An example of a simile used to explain a scientific idea:
"…the governor. This consists of an upright spindle, which is kept constantly turning, by being connected with a certain part of the machinery, and from which two balls are suspended in opposite directions by rods, attached by joints, somewhat in the manner of the legs of a pair of tongs. As long as the motion of the engine is uniform, that of the spindle is so likewise, and the balls continue steadily revolving at the same distance from each other. But as soon as any alteration in the action of the piston takes place, the balls, if it has become more rapid, fly further apart under the influence of the increased centrifugal force which actuates them – or approach nearer to each other in the opposite circumstances."
George L. Craik (1830) James Watt, reprinted in Cochrane, R. (Ed.). (1897). Heroes of Invention and Discovery. Lives of eminent inventors and pioneers in science. W. P. Nimmo, Hay, & Mitchell.
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The passage also contains an alternative conception as the force which causes the balls to rotate is centripetal (causing an acceleration towards the centre) not centrifugal. Confusing the governor is also widely known as both the centripetal governor and (inappropriately as) the centrifugal governor.
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