The Library Magazine of Select Foreign Literature Part 13

But times changed. Up to the last, Takezawa held out against the introduction of foreign innovations in the mode and manner of conducting the affairs of the firm; other houses might employ foreign steamboat companies as carriers for their produce from port to port, might import foreign goods, and even go so far as to allow the better paid of their clerks to dress themselves as they liked in foreign costume; but Takezawa and Co. were patriotic j.a.panese merchants, and resolved to run on in the old groove of their ancestors.

But times still changed, and the great house, running on in its solid old-fas.h.i.+oned manner, found itself left in the lurch by younger and more enterprising firms. This would never do. So Takezawa consulted with his partners, patrons, clients, and friends, and after much worthy discussion, and much vehement opposition on the part of the old man, it was resolved to keep pace with the times, as much as possible, without absolutely overturning the old status of the house.

Well, Takezawa and Co. had still a very fair share of the export rice and silk business; but their slow, heavy-sterned junks were no match for the swift, foreign-built steamers employed by other firms; so, with a tremendous wince, and not without a side thought at "Hara Kiri"--(the "Happy Despatch")--Takezawa consented to the sale of all his junks, and the purchase with the proceeds of a big foreign steamer.

The steamer was bought--a fine three-masted, double-funnelled boat, complete with every appliance, newly engined, and manned by European officers and leading seamen. From the dock at Yokoska, where she was lying, a preliminary trip was made; and so smoothly did everything work, and so easily did everything seem to act, under the guidance of the Europeans, that Takezawa considered his own mariners perfectly competent to handle the vessel after an hour's experience on board. So the Europeans were discharged with six months' salaries--about six times as much as they would have received at home--and Takezawa fixed a day when the s.h.i.+p should be rechristened, and should make her trial trip under j.a.panese management.

It was a beautiful day in autumn--the most glorious period of the year in j.a.pan--when Takezawa and a distinguished company a.s.sembled on board the steamer, to give her a new name, and to send her forth finally as a j.a.panese steamer. The s.h.i.+p looked brave enough as she lay in the dock--ports newly painted, bra.s.s-work s.h.i.+ning, yards squared, and half buried in bunting. At the mizen floated the empire flag of j.a.pan--a red sun on a white ground--and as Takezawa gazed fore and aft, and his eyes rested on brightness, cleanliness, and order everywhere, he wondered to himself how he could have been such a fool as to stand out so long against the possession of such a treasure, merely on the grounds of its not being j.a.panese. A fair daughter of one of his partners dashed a cup of "sake"

against the bows of the vessel, and the newly named "Lightning Bird"

dashed forward into the ocean. Her head was made straight for Yokohama (Takezawa had seen the Englishmen at the wheel manipulate her in that course on her trial trip, so he knew she couldn't go wrong). And straight she went. Every one was delighted; sweetmeats and wine were served round, whilst on the quarterdeck a troupe of the best "Geyshas" or singing-girls in Yedo mingled their shrill voices and their guitar notes with the sound of the fresh morning breeze through the rigging.

The engines worked magnificently: coals were poured into the furnaces by the hundredweight, so as to keep a good uniform thick cloud of smoke coming from the funnels--if the smoke lacked intensity for a minute, Takezawa, fearful that something was wrong, bellowed forth orders for more coal to be heaped on, so that in a quarter of an hour's time the "Lightning Bird" consumed as much fuel as would have served a P. and O.

steamer for half a day. On she went, everybody pleased and smiling, everything taut and satisfactory. Straight ahead was Treaty Point--a bold bluff running out into the sea. The "Lightning Bird" was bound for Yokohama--Yokohama lies well behind Treaty Point--but at the pace she was going it was very apparent that, unless a sudden and rapid turn to starboard was made, she would run, not into Yokohama, but into Treaty Point.

The singing and feasting proceeded merrily on deck, but Takezawa was uneasy and undecided on the bridge. The helm was put hard a-port, the brave vessel obeyed, and leapt on straight for the line of rocks at the foot of the Point, over which the waves were breaking in cascades of foam.

