one another—in fact, part of the same system. These observations were followed out, and it was found that in these immenselydistant stars, so distant from us that light travelling 12,000,000 of miles a minute would take 15, 20, 30, 40, 50, 100, and 150 years to reach us-it was found that these movements were regulated by gravitation; that, in fact, the law of gravitation is a law extending through the entire universe- -a law inherent in all matter, acting not only in our system, but in those far, far-off systems to which I referred last night. This discovery was an immense step forward, but did not entirely resolve the question. It led to the supposition that there must be a movement among the fixed stars, because such a movement as this was one of the natural consequences of the law of gravitation.

And here I must briefly explain the law of gravitation. The law of gravitation is the law by which every particle of matter attracts every other particle of matter in inverse proportion to the square of their distances; and where these particles are massed together, as in our own system, then all bodies attract directly as their size, and inversely as the of their distances. So that the earth attracts square the sun, as well as the sun attracts the earth; only the sun being so much larger than the earth, the attraction which the earth exercises on the sun is as nothing compared with the attraction the sun exercises upon the earth; so that we may speak, as we do, of the earth and all the planets being attracted by the sun, and revolving round the sun, as they do; but revolving round the sun not only by the force of this gravitation, but by virtue of that motion they have already impressed upon them. If they were acted upon by the force of gravitation alone, and not by the force of impetus they possess by themselves, they must be drawn to the sun; and, falling into the sun, would become part of the sun's mass. So it would be with the stars, too. If they had no motion of their own, they would be drawn to one another-they would attract one another directly as their masses-the whole universe would rush together, and form one immense heap of ruins. These considerations lead us entirely to the supposition that this movement of revolution round one another is not confined to these multiple

stars, but that it extends also to the other stars which altogether form our universe. In this enquiry, the first step was to endeavour to ascertain whether our own central star can be shown to have such a motion.

THE SUN'S MOTIONS.

Has the sun a forward motion in space, as well as a motion of rotation? How can that question be settled? Because, if the sun has a forward movement in space, it is carrying all the planets along with it; and therefore their motions relative to it are not affected at all by its movements. If you are travelling in a railway carriage, and if you were to look out behind you, your eyes would see all objects appearing to draw together. On the other hand, those objects towards which you are going would appear to be receding from one another--the arches of bridges widen out-houses stand further apart-the trees not only increase in size, but appear to separate from one another. Now, if our sun has that forward movement in space, and if our earth is being carried with it, we should observe something of the same kind-only, of course, in much less degree. Well, careful examinations have been made on this subject, not only with regard to stars directly above or before, but behind; and by these careful examinations it has been discovered that the sun is moving towards a certain constellation-the constellation Hercules; and not only that, but the precise point toward which he is moving has been ascertained; nay, we are able to fix upon a particular star the star called by a Greek letter Pi-Hercules. Well, now the question arises, too-Is this the case with regard to the other stars? As we have found out that the force of gravitation exists throughout the universe, as we have found it prevailing with regard to multiple stars, so it prevails with regard to stars in general. Observations upon an enormous number of stars, by the most beautifully-constructed instruments, leave no doubt that there are motions taking place among the fixed stars, and we have even been able to ascertain the rate at which these suns, or fixed stars, are moving. Our own sun is moving at the rate of 150,000,000 of miles in the course of the year; but you need not be alarmed lest the sun should come into collision with this star 7 Hercules: for it will take-we cannot ascertain the point exactly-considerably

more than a million of years for the sun to travel over that space at that rate before it reaches this star, even supsing the star Pi-Hercules to be waiting for it, and standing in our way; so that we shall have plenty of time to make our wills before that collision occurs. But astronomers are ambitious; when they have arrived at one point of knowledge, they always want to make that the basis of another discovery; and thus, having ascertained that the stars are moving forward in space, they next attempted to trace their paths. If they are moving forward under the influence of this force of gravitation, then there must be some central point which is the centre of gravitation in the universe, around which they are being carried; and so they endeavour to find out some central sun which must be the centre of gravitation, by which all these stars are attracted. Of course, it is quite impossible in a lecture like this to explain the nature of those calculations into which astronomers enter. But the attempt was made; and, after various approximations, they at last fixed upon that group of stars called the Pleiades, and out of that group they fixed upon a certain star, Alcyone, and determined that that was the fixed central star, around which all the other suns of the universe and their systems of planets are revolving. But they found that this star was so far distant from us, that light, travelling as it does at the rate of 12,000,000 miles a minute, would take 537 years to traverse the space which lies between this central star, Alcyone, and our sun. If this star, Alcyone, be the central sun, the movements taking place in its neighbourhood would be greater in the amount of their velocity, the motions more rapid, than those taking place in other parts of the heavens. And this is confirmed by actual observation. So that there is every reason to believe that we have succeeded in reducing the universe to a system in which there is one great central star, around which all the other stars of the universe revolve.

