The lunar landscape by night needs little description: it would be lit by the earth-moon sufficiently to allow salient features, even at a distance, to be easily made out, for its moon (i.e. the earth) has thirteen times the light-reflecting area that our’s has. But the night illumination will change in intensity, since the earth-moon varies from half-full to full, and again to half-full, between sunset and the next sunrise. The direction of the light, and hence the positions of the shadows, will scarcely alter on account of the apparent fixity of the earth in the lunar sky. A slight degree of warmth might possibly be felt with the reflected earth-light; but it would be insufficient to mollify the intensity of the prevailing cold. The heat accumulated by the ground during the three hundred hours’ sunshine radiates rapidly into space, there being no atmospheric coat to retain it, and a cooling process ensues that goes on till, all warmth having rapidly departed, the previously parched soil assumes a temperature approaching that of celestial space itself, and which has been, as we have stated, estimated at about 200° below the Fahrenheit zero. If moisture existed upon the moon, its night-side would be bound in a grip of frost to which our Arctic regions would be comparatively tropical. But since there is no water, the aspect of the lunar scenery remains unmodified by effects of changing temperature.
Such, then, are the most prominent effects that would manifest themselves to the visual and other senses of a being transported to the moon. The picture is not on the whole a pleasant one, but it is instructive; and our rendering of it, imperfect though it be, may serve to suggest other inferences that cannot but add to the interest which always attaches to the contemplation of natural scenes and phenomena from points of view different from those which we ordinarily occupy.
PLATE XXIII.
GROUP of LUNAR MOUNTAINS. ideal lunar landscape.
CHAPTER XIV.
THE MOON AS A SATELLITE: ITS RELATION TO THE EARTH AND MAN.
Apart from the recondite functions of the moon considered as one of the interdependent members of the solar family, into which it would be beyond our purpose to inquire, there are certain means by which it subserves human interests and ministers to the wants of civilized man to which we deem it desirable to call attention, especially as some of them are not so self-apparent as to have attracted popular attention.
The most generally appreciated because the most evident of the uses of the moon is that of a luminary. Popular regard for it is usually confined to its service in that character, and in that character poets and painters have never tired in their efforts to glorify it. And obviously this service as a “lesser light” is sufficiently prominent to excite our warmest admiration. But moonlight is, from the very conditions of its production, of such a changeable and fugitive nature, and it affords after all so partial and imperfect an alleviation of night’s darkness, that we are fain to regard the light-giving office of the moon as one of secondary importance. Far more valuable to mankind in general, so estimable as to lead us to place it foremost in our category of lunar offices, is the duty which the moon performs in the character of a sanitary agent. We can conceive no direful consequences that would follow from a withdrawal of the moon’s mere light; but it is easy to imagine what highly dangerous results would ensue if the moon ceased to produce the tides of the ocean. Motion and activity in the elements of the terraqueous globe appear to be among the prime conditions in creation. Rest and stagnation are fraught with mischief. While the sun keeps the atmosphere in constant and healthy circulation through the agency of the winds, the moon performs an analogous service to the waters of the sea and the rivers that flow into them. It is as the chief producer of the tides—for we must not forget that the sun exercises its tidal influences, though in much lesser degree—that we ought to place the highest value on the services of the moon: but for its aid as a mighty scavenger, our shores, where rivers terminate, would become stagnant deltas of fatal corruption. Twice (to speak generally) a day, however, the organic matter which rivers deposit in a decomposing state at their embouchures is swept away by the tidal wave; and thus, thanks to the moon, a source of direful pestilence is prevented from arising. Rivers themselves are providentially cleansed by the same means, where they are polluted by bordering towns and cities which, from the nature of things, are sure to arise on river banks; and it seems to be also in the nature of things that the river traversing a city must become its main sewer. The foul additions may be carried down by the stream in its natural course towards the ocean, but where the river is large there will be a decrease in velocity of the current near the mouth or where it joins the sea, thus causing partial stagnation and consequent deposition of the deleterient matters. All this, however, is removed, and its inconceivable evils are averted by our mighty and ever active “sanitary commissioner,” the moon. We can scarcely doubt that a healthy influence of less obvious degree is exerted in the wide ocean itself; but, considering merely human interests, we cannot suppress the conviction that man is more widely and immediately benefited by this purifying office of the moon than by any other.
