Anemometer
2 of the 6 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.
What Has Changed in the Last Century
Both accounts describe the Robinson cup anemometer as the standard, and Collier's states in 1921 that the cups turn "at the rate of exactly one-third the velocity of the wind" — Aiton's 1910 entry is more cautious, giving a quarter to a third. The ratio is in fact not fixed at all: it varies with cup size, arm length and wind speed, and every instrument must be individually calibrated, which is why John Patterson's three-cup design of the 1920s displaced the four-cup pattern. Cup and vane instruments are still made, but the US Weather Bureau that Aiton's cites became the National Weather Service in 1970, and its automated surface stations now largely use ultrasonic anemometers, which time sound pulses between fixed transducers and have no moving parts. Hot-wire and laser Doppler instruments serve laboratory and wind-tunnel work, while satellite scatterometers have measured wind over the open ocean since the 1970s. Collier's credit to "Wolfins in 1709" also sits oddly with the usual attribution of the first anemometer to Leon Battista Alberti around 1450.
Written for this edition, 2026.