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MARE VAPORUM AND THE LUNAR CLEFTS
BY THE REV. T. W. WEBB, A.M., F.R.A.S.
With the object of comprehending in a continuous description the whole extent of the Lunar Apennines, our guides have carried us considerably beyond the 1st Meridian, and into the 2nd Quadrant of the Moon. They now bring us back towards the W. to complete the Quadrant with which they commenced, and introduce us to the Mare Vaporum (F in our map). This is a level surface, of a lighter tone than the other Maria, and possessing no very definite boundary, but lying in a general sense between the S.W. slope of the Apennines and the crater Agrippa (26). There would be little to make the region worthy of especial notice were it not that its S. part contains a full development of one of the most curious of the lunar features, the system of CLEFTS (or Rills). Fortunately for the telescopic observer, this district, while it lies in so central a position as to be very little affected by foreshortening. contains two of the largest and most obvious of these mysterious formations, “in which," say B. and M., “an attentive observation can recognize something more than their mere existence.” Before we proceed, however, to describe them, it may be premised that there is a difficulty, not merely in comprehending their nature, but even in providing for them a suitable name. This arises from the fact that we are so little familiar with anything analogous to them on the surface of our own globe. It is true that features do exist, which, viewed at a corresponding distance, might have a somewhat similar aspect. We might refer to the transverse fractures of the Balkan range, and of some of the chains of Greece, the Barrancas of Mexico, or the marvellous cañons of the Colorado River (described in Int. Obs. iv. 309), which may, perhaps, exhibit as great a similarity to the clefts of our satellite as the terrestrial does to the lunar crater; and more could not in fairness be expected. But still, the fissures of our globe are too exceptional to constitute a system such as obtains on the moon, and consequently to have received any generally accepted name. Astronomers, therefore, are somewhat at a loss how to designate the lunar crack, if such it may be termed. Schr., the first discoverer of these objects, in 17-8, called them “canals,” or “rills " : the former term is obviously unsuitable to a dry surface, and has gone out of use; the latter has held its ground among the German observers, and very deservedly so, as far as their language is concerned, in which it signifies a small furrow, or groove. Its adoption, however, by ourselves, though sanctioned by some great names, does not seem free from objection, on account of the existence of a similar, but by no means equivalent English word, the derivation of which, from the Latin, rivulus, and its meaning as given by Johnson, " a small brook, , a little streamlet,” refer unequivocally to the presence of water. It is, however, frequently easier to find a fault than to mend it; and in the present instance (furrow having been rejected, on high authority, as not sufficiently comprehensive), if the employment of the term cleft is suggested, it need only be looked upon as a provisional arrangement, till something more appropriate has been brought forward in its stead. It would indeed be a gain to selenography, if the whole subject of lunar terminology were to undergo a careful revision at the hands of those whose geological as well as astronomical acquirements would enable them to frame such a nomenclature as would command universal acceptance.
But, to proceed from the name, to the objects which it designates. The lunar clefts are characterized by B. and M. as very narrow and long depressions either in a straight or moderately curved direction ; occasionally serpentine or hooked in form : with very steep,* parallel sides, and usually without any external rainpart : in Full Moon they appear as delicate white lines ; near the terminator usually as black ones, as we perceive only the shaded interior. They sometimes pass through or close by small craters, or come to an end in them; at others they lie isolated in level surfaces, without any marked termination; in fact, one of their frequent characteristics is this want of apparent object, as we might speak with reference to the handywork of intelligent beings. They are frequently hemmed in by mountains, but do not appear to run straight over them; occasionally they are found to unite like veins, and even to intersect one another. Their individual breadth varies but little; if there is any enlargement, it is somewhere in the middle, never at the end [rather an inattentive assertion, by the way). They occur in every kind of surface, excepting on lofty summits, or in the centre of the great plains; being of course less distinguishable towards the limb. A few of them are only from 9 to 14 miles in length; the generality 45 to 70; some, again, reach a maximum of 115 to 140 m.: their breadth is in places not inconsiderable, a mile or upwards; but they may be traced down to of that size.t Connecting links with other formations may often be perceived: the enlargements occasionally noticed take the fürm of longish craters, or if these are frequent, the whole cleft appears almost like a chain of minute craters, with a lateral communication throughout: on the other hand, there are regions where contiguous mountains form, with their straight and parallel sides, gorges much resembling these clefts, and where long straight valleys are distingnished from them chiefly by their greater proportional breadth and inferior steepness ; and several valleys of this kind are to be met with in the immediate neighbourhood of true clefts, and parallel to them. From these and other indications, such as their want of connection with any definite object, their occasional repetition in parallel lines within a small distance, and more especially their magnitude (as Schr. had already perceived), the idea of an artificial origin must be aid aside.
* Schmidt considers them pot extremely steep. + Schmidt gives their depth from 100 yards to mile.