But the G.o.ds would not see a vessel, making her first run under j.a.panese auspices, maltreated and destroyed by simple waves and rocks; so, just in time to save an ignominious run aground, the helm was put hard over, fresh fuel was piled on to the furnaces, and by barely half a s.h.i.+p's length the "Lightning Bird" shaved the Point, and stood in straight for Yokohama bay.

Takezawa breathed freely for the moment; but, as he saw ahead the crowd of European s.h.i.+ps and native junks through which he would have to thread his way, he would have given a very large sum to have had a couple of Europeans at the wheel in the place of his own half-witted, scared mariners.

However, there was no help for it; the s.h.i.+p sped on, and the guests on board, many of whom were thorough rustics, were in raptures at the distant views of the white houses on the Yokohama Bund, at the big steamers and the graceful sailing vessels on all sides. To avoid the chance of a collision, Takezawa managed to keep his steamer well outside; they nearly ran down a fis.h.i.+ng junk or two, and all but sunk the lights.h.i.+p; still, they had not as yet come to absolute grief. Round they went for a long half-hour; many of the guests were suffering from sickness, and Takezawa thought that he might bring the trip to an end. So he bellowed forth orders to stop the engines, and anchor. The anchor was promptly let go, but stopping the engines was another matter, for n.o.body on board knew how to do so--there was nothing to be done but to allow the vessel to pursue a circular course until steam was exhausted; and she could go no farther. It was idle to explain to the distinguished company that this was the course invariably adopted by Europeans, for under their noses was the graceful P.

and O. steamer, a moment since ploughing along at full steam, now riding at anchor by her buoy. So round and round went the "Lightning Bird," to the amazement of the crews of the s.h.i.+ps in harbour and of a large crowd gathered on the "Bund;" the brave company on board were now a.s.sured that the judgment of the G.o.ds was overtaking them for having ventured to sea in a foreign vessel, and poor Takezawa was half resolved to despatch himself, and wholly resolved never to make such an experiment as this again. He cursed the day when he was finally led to forsake the groove so honourably and profitably grubbed along by his fathers, and strode with hasty steps up and down the bridge, refusing to be comforted, and terrifying out of their few remaining wits the two poor fellows at the wheel. After a few circles, an English man-of-war sent a steam launch after the "Lightning Bird," and to the intense disgust of the great j.a.panese people on board, who preferred to see eccentricity on the part of their countrymen, to interference by foreigners, but to the great delight of the women and rustics, who began to be rather tired of the fun, the engines were stopped. Takezawa did not hear the last of this for a long, long time; caricatures and verses were constantly being circulated bearing upon the fiasco, although it would have been as much as any man's life was worth to have taunted him openly with it. But it was a salutary lesson; and although he still kept the "Lightning Bird," he engaged Europeans to man her, until his men proved themselves adepts, and she afterwards became one of the smartest and fastest craft on the coast.--_Belgravia._

SUPPOSED CHANGES IN THE MOON.

In this Magazine for August last I considered the moon's mult.i.tudinous small craters with special reference to the theory that some among those small craters may have been produced by the downfall of aerolithic or meteoric ma.s.ses upon the moon's once plastic surface. Whether it be considered probable that this is really the case or not with regard to actually existent lunar craters, it cannot be doubted that during one period of the moon's history, a period probably lasting many millions of years, many crater-shaped depressions must have been produced in this way.

As I showed in that essay, it is absolutely certain that thousands of meteoric ma.s.ses, large enough to form visible depressions where they fell, must have fallen during the moon's plastic era. It is certain also that that era must have been very long-lasting. Nevertheless, it remains possible (many will consider it extremely probable, if not absolutely certain) that during sequent periods all such traces were removed. There is certainly nothing in the aspect of the present lunar craters, even the smallest and most numerous, to preclude the possibility that they, like the larger ones, were the results of purely volcanic action; and to many minds it seems preferable to adopt one general theory respecting all such objects as may be cla.s.sed in a regular series, than to consider that some members of the series are to be explained in one way and others in a different way. We can form a series extending without break or interruption from the largest lunar craters, more than a hundred miles in diameter, to the smallest visible craters, less than a quarter of a mile across, or even to far smaller craters, if increase of telescopic power should reveal such. And therefore many object to adopt any theory in explanation of the smaller craters (or some of them) which could manifestly not be extended to the largest. Albeit we must remember that certainly if any small craters had been formed during the plastic era by meteoric downfall, and had remained unchanged after the moon solidified, it would now be quite impossible to distinguish these from craters formed in the ordinary manner.

