RECENT SCIENCE

I

THE beautiful big telescopes which are now at work at several observatories have rendered a new service to astronomy. They have given a fresh impulse to lunar studies, and once again the old questions as to the existence of air and water and the possibility of organic life on the surface of our satellite are discussed-this time with some prospects of a definite solution.

For some time past lunar studies have been decidedly falling into neglect. The immense, apparently lifeless plains of the Moon, which still retain the name of 'seas,' or 'maria,' although no traces of present or past marine action can be detected on their surfaces; its immense circus-shaped craters, which have no rivals in size on our own planet; its high chains of mountains and deep rents-all these had been minutely measured and mapped down to the smallest craterlets, with the hope of discovering some signs of life, or of change going on on the Moon's surface; and yet no such signs were forthcoming, at least in a definite form. There was, of course, a small group of devoted selenographers, Neison in this country, Klein in Germany, Oscar Schmidt at Athens, who continued to give their lives to a minute study of the visible surface of the Moon. With instruments of a modest power they achieved real wonders in delineating the minor details of lunar topography, and from time to time they caught glimpses of such appearances as seemed to indicate the presence of water in certain cavities, or a periodical growth of some vegetation, or, at least, a still continuing volcanic activity. But each time such appearances were studied in detail, it became evident that unless more powerful instruments were directed towards our satellite, there was little hope of solving those questions relative to life which, in astronomy as everywhere else, chiefly fascinate man. Gradually it began to be said that we already know about the Moon all that can be known, and interest in lunar studies waned amongst astronomers.

And yet, in reality, our knowledge of the Moon is still very limited. Our best map of its visible surface, although it is a marvel of accuracy, represents it only on a scale of 1 to 1,780,000, which is quite insufficient to show even such changes as are still going on on our own

globe. We know, indeed, that in our lifetime many changes have taken place in the shapes of our hills, valleys, river courses, and ocean shores; but what could we know of such changes if we only had small maps to compare? Moreover, only now, with such big instruments as the Lick telescope, which has a glass lens thirty-six inches in diameter, or the admirable Paris telescope (twenty-four inch lens), we can distinguish, under the most favourable circumstances, the valleys and the hillocks, which are from 650 to 1,000 feet in width; but until quite lately, all we could see was objects over one or two miles wide; so that it has been truly said that if all the knowledge of the Earth by a man in the Moon were of the same sort, he also might represent our planet as an arid, dreary body with no traces of life upon it.2

Photography undoubtedly supplied astronomers with a precious aid. Already in the admirable photograph of the Moon, which was made in 1865 by Rutherford, and still more in modern photographs, the circuses, the plains, and the mountains appeared with a relief and reality of which the best maps gave not the faintest idea. But lunar photography is beset with so many difficulties, chiefly on account of the irregular proper movements of the Moon, that up till now the largest photographs obtained were less than eight inches in diameter. And it was only quite lately that they could be enlarged ten, twenty, and even thirty three times, without the details being blurred. Some of the negatives obtained at the Lick Observatory, and at Paris by the brothers Henry, in an especially quiet and favourable atmosphere, were even so clear, that it was found advisable to carefully examine them under the microscope, and to make with the hand detailed enlarged drawings from the best of them.3

3

The Moon has on this map a diameter of 75 inches, while its real diameter is 2,160 miles.

2 There is no lack of excellent works, most attractively written, in which all information about the Moon may be obtained. Suffice it to name the following :Gwyn Elger's The Moon, London, 1895; Edward Neison's The Moon and the Condition and Configurations of its Surface, London, 1876; the English translation of Flammarion's Astronomie Populaire; and the excellent work of Miss Agnes Clerke, A Popular History of Astronomy during the Nineteenth Century, 3rd edition, London, 1893.

