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ART. VIII.-On the Moon's Figure, Rotation, and Surface. By the Rev. JAMES GLASGOW, D.D.

Presented 10th October 1861.

*

You were, perhaps, too young to have paid much attention to a discussion, displaying more of vanity than science, which went the round of newspapers some four or five years ago. The Moon's rotation was positively denied; and it was urged that, because she always keeps the same face next the earth, therefore she does not rotate; while the obvious and mathematically certain inference is, therefore she does rotate, though, as I propose to show, her rotation depends on a different cause from that of the earth. It was said the moon is fixed in relation to the earth, as if attached by a wire, or as if she were a part of the earth. But as every part of the earth rotates, so, if the moon were a part of the earth, she would share the earth's rotation. It was said this would constitute rotation about the earth's centre; but the word rotate properly applies only to a wheel-like motion within the body. The moon's rotation is round her own centre, which, however, is constantly shifting her position or describing the lunar orbit.

To show this,-let a (Pl. IV. fig. 1) be the centre of the earth, B that of the moon, and c that of the moon after she has revolved to some distance in her orbit. Produce AB, AC, to DE. Draw the tangent DH, which, after the revolution of the centre в to C, takes the position EG. Now, as the line AD revolves to AE, the diameter ID, being still directed to the earth, takes the position KE, and DF takes the position EG, which produced cuts DH in F. The moon therefore has rotated by the angle HFE, or the central angle a.

Among the questions which have been raised rather than settled by astronomers, one relates to the presence or absence of fluid matter as

* Some queries by this juvenile friend respecting the asteroids, became the immediate occasion of my putting into a written form my previous ideas, as referred to in p. 126. The first copy having been addressed and sent to him, the epistolary form, though perhaps not the most eligible, is retained. It scarcely appears afterwards, and affects not the mode of proof,

part of the moon's superficial mass. The telescopic observations of former times led astronomers to map out the disk of the moon, denominating the duskier parts seas, and assigning them names. This was too hasty; but some later astronomers have been perhaps equally hasty in the opposite direction. They have asserted that the moon contains neither water nor air. The facts are that clouds do not appear, and that when the moon is about to cause an occultation of a star, no atmosphere is detected. But from these facts the true conclusion is no more than that the quantity of air is, if existent, not sufficient to be detected by present telescopic means. Its existence may appear improbable to some; but more cannot be scientifically alleged, and late astronomers are not unanimous in supposing so much. The telescopic observations of Schroster are thought by some of them to have proved the existence of a very rare atmosphere.

Perhaps none has gone nearer to a negation of fluids on the moon than Sir J. Herschel in his volume on Astronomy in Lardner's Cyclopædia. He says (p. 229), "what is, moreover, extremely singular in the geology of the moon is, that, although nothing having the character of seas can be traced, yet there are regions perfectly level, and apparently of a decidedly alluvial character. The moon has no clouds nor any other indications of an atmosphere. Were there any, it could not fail to be perceived in the occultation of stars, and the phenomena of solar eclipses." But in the same and next page, he asserts that decisive marks of volcanic stratification, arising from successive deposits of ejected matter, may be clearly traced with powerful telescopes." Now with volcanic action we usually associate ignition and combustion; and we know chemically, that the general supporter of these is oxygen; and, though chlorine may also support some combustion, as of metals, yet that equally with oxygen supposes an atmosphere of some gas or gases. It is also true that volcanoes, without ignition, as those of mud and water, are found on the earth; yet I must ask, what conception can we form of volcanic action, without any of those agents, — fire, gases, or water? But it is admitted on all hands that the surface of the moon is honeycombed with volcanoes which, if not now, were formerly active. The appearances attributed to fire are as strenuously explained away as those supposed to prove the presence of air. It is indeed correct to say an atmosphere may have formerly existed; but it is surely no absurd idea to suppose that some portion of it may yet remain. He also says (p. 231), "Whatever moisture may exist on its surface must be constantly transferred by distillation in vacuo, &c. The consequence must be absolute aridity below the vertical sun,

constant accretion of hoar-frost in the opposite region, and perhaps a narrow zone of running water at the borders of the enlightened hemisphere." So then, according to Sir J. Herschel, there may after all be water, and consequently oxygen,-an ingredient common to air and water, the only question being that of quantity.

