physical science have not been, at all periods, distinguished for their unanimity and moderation. But we quit this argument, forced on us by the imputations we have been considering, to return to one more general and more likely to conciliate.

If the intellectual faculties are generally referrible to those of judgment and imagination, and the predominance of one class over the other primarily determines each individual mind to those pursuits which more particularly demand the exercise of that one: then the question before us is at once freed from all considerations of imagination, since its influence, when felt, is too powerful to admit of more than control, and is little susceptible of cultivation. When, therefore, the individual is not endowed with that rare genius which renders him a benefactor to mankind, by the happiness his creations as a poet, a painter, or a musician, excite in his own and future ages; the aim of the liberal education he gives himself, that is of his pursuits after he has passed through the discipline of school, should be the improvement of his judgment, so as to preserve his imagination subordinate without suppressing it: by so doing, he will most probably secure the maximum of happiness; the undue influence of ordinary imaginations, when unchecked by the judgment, being the source of all the evils which disturb the peace of the individual, as surely as its dominion, when it is of an exalted nature, is productive of the greatest good to society.

By not suffering any extrinsic cause to induce him to pursue one line of studies in preference to another, but by adopting that for which he feels an especial inclination, he will most certainly attain the end in view, and be secured from the exclusive effects of one study by the invariable tendency of each, if pursued with steadiness and energy, to lead to and connect itself with others. The endeavour to exalt the importance of one line of studies at the expense of another, is particularly to be deprecated, not only as causing an undue influence in determining the choice of pursuits, which, in our opinion his own feelings-the suggestions of the individual's mind,—ought alone to decide; but as fostering those bickerings and animosities so detrimental to the general intellectual advancement. Instead of exciting the philologist, the historian, the metaphysician, the naturalist, the politician, and the mathematician, to regard with jealousy, or to treat with contempt, the pursuits of the others, true philosophy should point out to each, that they are all equal in dignity if they contribute to the real happiness of the individual, by alluring him from the debasing tendency of sensual pleasures; and the contest among the different cultivators of knowledge should be to prove the beneficial effects of each species on the mind, by sedulously cherishing courtesy and charity towards their fellow-labourers in the vineyard, and not to flatter their own indolence or presumption by striving to depreciate the utility of exertions of which they are incapable, or to which they are averse.

VOL. II.

H

8

98

A POPULAR COURSE OF ASTRONOMY.

No. IV.

In the last chapter was pointed out to the reader a method readily applicable to the determination of the difference of latitude of any two places on the earth's surface. It was simply to observe the number of degrees through which any of the fixed stars had been made apparently to ascend or descend on the vault of the heavens by the change of the position of the observer.

But every star has another apparent motion besides that which arises out of an alteration in the place from which it is observed. There is an apparent motion from east to west, which is common to the whole host of the stars, and by which each star is made to describe about 15 degrees of the circle, which is its apparent path, every hour. Hence, whilst an observer is changing his place and his latitude, the star which he has observed will itself have had an apparent motion towards the east, arising from a cause quite independent of his changed place, and out of the whole apparent motion of the star he will find a difficulty in ascertaining how much is, and how much is not, due exclusively to this change of place.

Fortunately, however, the star will return after 24 sidereal hours, or 23 56′ 44:09′′ mean solar time, exactly to the same place in which it was at the period of his first observation; and if he repeat his observation at that moment, the result will be precisely the same as though the stars had not moved at all in the interval.

Thus, then, if he makes his second observation precisely 23h 56' 44.09" after the first, he knows that any change in the apparent altitude of the star must arise from his motion, and not from any apparent motion proper to it. And this remark applies to all that has been said before of that apparent motion of the stars which is produced by a change in the position of the observer.

This subject will, however, be better understood, when the reader's attention shall have been called more specifically to the diurnal motion of the earth.

It has been shown that the earth is a huge isolated mass, having no contact with any other, but self-supported in space.

