CHAPTER XV.

CONCLUDING SUMMARY.

Having arrived at the conclusion of our subject, it appears to us desirable that we should recall to the reader, by a rapid review, its salient features.

Our main object being to attempt what we conceived to be a rational explanation of the surface details of the moon which should be in accordance with the generally received theory of planetary formation, and with the peculiar physical conditions of the lunar globe—the opening of our work was a summary of the nebular hypothesis as it was started by the first Herschel and systemised by Laplace. Following these philosophers we endeavoured to show how a chaotic mass of primordial matter existing in space would, under the action of gravitation, become transformed into a system of planetary bodies circulating about a common centre of gravity; and further, how, in some cases, the circulating planetary masses would themselves become sub-centres of satellitic systems; our earth being one of these sub-centres with only one satellitic attendant—to wit, the moon, the subject of our study.

The moon being thus considered as evolved from the parent nebulous mass, and existing as an isolated and compact body, we had next to consider what was the effect of the continued action of the gravitating force. By the light of the beautiful “mechanical theory of heat” we argued that this force, not being _destructible_, but being _convertible_, was turned into heat; and that whatever may have been the original condition of the parent nebulous mass, as regards temperature, its planetary offspring became elevated to an intense degree of heat as they assumed the form of spheres under the influence of gravitation.

The incandescent sphere having attained its maximum degree of heat by the total conversion thereinto of the gravitating force it embodied, we explained how there must have ensued a dispersion of that heat by radiation into surrounding space, resulting in the cooling and consequent solidification of the outermost stratum of the lunar sphere, and subsequently in the continuation of the cooling process downwards or inwards to the centre. And here we essayed to prove that in this second stage of the cooling process, when the crust was solid and the subjacent portion of the molten sphere was about to solidify, there would come into operation a principle which appears to govern the behaviour of certain fusible substances, and which may be concisely termed the principle of pre-solidifying expansion. We adduced several examples of the manifestation of this principle, soliciting for it the careful consideration of physicists and geologists, and looking to it as furnishing the key to the mystery of volcanic action upon the moon, since, without needing recourse to aqueous or gaseous sources of eruptive power, it afforded a rationale of the ejection of the fluid and semifluid matter of the moon through the solidified crust thereof, and also of the dislocations of that crust, unattended by actual ejection of subsurface matter, of which our satellite presents a variety of examples, and which the earth also appears to have experienced at some period of its formative history.

Arrived at this stage of our subject we thought it needful to introduce some pages of data and descriptive detail. Accordingly in one chapter we discussed the form, magnitude, weight, and density of the moon, and the force of gravity at its surface: and the more soundly to fix these data in the mind, we devoted a few lines to explanation of the methods whereby each has been ascertained. We then examined the question (so important to our subject) of the existence or non-existence of a lunar atmosphere, giving the evidence, which may be regarded as conclusive, in proof of the absence of both air and water from the moon, and, therefore, refuting the claim of these elements to be considered as sources or influants of the moon’s volcanic manifestations. A general _coup-d’œil_ of the lunar hemisphere facing the earth next engaged our attention, and we considered the aspect of the disc as it is viewed by the naked eye and with telescopes of various powers. From this general survey we passed to the topography of the moon, tracing briefly the admirable labours of those who have advanced this subject, and, by aid of picture and skeleton maps and a table of position co-ordinates, placing it within the reader’s power to become more than sufficiently acquainted for the purposes of this work with the names and positions of detail objects and features of interest. Special descriptions of interesting and typical spots and regions were given in some few cases where such appeared to be called for.

