The Moon Part 1

The Moon.

by Thomas Gwyn Elger.

PREFACE

This book and the accompanying map is chiefly intended for the use of lunar observers, but it is hoped it may be acceptable to many who, though they cannot strictly be thus described, take a general interest in astronomy.

The increasing number of those who possess astronomical telescopes, and devote more or less of their leisure in following some particular line of research, is shown by the great success in recent years of societies, such as the British Astronomical a.s.sociation with its several branches, the Astronomical Society of the Pacific, and similar inst.i.tutions in various parts of the world. These societies are not only doing much in popularising the sublimest of the sciences, but are the means of developing and organising the capabilities of their members by discouraging aimless and desultory observations, and by pointing out how individual effort may be utilised and made of permanent value in almost every department of astronomy.

The work of the astronomer, like that of the votary of almost every other science, is becoming every year more and more specialised; and among its manifold subdivisions, the study of the physical features of the moon is undoubtedly increasing in popularity and importance. To those who are pursuing such observations, it is believed that this book will be a useful companion to the telescope, and convenient for reference.

Great care has been taken in the preparation of the map, which, so far as the positions of the various objects represented are concerned, is based on the last edition of Beer and Madler's chart, and on the more recent and much larger and elaborate map of Schmidt; while as regards the shape and details of most of the formations, the author's drawings and a large number of photographs have been utilised. Even on so small a scale as eighteen inches to the moon's diameter, more detail might have been inserted, but this, at the expense of distinctness, would have detracted from the value of the map for handy reference in the usually dim light of the observatory, without adding to its utility in other ways. Every named formation is prominently shown; and most other features of interest, including the princ.i.p.al rill-systems, are represented, though, as regards these, no attempt is made to indicate all their manifold details and ramifications, which, to do effectually, would in very many instances require a map on a much larger scale than any that has yet appeared.

The insertion of meridian lines and parallels of lat.i.tude at every ten degrees, and the subst.i.tution of names for reference numbers, will add to the usefulness of the map.

With respect to the text, a large proportion of the objects in the Catalogue and in the Appendix have been observed and drawn by the author many times during the last thirty years, and described in _The Observatory_ and other publications. He has had, besides, the advantage of consulting excellent sketches by Mr W.H. MAW, F.R.A.S., Dr. SHELDON, F.R.A.S., Mr. A. MEE, F.R.A.S., Mr. G.P. HALLOWES, F.R.A.S., Dr. SMART, F.R.A.S., Mr. T. GORDON, F.R.A.S., Mr. G.T. DAVIS, Herr BRENNER, Herr KRIEGER, Mr. H. CORDER, and other members of the British Astronomical a.s.sociation. Through the courtesy of Professor HOLDEN, Director of the Lick Observatory, and M. PRINZ, of the Royal Observatory of Brussels, many beautiful photographs and direct photographic enlargements have been available, as have also the exquisite heliogravures received by the author from Dr. L. WEINEK, Director of the Imperial Observatory of Prague, and the admirable examples of the photographic work of MM. PAUL and PROSPER HENRY of the Paris Observatory, which are occasionally published in _Knowledge_. The numerous representations of lunar objects which have appeared from time to time in that storehouse of astronomical information, _The English Mechanic_, and the invaluable notes in "Celestial Objects for Common Telescopes," and in various periodicals, by the late REV. PREBENDARY WEBB, to whom Selenography and Astronomy generally owe so much, have also been consulted.

As a rule, all the more prominent and important features are described, though very frequently interesting details are referred to which, from their minuteness, could not be shown in the map. The measurements (given in round numbers) are derived in most instances from NEISON'S (Nevill) "Moon," though occasionally those in the introduction to Schmidt's chart are adopted.

THOMAS GYWN ELGER.

BEDFORD, 1895.

