Part 16

However, there was no help for it; the ship 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 fishing junk or two, and all but sunk the lightship; 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 nobody 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 ships in harbour and of a large crowd gathered on the "Bund;" the brave company on board were now assured that the judgment of the gods 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 Japanese 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 multitudinous 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 masses 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 masses, 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 classed 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 multitudes of small lunar craters, say from a quarter of a mile to two miles in diameter, may have been formed by falling meteoric masses 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 space. 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 quantities 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 analogy 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 analogy 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 analogy.

Assuming 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 assume 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 circumstance 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 quantity 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 mass. 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 masses, consider what would happen if the earth drew down to its own surface a meteoric mass 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 mass 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 assured 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 quantity 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 mass 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 mass would be only one forty-ninth part of the earth's. But her mass being so much less, we must assume 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, assume 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 assumptions, 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 mass (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 circumstances 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 passing 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 assume that the rate of cooling is ten times greater than Bischoff supposed. And we must of course extend the same assumption 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.