it would not reach the ground at the completion of this circuit, but would go on to describe part of another revolution round the Earth. When this condition is once attained, the ball might, by a due increase of the projectile force, continue to revolve for ever about the Earth, and we should thus have a cast-iron satellite moving around us.

Let not the reader smile at the absurdity of supposing a cannon-ball to travel twenty-five thousand miles; for it will lead us to important results. If we suppose that the Sun were stationary in the universe, and that La body very much smaller than the Sun were to be projected with immense velocity, in a direction at right angles to a line joining the Sun and the other body, then the latter (which would proceed in a straight line if the Sun were not present), is drawn by the Sun into a curved path, the concave or hollow side of which is always towards the Sun. If the velocity with which this body were propelled were below a certain limit, it would move in a spiral which would gradually end at the Sun himself, to which, therefore, the body would fall: if the velocity were beyond a certain limit, the body would describe a spiral round the Sun, but gra'dually receding from it, and thus would continue through infinite ages to recede further and further from the Sun: but if a certain velocity, of a corresponding ratio with the attractive power of the Sun, were imparted to this body in the first instance, it would move constantly round the Sun, arriving at every revolution at the point from whence it started.

We are now in a condition to explain in some degree how we are to regard the motions of the planets round the Sun. When the Almighty had created the various

bodies which compose the universe, He exercised His infinite power and wisdom by imparting to them various velocities of motion. The Sun being made very much larger than all the planets put together, exerted a more powerful attraction on them than they could exert on him; consequently, the planets were drawn towards the Sun out of their original paths, and made to revolve round the larger body. But how shall we sufficiently admire the exquisite skill with which the various velocities were adapted to the size of the various bodies! What parallel can we find in the poor and imperfect works of man, to that surpassing power of adjustment by which the velocities of the planets are regulated! The Earth moves some hundreds of thousands of miles in a single day; and yet, if her velocity were to deviate by a small fraction, either more or less, the earth would, in the first case, gradually recede from the Sun, and never again approach near him; and in the second case, it would approach to, and fall upon, the Sun. It is when such results as these are obtained, that science enables us to appreciate the striking truth, that man's efforts, even in his proudest moments, are but poor and humble attempts to follow after, or to imitate, that which the Great Being performs with such boundless perfection. We applaud, and we give rewards to the man, who can make a chronometer which will be accurate within a few seconds in the year; and well may we do So, for it is a signal instance of human industry and ingenuity to produce such an instrument. Yet a mere fraction of such an error in the movements of the bodies composing the solar system, would be fatal to its stability. Truly wonderful, indeed, is it, that the eleven planets should revolve round the Sun in periods differ

ing greatly one from another, and at such various distances from him; and yet, that each one should have a velocity so exquisitely adjusted to its size and position as to bring it precisely round to the same point after every entire revolution round the Sun. The Sun, then, the golden magnet which thus draws all the other planets towards itself, is surrounded with whirling worlds, which borrow their light from him and share it with one another. Well might the poet of the Seasons exclaim

Thou, O sun!

Soul of surrounding worlds! in whom best seen
Shines out thy Maker! May I sing of thee?
'Tis by thy secret, strong, attractive force,
As with a chain indissoluble bound,
Thy system rolls entire: from the far bourne
Of utmost Saturn, wheeling wide his round
Of thirty years, to Mercury, whose disk
Can scarce be caught by philosophic eye,
Lost in the near effulgence of thy blaze.

* Thomson had written this before the year 1730; and the planet Uranus was not discovered until 1781.

37

CHAPTER III.

FORM OF THE ORBITS OF THE PLANETS, AND THEIR MOTIONS THEREIN. MOTIONS OF COMETS AND SATELLITES. GENERAL VIEW OF THE SOLAR SYSTEM.

OUR remarks hitherto have been so expressed as to lead to the conclusion that the planets move in perfect circles round the Sun at all times, and under all circumstances. Such, however, is not strictly the case. The paths which they describe are oval or elliptical. Most persons know the form which is meant by the term "oval." If we hold an egg in the hand, and look at its outline, it will give a near approach to this form: and, indeed, the word oval is derived from ovum, the Latin for an egg. Such, then, is the form of the paths in which the planets move. Now we may inquire whether the Sun is exactly in the middle of this oval, or near either end of it. To this it must be answered that the Sun is not precisely in the middle, but that he is a little nearer to one side than the other. It will be useful to give an idea of the position which the Sun occupies. Suppose A B (fig. 6) represent the orbit or path in which a planet (such as the Earth) moves round the Sun. (We have made this a larger oval in proportion than the Earth's orbit really is, in order that our meaning may be more conspicuous.) There are two points F and s (called foci, plural of the Latin word focus, signifying a fire-place), which have

peculiar properties. If we stick a pin in each of the points F and s, and fasten the two ends of a bit of thread to them, (taking care that the thread is just long enough to reach to any one part of the circumference, as at P,) we shall find that we shall be able to make the thread exactly touch every other part of the oval, by stretching it out; but that we cannot make it extend beyond the oval in any direction.

Now it is in one of such points F S that the Sun is situated, in the Earth's orbit. The oval, if we could possibly see it at once with the eye, would scarcely appear to us to deviate from a circle: it being rather a round, not a long oval. We here speak only of the Earth's orbit, but the same remark applies to the orbits of all the planets, which orbits are all more or less oval. We use the term oval in preference to the term elliptical, because it is more familiarly known; the meaning of each is, however, the same.

We may now be asked, whether the planets move equably in every part of their orbits, that is, If a planet move at the rate of so many miles in an hour at one part of its orbit, will it move with the same velocity at another part? This question, on account of the oval form of the paths in which the planets move, must be answered in the negative. They do not move equably in different points of their orbits. Suppose that in the following figure, the point s were like the axle of a wheel, and that twelve equidistant spokes, or radii, reached from it to the boundary of the oval, then the Earth, in passing by the end of each spoke during her revolution, would not pass from one to another in exactly equal times, but would take a longer time to pass from spoke to spoke at one part of her revolution