Part 8

I do not, however, in this place enter specially into the consideration of the moon's stage of volcanic activity, because already, in the pages of my Treatise on the Moon (Chapter VI.), I have given a full account of that portion of my present subject. I may make a few remarks, however, on the theory respecting lunar craters touched on in my work on 'The Moon.' I have mentioned the possibility that some among the enormous number of ring-shaped depressions which are seen on the moon's surface may have been the result of meteoric downfalls in long past ages of the moon's history. One or two critics have spoken of this view as though it were too fantastic for serious consideration. Now, though I threw out the opinion merely as a suggestion, distinctly stating that I should not care to maintain it as a theory, and although my own opinion is unfavourable to the supposition that any of the more considerable lunar markings can be explained in the suggested way, yet it is necessary to notice that on the general question whether the moon's surface has been marked or not by meteoric downfalls scarcely any reasonable doubts can be entertained. For, first, we can scarcely question that the moon's surface was for long ages plastic, and though we may not assign to this period nearly so great a length (350 millions of years) as Tyndall--following Bischoff--assigns to the period when our earth's surface was cooling from a temperature of 2000° C. to 200°, yet still it must have lasted millions of years; and, secondly, we cannot doubt that the process of meteoric downfall now going on is not a new thing, but, on the contrary, is rather the final stage of a process which once took place far more actively. Now Prof. Newton has estimated, by a fair estimate of observed facts, that each day on the average 400 millions of meteors fall, of all sizes down to the minutest discernible in a telescope, upon the earth's atmosphere, so that on the moon's unprotected globe--with its surface one thirteenth of the earth's--about 30 millions fall each day, even at the present time. Of large meteoric masses only a few hundreds fall each year on the earth, and perhaps about a hundred on the moon; but still, even at the present rate of downfall, millions of large masses must have fallen on the moon during the time when her surface was plastic, while probably a much larger number--including many much larger masses--must have fallen during that period. Thus, not only without straining probabilities, but by taking only the most probable assumptions as to the past, we have arrived at a result which compels us to believe that the moon's surface has been very much marked by meteoric downfall, while it renders it by no means unlikely that a large proportion of the markings so left would be discernible under telescopic scrutiny.

I would, in conclusion, invite those who have the requisite leisure to a careful study of the distribution of various orders of lunar marking. It would be well if the moon's surface were isographically charted, and the distribution of the seas, mountain ranges, and craters of different dimensions and character, of rills, radiating streaks, bright and dark regions, and so on, carefully compared inter se, with the object of determining whether the different parts of the moon's surface were probably brought to their present condition during earlier or later periods, and of interpreting also the significance of the moon's characteristic peculiarities. In this department of astronomy, as in some others, the effectiveness of well-devised processes of charting has been hitherto overlooked.

FOOTNOTES:

A NEW CRATER IN THE MOON.