While we thus recognise the possibility, at any rate, that mult.i.tudes of small lunar craters, say from a quarter of a mile to two miles in diameter, may have been formed by falling meteoric ma.s.ses hundreds of millions of years ago, and may have remained unchanged even until now, we perceive that on the moon later processes must have formed many small craters, precisely as such small craters have been formed on our own earth. I consider, at the close of the essay above mentioned, the two stages of the moon's development which must have followed the period during which her surface was wholly or in great part plastic. First, there was the stage during which the crust contracted more rapidly than the nucleus, and was rent from time to time as though the nucleus were expanding within it. Secondly, there came the era when the nucleus, having retained a greater share of heat, began to cool, and therefore to contract more quickly than the crust, so that the crust became wrinkled or corrugated, as it followed up (so to speak) the retreating nucleus.

It would be in the later part of this second great era that the moon (if ever) would have resembled the earth. The forms of volcanic activity still existing on the earth seem most probably referable to the gradual contraction of the nucleus, and the steady resulting contraction of the rocky crust. As Mallet and Dana have shown, the heat resulting from the contraction, or in reality from the slow downfall of the crust, is amply sufficient to account for the whole observed volcanian energy of the earth. It has indeed been objected, that if this theory (which is considered more fully in my "Pleasant Ways in Science") were correct, we ought to find volcanoes occurring indifferently, or at any rate volcanic phenomena of various kinds so occurring, in all parts of the earth's surface, and not prevalent in special regions and scarcely ever noticed elsewhere. But this objection is based on erroneous ideas as to the length of time necessary for the development of subterranean changes, and also as to the extent of regions which at present find in certain volcanic craters a sufficient outlet for their subterranean fires. It is natural that, if a region of wide extent has at any time been relieved at some point, that spot should long afterwards remain as an outlet, a sort of safety-valve, which, by yielding somewhat more quickly than any neighbouring part of the crust, would save the whole region from destructive earthquakes; and though in the course of time a crater which had acted such a part would cease to do so, yet the period required for such a change would be very long indeed compared with those periods by which men ordinarily measure time. Moreover, it by no means follows that every part of the earth's crust would even require an outlet for heat developed beneath it. Over wide tracts of the earth's surface the rate of contraction may be such, or may be so related to the thickness of the crust, that the heat developed can find ready escape by conduction to the surface, and by radiation thence into s.p.a.ce. Nay, from the part which water is known to play in producing volcanic phenomena, it may well be that in every region where water does not find its way in large quant.i.ties to the parts in which the subterranean heat is great, no volcanic action results. Mallet, following other experienced vulcanologists, lays down the law, "Without water there can be no volcano;" so that the neighbourhood of large oceans, as well as special conditions of the crust, must be regarded as probably essential to the existence of such outlets as Vesuvius, Etna, Hecla, and the rest.

So much premised, let us enquire whether it is antecedently likely that in the moon volcanic action may still be in progress, and afterwards consider the recent announcement of a lunar disturbance, which, if really volcanic, certainly indicates volcanic action far more intense than any which is at present taking place in our own earth. I have already, I may remark, considered the evidence respecting this new lunar crater which some suppose to have been formed during the last two years. But I am not here going over the same ground as in my former paper ("Contemporary Review"

for August, 1878). Moreover, since that paper was written, new evidence has been obtained, and I am now able to speak with considerable confidence about points which were in some degree doubtful three months ago.