* Great doubts were expressed at the outset as to the advantages offered by photography for the study of the Moon. The advantage of the relief-representation of the surface is, however, self-evident. Besides, with the aid of photography, a continuous record of the Moon's aspect is kept, and every modification of detail which may occur in it will be settled for subsequent reference by unimpeachable testimony. It hardly need be said that in one favourable night many photographs are taken, and that such a mass of details is thus recorded that it would take one's lifetime to map them by hand. As to the enlarged drawings which were made by Dr. Weineck, who is well known for his skill in that sort of work, they have met with a good deal of criticism (from Dr. Klein in Sirius, 1894 and 1895, and the Belgian professor W. Prinz in Ciel et Terre, ii. 1895, p. 449), but it may now be taken as -certain that they really contain a mass of details which may be seen directly even with smaller telescopes, but had been overlooked; while the appearance of

5

An examination of such enlarged photographs, which permits us to embrace with the eye a large surface, filled with a mass of naturetrue details, has led MM. Loewy and Puiseux to some interesting suggestions concerning the origin of the so-called 'rills' or groups of parallel rents in the Moon's crust. And on the other side of the Atlantic, the direct observations lately made by Professor W. Pickering under the clear sky of Peru, as well as his studies of the American photographs, have produced such new data concerning the atmosphere of the Moon, and the possible existence of water on its surface, as are sure to give a quite fresh interest to lunar studies."

The Moon is so small in comparison with the Earth (its weight is eighty-one times less), and consequently the force of gravity is so much smaller on its surface, that, even if it had an atmosphere of the same composition as ours, its density in its lowest parts would be from thirty to fifty times less than the density of our atmosphere at the sea-level. But it appears from Dr. Johnstone Stoney's 7 investigations, that even if the Moon was surrounded at some time of its existence with a gaseous envelope consisting of oxygen, nitrogen, and water vapour, it would not have retained much of it. The gases, as is known, consist of molecules, rushing in all directions at immense speeds; and the moment that the speed of a molecule which moves near the outward boundary of the atmosphere exceeds a certain limit (which would be about 10,600 feet in a second for the Moon), it can escape from the sphere of attraction of the planet. Molecule by molecule the gas must wander off into the inter-planetary space; and, the smaller the mass of the molecule of a given gas, the feebler

the same detail on two or three negatives settles all possible doubts as to its reality. (L. Weineck and E. S. Holden, Selenographical Studies' in Publications of the Lick Observatory, 1894, vol. iii.; Loewy and Puiseux, in Comptes Rendus, tome cxix. p. 254, and tome cxxi. pp. 6, 79; Folie, Bulletin of the Belgian Academy, 1895, vol. xxix. No. 1; and Dr. Klein in Sirius, 1895, p. 112.)

Comptes Rendus, 8 juillet 1895, tome cxxi. p. 79.

• To explain the origin of these rents, Loewy and Puiseux look for the time when the rocks were in an igneous half-liquid state and floating islands of consolidated scoria were formed on the surface of the molten rocks and drifted like the ice-floes in the Arctic Ocean. Remaining in that sphere of ideas, it may, however, be remarked that the same rents might have originated when the whole crust was already solidified. When Lake Baikal is covered with a thick sheet of ice, and the level of the water goes slightly down in the winter, the ice is intersected by long rents, one to ten yards wide, which usually appear in about the same places and in the same directions. They run in straight lines, have vertical sides, and when the water at their bottom is frozen, they become miniature models of lunar rents.

• William H. Pickering, 'Investigations in Astronomical Photography,' in Annals of the Astronomical Observatory of Harvard College, vol. xxxii. part 1 (Cambridge, Mass., 1895). See also Dr. Klein's analysis of the same (Sirius, 1895, Hefte 7, 8, und 9).

On the Physical Constitution of the Sun and Stars,' in Proceedings of the Royal Society for 1868; and paper On the Cause of Absence of Hydrogen from the Earth's atmosphere, and of Air and Water from the Moon,' read on the 20th of April, 1892, before the Royal Dublin Society.

the planet's attraction, and the higher the temperature at the boundary of its atmosphere, the sooner the escape of the gas must take place. This is why no free hydrogen could be retained in the Earth's atmosphere, and why the Moon could retain no air or water vapour.