Dr. Klein, a German astronomer, has recently called the attention of astronomers to a lunar crater some three miles wide, which had not before been observed, and which, he feels sure, was not in existence two years ago. Astronomers have long since given up all hope of tracing either the signs of actual life upon the moon or traces of the past existence of living creatures there. But there are still among them those who believe that by sedulous and careful scrutiny processes of material change may be recognised in that seemingly inert mass. In reality, perhaps, the wonder rather is that signs of change should not be often recognised, than that from time to time a new crater should appear or the walls of old craters fall in. The moon's surface is exposed to variations of temperature compared with which those affecting the surface of our earth are altogether trifling. It is true there is no summer or winter in the moon. Sir W. Herschel has spoken of the lunar seasons as though they resembled our own, but in reality they are very different. The sun's midday height at any lunar station is only about three degrees greater in summer than in winter; whereas our summer sun is 47° higher in the sky at noon than our winter sun. In fact, a midsummer's day on the moon does not differ more from a midwinter's day, as far as the sun's actual path on the sky is concerned, than with us the 17th of March differs from the 25th, or the 19th of September from the 27th. It is the change from day to night which chiefly affects the moon's surface. In the lunar year of seasons, lasting 346-2/3 of our days, there are only 11-3/4 lunar days, each lasting 29-3/4 of ours. Thus day lasts more than a fortnight, and is followed by a night of equal length. Nor is this all. There is neither air nor moisture to produce such effects as are produced by our air and the moisture it contains in mitigating the heat of day and the cold of night. Under the sun's rays the moon's surface becomes hotter and hotter as the long lunar day proceeds, until at last its heat exceeds that of boiling water. But so soon as the sun has set the heat thus received is rapidly radiated away into space (no screen of moisture-laden air checking its escape), and long before lunar midnight a cold exists compared with which the bitterest weather ever experienced by Arctic voyagers would be oppressively hot. These are not merely theoretical conclusions, though even as such they could be thoroughly relied upon. The moon's heat has been measured by the present Lord Rosse (using his father's splendid six-feet mirror). He separated the heat which the moon simply reflects to us from that which her heated surface itself gives out (or, technically, he separated the reflected from the radiated heat), by using a glass screen which allowed the former heat to pass while it intercepted the latter. He thus found that about six-sevenths of the heat we receive from the moon is due to the heating of her own substance. From the entire series of observations it appeared that the change of temperature during the entire lunar day--that is, from near midnight to near midday on the moon--amounts to fully 500° Fahrenheit. If we assume that the cold at lunar midnight corresponds with about 250° below zero (the greatest cold experienced in Arctic travelling has never exceeded 140° below zero), it would follow that the midday heat is considerably greater than that of boiling water on the earth at the sea-level. But the range of change is not a matter of speculation. It certainly amounts to about 500°, and in whatever way we distribute it, we must admit, first, that no such life as we are familiar with could possibly exist on the moon; and, secondly, that the moon's crust must possess a life of its own, so to speak, expanding and contracting unceasingly and energetically. Professor Newcomb, by the way, in his fine work on Popular Astronomy, rejects the idea that the expansions and contractions due to these great changes of temperature can cause any disintegration at the present time. There might, he says, be bodies so friable that they would crumble, 'but whatever crumbling might thus be caused would soon be done with, and then no further change would occur.' For my own part, I cannot consider that such a surface as the moon at present possesses can undergo these continual expansions and contractions without slow disintegration. It seems to me also extremely probable that from time to time the overthrow of great masses, the breaking up of arched crater-floors, and other sudden changes discernible from the earth, might be expected to occur. Professor Newcomb has, I conceive, omitted to consider the enormous volumetric expansion as distinguished from mere lateral extension, resulting from the heating of the moon's crust to considerable depths. On a very moderate computation, the surface of the central region of the full moon must at that time rise above its mean position to such a degree that hundreds, if not thousands, of cubic miles of the moon's volume lie above the mean position of the surface there. At new moon--that is, at lunar midnight for the same region--the same enormous quantity of matter is correspondingly depressed. And though the actual range in vertical height at any given point may be small, we cannot doubt that the total effect produced by these constant oscillations is considerable. Years or centuries may pass without any great or sudden change, but from time to time such catastrophes must surely occur. I believe that all the cases of supposed change in the moon, if all were regarded as proved, could be thus fully accounted for without any occasion to assume the action of volcanic forces properly so called.

Before considering the evidence for the new lunar volcano to which Dr. Klein has recently called the attention of astronomers, it may be well briefly to describe the condition of the moon's surface.

This surface, which is equal in extent to about that of the American Continent, or to Europe and Africa together (without their islands), is divided into two chief portions--the higher levels, which are in the main of lighter tint, and the lower levels, which are, almost without exception, dark. It may be remarked in passing that very erroneous ideas are commonly entertained respecting the moon's general colour. The moon is very far from being white, as many suppose. On the contrary, she is far more nearly black than white. It has been well remarked by Tyndall that if the moon were covered with black velvet (14,600,000 square miles of that material would be required for the purpose), she would still appear white to us, for we should see her disc projected on the blackness of star-strewn space. The actual tint of the moon as a whole is nearly the same as that of gray weathered sandstone. The brightest parts, however, are much whiter; Zöllner infers from his photometric experiments that they can be compared with the whitest of terrestrial substances. On the other hand, the darkest parts of the moon are probably far darker than porphyry, even if they are not so dark that on earth we should call them black. The fact that the low-lying parts of the moon are much darker than the higher regions is full of meaning, though hitherto its significance does not seem to have been much noticed. Either we must assume that these lower regions, the so-called seas (certainly now dry), are old sea bottoms, and owe their darkness to the quality of the matter deposited there in remote ages, or else we must suppose that the matter which last remained fluid when the moon's surface was consolidating was of darker material than the rest. For such matter would occupy the lowest lunar regions. There is here room for a very profitable study of the moon's aspect by geologists. I doubt not that, however different the general past history of our earth may have been from the moon's, terrestrial regions exist where the characteristic features of the moon's surface are more or less closely illustrated. On the American continent, for example, there are peculiarities of geological formation which seem to correspond closely with some of the features of the lunar globe, presently to be noticed; and it seems to me not improbable that geologists might find in the study of certain regions in North America the means of interpreting the difference of tint between higher and lower levels on the moon. If so, light would probably be thrown on very difficult questions relating to the remote past, not only of the moon, but of our own earth.