Let us consider, in the first place, what is the moon's probable age, not in years, but in development. Here we have only probable evidence to guide us, evidence chiefly derived from the a.n.a.logy of our own earth. At least, we have only such evidence when we are enquiring into the moon's age as a preliminary to the consideration of her actual aspect and its meaning. No doubt many features revealed by telescopic scrutiny are full of significance in this respect. No one who has ever looked at the moon, indeed, with a telescope of great power has failed to be struck by the appearance of deadness which her surface presents, or to be impressed (at a first view, in any case), with the idea that he is looking at a world whose period of life must be set in a very remote antiquity. But we must not take such considerations into account in discussing the _a priori_ probabilities that the moon is a very aged world. Thus we have only evidence from a.n.a.logy to guide us in this part of our enquiry. I note the point at starting, because the indicative mood is so much more convenient than the conditional, that I may frequently in this part of my enquiry use the former where the actual nature of the evidence would only justify the latter. Let it be understood that the force of the reasoning here depends entirely on the weight we are disposed to allow to arguments from a.n.a.logy.

a.s.suming the planets and satellites of the solar system to be formed in some such manner as Laplace suggested in his "Nebular Hypothesis," the moon, as an orb travelling round the earth, must be regarded as very much older than she is, even in years. Even if we accept the theory of accretion which has been recently suggested as better according with known facts, it would still follow that probably the moon had existence, as a globe of matter nearly of her present size, long before the earth had gathered in the major portion of her substance. Necessarily, therefore, if we a.s.sume as far more probable than either theory that the earth and moon attained their present condition by combined processes of condensation and accretion, we should infer that the moon is far the older of the two bodies in years.

But if we even suppose that the earth and moon began their career as companion planets at about the same epoch, we should still have reason to believe that these planets, equal though they were in age so far as mere years are concerned, must be very unequally advanced so far as development is concerned, and must therefore in that respect be of very unequal age.

It was, I believe, Sir Isaac Newton who first called attention to the circ.u.mstance that the larger a planet is, the longer will be the various stages of its existence. He used the same reasoning which was afterwards urged by Buffon, and suggested an experiment which Buffon was the first to carry out. If two globes of iron, of unequal size, be heated to the same degree, and then left to cool side by side, it will be found that the larger glows with a ruddy light after the smaller has become quite dark, and that the larger remains intensely hot long after the smaller has become cool enough to be handled. The reason of the difference is very readily recognised. Indeed, Newton perceived that there would be such a difference before the matter had been experimentally tested. The quant.i.ty of heat in the unequal globes is proportional to the volume, the substance of each being the same. The heat is emitted from the surface, and at a rate depending on the extent of surface. But the volume of the larger exceeds that of the smaller in greater degree than the surface of the larger exceeds the surface of the other. Suppose, for instance, the larger has a diameter twice as great as that of the smaller, its surface is four times as great as that of the smaller, its volume eight times as great.

Having, then, eight times as much heat as the smaller at the beginning, and parting with that heat only four times as fast as the smaller, the supply necessarily lasts twice as long; or, more exactly, each stage in the cooling of the larger lasts twice as long as the corresponding stage in the cooling of the smaller. We see that the duration of the heat is greater for the larger in the same degree that the diameter is greater.

And we should have obtained the same result whatever diameters we had considered. Suppose, for instance, we heat two globes of iron, one an inch in diameter, the other seven inches, to a white heat. The surface of the larger is forty-nine times that of the smaller, and thus it gives out at the beginning, and at each corresponding stage of cooling, forty-nine times as much heat as the smaller. But it possesses at the beginning three hundred and forty-three (seven times seven times seven) times as much heat. Consequently, the supply will last seven times as long, precisely as a stock of three hundred and forty-three thousand pounds, expended forty-nine times as fast as a stock of one thousand pounds only, would last seven times as long. In every case we find that the duration of the heat-emission for globes of the same material equally heated at the outset is proportional to their diameters.

Now, before applying this result to the case of the moon, we must take into account two considerations:--First, the probability that when the moon was formed she was not nearly so hot as the earth when it first took planetary shape; and secondly, the different densities of the earth and moon.