THE STELLAR SYSTEM.

Now we are able to take a view of our stellar system. We see that it is composed of a vast number of suns revolving about one central sun, but performing their revolutions in enormous spaces of time. For instance, the period o revolution of our sun around this central sun, has been

calculated to be eighteen millions two hundred thousand years; that is, it would take that time before our sun would have gone once round this central sun of the universe. So far we have succeeded in framing some sort of view of our universe.

LAPLACE'S COSMOGONY.

I now come to the speculations put forward to account for all these motions of the heavenly bodies, by what is termed the Cosmogony of Laplace. We will begin by considering it in reference to our own system, and then we shall be able to extend it to the systems which outlie ours. We know something with regard to the force of gravitation. We know also the fact that there is another force at work which causes the planets to revolve round the sun-that is, the momentum which they derive from the fact of their being in motion. It is an established law of matter that any body moving in vacant space will continue to move on in the same space until something occurs to stop it. If I throw a stone forward, that stone would go on for ever with the same speed I impressed upon it when I flung it out of my hand, if something or other did not stop it. That stone has to encounter the resistance of the air and the attraction of the earth; but if it were not exposed to these two resisting powers, the stone, when flung from my hand, would go on for ever. Well, the planets do not meet with any such resisting power, and thus they keep on with the motion originally impressed upon them. Now arises the question-How came this forward motion in space to be impressed upon them? Astronomers have tried to resolve that question. Perhaps that question is altogether beyond the reach of our minds. But still you will see there are causes which may be assigned to it, which at any rate lead us into very interesting speculations. I need hardly remind you that matter exists in three different states-solid, liquid, and gaseous. We are apt to regard the states in which we see these three substances as being their natural states; but that is not the case. Reduce the temperature a little, and water becomes ice, a solid as glass; elevate the temperature, and the water becomes steam. And this is not only the case with regard to water, but to all other substances that exist; so that all fluids, solids, and gases, are in a state of fluidity, solidity, a. gaseity, simply

on account of the temperature to which they are exposed. There is good reason to believe that if we were able to raise the heat sufficiently high, we should be able to bring all the substances on the earth, or in the earth, into a gaseous state. There seems to be but

little doubt that our earth has been progressively cooling down through long ages. There is reason to sup

pose that ages ago it was at a very much higher temperature than it is now; and if we could go back further still into that past time, we should probably find the temperature higher even still. But what will that lead us to? We have seen if we heat a substance up to a certain point, we make it fluid; if to a still greater amount, we make it a gas or a vapour. Thus we have only to apply a sufficient heat to the earth, to bring its most solid parts to a vapour; and we have only to trace this process back sufficiently far, to suppose that the earth once was in that vaporous state of which we have been speaking. When we change a substance into a vapour, it occupies a far greater space than it did as a solid substance. If, therefore, the earth were at such a heat as it is supposed it was at some remote period, it is clear that it must have occupied an enormously greater space than it does now; and if we suppose the same to have been the case with the other bodies which compose the solar system, then we may conceive the time when the heat was far different to what it is at present, and when the whole space comprehended in our system was filled by a diffused gaseous matter, by what has been termed a fire-cloud. Well, supposing our system was once in this state, the laws of physics tell us what would take place. They tell us, that as this fire-cloud cooled down, it would contract, but that it would not contract equably towards the centre; that this cooling down would produce a slow whirling motion, which, as the cooling went on, would become more and more rapid. Then, as this cooling and condensation went on, there would be a tendency in various parts to separate from one another. These separated parts would form themselves into vast rings, which would still retain the circular motion impressed upon them when forming parts of the original mass. As this cooling still went on, these rings, which we will suppose to be circulating round a centre of