place on the earth's surface. Since the star, P, is infinitely distant, the lines A P, B P, CP, may be considered parallel. Suppose the meri gani, m, of this place to be in the position a of the earth, in the act of passing over this star; then, since there is an interval of a sidereal day between the positions A and B, the meridian of the same place will be passing over the star in the position B, and similarly in the position c. Now, let s be the sun, supposed to lie about the centre of the earth's orbit, and at a finite distance from it as compared with the distance of the fixed stars. Let

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the small sphere described round A, as a nucleus, represent the sphere of the visible heavens in that position of the earth, or let it represent that sphere to which a spectator refers the positions of all the heavenly bodies, and on which he imagines them to be distributed. And let the spheres round B and c be similarly interpreted. At A the observer, at the place which we have supposed, will see the star p on his meridian at m*, and the sun at the same time on the meridian; at B he will see the star at n, and the sun at K; and at o the sun will be at L, and the star at R; thus it is manifest that every time the star comes upon the meridian, the sun will appear to have receded further from it than at the preceding transit, describing arcs N K, L R, of a circle, formed by the intersection of the plane of the earth's motion with the surface of the visible heavens.

It will be observed that although the spheres to which an observer refers the positions of the heavenly bodies, and which constitute his' visible heavens, are in different positions of the earth different; nevertheless, they appear to him the same, because the stars which cover them, by reason of their immense distance, do not alter their relative positions upon them. Thus, then, the sun will appear to describe a circle among the stars, which circle is, in point of fact, the intersection of the plane of the earth's orbit with that sphere of the visible heavens, which, in every one of its positions, appears to surround the



* He is supposed to have a telescope powerful enough to show him the stars in the day-time.

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observer. This circle being then supposed to be the ecliptic,—that is, the intersection of the plane of the earth's orbit with the sphere of the heavens being supposed to be the ecliptic,—all the phenomena of the apparent annual motion of the sun through this ecliptic are explained by an annual revolution of the earth about the sun in that orbit.

Here, then, as in the case of the solar day, are two hypotheses, both of which explain the phenomena of the solar year: according to one, the sun revolves about the earth every year, in the middle of that band of the heavens called the zodiac;—according to the other, the earth revolves about the sun every year, having for the plane* of its revolution a plane which cuts the sphere of the heavens in the ecliptic.

Between these two hypotheses we have to choose. To that of the annual revolution of the sun about the earth, there are objections precisely similar to those which were adduced in the case of its apparent diurnal revolution. It is ninety-five millions of miles from us, and its bulk is more than one million times greater than that of the earth. Now, if it revolve round the earth in a year, this huge mass must sweep through space at the rate of about 1200 miles in every minute of time. This rapid revolution of an immensely large body about one which is in comparison with it infinitely small, at once strikes us as an improbability; it is more than this, it is a mechanical impossibility, as will plainly be perceived when we come to treat of the laws of physical astronomy.

Again, such are the motions of those other bodies which we have called wandering stars or planets, and which we shall discuss hereafter, as incontrovertibly to prove that these revolve each in an orbit about the sun; also their magnitudes may be ascertained by direct trigonometrical admeasurement, and many of them are thus found to be greatly larger than the earth. On the hypothesis we are discussing, not only, then, must the huge sun revolve continually with prodigious velocity about our earth, but the whole host of planets which accompany and revolve continually round him, and which are, many of them, as it respects magnitude, far more important elements of the material universe than our earth is, must nevertheless, together with the sun, and in combination with their motion round it, sweep with it in a perpetual circle round the earth.

It is a curious psychological fact, strongly illustrative of that waywardness and perversity of judgment of which we are all more or less the victims, that a philosopher, a laborious observer, and a man of considerable mathematical learning, was once found to take up this strange complicated hypothesis. The phenomena of the heavens were thus explained by Tycho Brahe, a Danish philosopher, the contemporary of Kepler, and the author of a very valuable catalogue of the fixed stars.

Again (to accumulate all the evidence on this point) it has been shown that the earth has a daily motion upon its axis. Now this fact of its daily motion renders also its annual motion highly probable. A probability founded on this other; that if there be two modes of explaining any phenomenon of nature, then cæteris paribus, that is the most probable which is the most simple. For by what we observe


By the plane of the revolution of the tions of the earth from its centre to the cenearth, is here meant a plane in which are tre of the sun. found all the lines drawn in different posi

in the creation around, we are forced upon the conviction that the Almighty acts in this respect with that economy of creative energy, which, although infinitely more perfect in its degree, has, nevertheless, its visible type in that husbandry of our resources, that disposition to economy in our efforts, which impels us always to avail ourselves of the simplest possible means of effecting all that we wish to do.

Thus, when, in reasoning upon any hypothesis, we are forced back upon final causes, it is sound philosophy to judge of the probability of that hypothesis according to the simplicity or complication of the final causes to which we are thus compelled ultimately to refer it. If, for instance, there be two hypotheses, by one of which we shall be compelled to fall back upon a double operation of the hand of the Almighty, whereas the other resolves itself into a single effort of his will, then is the latter hypothesis, according to the analogy of nature, more probable than the former, and that INFINITELY.

Now, as has been before explained, a motion of rotation having been communicated to the earth, it must also, in consequence of the force applied to communicate this motion, have had further a motion of translation, unless another, or second force, had been communicated to it in a direction through its centre, precisely equal to the first force, and parallel to it, but in an opposite direction; thus having as it has a motion of rotation, if in other respects it be at rest, the earth must, in the beginning, have had two distinct impulses communicated to it from without, in opposite and parallel directions, and at different, but not opposite points of its surface*.