The following statements of Dr. Dick also merit attention (Chr. Phil. p. 283), "Certain luminous spots which have been occasionally seen, seem to demonstrate that fire exists in that planet.* Sir W. Herschel and several other astronomers suppose that they are volcanoes in a state of eruption. It appears highly probable, from the observations of Schroster, that the moon is encompassed with an atmosphere, but no clouds or rain or snow seem to exist in it. Also (Cel. Sem. p. 241): "It has been observed on different occasions that the moon in a solar eclipse was surrounded with a luminous ring, which was most brilliant on the side nearest the moon; that the sharp horns of the solar crescent have been blunted at their extremities during total darkness; that, preceding the emersion, a long, narrow streak of dusky light has been seen to colour the western limb of the moon,—and that the circular figure of Jupiter, Saturn, and the fixed stars has been changed into an elliptical one, when they approached either the dark or enlightened limb of the moon. The celebrated Schroster, of Lilienthal, discovered near the moon's cusps a faint grey light of a pyramidal form, extending from both cusps into the dark hemisphere, which, being the moon's twilight, must necessarily arise from its atmosphere. The following are the general conclusions :—that the inferior and more dense part of the moon's atmosphere is not more than 1,500 feet† (less than of an English mile) high; that the height of the atmosphere, where it could affect the brightness of a fixed star, or deflect the solar rays, does not exceed 5,742 feet. A fixed star will pass over this space in less than two seconds, and if it emerge at a part of the moon where there is a ridge of mountains (higher perhaps than such atmosphere), scarcely any observation can be perceptible." It is difficult to avoid the conclusion, unless the latest observations should have proved these totally

Scientific friends having kindly furnished interesting remarks and facts, I shall make use of some of them. One, for instance, remarks that "the bright spots are believed to have been portions of the shining rays, or bright veins, that form so very remarkable a feature in selenology." But these are obviously luminous, else on the dark part of the moon they would be invisible. Do we know any luminosity on an opaque body without flre? To say "phosphoric" is of no avail, for that requires atmospheric air.

A little more.

erroneous, which can scarcely be supposed. They are clearly in favour of the existence of an atmosphere very small in quantity, and to rough observation not perceptible.

But in determining the question of the possibility of observing fluids on this satellite, we must ascertain its figure. Even apart from this, it is unscientific to assert that such fluids do not exist, merely because they are not sufficient to come within observation. With Lord Rosse's great telescope a river would not be observable, even as a line, unless it were 60 feet broad. A lake within 60 feet diameter would not make a visible dot. In regions, such as some considerable districts in India, with only very small streams, water sufficient for irrigation is procurable from wells; and though the supply is kept up by the annual rains, yet, under different cosmical arrangements, the water night be perpetually restored to the land in invisibly condensed vapours or dews. And how know we that, without large masses of water in the moon, there may not be sufficient for the purposes of lunar life, animal and vegetable? The utmost science warrants us to do is, beyond observed facts, to leave the question open. We have no right indeed, without proof, to assert the existence of fluids, any more than geologists have to assert the existence of human fossils in particular strata, without producing any. In this case, the general opinion is that such fossils do not exist; but this rests not on the mere want of facts, which would only warrant doubt, but on the progress of civilization, language, learning, and history, especially the inspired, all seeming to assign a definite and not very remote origin to the human race.*

Men are making strenuous endeavours to prove the greater antiquity of man, by producing, from the Egyptian mud deposit, and from drift in Europe, some rude specimens of art, which some of them by a verbal trick are calling "human remains; "while, if human, they should be called "human works." Animal remains are the fossilised bones of animals. In this manner lexicographers define the word remains. But, suppose it proved that these works of art are older than the human race, what follows? Obviously, that some other order of beings capable of producing them existed on earth, and passed away before our progenitor appeared. "Oh! then," some may say, as one once did: "You advance the theory of monkeys." I advance no theory, I merely show the flaw in the reasoning of those who assume that very ancient and very rude works of art must, of course, be human. For aught science can tell, demons or angels may have inhabited the earth in some era, and the arrow heads and the pottery may be their work. To discuss this question is no part of my present task. I only notice it for illustration. It is a question not for geologists, but for biblical translators and interpreters, whether the man mentioned in the first chapter of Genesis and the man mentioned in the second represent the same or different races. Let human fossils be produced in situ, and let the rock in which they are embedded be proved beyond debate to be more ancient than the human

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