But the sanitary service is not the only one that the moon performs through the agency of the tides. There is the work of tidal transport to be considered. Upon tidal rivers and on certain coasts, notwithstanding wind and the use of steam, a very large proportion of the heavy merchandize is transported by that slow but powerful “tug” the flood-tide; and a similar service, for which, however, the moon is not to be entirely credited, is done by the down-flow of the ebb-tide. Large ships and heavily-laden rafts and barges are quietly taken in tow by this unobtrusive prime mover, and moved from the river’s mouth to the far-up city, and from wharf to wharf along its banks; and a vast amount of mechanical work is thus gratuitously performed which, if it had to be provided by artificial means, would represent an amount of money value which for such a city as London would have to be counted by thousands, possibly millions, of pounds yearly. For this service we owe the moon the gratitude that we ought to feel for a direct pecuniary benefactor.
In the existing state of civilization and prosperity, we do not, however, utilize the power of the tides nearly to the extent of their capabilities. Our coal mines, rich with “the light of other days”—for coal was long ago declared by Stevenson to be “bottled sunshine”—at present furnish us with so abundant a supply of power-generating material that in our eagerness to use it upon all possible occasions we are losing sight, or putting out of mind, many other valuable prime movers, and amongst them that of the rise and fall of the waters, which can be immediately converted into any form of mechanical power by the aid of tide-mills. Such mills may be found in existence here and there, but for the present they are generally out-rivalled by the steam-engine with all its conveniences and adaptabilities; and hence they have not shared the benefits of that inventive ingenuity which has achieved such wonders of mechanical appliance while steam has been in the ascendant. But it must be remembered that in our extravagant use of coal we are drawing from a bank into which nothing is being paid. We are consuming an exhaustive store, and the time must come when it will be needful to look around in quest of “powers that may be.” Then an impetus may be given to the application of the tides to mechanical purposes as a prime mover.[19] For the people of the British Islands the problem would have an especial importance, viewing the extent of our seaboard and the number of our tidal rivers. The source of motion that offers itself is of almost incalculable extent. There is not merely the onward flowing motion of streams to be utilized, but also the lift of water, which, if small in extent, is stupendous in amount; and within certain limits it matters little to the mechanician whether the “foot-pounds” of work placed at his disposal are in the form of a great mass lifted to a small height or a small mass lifted to a great height. There is no reason either why the utilization of the tides should be confined to rivers. The sea-side might well become the circle of manufacturing industry, and the millions of tons of water lifted several feet twice daily on our shores might be converted, even by schemes already proposed, to furnish the prime movement of thousands of factories. And we must not forget how completely modern science has demonstrated the inter-convertibility of all kinds of force, and thus opened the way for the introduction of systems of transporting power that, in such a state of things as we are for the moment considering, might be of immense benefit. Gravity, for instance, can be converted into electricity; and electricity gives us that wonderful power of transmitting force without transmitting (or even moving) matter, which power we use in the telegraph, where we generate a force at one end of a wire and use it to ring bells or deflect needles at the other end, which may be thousands of miles away. What we do with the slight amount of force needful for telegraphy is capable of being done with any greater amount. A tide-mill might convert its mechanical energy by an electro-magnetic engine, and in the form of electricity its force could be conveyed inland by proper wires and there reconverted back to mechanical or moving power. True, there would be a considerable loss of power, but that power would cost nothing for its first production. Another means ready to hand for transporting power is by compressed air, which has already done good service; another is the system so admirably worked out by Sir W. Armstrong, of transmitting water-power through the agency of an “accumulator,” now so generally used at our Docks and elsewhere, for working cranes and such other uses. And as the whole duty of the engineer is to convert the forces of nature, there is a rich field open for his invention, and upon which he may one day have to enter, in adapting the pulling force of the moon to his fellow man’s mechanical wants through the intermediation of the tides.