These descriptive matters disposed of, we proceeded to discuss the various classes of surface features with a view to explaining the precise actions which appear to us to have led to their formation. Naturally the craters first demanded our attention. We pointed out the reasons for regarding the great majority of the circular formations of the moon as craters, as truly volcanic as those of which we have examples, modified by obvious causes, upon the earth; and, tracing the causative phenomena of terrestrial volcanoes, we showed how the explanations which have been offered to account for them scarcely apply to those of the moon: and thus, driven to other hypotheses, we endeavoured to demonstrate the probability of the lunar craters having been produced by eruptive force, generated by that pre-solidifying expansion of successive portions of the moon’s molten interior, which we enunciated in our third chapter. The precise course of phenomena which resulted in the production of a crater of the normal lunar type, with or without the significant central cone, were then illustrated by a series of step-by-step diagrams with accompanying descriptive paragraphs. And after treating of craters of the normal type we pointed out and explained some variations thereupon that are here and there to be met with, and likewise those curious complications of arrangement which exhibit craters superimposed one upon another and intermingled in strange confusion.

From craters manifestly volcanic we passed to the consideration of those circular formations which, from their vastness of size, scarcely admit of satisfactory explanation by a volcanic hypothesis. We summarized several proffered theories of their origin, and pointed out what we considered might be a possible key to the solution of the selenological enigma which they constitute, without, however, expressing ourselves entirely satisfied with the validity of our suggestion. The less mysterious features presented by peaks and mountain ranges were then discussed to the extent that we considered requisite, viewing their comparatively simple character and the secondary position they occupy in point of numerical importance upon the moon. At greater length we dealt with the cracks and chasms and the allied phenomena of radiating streaks, pointing out with regard to these latter the strikingly beautiful correspondence in effect (and therefore presumably in cause) between them and crack-systems of a glass globe “starred” by an expanding internal medium.

The more notable objects and features of the lunar surface being disposed of, we had next to say a few words upon some residual phenomena, chiefly upon the colour of lunar surface details, and upon their various degrees of brightness or reflective power. And, inasmuch as varying brightness seemed to us to be related to varying antiquity, we were thence led to the question of the chronology of selenological formations, and to the disputation upon the continuance of volcanic action upon the moon in recent years. We regarded this question from the observational and the inferential points of view, and were led to the conclusion that the moon’s surface arrived at its terminal condition ages ago, and that it is next to hopeless to look for evidence of existing change.

Thus far our work dealt with the moon as a planetary body merely. It occurred to us, however, that we might add to the interest attaching to our satellite were we to regard it for a time as a world, and consider its conditions as respects fitness for habitation by beings like ourselves. The arguments against the possibility of the moon being thus fitted for human creatures, or, indeed, for any high organism, were decisive enough to require little enforcing. It appeared to us, nevertheless, that much might be learnt by imagining one’s self located upon the moon during a period embracing one lunar day (a month of our reckoning), with power to comprehend the peculiar circumstances and conditions of such a situation. We therefore attempted a description of an imaginary sojourn upon the moon, and pointed out some of the more striking aspects and phenomena which we know by legitimate inference would be there manifested. We trust, that while our modest efforts in the chapter referring to this branch of our subject may prove in some degree entertaining, they may be in a greater degree instructive, inasmuch as certain facts are brought into prominence which would not unnaturally be overlooked in contemplating the moon from the earth, the only _real_ stand-point that is available to us.

