Thus, while on a rough road or sea, the motion of a carriage or vessel continually reminds us of its existence, on a smooth one we often forget it; and whenever we do, the objects which we are passing appear in motion in the opposite direction to our own. This observation is within the experience of almost every one; and it furnishes by itself complete proof of the possibility of being unconscious even of a motion which, when our attention is called to it, we can easily discover. Perhaps a still more remarkable illustration is afforded by an experiment which every one can easily try. If there is any motion of which it would appear difficult to be unconscious, it would be one performed by our own active exertions, as that of walking; and, consequently, no one as he walks along attributes the general changes in the positions of objects to anything but his own motion. If, however, a man walks close along the side of a wall or railing, nearly of his own height, so that the outline of it may be about on a level with his eye and near it, he will see it continually dancing up and down as he moves, and he will find the sensation produced to be that of a vertical motion in the wall or railing itself. Reflection, indeed, will at once teach him that this cannot be so; and a little consideration will inform him that, as at every step he rises on the ball of his foot, and sinks again as he sets his foot down, the eye is at different elevations at different periods, and consequently the relative elevation of the outline different. The experiment may best be tried with a wall or paling of the height mentioned, because from its nearness to the eye the variations of the relative elevation are greater, and therefore more striking than in any other case; and a darkish night is, perhaps, best suited for the trial, because then we perceive the outline distinctly, and yet are not diverted by the presence of other observable objects. The natural observation to be made is this. Our progress forward is laborious in itself; we are, therefore, fully conscious of its existence, and we habitually consider that, in leaving objects behind us, we pass them, and not they us. But the vertical motion is only incidental to this; it therefore escapes our notice entirely fill our attention is called to it by some foreign circumstance; and while it does so, we find that a motion which we absolutely perform by the muscular action of our own bodies,

eludes our observation, and that the appearance is of motion in the objects at which we look. It is easy to vary our experiments, always with similar results; and we conclude that we are insensible of motion, unless made aware of it by mechanical obstructions or personal exertion.

Now it is obvious that when the question is whether the earth and all that is upon it move, no evidence can be derived from personal exertion; and no mechanical obstructions can be shown to exist. There is, therefore, no reason why we should not suppose its motion, if there be any, to be perfectly smooth and uninterrupted; and if so, experience leads us to believe that we should be unconscious of this motion, because the more nearly we can approach experimentally to such a motion, the more difficult do we find it to perceive the existence of any, and the more apt are we to attribute the effects of our own motion to the existence of motion in other objects.

It was, indeed, once imagined that the consequence of supposing the earth to move would be to show that bodies near it would continually have motions with respect to it materially different from those which would exist on the supposition of its being at rest; for instance, that if a body were let fall from the top of a tower, it would fall in a line joining its place with the earth's centre at that instant, and continue to move in this line; that the bottom of the tower, however, a point on the earth, would, by virtue of the earth's motion, move away from that line; and consequently that, when the body had fallen through the given height, the base of the tower would be found to be far from the line along which the body fell; while, in fact, it was found to coincide with it. The fallacy of this reasoning, however, is obvious, although it was long and much relied on as an objection to the theory of the earth's motion. If the body were at rest relatively to the earth before it began to fall, in which case only we find it fall at the bottom of the tower, it would only be so by having itself the motion of the earth. Its actual motion, therefore, when it fell, would be compounded of this motion and its motion of falling, and would be a curve in consequence; but the motion of falling is the only one which produces change of situation relatively to the earth, and this only therefore could we observe. The

combination of the two, however, prevents the earth from leaving the body behind, and it falls, therefore, at the foot of the tower; just as a ball let fall from the top of the mast of a ship in motion shares in the motion of the ship, and falls at the foot of the mast exactly as it would have done had the ship been at rest.

These are the objections which have been urged against the supposition of the earth's motion; and we conclude that there is no reason to disbelieve it, either on the ground that we are unconscious of that motion, or that the observable motions of bodies on the earth's surface and near it are the same that they would be on the supposition that the earth is at rest. This supposition, then, is an admissible one; for the appearances of the heavens may be explained by it, and there is no evidence whatever to contradict it.

The next stage of the inquiry, therefore, as this supposition and that of the motion of the sun and sphere are equally admissible, is which of the two has the stronger arguments in its fa

vour.

SECTION IV.-Probability of the

Earth's Motion.

THE first argument in favour of the earth's motion is derived from the much greater simplicity of such a supposition, and the less amount of motion thus introduced into the system.