INTRODUCTION

We know, both by tradition and published records, that from the earliest times the faint grey and light spots which diversify the face of our satellite excited the wonder and stimulated the curiosity of mankind, giving rise to suppositions more or less crude and erroneous as to their actual nature and significance. It is true that Anaxagoras, five centuries before our era, and probably other philosophers preceding him, --certainly Plutarch at a much later date--taught that these delicate markings and differences of tint, obvious to every one with normal vision, point to the existence of hills and valleys on her surface; the latter maintaining that the irregularities of outline presented by the "terminator," or line of demarcation between the illumined and unillumined portion of her spherical superficies, are due to mountains and their shadows; but more than fifteen centuries elapsed before the truth of this sagacious conjecture was unquestionably demonstrated.

Selenography, as a branch of observational astronomy, dates from the spring of 1609, when Galileo directed his "optic tube" to the moon, and in the following year, in the _Sidereus Nuncius_, or "the Intelligencer of the Stars," gave to an astonished and incredulous world an account of the unsuspected marvels it revealed. In this remarkable little book we have the first attempt to represent the telescopic aspect of the moon's visible surface in the five rude woodcuts representing the curious features he perceived thereon, whose form and arrangement, he tells us, reminded him of the "ocelli" on the feathers of a peac.o.c.k's tail,--a quaint but not altogether inappropriate simile to describe the appearance of groups of the larger ring-mountains partially illuminated by the sun, when seen in a small telescope.

The bright and dusky areas, so obvious to the unaided sight, were found by Galileo to be due to a very manifest difference in the character of the lunar surface, a large portion of the northern hemisphere, and no inconsiderable part of the south-eastern quadrant, being seen to consist of large grey monotonous tracts, often bordered by lofty mountains, while the remainder of the superficies was much more conspicuously brilliant, and, moreover, included by far the greater number of those curious ring- mountains and other extraordinary features whose remarkable aspect and peculiar arrangement first attracted his attention. Struck by the a.n.a.logy which these contrasted regions present to the land and water surfaces of our globe, he suspected that the former are represented on the moon by the brighter and more rugged, and the latter by the smoother and more level areas; a view, however, which Kepler more distinctly formulated in the dictum, "Do maculas esse Maria, do lucidas esse terras." Besides making a rude lunar chart, he estimated the heights of some of the ring- mountains by measuring the distance from the terminator of their bright summit peaks, when they were either coming into or pa.s.sing out of sunlight; and though his method was incapable of accuracy, and his results consequently untrustworthy, it served to demonstrate the immense alt.i.tude of these circ.u.mvallations, and to show how greatly they exceed any mountains on the earth if the relative dimensions of the two globes are taken into consideration.

Before the close of the century when selenography first became possible, Hevel of Dantzig, Scheiner, Langrenus (cosmographer to the King of Spain), Riccioli, the Jesuit astronomer of Bologna, and Dominic Ca.s.sini, the celebrated French astronomer, greatly extended the knowledge of the moon's surface, and published drawings of various phases, and charts, which, though very rude and incomplete, were a clear advance upon what Galileo, with his inferior optical means, had been able to accomplish.

Langrenus, and after him Hevel, gave distinctive names to the various formations, mainly derived from terrestrial physical features, for which Riccioli subsequently subst.i.tuted those of philosophers, mathematicians, and other celebrities; and Ca.s.sini determined by actual measurement the relative position of many of the princ.i.p.al objects on the disc, thus laying the foundation of an accurate system of lunar topography; while the labours of T. Mayer and Schroter in the last century, and of Lohrmann, Madler, Neison (Nevill), Schmidt, and other observers in the present, have been mainly devoted to the study of the minuter detail of the moon and its physical characteristics.

As was manifest to the earliest telescopic observers, its visible surface is clearly divisible into strongly contrasted areas, differing both in colour and structural character. Somewhat less than half of what we see of it consists of comparatively level dark tracts, some of them very many thousands of square miles in extent, the monotony of whose dusky superficies is often unrelieved for great distances by any prominent object; while the remainder, everywhere manifestly brighter, is not only more rugged and uneven, but is covered to a much greater extent with numbers of quasi-circular formations, differing widely in size, cla.s.sed as walled-plains, ring-plains, craters, craterlets, crater-cones, &c.

