We shall close this article with a mention of the actual proofs of the motion of the earth.
I. It is difficult to believe, in the present state of mechanical know ledge, that any heavenly body is at rest, and the burden of proof must lie upon those who assert, rest, and not upon those who believe in motion, which a person instructed in mechanics must do, until the contrary is proved.
2. If a motion existed, a centrifugal force would arise, which would produce an effect on the oscillation of a pendulum tried in different parts of the earth. [CENTRIFTJOAL Ponce; PriseuLusst Such an effiset is found to be produced corresponding to that whie should he produced by the earth's rotation ; nor have those who deny that rotation ever produced any explanation of the phenomenon.
3. An experiment has been tried, which it will bo worth while to describe, and which Delarnbro says had " a sort of emcee's." When we say that a 'stone let fall froth the top of a high tower should fall precisely under the point from which it started, we say that which ought not to he perfectly true; the reason is as follows :—The starting point of the stone, being at a greater distance. from the centre than the point directly under on the earth, describes a somewhat Larger circle, and moves a little quicker. The stone therefore at the coin menceinent of its fall has a motion from west to east, a little more rapid than the under point of the earth. The resistance of the air, though It exists with respect to the fall of the stone, does not exist with respect to the motion from west to cast since the air, earth, and stone are carried together : consequently the stone should fall a little east of the foot of the tower. This experiment was tried at Bologna by Guglielmini, who published his results in a work called De Motu Terry diurno,' Bologna, 1792; it was repeated at Hamburg : the heights were respectively 241 and 235 feet (French).
In every instance the projected body fell a fraction of an inch 10 the east, and never to the west of the point directly under the point of projection. Had this departure been an accidental effect, it is in credible that it should always have taken place in one direction. Delambre does not state the number of experiments made; but if it were only six, and if the chances of departure east or west had been equal, it would have been 63 to 1 against the departure being always in one direction. In the last two proofs we are made to perceive the earth's rotation, by phenomena which can be explained on no other hypothesis that is worth consideration.
4. We perceive the earth's orbital motion only in the phenomenon of ABERRATION, from which one of two things must be true ; either the earth moves round the sun, or light does not move in a straight line ; and what is more, the light from every star, in whatever part of the heavens it may be found, changes its course with the position of the sun. The change, it is true, is minute, but it is as well established
as it would be if it were visible to the naked eye; and it must be remembered that twenty seconds is not a small quantity when the eye is applied to au instrument capable of measuring one second. Were there no other phenomenon by which to test it, the orbital motion would be conclusively proved by that in question.
5. The next 'argument is analogy, which, though not perfectly con clusive itself, lends a great additional force to the rest. The planets all exhibit motion round the sun ; this can be proved; and the only question that remains is, whether the sun move round the earth, carrying the planets round itself, or whether the earth be itself a planet moving round the sun. The planets also, in most area, revolve round axes visibly, and there is no proof that any one does not.
6. The last argument is authority, properly used. There are many who do not know enough of the subject to decidd even between Newton and the worthy Frenchman, whose name is unnecessary to mention, who thinks he proves the planets to be reflections of the sun upon the polar ice, and the southern hemisphere of stars to be a reflec tion of the northern upon a very curious crystal plane, but how placed we do not exactly know. With such persons authority must decide, if there be any decision at all in their minds; and it is of some import ance to them to know what sort of authority they trust to. The argu ment from authority may be thus summed up :-1. The motions of the heavenly bodies are irregular, particularly those of the moon, which, when closely examined, exhibit irregularities, the cycles of which never were determined from observation alone. 2. At the time when the contro versy about the earth's motion took place, the time of the moon's tran sit over the meridian, for instance, could not be predicted within several minutes. 3. By means of the labours of Newton and his suc cessors in theory, and Flamsteed and his successors in observation, the prediction now rarely differs from the result by more than half a second of time. 4. It has been the unanimous opinion of those con cerned in bringing astronomy to this state, not merely that the earth has a motion both of rotation and orbital progression, but that the proofs are such as to leave no doubt whatsoever on the subject ; nor is it in history that any person who was mathematician enough to read the writings of Newton ever entertained any hesitation upon the 7. For the recent experimental proofs—proofs to those who have the requisite knowledge of dynamics—of the motion of the earth, see GYROSCOPE; PENDULUM.