Anemometer
2 of the 7 encyclopedias on this shelf carry an entry for Anemometer. Both are reproduced below, so you can see where they agree and where they differ.
Aiton's Encyclopedia (1910)
(Gr. anemos, wind, and meter, measure), a scientific instrument for measuring the velocity of wind. The kind in use by the United States Weather Bureau consists of four hemispherical cups facing in the same circular direction at the ends of horizontal arms or spokes of a light wheel. The wind catches with more force in the opening or concave face of the cup at one end of a diameter than on the bottom or convex face of the cup at the other end of the same diameter, thus causing the wheel to rotate continuously in the same direction. The higher the wind, the faster the wheel turns. A system of clock work shows the number of rotations per minute, from which the velocity of the wind is computed readily. It is interesting to know that the inequality of wind pressure on the opposing pair of cups results in a rotary speed of from one-fourth to one-third of the velocity of the wind. Another anemometer consists of a lightly rotating, delicate wheel, each spoke of which is a flat blade set obliquely like the sail of a windmill. Such an instrument is held in the hand and is used chiefly by engineers and architects to measure the rate of air currents in mines or in ventilating shafts.
Collier's New Encyclopedia (1921)
an instrument designed to measure the velocity of the wind. It was invented by Wolfins in 1709. The anemometer most commonly in use consists of four light metallic hemispheres, called (from Dr. Robinson, who first employed them) Robinson's cups, which are made to revolve like a vane or weather-cock, and are found to do so at the rate of exactly one-third the velocity of the wind. The result is then recorded in pencil marks by a self-registering apparatus.