The lunar feature which comes next in importance to the difference of tint between the so-called 'seas' and the higher lands is the existence of remarkable series of radiating streaks extending from certain important craters--centres probably of past disturbance. It is impossible to contemplate the disc of the full moon, as seen with a powerful telescope, without feeling that these systems of rays must have resulted from the operation of forces of the most stupendous nature, though as yet their true meaning is hid from us. They would be marvellous phenomena, even if they were not so mysterious--marvellous in their enormous extension, their singular brightness, and their manner of traversing 'seas,' craters, and mountain-ranges indifferently. But their chief marvel resides in the mysterious manner of their appearance as the moon approaches her full illumination. Other lunar features are most clearly recognised when the moon is not full, for then the shadows which afford our only means of estimating the height of lunar irregularities are clearly seen along the border between the bright and dark parts of her face, and we have only to wait until this border passes over any object we wish to study to obtain satisfactory evidence of its nature. It is quite otherwise with the rays. The regions occupied by these radiating streaks are neither raised nor depressed in such sort as either to throw shadows or to lie in shadow when surrounding regions are in sunlight. But when the moon approaches her full illumination, the radiating regions come into view, as bright streaks--bright even on the light-tinted lunar uplands. A mighty system of rays can be seen extending from the great crater Tycho in all directions. Other systems, scarcely less wonderful, extend from the battlemented crater, Copernicus, the brilliant Aristarchus, and the solitary Kepler. One ray from Tycho can be traced round nearly an entire hemisphere of the moon's surface. It is specially noteworthy of this great ray that, where it crosses the lunar Sea of Serenity, that great plain seems to be divided as by a sort of ridge line, the slope of the plain from either side of the ray's track being clearly discernible when the moon is near her first quarter.

What are these mysterious ray systems? How are we to explain the circumstance that though only the most tremendous forces seem competent to account for bands such as these, many miles broad and many hundreds of miles in length, there are yet none of the usual signs of the action of volcanic forces? If mighty rifts had been formed in the moon's crust by the outbursting action of a hot nucleus, or through the contraction of the crust on an unyielding nucleus (for the effect would be the same in either case), we should scarcely expect to find that after such rifts had formed no signs of any difference of level would appear. If lava flowed out all along the rift, one would imagine that it would form a long dyke which would throw an obvious shadow, except under full solar illumination. If the rift were not filled with lava, the bottom of the rift would certainly remain in shadow long after the surrounding region was illuminated. That lava should exactly fill the rift along its entire length seems incredible. This might happen by a strange chance in the case of a single rift, but not with all the rifts of a radiating system, still less with all the rifts of all the radiating systems. Yet I believe that neither astronomers nor geologists can form any other opinion respecting these mysterious ray-systems than that they were caused by what Humboldt (speaking of the earth) calls the reaction of the interior on the crust. Nasmyth has admirably indicated their appearance, or rather their radiating form, by filling a globular glass shell with water, hermetically closed, and then freezing the water. The expansion of the water bursts the glass shell, and the lines of fracture are found to extend in a series of rays from the part of the shell which first gave way. But this experiment of itself does not explain the mystery of the lunar rays. Accepting the theory that the moon's crust yielded in some such way, we have still to explain how the rifts which were thus formed came to be covered over with matter lying nearly at the same level as the surrounding surface. It appears to me that the only available way of explaining this is somewhat as follows. First, from the way in which the streaks are covered like the surrounding region with craters, we may conclude that the streaks are older than any except the largest craters; from the great extension of many of them, we may safely infer that the lunar crust possessed a large measure of plasticity when they were formed (for otherwise it would have yielded over a smaller area). It was, therefore, probably still hot during the era (which may have lasted millions of years) to which the formation of the rifts belonged: accordingly the lava which flowed out through the rifts remained liquid for a considerable time, and was thus able to spread widely on either side of the rift, forming a broad band of lava-covered surface, instead of a steep and narrow dyke. This seems not only to account for the most striking peculiarity of the bands, but to accord well with all that is known about them, and even to suggest explanations of some other lunar features which had appeared perplexing. I understand that in certain regions of North America there are lava-covered rifts large enough to form geographical features, and, therefore, fairly comparable with the lunar radiating streaks. But as yet American geologists have not presented in an accessible form a description of the peculiar features of the American continent; in fact, it may be doubted whether as yet the materials for such a description exist.

The mountain-ranges of the moon do not differ to any marked degree from those of our own earth. They are few in number; in fact, mountain-ranges form a less important feature of lunar than of terrestrial geography. On the other hand, the lunar ring-mountains and craters far exceed those of our earth in size and importance. The large craters may, in fact, be regarded as characteristic features of lunar scenery. There are several craters exceeding 100 miles in diameter. It is strange to consider that though the ringed wall surrounding some of these larger craters exceeds 10,000 feet in height, no trace of the highest peaks of such a wall would be visible from the middle of the enclosed plain. Conversely, an observer standing on one of the highest peaks beside one of these great craters, would not see half the floor of the crater, while more than half the horizon line around him would belong to the enclosed plain, and would appear as level as the horizon seen from a height overlooking a great prairie. These ringed plains and larger craters seem to belong to the third great era of the moon's history. The bright high regions and dark low levels called seas must have been formed while the greater part of the crust was intensely hot. The contraction of the cooling crust on the nucleus, which cooled far less slowly, led to the formation of the great ray systems. But though such systems extend from great craters, these craters themselves probably attained their present form far later. The crust having in great part cooled, the nucleus began in turn to shrink more quickly than the crust, having more heat to part with. Thus the crust, closing in upon the shrinking nucleus, formed the corrugations and wrinkles which can be seen under telescopic scrutiny in nearly every part of the visible lunar surface. The process was accompanied necessarily by the development of great heat--the thermal equivalent of the mechanical process of contraction. Mallet has shown that the process of contraction at present occurring in the earth's crust gives rise to the greater part of the heat to which volcanic phenomena are due. If this is so in the earth's case at present, how tremendous must have been the heat evolved by the far more rapid contraction of the moon's mass in the remote era we are considering, when probably her heat passed into space unchecked by the action of a dense moisture-laden atmosphere! We can well understand that enormous volumes of heated gas would be formed--including steam, for there is good reason to believe that water is present in large quantities in the moon's interior. The imprisoned gas would find an outlet at points of least resistance, the centres, namely, of the great radiating systems of streaks. These centres would certainly be regions of outlet. But they would not be sufficient. We can understand then why every ray system extends from a great crater, though that crater was really formed after the system of radiating streaks; and we can equally understand why these central craters are not the only or even the chief of the great craters in the moon. Here again I would suggest that possibly the careful study of American geology might disclose features illustrating the great lunar craters.