In explaining this argument, we will begin with the diurnal motions. We have seen that these may all be explained by supposing the earth to revolve round an axis in twenty-four sidereal hours. The motion in this case is of one body, and its extreme velocity about 25,000 miles in this period. If, on the other hand, the earth is at rest, all the bodies visible in the heavens must have a motion of revolution in twenty-four hours; and the circles of their revolution will exceed that of a point on the earth's surface in the proportion of their radii, or of the distances of the bodies from the earth's axis. Instead, therefore, of the motion of one body, we have to suppose that of an incalculable number, scattered at all distances from the moon at sixty times the earth's radius to the sun at nearly 24,000 times the same quantity, to the remoter planets at yet greater and varying distances, and to the fixed stars at distances altogether

inappreciable on account of their magnitude. Many of these bodies, again, are of magnitude very far surpassing the earth; and all these bodies, so vast and so immeasurably distant, are to be supposed to revolve round a comparatively small body, with which they do not appear to have any other assignable connexion whatever. The force of the argument will, perhaps, appear more strongly by an instance. The sun is nearly 24,000 times more distant from the axis of the earth than a point on its equator is: the daily circle, therefore, that it would describe, when in the equinoctial, would be 24,000 times greater than that described by such a point. Its magnitude, also, is about 1,331,000 times greater than that of the earth; if, therefore, we were to suppose all the earth to revolve at the same rate as a point on its surface, yet the whole motion of the sun, if it moves, would be greater than that of the earth, if it revolves, in the proportion of 24,000 × 133,100, or 3,194,400,000, to 1. The motion of a point in the equator is greater than that of any other in the earth, and consequently the whole motion of the earth is much less than here represented: on the other hand, the sun's density is probably very far inferior to that of the earth, and the actual proportion of the motions on the two suppositions is not very far from the truth. Monstrous, however, as the notion of so enormous a motion seems, it is but a small part of the difficulty involved in the supposition of the diurnal revolution of all the heavenly bodies round the earth; for the sun is only one, and one of the very nearest of their number. Nature, however, as far as we can observe her, works always frugally, and employs no more exertion than is necessary; and the mind at once recognizes the superior probability of the comparatively trivial motions supposed in the earth, over the inconceivably great and rapid motions which must otherwise be supposed to exist in all the heavenly bodies.

The same argument applies, and really with equal force, though the disproportion between the motions introduced on the two suppositions is not quite so overwhelming in support of the theory, to the supposition that the earth moves round the sun. If only the earth and sun were in existence, the whole motion thus introduced would be less than that involved in the supposition of the sun's motion in the proportion of their

respective masses, or of 1 to 133,100; or, after allowing for the difference of density, probably about 1 to 30,000; a difference quite sufficient to induce us to prefer the former hypothesis. But, besides this, if the sun moves round the earth, he is accompanied by ten planets, and seventeen satellites attending them, all partaking in his motion, and consequently moving round the earth also. Some of these bodies, although very inferior to the sun, are themselves of vast magnitude. The earth, on the contrary, is attended only by one satellite, the moon. Her motions, indeed, are to be added to those of the earth; but the additional motion thus introduced is very inferior to that introduced by the motion ascribed even to one of the larger planets, and vastly less, indeed, than that of all the bodies implicated in the supposition of the sun's motion; and thus, also, the superior simplicity of the supposition of the earth's motion is evinced.

In all our other observations of the heavenly bodies, the smaller seems uniformly, to attend and depend on the larger the moon on the earth, the satellites on their respective planets, the planets on the sun. If the earth is a planet, and moves round the sun, her motions are an instance of this observation: if the contrary supposition is adopted, the motion of the sun is an exception to it.

This introduces the mention of what may, perhaps, be considered almost as a general law of the human mind; the readiness to believe in uniformity. Whenever we observe a series of things agreeing with each other in all the circumstances presented to our notice, we feel inclined to conjecture that they will agree, also, in other circumstances; and whenever we find that a fact, of the nature of which we are ignorant, can be reduced to some class of facts with which we are well acquainted, we are impelled to refer it to that class, and consider it to be occasioned in the same manner. These conclusions are often drawn too hastily, but the number of such errors which have been committed, only shows the more strongly the propensity of the human mind thus to generalize and to abstract. This is not the statement of the mere rule, that like causes produce like effects; it is rather the principle on which all our notions of cause and

effect depend; for the very belief that, when one event is found uniformly to

follow another, the earlier produces the latter, is itself merely an instance of the principle in question.*

• There are some very curious instances of this propensity to generalize connected with the history of Astronomy. Kepler deduced his laws merely by it; he found them to subsist in the planets which he observed, and boldly announced them as general truths; but he was unable to demonstrate that they were necessarily true universally, if at all. This was reserved for Newton. The confirmation which his researches gave them, fixed them as undoubted laws of nature; till they received this, they were liable to be questioned and even exploded, like many other suppositions of their fanciful, though most ingenious author, which he propounded with equal confidence. In all instances, however, whether he was right or

wrong, he acted on the same principle which we are now discussing, that of believing in the generality of rules which he found to obtain in a few instances.

nature is contained in a singular analogy which Another very remarkable guess of the same Professor Bode, of Berlin, found to subsist between the major axes of the different planetary orbits. He found that the following table, the mode of the construction of which is obvious, very nearly expressed their relative distances, taking

that of the earth as 10:Mercury's distance, Venus's

Earth's

Mars's

Vesta, Juno, Ceres and Pallas =4+3.28 = 28 Jupiter's

Saturn's

Uranus's

"

