ends of equal horizontal arms, forming a horizontal cro which turns freely about a vertical axis B . By means of an endle screw carried by the axis a train of wheelwork is set in motion; and the indication is given by a hand which moves round a dial; or in some instruments by several hands moving round different dials like those of a gas-meter. It is found that the centre of each cup moves with a velocity which is almost exactly one-third of that of the wind. There are various other forms of the instrument, one of which is portable, and is especially intended for measuring the velocity of currents of air pa ing through mines, and the ventilating spaces of hospitals and other public buildings. The direction of the wind as indicated by a vane can also be made to leave a continuous record by various contrivances; one of the most common being a pinion carried by the shaft of a vane, and driving a rack which carries a pencil.