(the latter bearing a great outward resemblance to some terrestrial volcanoes), and mountain ranges of vast proportions, isolated hills, and other features.

Though nothing resembling sheets of water, either of small or large extent, have ever been detected on the surface, the superficial resemblance, in small telescopes, of the large grey tracts to the appearance which we may suppose our terrestrial lakes and oceans would present to an observer on the moon, naturally induced the early selenographers to term them Maria, or "seas"--a convenient name, which is still maintained, without, however, implying that these areas, as we now see them, are, or ever were, covered with water. Some, however, regard them as old sea-beds, from which every trace of fluid, owing to some unknown cause, has vanished, and that the folds and wrinkles, the ridges, swellings, and other peculiarities of structure observed upon them, represent some of the results of alluvial action. It is, of course, possible, and even probable, that at a remote epoch in the evolution of our satellite these lower regions were occupied by water, but that their surface, as it now appears, is actually this old sea-bottom, seems to be less likely than that it represents the consolidated crust of some semi- fluid or viscous material (possibly of a basaltic type) which has welled forth from orifices or rents communicating with the interior, and overspread and partially filled up these immense hollows, more or less overwhelming and destroying many formations which stood upon them before this catastrophe took place. Though this, like many other speculations of a similar character relating to lunar "geology," must remain, at least for the present, as a mere hypothesis; indications of this partial destruction by some agency or other is almost everywhere apparent in those formations which border the so-called seas, as, for example, Fracastorius in the Mare Nectaris; Le Monnier in the Mare Serenitatis; Pitatus and Hesiodus, on the south side of the Mare Nubium; Doppelmayer in the Mare Humorum, and in many other situations; while no observer can fail to notice innumerable instances of more or less complete obliteration and ruin among objects within these areas, in the form of obscure rings (mere scars on the surface), dusky craters, circular arrangements of isolated hills, reminding one of the monoliths of a Druidical temple; all of which we are justified in concluding were at one time formations of a normal type. It has been held by some selenologists --and Schmidt appears to be of the number,--that, seeing the comparative scarcity of large ring-plains and other ma.s.sive formations on the Maria, these grey plains represent, as it were, a picture of the primitive surface of the moon before it was disturbed by the operations of interior forces; but this view affords no explanation of the undoubted existence of the relics of an earlier lunar world beneath their smooth superficies.

MARIA.--Leaving, however, these considerations for a more particular description of the Maria, it is clearly impossible, in referring to their level relatively to the higher and brighter land surface of the moon, to appeal to any hypsometrical standard. All that is known in this respect is, that they are invariably lower than the latter, and that some sink to a greater depth than others, or, in other words, that they do not all form a part of the same sphere. Though they are more or less of a greyish-slaty hue--some of them approximating very closely to that of the pigment known as "Payne's grey"--the tone, of course, depends upon the angle at which the solar rays impinge on that particular portion of the surface under observation. Speaking generally, they are, as would follow from optical considerations, conspicuously darker when viewed near the terminator, or when the sun is either rising or setting upon them, than under a more vertical angle of illumination. But even when it is possible to compare their colour by eye-estimation under similar solar alt.i.tudes, it is found that not only are some of the Maria, as a whole, notably darker than others, but nearly all of them exhibit _local_ inequalities of hue, which, under good atmospheric and instrumental conditions, are especially remarkable. Under such circ.u.mstances I have frequently seen the surface, in many places covered with minute glittering points of light, s.h.i.+ning with a silvery l.u.s.tre, intermingled with darker spots and a network of streaks far too delicate and ethereal to represent in a drawing. In addition to these contrasts and differences in the sombre tone of these extended plains, many observers have remarked traces of a yellow or green tint on the surface of some of them. For example, the Mare Imbrium and the Mare Frigoris appear under certain conditions to be of a dirty yellow-green hue, the central parts of the Mare Humorum dusky green, and part of the Mare Serenitatis and the Mare Crisium light green, while the Palus Somnii has been noted a golden-brown yellow. To these may be added the district round Taruntius in the Mare Foecunditatis, and portions of other regions referred to in the catalogue, where I have remarked a very decided sepia colour under a low sun. It has been attempted to account for these phenomena by supposing the existence of some kind of vegetation; but as this involves the presence of an atmosphere, the idea hardly finds favour at the present time, though perhaps the possibility of plant growth in the low-lying districts, where a gaseous medium may prevail, is not altogether so chimerical a notion as to be unworthy of consideration. Nasmyth and others suggest that these tints may be due to broad expanses of coloured volcanic material, an hypothesis which, if we believe the Maria to be overspread with such matter, and knowing how it varies in colour in terrestrial volcanic regions, is more probable than the first. Anyway, whether we consider these appearances to be objective, or, after all, only due to purely physiological causes, they undoubtedly merit closer study and investigation than they have hitherto received.