Another of the high functions of the moon is that by which she subserves the wants of the navigator, and enables him to track his course over the pathless ocean. Of the two co-ordinates, Latitude and Longitude, that are needful to determine the position of a ship at sea (or of any standpoint upon the earth’s surface) the first is easily found, inasmuch as it is always equal to the altitude of the celestial pole at the place of observation. But the determination of the longitude has always been a difficult problem, and one upon which a vast amount of ingenuity has been expended. When it was first attacked it was soon discovered that the moon was the object of all others by which it could be most accurately and, all things considered, most readily determined. We must premise that the longitude of one place from another is in effect the difference between the local times at the two places, so that when we say that a place or a ship is, for instance, seven hours, twenty-four minutes, ten seconds, west of Greenwich, we mean that the time-o’-day at the place or ship is seven hours twenty-four minutes ten seconds earlier than that at Greenwich. Hence, finding the longitude at sea or at any place and moment means finding what time it is at Greenwich at that moment. Of course this could be most easily done if we could set a timekeeper at Greenwich and rely upon its keeping time during a long sea voyage; and this plan appeared so feasible that our Government long ago offered a prize of £20,000 for a timekeeper which would perform to a stated degree of accuracy after a certain sea voyage. One John Harrison did make such a timekeeper, that actually satisfied the conditions, and obtained the prize: and chronometers are now largely used for longitude, their construction having been brought to great perfection, especially in England, owing to a continuance (in a less liberal degree, however,) of Government inducement. But chronometers are not entirely to be relied on, even where several are carried, which in other than Government ships is rarely the case: recourse must be had to the heavenly bodies for check upon the timekeeper. And the moon is, as we have said, the body that best serves the requirements of the problem.
The lunar method for longitude amounts practically to this. The stars are fixed; the sun, moon, and planets move amongst them; the sun and planets with very slow rates of apparent motion, the moon with a very rapid one. If, then, it be predicted that at a certain instant of Greenwich time the moon will be a certain distance from a fixed star, and if the mariner at sea observes when the moon has that exact distance, he will know the Greenwich time at the instant of his observation.[20] The moon thus becomes to him as the hand of a timepiece, whereof the stars are the hour and minute marks, the whole being, as it were, set to Greenwich time. Then if he knows (which he does by other observations easily obtained) the local time at his ship, he can take the difference between the Greenwich time and his time, which difference is in fact his longitude from Greenwich. The requisite predictions of the distance of the moon from several fixed stars near her are given to the utmost exactness for every three hours of every day and night (when the moon can possibly be seen) in the navigator’s vade mecum, the “Nautical Almanac,” and from these given distances the navigator can, by a simple process of differencing, obtain the distance, and hence the Greenwich time, for any intermediate instant at which he may chance to make his observation. Whenever he can see the moon he can obtain Greenwich time. Of course the whole value of this method depends upon the exactitude of the predicted distances corresponding to the given Greenwich times. These distances are obtained by tables of the moon’s motions, which must be found from observations. The motions in question are of an intricacy almost past comprehension, on account of the disturbing forces to which the moon is subjected by the sun and planets. The powers of the profoundest mathematicians, from Newton downwards, have been severely exercised in efforts to group them into a theory, and represent them by tables capable of furnishing the requisite exact predictions of lunar positions for nautical purposes. Accurate observations of the moon’s place night after night have, from the dawn of this lunar method for longitude, been in urgent request by mathematicians for the purposes specified, and it was solely to procure these observations that the Observatory at Greenwich was established, and mainly for their continued prosecution (and for the stellar observations necessary for their utilization) that it is sustained. For two centuries the moon has been unremittingly observed at Greenwich, and the tables at present used for making the “Nautical Almanac” (those formed by Prof. Hansen) depend upon the observations there obtained. The work still goes on, for even now the degree of exactitude is not what is desired, and astronomers are looking forward with some interest to new lunar tables which were left complete by the late M. Delaunay, formerly the head of astronomy in France, based upon a theory which he evolved. This use of the moon is the grandest of all in respect of the results to which it has led.