The original heat of every member of the solar system, including the sun, depended on the gravitating energy of its own ma.s.s. The greater that energy, the greater the heat generated either by the process of steady contraction imagined in Laplace's theory, or by the process of meteoric indraught imagined in the aggregation theory. To show how very different are the heat-generating powers of two very unequal ma.s.ses, consider what would happen if the earth drew down to its own surface a meteoric ma.s.s which had approached the earth under her own attraction only. (The case is of course purely imaginary, because no meteor can approach the earth which has not been subjected to the far greater attractive energy of the sun, and does not possess a velocity far greater than any which the earth herself could impart). In this case such a ma.s.s would strike the earth with a velocity of about seven miles per second, and the heat generated would be that due to this velocity only. Now, when a meteor strikes the sun full tilt after a journey from the star depths under his attraction, it reaches his surface with a velocity of nearly three hundred and sixty miles per second. The heat generated is nearly fifty times greater than in the imagined case of the earth. The moon being very much less than the earth, the velocity she can impart to meteoric bodies is still less. It amounts, in fact, to only about a mile per second. The condensing energy of the moon in her vaporous era was in like manner far less than that of the earth, and consequently far less heat was then generated. Thus, although we might well believe on _a priori_ grounds, even if not a.s.sured by actual study of the lunar features, that the moon when first formed as a planet had a surface far hotter than molten iron, we must yet believe that, when first formed, the moon had a temperature very much below that of our earth at the corresponding stage of her existence.

On this account, then, we must consider that the moon started in planetary existence in a condition as to heat which our earth did not attain till many millions, probably hundreds of millions of years after the epoch of her first formation as a planet.

As regards the moon's substance, we have no means of forming a satisfactory opinion. But we shall be safe in regarding quant.i.ty of matter in the moon as a safer basis of calculation than volume, in comparing the duration of her various stages of development with those of our own earth.

When, in the August number of this Magazine, I adopted a relation derived from the latter and less correct method, it was because the more correct method gave the result most favourable to the argument I was then considering. The same is indeed the case now. Yet it will be better to adopt the more exact method, because the consideration relates no longer to a mere side issue, but belongs to the very essence of my reasoning.

The moon has a ma.s.s equal to about one eighty-first part of the earth's.

Her diameter being less than the earth's, about as two to seven, the duration of each stage of her cooling would be in this degree less than the corresponding duration for the earth, if her density were the same as the earth's, in which case her ma.s.s would be only one forty-ninth part of the earth's. But her ma.s.s being so much less, we must a.s.sume that her amount of heat at any given stage of cooling was less in similar degree than it would have been had her density been the same as the earth's. We may, in fact, a.s.sume that the moon's total supply of heat would be only one eighty-first of the earth's if the two bodies were at the same temperature throughout.[63] But the surface of the moon is between one-thirteenth and one-fourteenth of the earth's. Since, then, the earth at any given stage of cooling parted with her heat between thirteen and fourteen times as fast as the moon, but had about eighty-one times as much heat to part with (for that stage), it follows that she would take about six times as long (six times thirteen and a-half is equal to eighty-one) to cool through that particular stage as the moon would.

If we take this relation as the basis of our estimate of the moon's age, we shall find that, even if the moon's existence as a planet began simultaneously with the earth's instead of many millions of years earlier, even if the moon was then as hot as the earth instead of being so much cooler that many millions of years would be required for the earth to cool to the same temperature--making, I say, these a.s.sumptions, which probably correspond to the omission of hundreds of millions of years in our estimate of the moon's age, we shall still find the moon to be hundreds of millions of years older than the earth.

Nay, we may even take a position still less favourable to my argument. Let us overlook the long ages during which the two orbs were in the vaporous state, and suppose the earth and moon to be simultaneously in that stage of planetary existence when the surface has a temperature of two thousand degrees Centigrade.

From Bischoff's experiments on the cooling of rocks, it appears to follow that some three hundred and twenty millions of years must have elapsed between the time when the earth's surface was at this temperature and the time when the surface temperature was reduced to two hundred degrees Centigrade, or one hundred and eighty degrees Fahrenheit above the boiling point. The earth was for that enormous period a ma.s.s (in the main) of molten rock. In the moon's case this period lasted only one-sixth of three hundred and twenty million years, or about fifty-three million years, leaving two hundred and sixty-seven million years' interval between the time when the moon's surface had cooled down to two hundred degrees Centigrade and the later epoch when the earth's surface had attained that temperature.