When we pass to the smaller craters, ranging in diameter from seven or eight miles to less than a quarter of a mile, even if there be not some far smaller and beyond the range of the most powerful telescopes man can construct, we find ourselves among objects resembling those with which the study of our own earth has rendered us familiar. When Sartorius's map of Etna and the surrounding region was first seen at the Geological Society's rooms, many supposed that it represented lunar features. The Vesuvian volcanic region, again, is presented side by side with a lunar region of similar extent in Nasmyth's fine treatise on the moon, and the resemblance is very close. Considering the part which water plays in producing terrestrial volcanic phenomena, it may reasonably be doubted whether there is in reality so close a resemblance as a superficial comparison (and we can make no other) would suggest. There are those, indeed, who believe that some of the multitudinous small craters of the moon have had their origin in the downfall of meteoric masses on her once plastic surface; and strange though the thought may seem, there would be considerable difficulty in showing how the surface of the moon could have remained without traces of the meteoric downfall to which during myriads of centuries she was exposed undefended by that atmospheric shield which protects our earth from millions of meteors yearly falling upon her. We could only attribute the smallest lunar craters, however, to this cause. It may be noticed in passing that Professor Newcomb, apparently referring to this suggestion, which some had thought too fanciful to be seriously advanced, says that 'the figures of these inequalities (the small craters) can be closely imitated by throwing pebbles upon the surface of some smooth plastic mass, as mud or mortar.' Craters, however, larger than a mile or so in diameter, and many also of smaller dimensions, must be regarded as due to the same process of contraction which produced the great craters, but as belonging to an era when this process went on less actively. In like manner another feature of the moon's surface, the existence of narrow furrows called rilles, which sometimes extend to a considerable distance, passing across levels, intersecting crater walls, and reappearing beyond mountain-ranges as though carried under like tunnels, must be regarded as due to the cracking of the crust thus slowly shrinking.

It is noteworthy that the signs of change which have been suspected during recent years belong to these smaller and probably more recent lunar formations. In November, 1866, Dr. Schmidt, chief of the Athens Observatory, announced that the crater Linné in the lunar Sea of Serenity was missing. To understand the importance of this announcement, let it simply be noted that the quantity of matter necessary to fill that crater up would be at least equal to that which would be required to form a mountain covering the whole area of London to a height of two miles! The crater was described by former lunar observers as at least five miles in diameter and very deep. It is not now actually missing, as Schmidt supposed, but it is certainly no longer deep. It is, in fact, exceedingly shallow. Sir J. Herschel's opinion was that the crater had been filled up from beneath by an effusion of viscous lava, which, overflowing the rim on all sides, poured down the outer slope so as to efface its ruggedness and convert it into a gradual declivity casting no stray shadows. But the stupendous nature of the disturbing forces necessary to produce such an overflow of molten matter has led most astronomers to adopt in preference the theory that the wall surrounding the crater has been overthrown, either in consequence solely of the processes of contraction and expansion described above, or from the reinforcement of their action by the effects due to sublunarian energies. Some consider that the descriptions of the crater by Mädler and Lohrmann (which slightly differ) were erroneous, and that there has been no real change. Others deny that any change has occurred, on the ground that Linné varies in aspect according to the manner of its illumination. This I perceive is Professor Newcomb's explanation, who considers such variations 'sufficient to account for the supposed change.' But since the time of Schmidt's announcement Linné has several times been observed under nearly the same conditions as by Mädler and Lohrmann, as the great shadows formerly seen in its interior have not reappeared. There seems to be great reason for believing that a change has really occurred there.