=

4+3.24 52 4+3.25-100 4+3.26-196

It will be found, on inspection, that these numbers very nearly correspond with those in the table in p. 122. It is a very remarkable circumstance in the history of this table, that it was formed before the discovery of the telescopic had led some persons to conjecture the existence planets, and that the void thus occurring in the series

of a planet between Mars and Jupiter, just about the distance at which the telescopic planets were afterwards discovered. A similar conjecture, if the earth's planetary character were unknown, would fix a planet at the distance actually occu

pied by the earth; and this adds a circumstance

of similarity to those stated in the text, although the force of an argument resting on so loose a foundation as this empirical law, cannot be very great.

The law itself has lately received a remarkable extension from Mr. Challis. This gentleman has shown (Cambridge Philosoph. Transact. vol. ill. p. 171), that it prevails not only between the distances of the planets from the sun, but between the distances of the satellites from their respective primaries. Thus in the case of the system of Jupiter, the respective distances of the satellites may be expressed without considerable error by the following law:

7. 7+4 7+4×24 7+4× (24)2

Empirical Values.

=7

=11

=17

=32

True Values.

If, however, it be an argument in favour of any explanation of a fact, that it reduces it into a class of known phenomena, the supposition of the earth's motion round the sun is most strongly recommended to our belief. The motion of the sun round the earth, if it exists, is a fact unlike any other with which we are acquainted. It is, indeed, an elliptic motion round the earth, as is, also, that of the moon; but there is not the proportion between their periodic times which we find to subsist in all other systems of bodies revolving round the same principal; and it is a motion of the larger round the smaller body, of which we find no other instance. In spite, therefore, of some circumstances of resemblance, it would have to be considered as a motion quite distinct from all others, and governed by different laws. The earth, on the other hand, if we suppose it to move round the sun, is at once included in a class of objects, the planets, whose motions are well known; and its motions are found to correspond in every particular with those of the other planets. The proportion of the distance to the periodic time is the same; the shape of the orbit the same; the direction of the motion, whether of rotation or in the orbit, is the same; the existence of a motion of rotation in the earth itself, and also in a satellite revolving round the earth, and the direction and duration of the satellite's rotation, all correspond to exactly similar phenomena observed in other bodies of the solar system, and furnish arguments in support of the supposition that the earth is really one of the same class. The conclusion is irresistible, that the supposition is true; for how great is the improbability that all these coincidences should happen by accident only, which they must do, if the

336+82×(2)3

336+82 x (23) ×3 336+(82) (23) x 32

= 336

= 418

= 500

529

= 664

632

= 6240

6436

With the exception here noticed, it may then be affirmed, that the planets and satellites arrange themselves about their primaries at mean distances, which observe approximately this progression, a, a+b, a+rb, a+r2 b, &c. The value of r is always one of the terms of the series 1, 14, 2, 24, 3, &c. We may add, that this ratio of b to a is generally expressed by very simple numbers: thus, for the planets it is nearly; for the system of Jupiter ; for that of Saturn, ; of Uranus,, early.

earth is a body of a different nature from the planets.*

CHAPTER VI.

On Aberration.

THE arguments already adduced are probably abundantly sufficient to induce the reader to adopt the supposition of the earth's motion. If, however, the earth moves, its situations at different periods of the year are very distant from each other; and we might therefore expect to find the apparent situations of the bodies we observe, different in consequence. Those of the planets are so, for the effects of this variation of situation are included in the account already given of their apparent motions. But we might expect to find also a perceptible difference in the situation of the fixed stars, which, although too distant to be affected by parallax, considered as the variation produced by the distance of places on the earth's surface from its centre, might well be so by this far greater change of position; and any such effect, if produced, would be of the same nature, and would therefore be accurately described as parallax; and going through all its changes in the course of a year, it may properly receive the name of annual parallax. In fact, however, none such can be detected, at least with any certainty, in any of the fixed stars. Astronomers have been much divided in opinion on this question; but the amount of this parallax, if any such is observable, is at least ascertained to be exceedingly small. This, however, furnishes no reason for disbelieving the existence of the earth's motion. It may be accounted for, as in p. 60, by supposing the distance of the stars to be indefinitely great, in comparison even with the diameter of the earth's orbit; and our conclusions in this case, with respect to the distance of

In these reasonings we have treated the motion of rotation of the earth as already established, and have used it as one of the circumstances of correspondence to prove the planetary nature of the earth. If, on the other hand, we consider it to be sufficiently established without resorting to this argument, that the earth is a planet, its motion of rotation would itself be confirmed by the analogy of the other planets which have corresponding motions. This argument, indeed, may have some weight, without introducing the question of the earth's planetary character, as all the bodies, whose magnitude and nearness give us the means of ascertaining their motions, have a similar motion,