I would not, however, insist on these numerical details. It has always seemed to me unsafe to base calculations respecting suns and planets on experiments conducted in the laboratory. The circ.u.mstances under which the heavenly bodies exist, regarding these bodies as wholes, are utterly unlike any which can be produced in the laboratory, no matter on what scale the experimenter may carry on his researches. I have often been amused to see even mathematicians of repute employing a formula based on terrestrial experiments, physical, optical, and otherwise, as though the formula were an eternal omnipresent reality, without noting that, if similarly applied to obtain other determinations, the most stupendously absurd results would be deduced. It is as though, having found that a child grows three inches in the fifth year of his age, one should infer not only that that person but every other person in every age and in every planet, nay, in the whole universe, would be thirty inches taller at the age of fifteen than at the age of five, without noticing that the same method of computation would show everyone to be more than fifteen feet taller at the age of sixty-five. It may well be that, instead of three hundred and twenty millions of years, the era considered by Bischoff lasted less than a hundred millions of years. Or quite as probably it may have lasted five or six hundred millions of years. And again, instead of the corresponding era of the moon's past history having lasted one sixth of the time required to produce the same change in the earth's condition, it may have lasted a quarter, or a third, or even half that time, though quite as probably it may have lasted much less than a sixth. But in any case we cannot reasonably doubt that the moon reached the stage of cooling through which the earth is now pa.s.sing many millions of years ago. We shall not probably err very greatly in taking the interval as at least two hundred millions of years.

But I could point out that in reality it is a matter of small importance, so far as my present argument is concerned, whether we adopt Bischoff's period or a period differing greatly from it. For if instead of about three hundred millions the earth required only thirty millions of years to cool from a surface temperature of two thousand degrees Centigrade to a temperature of two hundred degrees, we must a.s.sume that the rate of cooling is ten times greater than Bischoff supposed. And we must of course extend the same a.s.sumption to the moon. Now, since the sole question before us is to what degree the moon has cooled, it matters nothing whether we suppose the moon has been cooling very slowly during many millions of years since she was in the same condition as the earth at present, or that the moon has been cooling ten times as quickly during a tenth part of the time, or a hundred times as quickly during one-hundredth part of the time.

We may, therefore, continue to use the numbers resulting from Bischoff's calculation, even though we admit the probability that they differ widely from the true values of the periods we are considering.

Setting the moon, then, as about two hundred and fifty millions of years in advance of the earth in development, even when we overlook all the eras preceding that considered by Bischoff, and the entire sequent interval (which must be long, for the earth has no longer a surface one hundred degrees Centigrade hotter than boiling water), let us consider what is suggested by this enormous time-difference.

In the first place, it corresponds to a much greater interval in our earth's history. During the two hundred and fifty millions of years the moon has been cooling at her rate, not at the earth's. According to the conclusion we deduced from the moon's relative ma.s.s and surface, she has aged as much during those two hundred and fifty million years as the earth will during the next fifteen hundred million years.

Now, however slowly we suppose the earth's crust to be changing, it must be admitted that in the course of the next fifteen hundred millions of years the earth will have parted with far the greater part, if not with the whole, of that inherent heat on which the present movements of her surface depend. We know that these movements at once depend upon and indicate processes of contraction. We know that such processes cannot continue at their present rate for many millions of years. If we a.s.sume that the rate of contraction will steadily diminish--which is equivalent, be it noticed, to the a.s.sumption that the earth's vulcanian or subterranean energies will be diminished--the duration of the process will be greater. But even on such an a.s.sumption, controlled by consideration of the evidence we have respecting the rate at which terrestrial contraction is diminis.h.i.+ng, it is certain that long before a period of fifteen hundred millions of years has elapsed, the process of contraction will to all intents and purposes be completed.