Now it will at once occur to the reader, that a mass, placed under these circumstances, whose surface had no other contiguous surface to rest against, no fixed pedestal or suspending-chain to keep it in its place, would necessarily move, if any external force were applied to it. And those who have studied the theory of mechanics know further, that any motion thus communicated to it would, since there is no friction or other opposing resistance to destroy it, continue for ever. And that this is true as to the fact, however great or however small may be the amount of the disturbing force, varying only in this respect as to the degree of the motion. It will occur to them, therefore, as quite possible that this ball should be, and should have been from all eternity, in motion, provided there be, or ever have been in existence, an external power capable of moving it. Nay, their speculations on the probability of the

case may be carried yet further. It is a principle of mechanics, that if motion be communicated, by impact or otherwise, to a mass in any other direction than through its centre of gravity, this mass, when left to itself, will have two motions, one a motion of translation, in which all its parts, including its centre of gravity partake in common-the other, a motion which will ultimately be a motion of rotation about a certain axis through its centre of gravity, in which only those parts of the body which are without this axis will partake. And it is a remarkable fact that these two motions of translation and rotation will be quite independent of one another, so that the motion of translation will be precisely the same as though the mass had been struck through its centre of gravity, and there had been no rotation, and the motion of rotation the same as though there had been no translation, the centre of gravity of the mass having been held at rest. Thus, were the mass a sphere, and had it been struck otherwise than through its centre, it would necessarily spin round one of its diameters, and at the same time move forward in a straight line, with a motion of translation. Also this spinning motion would be the same as if the axis about which it takes place had been kept at rest like that of a globe, and the motion of translation the same as though the ball had been struck through its centre, and had not therefore spun at all on its axis.

And all this is true, however slight the impulse which might be given to it.

To put this fact in a more striking light, let us suppose the force of gravity on the earth's surface for an instant to be destroyed, and let the reader be imagined to have constructed a sphere of clay, and having done so, to hold it up in his hand, and then to unloose his grasp from it. It would immediately begin to spin upon one of its diameters, and to move onward through space with an uniform motion, which would never of its own accord alter its direction, or cease. There being no force of gravity to draw it downwards, had no force whatever been communicated to it when it was set free, it would have rested; but it will have been found impossible to set it free from the hand without communicating some motion to it, and it is an infinity of chances, that the direction of that motion shall not have been precisely through its centre; in which case there will, of necessity, have resulted a motion of translation, and one of rotation.

It is scarcely necessary to apply this illustration to the case now under our discussion: that the Hand by which the materials of our globe were brought together could have been withdrawn, and yet that mass left quiescent in space no one ventures to deny; but that it should move is the simpler case, and that the same Hand, when it had spread upon the face of the earth its glorious covering of green herbage, of flowers, and of forest-trees, and sent forth the cattle on a thousand hills, should then have imparted to it that impulse in space, whence should result the alternations of day and night for the repose of every living animal, and the periodical changes of heat and cold, whereby every variety of vegetable life should be made to bring forth its fruit in due season, is by far the more probable of the two suppositions.

That this earth, then, which we know to exist, isolated in space,

should be in motion, that it should revolve continually and uniformly round one of its axes, and at the same time with a motion of translation forward in space, will not, therefore, seem improbable. In fact it is seen on the whole to be more probable than that it should be at rest.

Let us now consider the matter in another light. In a former chapter, an observer was supposed to set out from the north, and travel southward round the earth; and it was shown that the horizon of such an observer must (to explain the phenomena) be supposed continually to roll with him, causing by its angular approach to some of the fixed stars as it thus rolled along, and its recession from others, an apparent approach of the stars northwards to the horizon, and their ultimate immersion beneath it, and the converse of all this southward. Now, instead of his moving southward from the north, let us suppose him to move eastward from the west. His horizon now, as before, rolling along with him, those stars which are behind him will continually appear to descend upon the vault of the heaven behind him, and those before him to ascend; thus they will appear to rise to the eastward, to revolve over his head, and to set in the west. Let him now be supposed to move thus with such rapidity as to describe in twenty-four hours the whole circumference of the earth, and to continue this gyration uniformly and unconsciously for ever. As his horizon is thus brought continually into different positions, with reference to the stars, and as he does not suspect the fact of the motion of his horizon, he will necessarily suppose the stars themselves to take up different positions with regard to the horizon, to ascend from beneath it, pass over the space above it, descend again beneath it, and every twenty-four hours, to make a complete revolution about him. Now, instead of the observer thus careering continually round the earth, let us suppose him to remain at rest, and the earth itself to move, carrying him round with it. The appearances of the heavens will manifestly be to him exactly the same as before.