Again, this hypothesis of the quiescence of the earth in space, involves necessarily the revolution of the sun about it; a third impulse, therefore, must be supposed to have communicated this motion to the sun. Reasoning, then, upon the hypothesis of the sun's annual revolution, we are obliged to fall back upon three final causes, three distinct operations of the Deity, whereas the opposite hypothesis of the annual revolution of the earth subjects the whole of the phenomena to one. One will, one impulse, one developement of the powers of Him who spake and it was done.

This argument (and indeed every one of those which have yet been set before the reader) is perhaps in itself conclusive. Different arguments in proof of the revolution of the earth, have been adduced rather because it may be considered necessary to a knowledge of astronomy, that the reader should be put in possession of all that has been said on the subject, than because it is thought that the arguments are in any way necessary to support one another. The accumulation of proofs, any one of which is sufficient, does not perhaps in reality consti

* It has been calculated by Bernouilli, | than the earth’s centre is, by about the jás that the single impulse by which the part of the earth's radius, or at a distance earth was made to revolve upon its axis about 25 miles further from the sun than in the time which we know it to revolve, the centre of the earth is. Similarly the and at the same time to move forward in impulse communicated to Mars must have space as we know it to move, must have been at a distance of air of its radius been communicated to it in a direction from its centre-that of Jupiter of 1's, and perpendicular to the line drawn from it to that of the moon at Išo. the sun, at a distance further from the sun

tute any accumulation of evidence; on the contrary, it is perhaps more according to experience, to assert that everything in the shape of proof which is added to that which is already proved, tends to weaken its authority. The evidence on this point is, however, too strong to be shaken by any method of arguing upon it, however illogical; yet another proof of the annual revolution of the earth will therefore now be added.

Whatever may be the true explanation of the phenomena of light, certain it is, that their origin and mode of operation is subject to the usual and known laws of mechanical action. The perception of light is the effect on impulse, somehow or another taking effect on the nerves which belong to the retina of the eye. Now this being the case, it is manifest that the effect of that impulse may be modified by the circumstances under which the eye is placed. If, for instance, the


be at rest, the effect of the impulse on the eye and the resulting perception of light, will be in the same direction as that in which the impulse is made. If it be in motion, and the velocity of its motion bear any finite relation to the velocity of the impulse of the light, then the effect on the eye,

and the consequent perception of light, will not be in the direction of the impulse of the light, but in a direction compounded of the direction of the eye's motion, and the direction of the impulse of the light.

Thus, if a person standing at rest be struck by a ball obliquely from above, he will feel the blow in the direction in which the ball moves ; but if he be in motion, the direction in which he feels the blow will be compounded of that of the ball, and that of his own motion.

We may thus ascertain what that compounded direction will be. Let us suppose a motion equal to that of the man to be communicated both to him and to the ball, at the same instant of time, but both in a direction precisely opposite to the man's actual motion—the same motion being communicated to both man and ball, the effect of the ball upon

the man will not be altered by this motion thus added to it. But the man, having now impressed upon him a motion, equal and opposite to that with which he is already moving, will thus be brought to rest, and the ball will have two motions communicated to it, in virtue of which it will move in the direction of the diagonal of the parallelogram whose adjacent sides represent these motions ; thus, then, its effect upon the man now brought to rest will be in the direction of that diagonal; also, by the hypothesis by which we have brought him to rest we have not altered the effect of the ball upon him ; restoring, therefore, the circumstances of our first hypothesis, the effect of the ball upon the man in motion, will be in the direction of the diagonal of the parallelogram, one of whose sides represents the motion of the ball, and the other the motion of the man.

If the direction of his motion be towards that from which the ball comes, the effect will therefore be in a direction still more inclined downwards than the actual direction of motion of the ball, and if it be from that direction it will be more inclined upwards. Thus let him be moving in the direction Ac, so as to describe ac in one second of time, and let the ball describe Ba in one second of time. Then communicating to the ball and to the man the motion ca, equal and opposite to Ac, the man will be brought to rest, and the ball will have the two motions ca


and BA communicated to it, and will move in the direction da. It is therefore in this direction, which is lower than its proper direction,

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that it actually takes effect. Similarly, if it had come from behind in the direction EA, it would have produced its effect, when combined with the motion of the man, in the direction fa, which is higher than its proper direction. Now for the impulse of the ball, let us substitute that of a wave of light, and let us suppose the spectator to be borne along with a velocity which has a certain finite proportion to that of the propagation of such a wave of light. The effect of the motion of the spectator on the direction in which the impulse of the light is perceived, will be precisely like that of the ball. If he is borne towards the object which is the origin of the wave, the direction in which the impulse is perceived will be lower, and if from it, higher than that in which it actually comes, and thus the objects towards which he is moving will appear lower to him, and those from which he is moving higher than they really are. If, then, the earth move in its orbit, and if the velocity of its motion bear any finite proportion to the velocity with which the light of the fixed stars is propagated to an observer on the earth's surface, then those towards which the earth is moving in her orbit will always appear to him lower than they really are, and those from which she is moving higher. And if she is not moving with any such velocity, then the light of the stars will

to come to him in the direction in which it actually does come, and the stars will not appear higher when the earth’s motion carries him from them, than when it carries him towards them. Now light travels at the rate of 192,000 miles per second, and the earth at about 19 miles per second. Thus, although the velocity of the earth bears but a small proportion to that of light, yet it does bear a certain finite and appreciable proportion. There will then be a finite and appreciable, though scarcely apparent depression, of those fixed stars towards which the earth is moving, and elevation of those in the opposite regions, provided the earth moves in its orbit as we assert; and if it do not, there will be no such depression or elevation. Now this difference of the true from the apparent position of the stars does exist; it was discovered by Dr. Bradley, and is called the aberration of light.


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