The discovery announced by Dr. Klein is of a different nature. Near the middle of the visible half of the moon there is a well-known though small crater called Hyginus, the neighbourhood of which has been often and carefully examined. While examining this part of the moon's surface with an excellent 5-1/2in. telescope, in May, 1877, Dr. Klein observed a small crater full of shadow, and apparently nearly three miles in diameter. It formed a conspicuous object on the Sea of Vapours. Having frequently observed this region during the last few years, he felt certain that no such crater existed there in 1876. He communicated his discovery to Dr. Schmidt, who stated, in reply, that in all the numerous drawings he had made of this lunar region no such crater was indicated. It is not shown in the great chart by Beer and Mädler, or in Lohrmann's map. Further observation showed that the crater is a deep, conical opening in the moon's surface. Soon after the sun has risen at that part of the moon, and, as later observations confirm, shortly before sunset there, the opening is entirely in shadow, and appears black. But when the sun is rather higher it appears grey, and with a yet higher sun it can no longer be distinguished. It can, however, be seen when the sun is very high on that part of the moon, appearing then somewhat brighter than the surrounding region, a circumstance which does not hitherto seem to have been noticed by either Klein or Schmidt.

The moon's surface has been so long and so carefully studied, that it is almost impossible to understand how such a crater as now certainly exists in the Sea of Vapours near Hyginus could have escaped detection. Craters of the kind exist, indeed, in hundreds on the moon's surface. But many astronomers have given years of their life to the study of such objects; and the centre of the moon's disc, for reasons which astronomers will understand, has been studied with exceptional care. It seems so unlikely that a deep crater three miles in diameter could escape recognition, that some astronomers have not hesitated to regard the newly-detected crater as certainly a new formation. For my own part, though it seems almost impossible to explain how such a crater could have remained so long unnoticed, I can regard the evidence of change as amounting only to extreme probability so far as it depends on the result of past telescopic scrutiny of the moon.

Admitting that a change had occurred, it would not follow that it had been produced by volcanic forces. It seems far more likely that a floor originally covering the conical hole now existing in the Sea of Vapours has given way at last under the effect of long-continued processes of expansion and contraction, which would operate with special energy over a region, like the Sea of Vapours, near the moon's equator.

But there remains to be mentioned a form of evidence respecting lunar features which could not be effectively applied to the case of the crater Linné, because the moon had only been subject to the necessary method of examination during a few years before that crater was missed. I refer to lunar photography. Many objects less than two miles in diameter are shown in the best photographs of our satellite by Rutherfurd, De la Rue, Ellery, and Draper; and as the moon has been photographed in every phase, some among the views might fairly be expected to show Klein's crater if it really existed before 1877. I do not find that in any lunar photographs the crater is shown as a black or dark gray spot. But in Rutherfurd's splendid photograph of the moon on March 6, 1865 (when the moon was about nine days five hours old), the place of Klein's crater is occupied by a small spot lighter than the surrounding 'sea.' This is the usual appearance of a small crater under a high sun; and though it may indicate only the existence of a flat crater floor in 1865 where now a great conical hole exists, it throws some degree of doubt on the occurrence of any change at all there. The case strongly suggests the necessity for continuing the work of lunar photography, which seems of late years to have flagged. Photographs of the moon should be taken in every aspect and in every stage of her librational swayings. Possessing such a series, we should be able to decide at once whether any newly-recognised crater was in reality a new formation or not.

THE NOVEMBER METEORS.

During November 13 and 14 the earth is passing through the region along which lies the course of the family of meteors called the Leonides, sometimes familiarly known as the November meteors. When at this time of the year the meteor region thus traversed by the earth is densely strewn with meteors, there occurs a display of falling stars, one of the most beautiful, and, rightly understood, one of the most remarkable of all celestial phenomena. Of old, indeed, when it was supposed that these meteors were purely meteorological phenomena, they were not thought specially interesting objects. They were held by some as mere weather-portents. It was only when a storm of wind was approaching, vento impendente, according to Virgil, that a shower of meteors was to be seen. Gross ignorance, indeed, has given to showers of falling stars an interest surpassing even that which has become attached to them through the discoveries of modern science, for they have been regarded as portending the end of the world. The shower of November 13, 1833, which was seen in great splendour in America, frightened the negroes of the Southern States nearly out of their wits. A planter of South Carolina relates that he was awakened by shrieks of horror and cries for mercy from 600 or 700 negroes. When he went out to see what was the matter, he found the negroes prostrate on the ground, 'some speechless, some with bitterest cries imploring God to spare the world and them.' There is, however, a grandeur in the interpretation placed by modern science upon these beautiful displays which dwarfs into littleness even such ideas as have been suggested by the terrors of superstition. We perceive that meteors are not mere terrestrial phenomena, nor of brief existence. They speak to us of domains in space compared with which the volume of our earth--nay, even the volume of the sun himself--is a mere point: of time-intervals compared with which the millions of years spoken of by geologists appear but as mere seconds.