We must a.s.sume, then, as altogether the most probable view, that the moon has reached this stage of planetary decrepitude, even if she has not become an absolutely dead world. We can hardly reject the reasoning which would show that the moon is far older than has been a.s.sumed when long stages of her history and our earth's have been neglected. Still less reasonable would it be to reject the conclusion that at the very least she has reached the h.o.a.r antiquity thus inferred. a.s.suming her to be no older, we yet cannot escape the conviction that her state is that of utter decrepitude. To suppose that volcanic action can now be in progress on the moon, even to as great a degree as on the earth, would be to a.s.sume that measurable sources of energy can produce practically immeasurable results.

But no volcanic changes now in process on the earth could possibly be discernible at the moon's distance. How utterly unlikely does it seem, then, that any volcanic changes can be now taking place on the moon which could be recognized from the earth! It seems safe to a.s.sume that no volcanic changes at all can be in progress; but most certainly the evidence which should convince us that volcanic changes of so tremendous a character are in progress that at a distance of two hundred and sixty thousand miles terrestrial telescopists can discern them, must be of the strongest and most satisfactory character.

Evidence of change may indeed be discovered which can be otherwise explained. The moon is exposed to the action of heat other than that which pervaded her own frame at the time of her first formation. The sun's heat is poured upon the moon during the long lunar day of more than a fortnight, while during the long lunar night a cold prevails which must far exceed that of our bitterest arctic winters. We know from the heat-measurements made by the present Lord Rosse, that any part of the moon's surface at lunar mid-day is fully five hundred degrees Fahrenheit hotter than the same part two weeks later at lunar midnight. The alternate expansions and contractions resulting from these changes of temperature cannot but produce changes, however slowly, in the contour of the moon's surface. Professor Newcomb, indeed, considers that all such changes must long since have been completed. But I cannot see how they can be completed so long as the moon's surface is uneven, and at present there are regions where that surface is altogether rugged. Mighty peaks and walls exist which must one day be thrown down, so unstable is their form; deep ravines can be seen which must one day be the scene of tremendous landslips, so steep and precipitous are their sides. Changes such as these may still occur on so vast a scale that telescopists may hope from time to time to recognise them. But changes such as these are not volcanic; they attest no lunar vitality. They are antecedently so probable, indeed, while volcanic changes are antecedently so unlikely, that when any change is clearly recognised in the moon's surface, nothing but the most convincing evidence could be accepted as demonstrating that the change was of volcanic origin and not due to the continued expansion and contraction of the lunar crust.

And now let us see how stands the evidence in the few cases which seem most to favour the idea that a real change has taken place.

We may dismiss, in the first place, without any hesitation, the a.s.sertion that regular changes take place in the floor of the great lunar crater Plato. According to statements very confidently advanced a few years ago, this wide circular plain, some sixty miles in diameter, grows darker and darker as the lunar day advances there until the time corresponding to about two o'clock in the afternoon, and then grows gradually lighter again till eventide. The idea seems to have been at first that some sort of vegetation exists on the floor of this mighty ring-shaped mountain, and that, as the sun's heat falls during the long lunar day upon the great plain, the vegetation flourishes, darkening the whole region just as we might imagine that some far-extending forest on the earth would appear darker as seen from the moon when fully clothed with vegetation than when the trees were bare and the lighter tints of the ground could be seen through them. Another idea was that the ground undergoes some change under the sun's heat corresponding to those which are produced in certain substances employed in photography; though it was not explained why the solar rays should produce no permanent change, as in the terrestrial cases adduced in ill.u.s.tration. Yet another and, if possible, an even stranger explanation, suggested that, though the moon has no seas, there may be large quant.i.ties of water beneath her crust, which may evaporate when that crust becomes heated, rising in the form of vapour to moisten and so darken the crust. Certainly, the idea of a moistening of the lunar crust, or of portions thereof, as the sun's rays fall more strongly upon it, is so daring that one could almost wish it were admissible, instead of being altogether inconsistent, as unfortunately it is, with physical possibilities.

