It was hardly to be expected that we should discover, in our own day, a new physical proof of the earth’s motion, yet so it has been. The experiments of M. Foucault have enabled us to see the Rotation of the Earth on its axis, as taking place, we may say, before our eyes. These experiments are, in fact, a result of what has been said in speaking of the Moon’s rotation: namely, That the mechanical causes of motion operate with reference to absolute, not relative, space; so that where there is no cause operating to change a motion, it will retain its direction in absolute space; and may on that account seem to change, if regarded relatively in a limited space.
In M. Foucault’s first experiment, the motion employed was that of a pendulum. If a pendulum oscillate quite freely, there is no cause acting to change the vertical plane of oscillation absolutely; for the forces which produce the oscillation are in the vertical plane. But if the vertical plane remain the same absolutely, at a spot on the surface of the revolving Earth, it will change relatively to the spectator. He will see the pendulum oscillate in a vertical plane which gradually [526] turns away from its first position. Now this is what really happens; and thus the revolution of the Earth in absolute space is experimentally proved.
In subsequent experiments, M. Foucault has used the rotation of a body to prove the same thing. For when a body rotates freely, acted upon by no power, there is nothing to change the position of the axis of rotation in absolute space. But if the position of the axis remain the same in absolute space, it will, in virtue of its relative motion, change as seen by a spectator at any spot on the rotating Earth. By taking a heavy disk or globe and making it rotate on its axis rapidly, the force of absolute permanence (as compared with the inevitable casual disturbances arising from the machinery which supports the revolving disk) becomes considerable and hence the relative motion can, in this way also, be made visible.
Mr. De Morgan has said (Comp. to Brit. Alm. 1836, p. 18) that astronomy does not supply any argument for the earth’s motion which is absolutely and demonstrably conclusive, till we come to the Aberration of Light. But we may now venture to say that the experiments of M. Foucault prove the diurnal motion of the Earth in the most conclusive manner, by palpable and broad effects, if we accept the doctrines of the Science of Mechanics: while Aberration proves the annual motion, if we suppose that we can observe the places of the fixed stars to the accuracy of a few seconds; and if we accept, in addition to the doctrines of Mechanics, the doctrine of the motion of light with a certain great velocity.
CHAPTER III.
Sequel to Copernicus.
English Copernicans.
PROFESSOR DE MORGAN has made numerous and interesting contributions to the history of the progress and reception of the Copernican System. These are given mainly in the Companion to the British Almanac; especially in his papers entitled “Old Arguments against the Motion of the Earth” (1836); “English Mathematical and Astronomical Writers” (1837); “On the Difficulty of Correct [527] Description of Books” (1853); “The Progress of the Doctrine of the Earth’s Motion between the Times of Copernicus and Galileo” (1855). In these papers he insists very rightly upon the distinction between the mathematical and the physical aspect of the doctrines of Copernicus: a distinction corresponding very nearly with the distinction which we have drawn between Formal and Physical Astronomy; and in accordance with which we have given the history of the Heliocentric Doctrine as a Formal Theory in [Book v.], and as a Physical Theory in [Book vii.]
Another interesting part of Mr. De Morgan’s researches are the notices which he has given of the early assertors of the heliocentric doctrine in England. These make their appearance as soon as it was well possible they should exist. The work of Copernicus was published, as we have said, in 1543. In September 1556, John Field published an Ephemeris for 1557, “juxta Copernici et Reinholdi Canones,” in the preface to which he avows his conviction of the truth of the Copernican hypothesis. Robert Recorde, the author of various works on Arithmetic, published among others, “The Pathway to Knowledge” in 1551. In this book, the author discusses the question of the “quietnes of the earth,” and professes to leave it undecided: but Mr. De Morgan (Comp. A. 1837, p. 33) conceives that it appears from what is said, that he was really a Copernican, but did not think the world ripe for any such doctrine.