In our final chapter we considered the moon as a satellite, and sought to enhance popular regard for it on account of certain high functions which it performs for man’s benefit on this earth; but which are in great risk of being overlooked. We showed that, notwithstanding the moon’s occasionally useful service as a nocturnal luminary, it fills a far higher office as a sanitary agent by cleansing the shores of our seas and rivers through the agency of the tides. We pointed out the vast amount of absolutely mechanical work and commercial labour which the same tidal agency executes in transporting merchandize up and down our rivers—an amount that, to take the port of London alone, represents a money value _per annum_ that may be reckoned in millions sterling, seeing that if our river was tideless all transport would have to be done by manual or steam power. We then hinted at the stupendous reservoir of power that the tidal waters constitute, a form of power which has not as yet been sufficiently called into operation, but which may be invoked by-and-by, when we have begun to feel more acutely the consequences of our present prodigal use of the fuel that was stored up for us by bountiful nature ages upon ages ago. The moon’s services to the navigator, in affording him a ready means of finding his longitude at sea; to the chronologist and historian, as a timekeeper, counting periods too vast for accurate reckoning by other means; to the astronomer and student of nature, in revealing certain wonderful surroundings of the solar globe, which, but for the phenomena of eclipses caused by the moon’s interposition, would never have been suspected to exist—these were other functions that we dwelt upon, all too briefly for their deserts; and, lastly, we spoke of the moon as a medal of creation fraught with instructive suggestions, which it has been our endeavour to bring to notice in the course of this work. And from uses we passed to abuses, directing attention to a few popular errors and widespread illusions relating to lunar influence upon and in connection with things terrestrial. This part of our work might have been considerably expanded, for, in truth, the moon has been a misunderstood and misjudged body. Some justice we trust we have done to her: we have brought her face to the fireside; we have analysed her features, and told of virtues that few of her admiring beholders conceived her to possess. We have traced out her history, fraught with wonderful interest, and doubtless typical of the history of other spheres that in countless numbers pervade the universe: and now, having done our best to make all these points familiar, we commend the moon to still further study and still more intimate acquaintance, confident that she will repay all attentions, be they addressed to her as

A PLANET, A WORLD, OR A SATELLITE.

THE END.

FOOTNOTES

[1]The melting temperature of iron is 1500° Centigrade.

[2]Mr. T. Heunter, Manager of the Iron-works of James Murray, Esq., of Dalmellington, Ayrshire. Another authority (Mr. Snelus, of the West Cumberland Iron Company), writes as follows: “I had a hole dug on the ‘cinder-fall,’ and allowed the running slag to flow through it so as to form a tolerably large pool and yet keep fluid. Any crust that formed was skimmed off. A portion of the same slag was cooled, and the solid lump thrown into the pool. It floated just at the surface.” Mr. Snelus adds, by the way, that he tried “Bessemer-Pig” in the same way, and that the solid pig sunk in the molten for a minute and then _rose and floated_ just at the surface, with about one-twentieth of its bulk above the level of the fluid.

[3]Irradiation is an ocular phenomenon in virtue of which all strongly illuminated objects appear to the eye to be larger than they really are. The impression produced by light upon the retina appears to extend itself around the focal image formed by the lenses of the eye. It is from the effect of irradiation that a white disc on a black ground looks larger than a black disc of the same size on a white ground.

[4]For the original photograph from which this plate was produced, and for permission to reproduce it, we owe our acknowledgments to Warren De la Rue and Joseph Beck, Esquires.

[5]The proper distance for realising the conditions under which the moon itself is seen will be that at which our disc is just covered by a wafer about a quarter of an inch in diameter, held at arm’s length. This will subtend an angle of about half a degree, which is nearly the angular diameter of the moon.

[6]The libratory movement has been taken advantage of, at the suggestion of Sir Chas. Wheatstone, for producing stereoscopic photographs of the moon. In the early days of stereoscopic photography the object to be photographed was placed upon a kind of turn-table, and, after a picture had been taken of it in one position, the table was turned through a small angle for the taking of the second picture; the two placed side by side then represented the object as it would have been seen by two eyes widely separated, or whose visual rays inclined at an angle equal to that through which the table was turned; and when the pictures were viewed through a stereoscope, they combined to produce the wonderful effect of solidity now familiar to every one. The moon, by its librations, imitates the turn-table movement; and, from a large number of photographs of her, taken at different points of her orbit and at different seasons of the year, it is possible to select two which, while they exhibit the same phase of illumination, at the same time present the requisite difference in the points of view from which they are taken to give the effect of stereoscopicity when viewed binocularly. Mr. De la Rue, the father of celestial photography, has been enabled to produce several such pairs of pictures from the vast collection of lunar photographs that he has accumulated. Any one of these pairs of portraits, when stereoscopically combined, reproduces, to quote the words of Sir John Herschel, “_the spherical_ form just as a giant might see it whose stature were such that the interval between his eyes should equal the distance between the place where the earth stood when one view was taken, and that to which it would have to be removed (our moon being fixed) to get the other. Nothing can surpass the impression of _real corporeal form_ thus conveyed by some of these pictures as taken by Mr. De la Rue with his powerful reflector, the production of which (as a step in some sort taken by man outside of the planet he inhabits) is one of the most remarkable and unexpected triumphs of scientific art.”

