adjustment of a cold crust to a cooling nucleus. It is therefore probable that the final shrinkage of nucleus was small, and the antecedent storage of heat correspondingly small. During the whole period of growth the body of the moon was cold."

After thus stating the hypothesis of Gilbert for the origin of the moon, in his own words, it is not needful to consider here in detail the numerous arguments which favour vulcanism, instead of impacts, as the cause of the moon's craters. The adoption of Gilbert's explanation of the physiography and development of the moon would go very far toward conclusive verification of the planetesimal hypothesis; but Chamberlin evidently thinks that volcanic origin of the lunar craters is more probable.

Gilbert considers the whole process of the moon's gathering its formerly scattered material to have been completed at least before the deposition of the earth's Paleozoic sediments, else they would here and there reveal evidences of collision of some of the portions of the previous ring matter, since these must have fallen not only on the moon but in like manner on the earth. Whether the craters of the moon resulted from meteoric aggregation or from vulcanism, the very steep and high mountains of the crater rims have doubtless remained through very long ages unaffected by agencies of erosion, because of the absence of atmosphere.

Geologic antiquity, as hitherto studied, falls far short of reaching back to the time of completion of the creation of these companion globes, the earth and its satellite, in nearly the same size and conditions which they have now. But in the new views opened by the hypotheses noticed in this paper the range of geologic inquiries and theories is extended almost inconceivably farther back, through the laying of "the foundation of the earth."

DISCUSSION.

Colonel MACKINLAY.-I understand Mr. Gilbert tells us on page 202, that what we call volcanoes in the moon are masses roughly comparable to the belt left on a wall when a snowball strikes it, and not volcanoes at all. I must confess it has always struck me as a very strange thing that the moon, which is so much smaller than the earth, is so much richer in volcanoes, and that they should be so very much larger than terrestrial ones. I never heard anyone give

an explanation of this difference. I think it is very difficult to believe they can be volcanoes at all, and I am glad to think there are theories to account for the mass of rings on the moon's surface.

The SECRETARY.-Sir Robert Ball and myself and my son paid a visit several years ago to the Auvergne district of Central France, a district of recently extinct volcanoes, and he made that journey with the special purpose of observing the extinct volcanoes and their apparent similitude to those of the moon. I am sure Sir Robert Ball is a strong believer in the crater-like forms on the moon's surface as being volcanic. They are very deep depressions because the shadows are deep. The terrestrial ones are smaller than the moon's, but some of those in the Pacific Ocean, the great volcanic islands are of enormous size-six or seven miles in diameter.

Mr. ROUSE. It occurred to me that the impression made by a snowball upon another ball, or upon a wall for that matter, would not have been like that of the volcanic walls on the moon, because there would have been an inward slope as well as an outward, whilst they present the appearance of a perpendicular wall without. If any soft body is hurled against another there will be an inner slope of considerable deposit. There will be an inner very considerable slope greater than the outer.

Then it has also occurred to me that if the moon itself was in at all a soft condition, as we may suppose it was at that time, that there would be also a depression in the moon-not only a flat appearance which looks like the continued level of the moon inside the volcanic wall, but there would be a hollow.

The SECRETARY.-There is one difference between the extinct volcanoes of Auvergne and those of the moon. In Auvergne the lava flows break down the walls of the circle, which is generally formed of volcanic ash, but through which molten lava is coming up and filling the great bowl gradually up. It has broken down that rim in some places of least resistance, and then you have a stream flowing out for several miles, and so little covered with vegetation that you might think it was only a few years since they had ceased to flow.

Rev. JOHN TUCKWELL, M.R.A.S.-It will not be possible for us to spend time enough to discuss this nebular hypothesis to anything like its full extent.