Then, too, regarding the moon as a timekeeper, we must not forget the service that it renders in furnishing a division of time intermediate between the day—which is measured by the earth’s rotation—and the year, which is defined by the earth’s orbital revolution. Notwithstanding the survival of lunar reckoning in our religious services, we, in our time and country, scarcely need a moon to mark our months; but we must not forget that with many ancient people the moon was, and with some is still, the chief timekeeper, the calendars of such people being lunar ones, and all their events being reckoned and dated by “moons.” To us, however, the moon is of great service in this department by enabling us to fix dates to many historical events, the times of occurrence of which are uncertain, by reason of defective records or by dependence upon such uncertain data as “lives of emperors,” years of this or that king’s reign, or generations of one or another family. The moon now and then clears up a mystery, or decides a disputed point in chronology, by furnishing the accurate date of an ancient eclipse, which was a phenomenon that always inspired awe and secured for itself careful record. The chronologer is continually applying to the astronomer for the date and place of visibility of some total eclipse, of which he has found an imperfect record, veritable as to the fact, but dated only by reference to some year of a so-and-so’s reign, or by some battle or other historical occurrence. The eclipses that occurred near the time are then examined, and when one is found that tallies with recorded conditions in other respects (such as the time of day and the place of observation), its indisputable date becomes a starting-point from which the chronologer works backwards and forwards in safety. There is one famous eclipse—that predicted by Thales six centuries before Christ, which put an end to the battle between the Medes and Lydians by the terror its darkness created in both armies—which is most intimately associated with ancient chronology, and has been used to rectify a proximate date (the first year of Cyrus of Babylon) which forms the foundation of all Scripture chronology. Sacred and profane history alike are continually receiving assistance from the accurate dates which the moon, by having caused eclipses of the sun, enables the astronomer to fix beyond cavil or doubt.
The mention of eclipses reminds us, too, of the use which the moon has been in increasing, through them, our knowledge of the physical condition of the sun. If the moon had never intervened to cut off the blinding glare of the solar disc, we should have been to this day left to assume that the sun is all-contained by the dazzling globe that we ordinarily see. But, thanks to the moon’s intervention, we now know that the sun is by no means the mere naked sphere we should have suspected. Eclipses have taught us that it is surrounded by an envelope of glowing gases, and that it has a vast vaporous surrounding, beyond its glowing atmosphere, which appears to be composed of matter streaming away from the sun into surrounding space. With these discoveries still in their infancy, it is impossible to foresee the knowledge to which they will eventually lead, but they can hardly be barren of fruit, and whatever they ultimately teach will be so much insight gained into the sublimest problem that human science has before it—the determination of the source and maintaining power of the light and heat and vivifying agency of the sun. In according our thankful reflections to the moon for these revelations, we must not forget that, should there be inhabitants upon our neighbouring worlds, Mercury, Venus, and Mars, which have no satellites, they, the supposed inhabitants, can gain no such knowledge upon the surroundings of the ruler of the solar system. On the other hand, any rational being who may be supposed to dwell upon Saturn or Jupiter, would, through the intervention of their numerous moons, have, in the latter case especially, far more abundant opportunities of acquiring the knowledge in question than we have.
Finally, there is a use of the moon which touches us, author and reader, very closely. It has taught us of a world in a condition totally different from our own; of a planet without water, without air, without the essentials to life development, but rather with the conditions for life destruction; a planet left by the Creator—for wise purposes that we cannot fully know—as it were but half-formed, with all the igneous foundations fresh from the cosmical fire, and with its rough-cast surface in its original state, its fire and mould-marks exposed to our view. From these we have essayed to resolve some of the processes of formation, and thus to learn something of the cosmical agencies that are called forth in the purely igneous era of a planet’s history. We trust that we, on our part, have shown that the study of the moon may be a benefit not merely to the astronomer, but to the geologist; for we behold in it a mighty “medal of creation” doubtless formed of the same material and struck with the same die that moulded our earth; but while the dust of countless ages and the action of powerful disintegrating and denuding elements have eroded and obliterated the earthly impression, the superscriptions on the lunar surface have remained with their pristine clearness unsullied, every vestige sharp and bright as when it left the Almighty Maker’s hands. The moon serves no second-rate or insignificant service when it teaches us of the variety of creative design in the worlds of our system, and exalts our estimation of this peopled globe of ours by showing us that all the planetary worlds have not been deemed worthy to become the habitations of intelligent beings.
