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In particular, Jean Richer travelled to Cayenne which is on the coast of South America, near the equator, to make observations that would help in the determination of the solar parallax. He took with him a pendulum that was designed to beat seconds in Paris and found that he had to shorten it by about 28 mm in order for it to keep time in Cayenne (Chapin (1995)). Richer's trip is described in detail by Olmsted (1942).

This deviation is contrary to what has always been supposed to be a very certain truth, namely, that the cord stretched by the plumb is directed straight toward the centre of the earth... Therefore, looking northward, should not the level line visibly descend below the horizon? This, however, has never been perceived and surely does not take place. And the reason for this, which is another paradox, is that the earth is not a sphere at all but is flattened at the two poles, nearly as an ellipse turning about its smaller axis would produce. This is due to the daily motion of the earth and is a necessary consequence of the deviation of the plumb line mentioned above. Because bodies by their weight descend parallel to the direction of this line, the surface of a fluid must put itself perpendicular to the plumb line, since else it

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