(Copyright 1888 by the Author.)

Otago Witness, Issue 1918, 24 August 1888, Page 32

(Copyright 1888 by the Author.)

The moon, regarded as an object of physical study, occupies a unique position among the members of our solar syscem. Some of the dther planets — for the moon be it remembered is a planet, a, companion world of the earth's, no mere satellite — tell us of the past of our earth's career ; others again tell us of the future; bub the moon speaks both of the past and future of world life. We can study her surface to learn what our earth will be like when old age and death overtake her ; but we can also study the moon's surface to 'learn something about our earth's condition in the past. ' She retains the record of stages of life through which she passed millions of years ago, and through which our earth has also passed, but from the earth's face the records of these past stages have been in great degree removed, or blurred and defaced till we are unable to read them aright. Let us consider the moon in both aspects, beginning with the lessons she teaches us respecting the past. It is easy to see why the moon's surface should be much less affected by atmospheric and aqueous wear and tear during the past ages, when she had an atmosphere and when she had seas. In the first place the period •of time during which she was thus clothed with air and water must have been much shorter than the corresponding period for the earth. As much the smaller orb, having in fact but the eighty-first part of the earth's mass, she would necessarily rool Far More Rapidly— probably (through I will not here enter on the calculations which indicate the probability) about six times faster than the earth ; so that if we set the past duration of the earth's fitness for life at 60,000,000 years, which geologists would regard as a moderate estimate for the duration of the Archraan, primary, secondary, tertiary and recent eras, we should have to regard 10,000,000 years as the duration of the corresponding part of the moon's life. The work of denudation which has gone on upon the earth during the past 60,000,000 years could not possibly have been accomplished upon the moon during 10,000,000 years, even though the work had proceeded, century by century, at the same rate. But it could not proceed nearly so fast on the moon as on the earth. " To begin with, there must have been much less air and water per square mile of the moon's surface than on the earth's in corresponding portions of the life of each. Assuming the air and water to be the same proportions of the total mass of each planet, fche earth had 81 times as much air and water as the moon, and the earth's atmosphere and water covered a surface not 81 times but only 13| times the surface of the moon : so that on"our reasonable assumption as to the quantity of air and water present on the two planets, the earth had six times as much air and water over each square mile of her surface as the moon had at the cor- ' responding stages of her life as a world. With a sixtli part only of these, the instruments of denudation, the moon had also but a sixth part of the energy with which to work them. Gravity on the earth is six times as strong as gravity on the moon ; and gravity on each planet was the hand by which air and water, the tools of denudation, were ' worked. With tools having but one-sixth the efficiency, the force of lunar gravity having but one-sixth, the energy of terrestrial gravity, and working for but a sixth part of the time, could do no such work in destroying the primeval features of the planet as has been accomplished on our earth. We may then expect to find in the -crust of the moon, stripped as it now is cf the air "and ' water which once surrounded it, the records of the original rock surface corresponding to the Arcbjean rocks' .of our earth/ Geologists admit that the condition of the earth's surface before the primary strata began to be- formed cannot be satisfactorily determined. Although certain parts of the still remaining crust are called Archscan, it is very doubtful whether they really represent the primeval crust. There is a much better chance that, in portions of the moon's surface the primeval rock will still remain in something like its primeval condition ; so that we may be able to read a portion of Tlic Past History of the Earth better in our neighbour world the moon than in our own, just as we can form a better idea of the nature of our savage ancestry by studying the ways and manners of less civilised races of to-day than from any evidence that we can obtain from the study oE the records of our own race. - Thus studying the moon we see the traces of the earlier vulcanian stages of her career, stages such as our earth and every planet passes through. We oec first how the still plastic crust was riven for distances of many hundreds of miles by the action of the interior forces ; and we can see where molten matter flowed through those rifts and spread in ribbon-like strips over the "hidden seams. Later, as the process of contraction bee'an chieily to afroct the crust, while the nuclear matter .retreated from it,, we see how immense novrugalions were formed, akin to those or" v wrinkled skin or of a shrivelled apple. •Of these adages of earth-life traces still remain. Geologists had indeed already shown