Reflections upon the uses of the moon not unnaturally lead our thoughts to some matters that may be regarded as abuses. These mainly take the form of superstitions, erroneous beliefs in the moon’s influence over terrestrial conditions, and occasionally of erroneous ideas upon the moon’s functions as a luminary. The first-mentioned are almost beneath notice, for they include such mythical suspicions as that the moon influences human sanity and other affections of mind and body; that the moon’s rays have a decomposing effect upon organic matter; that they produce blindness by shining upon a sleeper’s eyes; that the moon determines the hours of human death, which is supposed to occur with the change of the tide, etc. All such, having no foundation on fact, are put beyond our consideration. The third matter we have mentioned may also be dismissed in a very few words. The erroneous ideas upon the moon’s functions as a luminary, to which we allude, are those which are manifested by poets and painters, and even historians, who do not hesitate to bring the moon upon a scene in any form and at any time they please without reference to actual lunar circumstances. It is no uncommon thing to see, in a picture representing an evening scene, a moon introduced which can only be seen in the morning—a waning moon instead of a waxing one; and astronomical critics have, indeed, caught artists so far tripping as to put a moon in a picture representing some event that occurred upon a date when the moon was new, and therefore invisible. Writers take the same liberties very frequently. A newspaper correspondent, during the Franco-Prussian war, described the full moon as shining upon a scene of desolation on a particular night, when really there was no moon to be seen. One of the most flagrant cases of this kind, however, occurs in Wolfe’s ballad on “The death of Sir John Moore,” where it is written that the hero was buried “By the struggling moonbeam’s misty light.” But the interment actually took place at a time when the moon was out of sight. We mention these abuses of the moon in the hope of promoting a better observance of the moon’s luminary office. They who wish to bring the moon upon a scene, not knowing ipso facto that it was there, should first take the advice of Nick Bottom in the “Midsummer Night’s Dream,” and make sure of their object by consulting an almanac.
The second of the specified abuses to which the moon is subject refers to its supposed influence on the weather; and in the extent to which it goes this is one of the most deeply rooted of popular errors. That there is an infinitesimal influence exerted by the moon on our atmosphere will be seen from the evidence we have to offer, but it is of a character and extent vastly different from what is commonly believed. The popular error is shown in its most absurd form when the mere aspect of the moon, the mere transition from one phase of illumination to another, is asserted to be productive of a change of weather; as if the gradual passage from first quarter to second quarter, or from that to third, could of itself upset an existing condition of the atmosphere; or as if the conjunction of the moon with the sun could invert the order of the winds, generate clouds, and pour down rains. A moment’s reasoning ought to show that the supposed cause and the observed effect have no necessary connection. In our climate the weather may be said to change at least every three days, and the moon changes—to retain the popular term—every seven days; so that the probability of a coincidence of these changes is very great indeed: when it occurs, the moon is sure to be credited with causing it. But a theory of this kind is of no use unless it can be shown to apply in every case; and, moreover, the change must always be in the same direction: to suppose that the moon can turn a fine day to a wet one, and a wet day to a fine morrow indiscriminately, is to make our satellite blow hot and cold with the same mouth, and so to reduce the supposition to an absurdity. If any marked connection existed between the state of the air and the aspect of the moon, it must inevitably have forced itself unsought upon the attention of meteorologists. In the weekly return of Births, Deaths, and Marriages, issued by the Registrar-General, a table is given, showing all the meteorological elements at Greenwich for every day of the year, and a column is set apart for noting the changes and positions of the moon. These reports extend backwards nearly a quarter of a century. Here, then, is a repertory of data that ought to reveal at a glance any such connection, and would certainly have done so had it existed. But no constant relation between the moon columns and those containing the instrument readings has ever been traced. Our meteorological observatories furnish continuous and unbroken records of atmospheric variations, extending over long series of years: these afford still more abundant means for testing the validity of the lunar hypothesis. The collation has frequently been made for special points in the inquiry, and certainly some connection has been found to obtain between certain positions of the moon in her orbit and certain instrumental averages; but so small are the effects traceable to lunar influence, that they are almost inappreciable among the grosser irregularities that arise from other and as yet unexplained causes.
