Up to the year 1672, no doubt had been entertained of the spherical figure of the earth, and, as a consequence, of the equality of all the degrees of the meridian; so that one being known, the whole circumference was determined.
In that year, the French Academy of Sciences, then occupied in the measurement of an arc in the meridian, sent the astronomer Richer to Cayenne, on the coast of South America, to make observations of the sun's altitude.
In the course of these observations he was surprised to find that a superior clock, furnished with a pendulum which vibrated seconds, was found to lose nearly two minutes and a half a day.
The astonishment created by the report of this fact in France was very great, particularly after the accuracy of the clock had been fully tested.
Other scientific men then visited different points on the coasts of Africa and South America, and were convinced of the absolute necessity of shortening the pendulum to make it vibrate seconds in those latitudes.
The phenomenon was explained by Newton in the Third Book of his Principia (1687)—see p. 409 et seq., American edition—where he shows it to be a necessary consequence of the earth's rotation on its axis, and of the centrifugal force created by it. That force, in modifying the gravity, gives to the earth an oblate spheroidal figure, more elevated at the equator than on the poles, and makes bodies fall and pendulums vibrate more slowly in low than in high latitudes.
There is, unfortunately, such a thing as national jealousy even in science, and to such a motive only can we ascribe the fact that Newton's explanation was not accepted in France until presented by Huyghens, several years afterward, in a different and less accurate form.
The Velocity And Aberration Of Light.
In the entire range of scientific literature, there are few chapters of greater interest than those which recount the rise and gradual development of all the principles involved in the triumphant demonstration of these two beautiful discoveries.