The special meteor family whose track the earth crosses on November 13-14 forms a mighty ellipse round the sun, extending more than 19 times farther from him than the track of our earth, which yet, as we know, lies more than 92,000,000 miles from the sun. Along this tremendous orbit the meteors speed with planetary but varying velocity, crossing the track of our earth with a velocity exceeding by more than a third her own swift motion of about 19 miles in every second of time. Coming down somewhat aslant, but otherwise meeting the earth almost full tilt, the meteors rush into our air at the rate of more than 40 miles per second. They are so intensely heated as they rush through it that they are turned into the form of vapour, insomuch that we never make acquaintance with the members of this particular meteoric family in the solid form. In this respect they resemble the greater number of our meteoric visitants. It is, indeed, a somewhat fortunate circumstance for us that this is so, for if Professor Newton, of Yale College (United States), is right in estimating the total number of meteors, large and small, which the earth encounters per annum at 400,000,000, it would be rather a serious matter if all or most of these bodies were not warded off. The least of them, even though a mere grain perhaps in weight, would yet, arriving with planetary velocity exceeding a hundredfold or more the velocity of a cannon-ball, prove an awkward missile if it struck man or animal. But the air effectually saves us from all save a few fire-balls which are large enough to remain in great part solid until they actually strike the earth itself.

The importance of the meteors in the planetary system will be recognised when we remember that the November group alone extends along its oval course in one complete system of meteors for a length of more than 1,700 millions of miles, with an average thickness of about a million miles (determined by noting the average time occupied by the earth in passing through the system on November 13-14), and an unknown cross breadth which probably does not fall short of three or four millions of miles. Other systems are, no doubt, far more important, for it has been found that meteors follow in the track of comets. Now the November meteors follow in the track of a comet (Tempel's comet of 1866), which was so small when last favourably placed for observation that it escaped detection by the naked eye. If so small a comet as this is followed by so large a meteoric system, in which also meteors are strewn so richly that during the passage of the earth through it, tens of thousands of meteors have been counted, how vast must be the numbers and how large probably the individual bodies following in the track of such splendid comets as Newton's, Donati's (1858), the comets of 1811, 1847, 1861, and others! For it should be remembered that we become cognisant of the existence of a meteoric system only when the earth threads its way through one, when those which she encounters may become visible as falling stars if it so chances that she encounters them on the dark or night half of her surface. But the earth is far smaller compared with a system like the November meteor-flight than a rifle-ball compared with the largest flight of birds ever yet seen. Such a ball fired into a very dense and widely extending flock of birds might encounter here and there along its course some five or six birds--not one in 10,000, perhaps, of the entire flight; and if the flock continued flying with unchanging course, a hundred rifle balls might be fired through it without seemingly reducing its numbers. Our earth has passed hundreds of times through the November meteor system, yet its meteoric wealth has scarcely been reduced at all, so exceedingly minute is the track of the earth through the meteor system compared with the extension of the system itself. The region through which the earth has passed is less than a billionth part of the entire region occupied by the system. But the November system is but one among several hundreds through which the earth passes--in other words, the systems which chance to be traversed by that mere thread-like ring in space traversed each year by the earth, are not a millionth, not a billionth, of the total number of such systems. It will be conceived, therefore, that the total amount of meteoric matter, travelling on orbits of all degrees of eccentricity and extension from the sun and inclined at all angles to the general plane of the solar system, must be enormously great. The idea once advanced by an eminent astronomer that the total quantity of unattached matter, so to speak, existing within the solar domain must be estimated rather by pounds than by tons is now altogether exploded. It would be truer to say that the totality of matter thus freely travelling around the sun must be estimated by billions of tons rather than by millions.

Whether it is likely that there will be a display of meteors to-night (or, rather, to-morrow morning), is a question to which most astronomers would be disposed, we believe, to reply definitely in the negative. The display of November 13-14, 1866, was very brilliant; that of 1867 (best seen in the United States) was almost equally so; but successive showers steadily diminished. In other words, the part of the system crossed by the earth in 1866 and 1867 was very rich, but the part which she crossed afterwards (the rich part having passed far on towards the remote aphelion of the system outside the orbit of Uranus) was less rich. For the last few years very few November meteors have been seen, though the few stragglers which have been seen, and have been identified as belonging to the family by their paths athwart the star-depths, have been almost as interesting to astronomers as the showers of such bodies seen in 1799, 1833, 1866, and 1867. But it is not altogether impossible that in the small hours 'ayont the twal' to-morrow morning a shower of meteors may be seen. For Schiaparelli (the Italian astronomer who first started the ideas which led when properly followed up to the discovery of the relations existing between meteors and comets) asserts that it has happened before now that the November meteors have appeared in great numbers in years lying midway between the times of maximum display. These times are separated on the average by about 33-1/4 years. Thus, in 1799, there was a great display of November meteors, a shower rendered specially celebrated by Humboldt's description. In 1833 there was another, the display which so terrified the negroes of South Carolina, but more interesting scientifically as described by Arago. In 1866 the shower again attained its maximum splendour, though the display of 1867 was little inferior. It will not be till 1899 that another great shower of November meteors may be confidently looked for. But if Schiaparelli be right, it is quite possible that there may be a shower this year, due to some scattered flight of the November meteors which, delayed accidentally (through some special perturbation) many hundreds of years ago, has come in the course of ages to travel nearly half a circuit behind the richest part of the system, the 'gem of the meteor-ring,' as it has been poetically called. Even, however, though no display of November meteors should be seen, yet the recognition of even a few scattered stragglers would be exceedingly interesting to astronomers. A single meteor seen to-night which could be regarded as certainly belonging to the November system would suffice to show the possibility that a whole flight of the November meteors might travel at a similar distance behind the main body. It would be more easy, however, to identify two such meteors than one, six than two, and a score than half-a-dozen. The only way in which a meteor can be questioned, so to speak, respecting the family it belongs to, is by noting its path across the sky. If this path tends directly from the constellation Leo (however remote Leo may be from the part of the heavens traversed by the meteor), the chances are that the meteor is a Leonid, or one of the November family. If the path tends from that particular part of the constellation Leo (near the end of the curved blade of the so-called Sickle in Leo), the probability of the meteor being a Leonid is increased. If two or more meteors are seen to-morrow morning (after 12.30) which both tend from the Sickle in Leo, even though they seem to tend in opposite directions, the chances are yet greater that they are travelling in parallel paths along the track of the November meteors, but some 2,000 million miles behind the main body. Should the number mount up to a score or so, the conclusion would be, to all intents and purposes, certain; and the possible occurrence of even a shower of Leonids at a time midway between the customary maxima of the meteoric displays would be placed beyond question.