[7]This is a point of some uncertainty. Dr. Young stated (Lectures Vol. II. p. 575) that “a minute is perhaps nearly the smallest interval at which two objects can be distinguished, although a line subtending only a tenth of a minute in breadth may sometimes be perceived as a single object.”

[8]Plate VIII.

[9]“Cosmos,” Bohn’s Edition, Vol. V. p. 322.

[10]_American Journal of Science, Second Series, Vol. II._

[11]“Volcanoes,” page 155.

[12]In reference to such prominences on the lunar surface as cast steeple-like shadows, it is well to remark that we must not in all cases infer, from the acute spire-like form of the shadow, that the object which casts the shadow is of a similar sharp or spire-like form, which the first impression would naturally lead us to suppose. A comparatively blunt or rounded eminence will project a long and pointed shadow when the rays of light fall on the object at a low angle, and especially so when the shadow is projected on a convex surface. We illustrate this with a copy of an actual photograph of the shadow cast by half a pea, Fig. 41.

[13]We meet a difficulty in reconciling this idea with the partial craters of which we have a conspicuous example in Fracastorius, No. 78, of our Map, which seem to be partially sunk below the contiguous surface. This looks as though the crater-rim belonged to an older epoch than the plain from which it rises.

[14]We are informed by a friend, who has lately visited Athens, that Schmidt’s detail drawings of the Moon, comprising the work of forty years, form a small library in themselves. The map embodying them is so large (6 ft. 6 in. in diameter) and so full of detail that there is small hope of its complete publication, unless there should be such a wide extension of interest in the minute study of our satellite as to justify the cost of reproducing it.

[15]It is conceivable that the alleged changes in the crater Linné may have been caused by a filling of the crater by some such crumbling action as we are here contemplating.

[16]Is it not conceivable that the protogerms of life pervade the whole universe, and have been located upon every planetary body therein? Sir William Thomson’s suggestion that life came to the earth upon a seed-bearing meteor was weak, in so far that it shifted the locus of life-generation from one planetary body to another. Is it not more philosophical, more consistent with our conception of Creative omnipotence and impartiality, to suppose that the protogerms of life have been sown broadcast over all space, and that they have fallen here upon a planet under conditions favourable to their development, and have sprung into vitality when the fit circumstances have arrived, and there upon a planet that is, and that may be for ever, unfitted for their vivification?

[17]Our remarks have general reference to a region of the moon near her equator; near the poles some of the conditions we shall describe would be somewhat modified.

[18]We see this reddening during an eclipse of the moon (when the event we are describing—an eclipse of the sun visible from the moon—really takes place). The blood-red colour has often struck observers very forcibly, and it has indeed been suggested that the appearance may be the innocent and oft-repeated fulfilment of the prophetic allusion to the moon being “turned into blood.”

[19]About 100 years ago London was supplied with water chiefly by pumps worked by tidal mills at London Bridge.

[20]The sun and planets are comparatively useless for this object, because of their slow movement among the stars; the change of their positions from hour to hour is so small as to render uncertain the Greenwich times deducible therefrom. Their use would be comparable to taking the time from the hour-hand of a clock.

BRADBURY, AGNEW, & CO., PRINTERS, WHITEFRIARS.

Albemarle Street, _December, 1873_.

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Transcriber’s Notes

—Retained copyright information from the printed edition: this eBook is public-domain in the country of publication.

—Silently corrected a few palpable typos.

—In the text versions only, text in italics is delimited by _underscores_.

End of Project Gutenberg's The Moon, by James Nasmyth and James Carpenter