On this subject B. and M. have some remarks, which, though referring to speculations which have never found great favour among ourselves, are still worthy of consideration. They observe that, but from inattention to the scale of the objects we are studying, and from the unreasonable expectations entertained at the end of the last century of a marvellous increase of magnifying power (some persons, in Germany we may presume, seem to have been anticipating the use of powers
of 15,000 !), the idea of these clefts being lines of road, or anything else of an artificial nature, would never have been seriously entertained.* In all such analogies, they judiciously remark, one very important point has been ignored—the relative proportion of gravity on the different bodies of the system. When this, as on the Moon, is 6 times inferior to that on the Earth, fresh relations are introduced between power, weight, and motion; and while we have no positive information as to artificial products on the Moon, it may be maintained with great probability that they would be totally unlike our own, and that, even if our optical means were capable of reaching them, they would not appear in any recognized or familiar form. Remarks of this kind are valuable if they check the vagaries of irrational fancy. We should take care, however, not to push them too far; it would be possible, though much less probable, to err in an opposite direction, and to impede the progress of discovery by an overweening estimate of our own previous conclusions.
The hypothesis of their being actually existing or dried-up water-courses may be disposed of with little trouble. Al modern observers are agreed as to the absence of water, at least in any noticeable quantity, from the Moon as it now is; but could we conceive, with Gruithuisen, that it had formerly found a place there, the traces of its smaller receptacles would be of a very different character. The river-systems of a driedup Earth, as contemplated from the Moon, would have a wholly dissimilar aspect, with their confluent branches, and their regular increase from end to end. The lunar cleft, on the contrary, often begins and ends on a high level, without ever reaching lower ground: or runs from mountain to mountain through a plain : it does not grow broader towards one end : it very seldom receives branches : it is very frequently straight: its proportional depth is too great, especially with the slight force of lunar gravity, to have been excavated by currents of water: it is often only ten or twelve times longer than it is broad: it frequently reappears on the further side of mountains which have interrupted it: or it keeps a regular curvature in passing through obstacles lying in every direction : occasionally it goes through craters of considerable size. All this -as well as the absence of any noticeable atmosphere-is inconsistent with the hypochesis of water-currents of any assignable date. The supposition of life in the Moon thus seems negatived. But why, ask our authorities, should we assume, as alone possible, the existence of that one form of life with which we are acquainted on our planet ? Even with us the most extraordinary differences in climates and modes of life are found, co-existent with the same altimate elements, with an uniform force of gravity, and an unvaried general density: why, then, should not entirely new arrangements as to life and motion have place under totally different conditions of existence in other worlds ? To these very sensible questions, the spirit of which had been in many points anticipated by Schr., may be added some reflections of the latter to the effect that our speculations ought to be confined to what is deducible in the strictest sense from observation, and that there is no necessity that we should denominate the Moon “an entirely dry mass of chalk, and that therewith at the same time every substitute for water, so beneficial to us, should be denied to those who, as well as ourselves, are indebted to the Universal Father of nature for the enjoyment of an active life. With the largest telescopes we still see the great works of God always in a remote background, and can only keep ourselves to sure observations and their immediate consequences.” He goes on to suggest (but in a very modest and rational manner) the possibility of some transparent fluid on the Moon, bearing an analogous proportion to her atmosphere, in point of density, to that existing between our water and air, and that it may perform a similar function in point of utility with regard to the needs of such organized life as may exist there. Till of late it might have been said that the analysis of aerolites, in the composition of which no new chemical element has ever been detected, and from which we should infer a similarity of matter throughout the solar system, strongly contravenes such speculations. But the recent discoveries of Huggins, rendering it highly probable that the atmospheres of Jupiter and Saturn contain elements or combinations foreign to our knowledge, again restore the balance, and enable us to call the worthy old Hanoverian into court again. He always deserves at least a hearing
* Gruithuisen referred them partly to dried river-beds, or natural clefts, partly to artiticial clearings through forest lands, used in either case as lines of communication.
Some other peculiarities of these clefts, pointed out by B. and M., in their “ Beiträge,” may be mentioned, as the result of their discovery, within six or seven years, of upwards of seventy, in addition to less than twenty previously known. Absolute straightness and singleness characterize the majority, and the average bearing falls between 8h. and ilh. of a clockdial, divided into 24h. There is an optical cause why they should be frequently found in the line of the meridian, but scarcely ever in a 6h. (or E. and W.) direction, namely, the full development in the former, the absence in the latter case, of that internal shadow by which they are most readily distinguished. But no such reason will explain why more than fifty of them should be directed between 8h. and Th., and only some thirty between lh. and 4h. It is remarkable, however, that the prevailing direction of all lunar objects is so much the same that in some regions scarcely any other tendency is perceptible; that in the majority it predominates; and is absent from none; so that ridges alike and clefts seem to indicate a similar origin.
As to that origin, B. and M. consider it to have been due to a modification of the same eruptive force, the “reaction of the interior,” as Humboldt would call it, which has so extensively modified the lunar surface. This would naturally press outwards along the line of least resistance, or, on the supposition of homogeneity of material, perpendicular to the surface; and during its earlier and intenser action would produce regularly circular cavities, or, if it encountered more coherent material, roundish mountains; as the activity decreased, and circumstances were changed by the great alteration of level, as well as of cohesive power, the direction of the force would be varied; it would take a horizontal course; and while in general it elevated the surface into those long ridges, or banks, of which instances are so abundant, it would occasionally burst it open, so as to form burrows or clefts. This speculation, which had been propounded by Schr. at a much earlier period, does not seem to commend itself by its plausibility. There is a difficulty in conceiving the continuous progress, perhaps for hundreds of miles, of an