proaching to circles, and the Comets in long narrow ovals. Such a view is represented by Plate 2, on which the orbits of all the planets are shewn, I believe, with accuracy, but the path of only one Comet (that of 1680,) has been given, because the addition of others would serve to introduce confusion, considering that phenomena so vast are exhibited on a scale of 600 millions of miles to an inch. If confusion be in any degree observable, it must be owing to the introduction of the paths of the four newly discovered planets; but I trust that these, though intersecting each other so frequently, will, on close inspection, be found sufficiently distinct. That part of each orbit which is above, or north of the Earth's orbit is shewn by a continuous line, and that which is below, or south of it, is dotted. I shall have occasion to refer again to this plate in treating of the plane of the Ecliptic.

I do not introduce to your notice either Planetarium or Orrery, which have been ingeniously constructed so as to make the several balls representing the planets revolve around another representing the Sun. I say ingeniously, because this has been done with much accuracy in so far as regards their respective periods of revolution. But in other respects, these instruments convey false ideas. They represent the planets as moving in circular orbits and on the same plane; the respective sizes of the planets are not correctly shewn; and they give false notions of the path of our Moon, and the moons of the other planets. So that whenever we look on an Orrery or a Planetarium, we must recollect that they justly represent only the comparative times of revolution. But methinks, some of the younger part of my audience may say to themselves, All this is truly astonish

ing. I am then to believe that the Georgium Sidus is 1800 millions of miles from the Sun, which makes the circumference of his orbit about 11,300 millions of miles; yet the whole circuit of the solar system must be far greater, because the orbits of some of the Comets extend beyond those of the planets to a distance that almost exceeds calculation.' And they may further say, 'But if I am to believe that all this is true, I should like, before I give my assent to it, to be shewn the evidence upon which these assertions rest, and by what means the sizes of the planets, and their distances from the Sun have been calculated.' To this, my reply is, that this inquiry is extremely reasonable, but that I feel it impossible wholly to comply with it, because the complete evidence of these truths is involved in geometry and the mathematics; yet I shall attempt some familiar illustrations as we proceed. But without this we have most intelligible evidence for the truth of the astronomer's assertions, to which I allude as having long carried to my own mind a decided conviction. I mean the precise manner in which all their calculations are made in regard to eclipses and other phenomena dependent on, or connected with, the movements of the planets. The transit of Venus over the disk of the Sun in 1761, and again in 1769, were foretold-and it is also foretold, by pursuing the calculation, that another transit of Venus will not happen till 1874. Besides we must all recollect the eclipse of the Sun which lately happened, owing to the passing of the Moon between us and a part of its disk; and when we reflect that the time of the beginning and end of this eclipse were foretold, as well as the portion of the Sun that was hidden from us, we cannot withhold our assent to the fact, that

the astronomer must be well acquainted with the motions of the celestial bodies. Indeed to such extreme nicety are astronomical tables calculated; that if, to-day, we were to direct a telescope towards a particular place in the starry sphere, we may decide by the help of these tables, at what day, hour, minute and second, many years to come, a given star will pass before the centre of the glass, and over a hair stretched across it: so that all the errors in these tables, may be said to be comprehended within the thickness of a hair. Surely we cannot want more substantial proofs of the astronomer's knowledge, more decided evidence that mathematical demonstration has raised this science to its utmost elevation.

An astronomer therefore, to a nice observation of the movements of the celestial bodies, must unite a profound knowledge of the mathematics, and must possess a mind of the most patient, persevering, and inquisitive kind. Such was the mind of our famous Newton. For though to him does not belong the honour of discovering the actual motions of the planets and comets about the Sun, he contributed largely to the perfecting of this science, of all others the most noble. For even a superficial knowledge of it, gives us large opportunity of contemplating the power, knowledge, and wisdom of the Deity, in the magnificence of our system, and in the harmonious march of the planetary bodies. That man should have been able to penetrate thus far into the works of Omnipotence, is truly astonishing. If it be not sufficient to raise his still feeble powers in his own estimation; it is at least sufficient to make him understand that comprehensive gifts of intellect have been bestowed upon him.

LECTURE II.

Of Gravity-Laws of the Planetary Motions.

WE Come now to the consideration of the impulse or power by which the bodies, constituting the Solar system, are kept in their paths or orbits, with a precision so nearly accurate and continual, as they are found to keep.

This is a purely philosophical inquiry. Our knowledge on the subject is drawn from analogy; or in other words, by assuming that the whole system is acted upon by a power, a force, or a law, call it whichsoever we may, that demonstrably exists in the Earth.

It is said that Newton, when sitting alone in his garden, saw an apple fall from a tree, and that this simple circumstance sufficed to awake in his powerful mind, ideas which, being followed up by the peculiar sagacity that characterized it, led to the discovery of a power which is termed Gravity; and reflecting on this power, he was led to inquire why, as it can be proved to exist in every part of the Earth, it should not extend to the Moon ;perhaps her motion might be affected by it ;—perhaps it might be the means of retaining her in her orbit. The conclusions arising from these suggestions are indeed most important.

If we were to throw up a stone, or were to project a cannon-ball in a straight line above our heads, it would return to that spot on the earth from which it arose; and

if the same were done on the opposite side of the Earth, at our antipodes, the same effect would ensue; the stone or the ball would return to the Earth. By the Earth, therefore, some power or force must be exerted-in it, some law must exist, by which two bodies, thrown from it in opposite directions, return to it again. This power, as has been said, is termed Gravity; it is also termed Attraction.-Attraction, because it is evident that the one body is attracted by the other-Gravity, because it is known that this attraction exists in bodies, in proportion to the quantity of matter in them, whatever may be their bulks or external forms.

Cavendish placed a small leaden ball at each end of a slender deal rod, which he then hung on a very fine wire through a hole in the centre of the rod. The balls balanced each other; when so balanced, he caused two balls of lead, each eight inches in diameter to approach the little balls, at the extremities of the rod. Immediately they were attracted from their places by the larger balls, but returned as soon as they were taken away.

Experiments on the same principle have since been tried upon the masses of large mountains. It was found that Schihallien in Scotland attracted a plumb line let fall from near its summit, 6 seconds of a degree from the right line, towards the mountain, and Chimboraço in America 7 seconds and a half. All these facts contribute to prove the great truth, that gravity or attraction exists in every particle of matter composing the Earth, and a combination of powerful reasons induces the conclusion that it is exerted upon all bodies on one great principle,namely, in regular proportions, according to the quantities of matter in them and their distances from each other. Preparatory to an inquiry into what effect gravity can