Before electronics became the pre-eminent technology for computing and control, analogue mechanical computers were used. They required precision design and construction to achieve acceptable accuracy, and were correspondingly expensive. They were therefore used only for the most important applications, such as fire control during WW2.

Left: Evaluating a function of one variable with a rotary cam. As with all mechanical computing elements, accuracy depends on the precision of original manufacture. It is also likely to degrade over time due to mechanical wear.

Left: Evaluating a function of two variables x and y with a 3D rotary cam.

Left: One method of mechanical multiplication The inputs are logged by the x and y cams, summed by the lever, and then anti-logged by means of the non-linear slot which translates the output into Z.

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Left: A Dreyer Fire Control deflection totaliser: 1906. This device was used in the Royal Navy for computing gunlaying data before the First World War. It uses only addition and subtraction, carried out by differential gears. The power for relaying the result is provided by the transmitting handle at the left, on a "follow-pointer" basis.

Left: A US Army Air Corps astro-navigation computer from 1938. Two sights are taken on the sun and a star, or on two stars, and turning the handle produced the answer. This replaced complicated and error-prone manual calculation. This unit was carried by Howard Hughes on a round-the-world seaplane flight. Photo from Popular Mechanics, Oct 1938

Left: The internals of the US astro-nav computer. I'm afraid I know nothing about its internal operation.

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