We must, however, admit that it seems less likely there will be anything like a display of Leonids to-night than that patient observers may be able to identify a few of these bodies, and thus--though by observations of a less attractive kind--to advance our knowledge of this interesting system. Far more likely is it that towards the end of the month there will be a display of meteors belonging to another and an entirely distinct family, a family scarcely less worthy to be called November meteors par excellence, but actually rejoicing in the classically unsatisfactory name of Andromeds.

EXPECTED METEOR SHOWER.

(From the Times of November 25, 1878.)

It is probable that during the next three nights some light may be thrown on one of the most perplexing yet most interesting of all the problems recently suggested to the study of astronomers. It is confidently expected that many of those November meteors called Andromeds will be seen on one or other of those nights, if not on all three. No meteor systems, not even the famous systems of August and November, are more remarkable than this singular family. To explain why astronomers regard the Andromeds with so much interest, it will be necessary to speak of an object which at first sight seems in no way connected with them--an object, in fact, which, so long as it was actually known to astronomers, was never supposed to be connected with any family of meteors--the celebrated lost comet called Biela's (or, by Frenchmen, Gambart's comet). In February, 1826, Biela discovered in the constellation Aries a comet which was found to be travelling in an oval path round the sun, in a period of about six years seven and a half months. Tracing its course backwards, astronomers found that it had been seen in 1772 by Montaigne, and observed for two or three weeks in that year by Messier, the great comet hunter. Nothing very remarkable was recognised regarding this comet in 1826, except the fact that its path nearly intersects that of our own earth; so that if ever the earth is to encounter a comet, here seemed to be the comet she had to fear. Great terror was, indeed, excited by the announcement that in 1832 the comet would cross the earth's track only four or five weeks before the earth came to the place of danger. But no harm happened. In that year, and again in 1839, the comet returned quietly enough, though in 1839 it was not observed, being so placed that it was lost in the splendour of the solar rays. In February, 1846, the comet was again seen, this being the third return since its discovery in 1826, or rather, since its recognition as a member of the solar system, the eleventh since it was first seen by Montaigne. At this time everything seemed to suggest that this comet, unless our earth at some future time should absorb it, would remain for a long time a steady member of the sun's comet family. But only a few days after its detection in February, 1846, the comet was found to have divided into two, which travelled side by side until both vanished from view with increasing distance. In 1852 the companion comets reappeared, and again both continued in view till their motion carried them beyond telescopic range. As the distance between the coupled comets had increased from about 160,000 miles in 1846 to about 1,250,000 miles in 1852, astronomers anticipated a most interesting series of observations at the successive returns of the double comet to the earth's neighbourhood. Unfortunately, in 1859 the comet's course carried it athwart a part of the sky illuminated by the sun's rays, so that astronomers could not then expect to see it. But in 1866 it was looked for hopefully. Its orbit had now been most carefully computed, and many observers, armed with excellent telescopes, were on the watch for it, with very accurate knowledge of the course along which it might be expected to travel, and even of its position from day to day and from hour to hour. But it was not seen. Nor, again, was it seen in 1872, when fresh computations had been made, and observations were extended over a wider range, to make sure, as was hopefully thought, that this time it should not escape recognition. Could it have come, asked Herschel in 1866--and in 1872 the same question might still more pertinently be asked--into contact or exceedingly close approach to some asteroid as yet undiscovered? or, peradventure, had it plunged into and got bewildered among the rings of meteorolites, which astronomers more than suspected?