"that the earth 'must have passed through them. But the evidence in the earth's case is recondite and ociscure. 1 In the moon's it is striking and obvious. Geologists had to study the structure of strata deep down! below those whioh form the main portions of mountain masses before they came on evidence of that primeval rifting, followed by the outflow of molten matter, which led to the sinking of the crust -in the parts thus broken and overloaded, and to their receiving, larger supplies of matter deposited on the trough-like portions of the sea floor thus formed. Of the original corrugations'formed later, as the ancient cru3t contracted, we have evidence in the existence of mountain ranges ; not that mountain ranges are themselves such corrugations, as was once supposed, but that it has been by the contraction of the crust, necessarily leading to a crumpling and wrinkling of its substance, that the 'material of the mountain ranges was raised above the sea' level. This material was deposited originally in the trough-like depressions just referred to, to depths amounting in some cases to 10 or 12 miles. But under the side pressures exerted on the contra'ctirjg cruse these immense seams of matter were shouldered, so fco speak, above the mean level, to greater and greater heights until those pressures had expended their energies. On the mooTi we can in many places see the corrugations themselves into which the crusfe was crumpled in the process of contraction; On the moon, again, we find An Immense Number of Craters, ranging in size from mere saucer-shaped depressions less than half a mile wide to gigan-, lie craters having a span of more than 10Q miles. In height the crater rings ran^e, from a few hundred yards to 4000 or 5000. Now on the earth there are many craters, but none which either resemble the great lunar craters in form or can be compared with them for size. Yet we cannot reasonably doubt that in earlier stages of her vulcanian past the earth had such craters as still remain on the moon. There are indeed parts of the earth where the tiaces of such craters still remain. In the Isle of Mull, for instance (in the Innei; Hebrides), there is the basal wreck of a great volcanic ring which must have been 30 miles wide, and 12 or 13 thousand feet high. I ought rather perhaps to say that the Isle of Mull itself is the basal wreck of that crater, long since reduced to a mere ring of hills by the denuding forces of air and water. And while the moon is thus interesting and instructive in regard to the past, she has much to tell us also respecting The Future of our Earth. To begin with, it is clear that the moon once had seas. We can see the floors of those ancient seas in the regions still called " seas," whose uniformity of level and peculiarities of tint can be explained in no other way. The mere fact of course that certain tracts of the moon are smoother and darker than others would mean little. But when we notice that these broad, dark, and relatively smooth tracts all lie at lower levels than the brighter or whiter tracts, we see this can not be a mere coincidence. If water were poured upon the moon now in sufficient quantity, the water would occupy just those parts of the moon which are dark and leave uncovered just those parts which are light. No reasonable explanation of this peculiarity can be found except the supposition that seas formerly existed on the moon, occupying those regions which thus attest by their darker tints, by their uniformity of surface, and by their lower level, that they were formerly the floors. of ancient lunar seas. From these regions the seas have retreated, soaked up, as ij; were, within the moon's interior. But the process musL have been a slow one, lasting doubtless for millions .of years, during which the lunar air also slowly (but in such long periods effectively) diminished in amount. When at length the process was completed, the moon had become the scone of desolation and of death which she presents now. She " being . dead yet speaketh," however, and the lesson she teaches is that our earth must likewise die. The moon is a heavenly record telling us that all created things, planet and sun as well as plant and animal, can last but for awhile, though the measure oE the lives of some be longer than the measure of the lives of others. It may be objected that we have no evidence to show that some forms of life may not still exist upon the moon, though none of the higher forms of life known upon the earth could endure under conditions so unfavourable. This of course must be admitted as a possibility. But since the theory of biological evolution. has. been established, there is a limit to the play of fancy in this direction. We are not to imagine beings specially created to be fitted for life on such a world as is our moon at present. All we can allow ourselves to conceive is the possibility that creatures may exist upon the moon who are the exceedingly remote descendants of creatures which existed. on her surface or in her seas in the time when she was a miniature world. We may ask ourselves whether, as our earth grows cold and dry and almost airless with advancing ages, the various forms of life existing on her surface, even man himself, might net steadily change to correspond, until at last when every trace of water had disappeared and the air had become so thin that no creatures known to us now could breathe it and live, Itaces Utterly Unlike Those Now Existing though descended from them, would people the earth with countless millions of living creatures, while possibly many forms of vegetable life corresponding with the changed conditions would have come into existence. But though this is possible it cannot be regarded as probable. We know that there are limits to the possibilities of life upon this earth, and that Leyond those limits ev%n the amazing powers 'of development given- to the various forms of animal and vegetable life cannot avail to keep them in existence; and we cannot but believe thai- these possibilities' lie far within the range of those seejningly decisive and life-destructive changes of which the moon's desolate surface speaks. Assuredly wo can glean no hope from the fancy, once supported by astronomers of repute, that though the moon's hither side be dead her farther t>ide may be dotted with air aud water, and so may be the abode of life. This theory never had any sufficieut evidence even for its bare possibility, when considered

only*' with reference to tKe existence of Water Bat when the question of an atmosphere is considered, the theory of an inhabited farther side of the moon is seen to be wholly untenable. Such an atmosphere could not be concealed from our ken even if the moon did not librate; but when we consider how the moon's librations bring large parts of her farther hemisphere into view, insomuch that instead of seeing but one-half we see about foursevenths of her surface, we perceive that an atmosphere even of the most moderate density could not by any possibility be so limited to a part of the moon's farther hemisphere that no traca of its existence would be recognised from the earth. But as a matter of fact Hansen's theory of the possible existence of water and air on the farther side of the mpon has long since been abandoned by' all except a few who are not sufficiently acquainted with physical laws to weigh this theory right. The fact that Sir John Herschel, who had at first been attracted by the theory, and has even introduced it into his " Outlines of Astronomy," eventually gave it up because he saw that the imagined atmosphere could not possibly remain concealed during the lunar librations, should be regarded as of itself sufficing to show how untenable the theory is. But that peculiarity of rotation which causes the moon always to turn the same face earthward, save for slight changes due to her librations, is of itself one of the points of evidence in regard to the moon's age and to her present condition. This peculiarity can only be explained as due to the gradual action of forces akin to those by which The Earth's Rotation-spin is llciu^ Slowly Reduced. This is chiefly due to the lrictional action of the tidal"wave in a direction contrary to that of the 1 earth's rotation. The effect is very small. Millions of years will be required before the length of our' clay will be increased into coincidence with the length of the lunar month. (Sir Robert flail speaks of an even greater change corresponding to the solar tidal wave, forgetting thao the lunar action, being the stronger, will prevent the solar action frcm producing any effect after once the moon's full effect has been produced.) Bui the moon shows us by her unvarying face that she has had t) obey the 1 I slowly acting forces exerted on her by the earth. And while she thus tell us how very, very old she is she assures for our earth corresponding changes (which are of themselves death dealing) when the earth has reached the corresponding age. Richard A. Pitocrcm.