The lunar influences upon our atmosphere most likely to be detected are those of a tidal character, and those due to the radiation of the heat which the moon receives from the sun. The first would be shown by the barometer, which may be called an “atmospheric tide gauge.” Some years ago Sir Edward Sabine instituted a series of observations at St. Helena, to determine the variations of barometric indications from hour to hour of the lunar day. The greatest differences were found to occur between the times when the moon was on the meridian, and when it was six hours away from the meridian; in other words, between atmospheric high tide and low tide. But the average of these differences amounted only to the four-hundredth part of an inch on the instrument’s scale; a quantity that no weather observer would heed, that none but the best barometers would show, and that can have no perceptible effect on weather changes. The distance of the moon from the earth varies, as is well known, in consequence of the elliptical form of her orbit: this variation ought also to produce an effect upon the instrument’s indications; but Colonel Sabine’s analysis showed that it was next to insensible; the mean reading at apogee differing from that at perigee by only the two-thousandth part of an inch. Schubler, a German meteorologist, had arrived at similarly negative results some years previously. Hence it appears that the great index of the weather is not sensibly affected by the state of the moon: the conclusion to be drawn with regard to the weather itself is obvious enough. As regards the heat received from the moon, we know, from the recent experiments of Lord Rosse in England, and Marie Davy in France, elsewhere alluded to, that a degree of warmth appreciable to the highly sensitive thermopile is exerted by the moon upon the earth near to the time of full moon, when the sun’s rays have been pouring their unmitigated heat upon the lunar surface continuously for fourteen days. And as it is improbable that the whole of the heat sent earthwards from the moon reaches the earth’s surface, we must infer that a considerable amount is absorbed in the higher atmosphere, and does work in evaporating the lighter clouds and thinning the denser ones. The effect of this upon the earth is to facilitate the radiation of its heat into space, and so to cool the lower atmospheric strata. And this effect has been shown to be a veritable one by an exhaustive tabulation of temperature records from various observatories, which was undertaken by Mr. Park Harrison. The general conclusion from these was, that the temperature at the earth’s surface is lower by about 2½ degrees at moon’s last quarter than at first quarter; the paradoxical result being what would naturally follow from the foregoing consideration. The tendency of the full moon to clear the sky has been remarked by several distinguished authorities, to wit, Sir John Herschel, Humboldt, and Arago; and in general the clearing may be accepted as a meteorological fact, though in one case of close examination it has been negatived. It cannot be doubted that a full moon sometimes shows a night to be clear that would in the absence of the moon be called cloudy.
When close comparisons are made between the moon’s positions and records of rain-fall and wind-direction, dim indications of relation exhibit themselves, which may be the feeble consequences of the change of temperature just spoken of; but in every case where an effect has been traced it has been of the most insignificant kind, and no apparent connexion has been recognized between one effect and another. Certainly there is nothing that can support the extensive popular belief in lunar influence on weather, and nothing that can modify the conviction that this belief as at present maintained is an absurd delusion. Yet its acceptance is so general, and runs through such varied grades of society, that we have felt it our duty to dwell upon it to the extent that we have done.
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
BRADBURY, AGNEW, & CO., PRINTERS, WHITEFRIARS.
Albemarle Street, December, 1873.
WORKS ON SCIENCE, &c.