Between 1866, when Sir John Herschel thus wrote, and 1872, when again Biela's comet was sought in vain, a series of strange discoveries had been made respecting meteors, which led astronomers to believe that, even though the missing comet might never again be seen as a comet, we might still learn something respecting its present condition. It had been noticed that the remarkable comet of 1862 (comet 11 of that year) crossed the earth's track near the place where she is on August 10-11, the time of the August meteors, called the Tears of St. Lawrence in old times, but now known as the Perseids, because they seem to radiate from the constellation Perseus. Later the idea occurred to Schiaparelli, an Italian astronomer, that the August meteors may travel along the path of that comet. He could not prove this, but he advanced very strong evidence in favour of the opinion, for he found that bodies travelling along the path of the comet of 1862 would seem to radiate from Perseus as they traversed the earth's atmosphere. It was as if a person suspected that a steam-cloud seen on a distant railway track belonged to a particular train, and, though unable actually to prove this, was yet able to show that, with the wind and weather then prevailing, that train, travelling at its customary rate, would leave a steam-cloud behind it precisely of the apparent length and position of the observed steam-cloud. This cloud might have the observed position though otherwise produced, yet the evidence would be thought strongly to favour the supposition that it came from the train in question. In like manner the August meteors might be travelling on any one of a great number of tracks intersecting the earth's orbit in the place occupied by the earth on August 10-11; yet it was at least a striking coincidence that a flight of meteors travelling in the orbit of the chief comet of 1862 would seem to radiate from the constellation Perseus, precisely as the August meteors do.

While astronomers were still discussing the ideas of Schiaparelli, Professor Newton of Yale College, in America, called their attention to the great display of November meteors which might be expected on November 13-14, 1866. The fine shower of that year was well observed, and the part--we may almost say the point--of the constellation Leo from which the meteors radiated was correctly determined. And now a strange thing happened. Those who believed in Schiaparelli's account of the August meteors supposed of necessity that the bodies forming that system travel in an orbit of enormous extent, for the comet of 1862 travels on a path extending much further from the sun than the path of Neptune. There was, therefore, nothing to prevent them from believing that the Leonides travel in a track carrying them far away from the sun. The recurrence of great displays of these meteors at intervals of about 33 years might be readily explained on such an assumption, for if the Leonides have a period of about 33 years, their path must extend far beyond the path of Uranus. But hitherto astronomers had not been ready to admit such an explanation of the periodic recurrence of great displays of the November meteors. They preferred theories (for several were available) which accounted for the 33-year period, while assigning to the Leonides paths of much less extent. Now that the idea of vast meteoric orbits had been fairly broached, some astronomers thought it might at least be worth while to calculate the path of the November meteors on the assumption that their true period is about 33-1/4 years. This was perfectly easy, because the period of a body travelling round the sun determines the velocity at any given distance from the sun, and knowing thus (at least, on this assumption) the true velocity of the Leonides as they rush into our air, while their apparent path is known, their true course is as readily determined as the true course of the wind can be determined by a seaman from the apparent direction and velocity with which it reaches his ship. When the path of the November meteors had been determined (on the assumption mentioned), it was found to be identical with the path of a comet which had only been discovered a few months before-the comet called Tempel's. That a comet which is invisible to the naked eye should have been discovered in the very year when first astronomers made exact observations of the meteors which travel in its track--for it will presently be seen that the assumption above mentioned was a just one--cannot but be regarded as a very singular coincidence. It was a most fortunate coincidence for astronomers, since there can be but little doubt that but for it Schiaparelli's theory would very soon have been forgotten. As that theory was itself suggested by the fortuitous recognition of another comet (only visible at intervals of more than a century) at a time when attention had been specially directed to the August meteors, it may fairly be said that the theory which now associates meteors and comets in the most unmistakable manner was suggested by one accident and confirmed by another. Albeit such accidents happen only to the zealous student of nature's secrets. We shall presently see that the fortunate detection of Tempel's comet in 1866 was not the last of the series of coincidences by which the theory of meteors was established.

Although the evidence favouring Schiaparelli's theory was now strong, yet it was well that at this stage still more convincing evidence was forthcoming. The date of the November display has changed since the Leonides were first recognised, in such sort as to show that the position of their path has changed. The change is due to the disturbing attractions of the planets. It occurred to our great astronomer Adams, discoverer with Leverrier of distant Neptune, to inquire whether the observed change accorded with the calculated effects of planetary attraction, if the Leonides are supposed to travel in any of the smaller paths suggested by astronomers, or could be explained only by the assumption that the meteors travel on the widely-extending path corresponding to the 33-1/4 years period. The problem was worthy of his powers--in other words, it was a problem of exceeding difficulty. By solving it, Adams made that certain which Schiaparelli and his followers had merely assumed. He showed beyond all possibility of doubt or question that of all the paths by which the periodic meteoric displays could be accounted for, the wide path carrying the November meteors far beyond the track of Uranus was the only one which accorded with the observed effects of planetary perturbation.