The only difference of the cases is this; instead of the observer having in every position a new horizon, occupying a different situation with regard to the region of the fixed stars, he will have everywhere the same horizon, which will be made to occupy in succession precisely the same positions as his different horizons did on the former supposition; and being here, as before, unconscious of the motion of his horizon, he will attribute the apparent ascent of the stars to the eastward, and their descent westward, to a proper motion of the stars themselves, and not to its true cause the alteration of the position of his own horizon with respect to them.

Thus, then, if the earth carried us round perpetually in space as it spun upon its axis, looking at the stars we should observe precisely the same phenomena as those which the appearances of the heavens daily present to us. The heavens would appear to turn round us. then to choose between two hypotheses, which equally well account for We have the observed facts of the apparent daily rotation of the vault of the heavens; these hypotheses are, that the host of heaven do daily revolve with a common motion round us, or that the earth revolves daily and uniformly round one of its diameters.

We have shown this last hypothesis to be in the highest degree probable from the fact, that the earth is a mass, separated and isolated from all others, and, as it were, self-supported in space. Certain, therefore, to retain any motion communicated to it, and if that motion were communicated otherwise than through its centre, certain to revolve for ever upon one of its axes, as well as to move forwards.

Now let us consider the probability of the other hypothesis, viz. that the heavens and all their host do really revolve round us every twenty-four hours as they appear to do. It has been shown in a preceding chapter, that the region of the fixed stars is distant from us by a space not less than one hundred thousand times the earth's diameter— in reality it is far more remote than so many times the diameter of the earth's orbit. Being thus distant, the magnitudes of the fixed stars must be enormous, or we should not be able to see them.

The hypothesis of a daily revolution of the heavens amounts then to this, that millions of immense bodies, stars innumerable, revolve each in its particular orbit, and each with a velocity greater than that of light round one of the axes of this earth of ours, which is but an atom in comparison with the least of them. There is a limit somewhere placed, beyond which that which is improbable identifies itself with that which is impossible, and this hypothesis seems to pass it.

The improbability may, however, yet be rendered stronger. The stars, called fixed, because they preserve always the same relative positions, are not the only stars seen in the heavens, there are other bodies, whose apparent positions in reference to one another, and to the fixed stars, are pepetually changing, "Palantia sidera cœlo." Besides, then, their daily revolution with the rest of the heavens, these must, if our hypothesis be true, have a continual motion among the other stars, and this of the most perplexing and extraordinary kind. The sun, for instance, must be supposed, besides his daily motion, to move in the same direction completely through that girdle of stars called the zodiac, once a year; and the moon once a month. The planets Mercury and Venus must be supposed always to accompany the sun in his motion, but sometimes to lag behind him, and at others, to press on before him, altering perpetually in brightness with each variety of motion. The planets Mars, Jupiter, and Saturn, must be supposed to have paths, subject to so complicated a law of change, as to appear to have their motions governed by a kind of caprice, and to render it difficult to say whether

'Sponte sua jussæne vagentur et errent.'

Sometimes we must suppose them to travel forwards on the vault of the heavens, then by an indirect and tortuous course to retrograde, at one time in opposition, at another, in conjunction with the sun, thus presenting the image of a wandering, unsettled, reeling, and lawless course through the sky; and all this motion, which passes through its changes slowly, and by periods of months, combined with the steady and regular daily motion, common to the whole region of the stars. The complexity of this hypothesis renders it next to impossible that it should proceed from the same Hand, of whose skilful operation we find such abundant evidence in the things that surround us.