A NATURALIST’S VOYAGE ROUND THE WORLD. By Charles Darwin, F.R.S. With Illustrations. Post 8vo. 9s.
THE ORIGIN OF SPECIES, BY MEANS OF NATURAL SELECTION; or, the Preservation of Favoured Races in the Struggle for Life. By Charles Darwin, F.R.S. With Illustrations. Post 8vo. 7s. 6d.
THE FERTILIZATION OF ORCHIDS THROUGH INSECT AGENCY. By Charles Darwin, F.R.S. With Woodcuts. Post 8vo. 9s.
THE VARIATION OF ANIMALS AND PLANTS UNDER DOMESTICATION. By Charles Darwin, F.R.S. With Illustrations. 2 vols. 8vo. 28s.
THE DESCENT OF MAN, AND ON SELECTION IN RELATION TO SEX. By Charles Darwin, F.R.S. With Illustrations. 2 vols. Crown 8vo. 24s.
THE EXPRESSION OF THE EMOTIONS IN MAN AND ANIMALS. By Charles Darwin, F.R.S. With Illustrations. Crown 8vo. 12s.
FACTS AND ARGUMENTS FOR DARWIN. By Fritz Muller. Translated by W. S. Dallas, F.L.S. With Illustrations. Post 8vo. 6s.
THE STUDENT’S ELEMENTS OF GEOLOGY. By Sir Charles Lyell, F.R.S. With 600 Woodcuts. Post 8vo. 9s.
PRINCIPLES OF GEOLOGY; or, the Modern Changes of the Earth and its Inhabitants, considered as illustrative of Geology. By Sir Charles Lyell, F.R.S. With Illustrations. 2 vols. 8vo. 16s. each.
THE ANTIQUITY OF MAN, FROM GEOLOGICAL EVIDENCES; with Remarks on Theories of the Origin of Species by Variation. By Sir Charles Lyell, F.R.S. With Illustrations. 8vo. 14s.
A GEOGRAPHICAL HANDBOOK OF ALL THE KNOWN FERNS, with Tables to show their distribution. By K. M. Lyell. Post 8vo. 7s. 6d.
A MANUAL OF SCIENTIFIC INQUIRY. For the Use of Travellers. Edited by Sir J. F. Herschel and Rev. Robert Main. Post 8vo. 3s. 6d.
THE CONNECTION OF THE PHYSICAL SCIENCES. By Mary Somerville. Post 8vo. 9s.
PHYSICAL GEOGRAPHY. By Mary Somerville. Post 8vo. 9s.
MOLECULAR AND MICROSCOPIC SCIENCE. By Mary Somerville. Illustrations. 2 vols. Post 8vo. 21s.
PERSONAL RECOLLECTIONS FROM EARLY LIFE TO OLD AGE OF MARY SOMERVILLE. With Selections from her Correspondence. By Her Daughter. With Portrait. Crown 8vo. 12s.
THE RIVERS, MOUNTAINS, AND SEA-COAST OF YORKSHIRE. With Essays on the Climate, Scenery, and Ancient Inhabitants of the Country. By John Phillips, F.R.S. Plates, 8vo. 15s.
RECORDS OF THE ROCKS; or, Notes on the Geology, Natural History, and Antiquities of North and South Wales, Devon, and Cornwall. By Rev. W. S. Symonds, F.G.S. With Illustrations. Crown 8vo. 12s.
RESEARCHES INTO THE EARLY HISTORY OF MANKIND, AND THE DEVELOPMENT OF CIVILIZATION. By E. B. Tylor, F.R.S. 8vo. 12s.
PRIMITIVE CULTURE; Researches into the Development of Mythology, Philosophy, Religion, Art, and Custom. By E. B. Tylor, F.R.S. 2 vols. 8vo. 24s.
SILURIA; a History of the Oldest Rocks in the British Isles and other Countries, with a Sketch of the Distribution of Native Gold. By Sir Roderick Murchison, F.R.S. With Illustrations. 2 vols. 8vo. 18s.