There are great difficulties in the way of the acceptance of this new hypothesis when compared with the older hypothesis, more or less modified, of Laplace. Matter in its original condition was no doubt extremely attenuated, and in this extremely attenuated condition it hardly appears possible for us to believe that it was heated. The temperature of cosmic space I think is said to be something like 460° or 470° F. below zero. When we look upon such objects as comets, their tails, which consist of matter in an extremely attenuated condition, certainly cannot be regarded as a fire mist or anything of that kind. With regard to its motion the rotating and spiral nebulæ are certainly very suggestive of the original motion which resulted in the formation of the central sun and planets. I do not think it is necessary to the older hypothesis that we should suppose that the whole mass of the original nebula formed into one compact whole with a flattened surface. We may still accept it together with the suggestion made here, that various nuclei became formed. But when we go right back to their origin, and to the character of the original motion of the nebulæ upon either hypothesis, we come to that state of things when we are obliged to suppose the assertion of the Infinite Will, and the Infinite Wisdom of an Infinite Person. We know of no source whence force could originate except in will. Force may be transformed from one nature or condition to another, but force so far as we know could only originate in will; and thus in the origination of force there is no correlation between the power of the will-the assertion of the power of the will, and the effect produced. In the case of the nebular hypothesis, as we have been accustomed to think of it, you have evidence of the original gaseous condition of matter. gaseous or nebulous condition may have preceded the granular state, if I may so call it, suggested by the hypothesis of Mr. Chamberlin. It is only necessary to apply a few simple laws of Nature to see at all events how development into subsequent conditions might have taken place from matter in its original and gaseous condition. I suppose in that original condition we must regard it as having been atomic. But whether these atoms were the atoms of one primitive substance or atoms possessing different qualities we do not know. By some means or other these atoms must have become combined into molecules, but by what means we do not know. That it did take place at some time or other must be taken for granted.

This

Then with regard to the formation of the planets out of the mass rotating around its central orb, which ultimately became our sun, we may perhaps blend the two hypotheses and suppose that various nuclei were formed which ultimately became planets. Then comes in the question of heat. When was heat evolved? There are two ways by which it might have been produced. Heat may have been produced spontaneously by the closer contact of the original atoms or molecules of the planets, for everyone knows that the closer the atoms or molecules of any substance are driven together the greater the heat is which is evolved. But another means is possible. We have recently had evidence of the way in which a gaseous world can become suddenly ignited. Some two or three years ago there was a world observed, since known as Nova Persei, which suddenly became incandescent. How this took place we do not know. Sir Robert Ball suggested that it may have come into contact with some other planet or with some large meteorite, and that the impact produced ignition.

Professor LOBLEY.-The subject of this paper to-day is an illustration of the very great activity of scientific men on the other side of the Atlantic, and especially is this the case in the subject of astronomy. During the last two or three decades the American astronomers have achieved very great results. It seems to me that this in a great measure is due to the support given by the rich men of America, and it is an example to the rich men of this country, if we wish our country to maintain its place in the van of science.

There are very many points in this paper; it bristles with points suggestive of remarks. It cannot be adequately discussed in a short time, but may I venture one or two remarks about one or two points?

I would like to refer to what has been said with regard to the theory of the rings on the moon being caused by impact. It seems a difficult thing to imagine that these were induced by a moonlet. Where has the moonlet got to? The moonlet did not sink into the moon and there is no evidence of its presence. If it had sunk into the moon it would have left a hollow. Professor Hull has very well referred to the remarkable region of Central France in which you have a number of extinct volcanoes. There is another region in Europe which even more resembles the moon's surface, and that is the Phlegræan fields near Naples, where you have a number of

craters quite resembling the moon's craters, but not on the extensive scale of the moon's surface, although a number of the craters on the moon's surface are small.

With respect to the large question of the nebular hypothesis I confess I must coincide with Mr. Tuckwell. It seems to me that Laplace's idea of a heated nebular mass is quite out of the question in cold regions of space. I quite conform to Mr. Chamberlin's theory of the mass being elemental, as it were, and that these atoms combined together would form molecular solids, and thus we get Mr. Chamberlin's original nebulæ. These would unite together and form a central nucleus, and attract more and more of those surrounding them, and thus rapidly grow.

With respect to the growth of the earth, as stated in this paper, I really cannot follow Mr. Upham. He speaks of primitive atmosphere when the earth was only half or less than half the size that it is. There was a gradual growth of the earth from that small mass to the greatness of a planet. In a nebula, such

as he assumes, a great number of small bodies that formed one mass would be aggregated to a very considerable size. It could draw and unite others and it would grow to its maximum dimensions in a very short time. It would not require enormous ages for bodies one after another to come into it, and it would rapidly aggregate to itself all that was available for it, with the exception of any small masses which are coming in down to the present day.

With regard to the volcanic hypothesis I must say that something is wanting. The two writers seem to assume that the cause of volcanic heat is internal heat, the central heat of the globe. I have disputed that for a long time. It is practically impossible for volcanic lava to come from 30 miles below the surface. It is impossible for lava to penetrate through solid rocks for that distance; and lava is not due to the central heat of the globe.

There is no mention in this paper of rock-fusing temperature except this. This rock-fusing heat is induced by internal heat, but when it produces the chemical action—which again produces heat-you have a rock fusing temperature obtained at a very short distance below the surface; and the lava comes from a very short distance below the surface, three or four miles at most.