But still more unfortunately, the fact supposed to have been observed, on which these ingenious speculations were based, has not only been called in question, but has been altogether negatived. More exact observations have shown that the supposed darkening of the floor of Plato is a mere optical illusion. When the sun has lately risen at that part of the moon, the ringed wall surrounding this great plain throws long shadows across the level surface. These shadows are absolutely black, like all the shadows on the moon. By contrast, therefore, the unshadowed part of the floor appears lighter than it really is; but the mountain ring which surrounds this dark grey plain is of light tint. So soon as the sun has pa.s.sed high above the horizon of this region, the ring appears very brilliant compared with the dark plain which it surrounds; thus the plain appears by comparison even darker than it really is. As the long lunar afternoon advances, however, black shadows are again thrown athwart the floor, which therefore again appears by contrast lighter than it really is. All the apparent changes are such as might have been antic.i.p.ated by anyone who considered how readily the eye is misled by effects of contrast.

To base any argument in favour of a regular change in the floor of Plato on evidence such as this, would be as unwise as it would be to deduce inferences as to changes in the heat of water from experiments in which the heat was determined by the sensations experienced when the hands were successively immersed, one hand having previously been in water as hot as could be borne, the other in water as cold as could be borne. We know how readily these sensations would deceive us (if we trusted them) into the belief that the water had warmed notably during the short interval of time which had elapsed between the two immersions; for we know that if both hands were immersed at the same moment in lukewarm water, the water would appear cold to one hand and warm to the other.

Precisely as in such a case as we have just considered, if we were obliged to test the water by so inexact a method, we should make experiments with one hand only, and carefully consider the condition of that hand during the progress of the experiments, so in the case of the floor of Plato, we must exclude as far as possible all effects due to mere contrast. We must examine the tint of the plain, at lunar morning, mid-day, and evening, with an eye not affected either by the darkness or brightness of adjacent regions, or adjacent parts of the same region. This is very readily done.

All we have to do is to reduce the telescopic field of view to such an extent that, instead of the whole floor, only a small portion can be seen.

It will then be found, as I can myself certify (the more apparently because the experience of others confirms my own), that the supposed change of tint does not take place. One or two who were and are strong believers in the reality of the change do indeed a.s.sert that they have tried this experiment, and have obtained an entirely different result. But this may fairly be regarded as showing how apt an observer is to be self-deceived when he is entirely persuaded of the truth of some favourite theory. For those who carried out the experiment successfully had no views one way or the other; those only failed who were certainly a.s.sured beforehand that the experiment would confirm their theory.

The case of the lunar crater Linn, which somewhere about November 1865 attracted the attention of astronomers, belongs to a very different category. In my article on the moon in the "Contemporary Review" I have fully presented the evidence in the case of this remarkable object. I need not therefore consider here the various arguments which have been urged for and against the occurrence of change. I may mention, however, that, in my anxiety to do full justice to the theory that change has really occurred, I took Mdler's description of the crater's interior as "very deep," to mean more than Mdler probably intended. There is now a depression several hundred yards in depth. If Mdler's description be interpreted, as I interpreted it for the occasion in the above article, to mean a depth of two or three miles, it is of course certain that there has been a very remarkable change. But some of the observers who have devoted themselves utterly, it would seem, to the lively occupation of measuring, counting, and describing the tens of thousands of lunar craters already known, a.s.sert that Mdler and Lohrman (who uses the same description) meant nothing like so great a depth. Probably Mdler only meant about half a mile, or even less. In this case their favourite theory no longer seems so strongly supported by the evidence. In some old drawings by the well-known observer Schrter, the crater is drawn very much as it now appears. Thus, I think we must adopt as most probable the opinion which is, I see, advanced by Prof. Newcomb in his excellent "Popular Astronomy,"

that there has been no actual change in the crater. I must indeed remark that, after comparing several drawings of the same regions by Schrter, Mdler, Lohrman, and Schmidt, with each other and with the moon's surface, I find myself by no means very strongly impressed by the artistic skill of any of these observers. I scarcely know a single region in the moon where change might not be inferred to have taken place if any one of the above-named observers could be implicitly relied upon. As, fortunately, their views differ even more widely _inter se_ than from the moon's own surface, we are not driven to so startling a conclusion.