However, neither these speculations, which are very likely to be true, nor Bessel's previous calculations, could convince practical astronomers of the absolute absence of any atmosphere round the Moon. A feeble twilight is seen on our satellite, and twilight is due, as is known, to the reflection of light within the gaseous envelope; besides, it had been remarked long since at Greenwich that the stars which are covered by the Moon during its movements in its orbit remain visible for a couple of seconds longer than they ought to be visible if their rays were not slightly broken as they pass near to the Moon's surface. Consequently, it was concluded that the Moon must have some atmosphere, perhaps only 200 times thinner than our own. Of course, a gaseous envelope so thin as that would only be noticeable in the deeper valleys, and it would attain its greatest density within the circus-like cavities whose floor, as a rule, lies deeper than the surrounding country. Towards the tops of the mountains it would be imperceptible. But nevertheless, as was shown by Neison, it would play an important part in the economy of life on the Moon's surface.

The observations made at Lick, at Paris, and at Arequipa, fully confirm this view. A twilight is decidedly visible at the cusps of the crescent-moon, especially near the first and the last quarter. It prolongs the cusps as a faint glow over the dark shadowed part, for a distance of about seventy miles (60"), and this indicates the existence of an atmosphere having on the surface of the Moon the same density as our atmosphere has at a height of about forty miles. A similar result is obtained when the slight flattening of the disc of Jupiter, which takes place when the planet is just going to be covered by the Moon, or emerges from behind it, is measured on the Arequipa photographs. Such an atmosphere is next to nothing, but there is another observation, namely, of a dark band appearing between Jupiter and the Moon's limb when the former begins to be covered by the latter; and Professor Pickering finds no other explanation for it than in some very light haze, partly due to water vapour, which would rise a few miles above the Moon's surface where it is illuminated by the rays of the Sun.

Such a supposition would have been met some time ago with great suspicion. But it must be said that the more the Moon's surface is studied in detail the more astronomers are inclined to think that, in some places at least, a haze, originated from water vapour, is the only possible means to explain certain curious occurrences. Thus, Dr. Sarling has lately reminded us that, in 1774, Eysenhard, a pupil of Lambert, saw the part of the shadow line which crossed one

of the plains (the Mare Crisium) brought in a wave-like movement which lasted for two hours and was seen by three different personsonly in this part of the lunar disc. Those undulations, which spread at a speed of 1,200 feet per second over a distance of eighty miles, could only be due-as Dr. Sarling truly remarks—to vapours floating over the plain. In several instances, the interiors of deep lunar circuses took a misty appearance at sunrise, and this misty appearance disappeared as the Sun rose higher above the same circus, while in other cases it persisted a considerable time after sunrise, even though all around was sharply marked and distinct. And so on. The temperature of the Moon's surface, when it is heated by the Sun's rays, being very near to the freezing point, as appears from Langley's last measurements, the evaporation of frozen water under the rays of the rising Sun is surely not at all improbable.

It remains, of course, to be seen whether a haze of this sort is not due in some cases to water ejected by volcanoes or geysers; the more so as some volcanic activity, remodelling until now the forms of the craters, seems to exist. There is, indeed, among astronomers a strong suspicion of a lunar crater, nearly three miles in diameter, being of recent formation. It was first discovered by Dr. Klein in 1876, in the plain named Mare Vaporum, after he himself and many others had previously so often examined that region without seeing the crater. Besides, the alternate appearance and disappearance of another crater (Linné), nearly four miles in diameter, can hardly be explained unless it is concealed from time to time by the vapours which it itself exhales. As to changes observed in the shapes of small lunar volcanoes, they are too numerous to be due to mere errors of observation.10

If free water thus exists occasionally, even now, on the Moon's surface, or has existed at a relatively recent period, it is natural to ask whether it has left no traces of its activity. Are there no rivervalleys which would bear testimony to its existence? Till lately, the majority of astronomers answered this question in the negative, even though their earlier predecessors, armed with feebler telescopes, were most affirmative on this point. The maria, or seas, are known to be plains on which no traces of aqueous action have been detected, and the clefts, or large 'rills,' are almost certainly rents produced in a solid surface.

However, beside these clefts, there are much finer formations which only lately have received due attention, and these finer rills have all the aspects of river-beds. They are not straight-lined, but wind exactly as rivers wind on our maps; they fork like rivers; they

• Sirius, 1895, vi. p. 134.

• Edw. Neison, The Moon and the Condition and Configurations of its Surface, p. 33 (London, 1876).

10 The Observatory, June 1892; Nature, vol. xlvi. p. 134.