There are twenty-three of these dusky areas which have received distinctive names; seventeen of them are wholly, or in great part, confined to the northern, and to the south-eastern quarter of the southern hemisphere--the south-western quadrant being to a great extent devoid of them. By far the largest is the vast Ocea.n.u.s Procellarum, extending from a high northern lat.i.tude to beyond lat.i.tude 10 deg. in the south-eastern quadrant, and, according to Schmidt, with its bays and inflections, occupying an area of nearly two million square miles, or more than that of all the remaining Maria put together. Next in order of size come the Mare Nubium, of about one-fifth the superficies, covering a large portion of the south-eastern quadrant, and extending considerably north of the equator, and the Mare Imbrium, wholly confined to the northeastern quadrant, and including an area of about 340,000 square miles. These are by far the largest lunar "seas." The Mare Foecunditatis, in the western hemisphere, the greater part of it lying in the south- western quadrant, is scarcely half so big as the Mare Imbrium; while the Maria Serenitatis and Tranquilitatis, about equal in area (the former situated wholly north of the equator, and the latter only partially extending south of it), are still smaller. The arctic Mare Frigoris, some 100,000 square miles in extent, is the only remaining large sea,--the rest, such as the Mare Vaporum, the Sinus Medii, the Mare Crisium, the Mare Humorum, and the Mare Humboldtianum, are of comparatively small dimensions, the Mare Crisium not greatly exceeding 70,000 square miles, the Mare Humorum (about the size of England) 50,000 square miles, while the Mare Humboldtianum, according to Schmidt, includes only about 42,000 square miles, an area which is approached by some formations not cla.s.sed with the Maria. This distinction, speaking generally, prevails among the Maria,--those of larger size, such as the Ocea.n.u.s Procellarum, the Mare Nubium, and the Mare Foecunditatis, are less definitely enclosed, and, like terrestrial oceans, communicate with one another; while their borders, or, if the term may be allowed, their coast-line, is often comparatively low and ill-defined, exhibiting many inlets and irregularities in outline. Others, again, of considerable area, as, for example, the Mare Serenitatis and the Mare Imbrium, are bounded more or less completely by curved borders, consisting of towering mountain ranges, descending with a very steep escarpment to their surface: thus in form and other characteristics they resemble immense wall-surrounded plains. Among the best examples of enclosed Maria is the Mare Crisium, which is considered by Neison to be the deepest of all, and the Mare Humboldtianum.

Though these great plains are described as level, this term must only be taken in a comparative sense. No one who observes them when their surface is thrown into relief by the oblique rays of the rising or setting sun can fail to remark many low bubble-shaped swellings with gently rounded outlines, shallow trough-like hollows, and, in the majority of them, long sinuous ridges, either running concentrically with their borders or traversing them from side to side. Though none of these features are of any great alt.i.tude or depth, some of the ridges are as much as 700 feet in height, and probably in many instances the other elevations often rise to 150 feet or more above the low-lying parts of the plains on which they stand. Hence we may say that the Maria are only level in the sense that many districts in the English Midland counties are level, and not that their surface is absolutely flat. The same may be said as to their apparent smoothness, which, as is evident when they are viewed close to the terminator, is an expression needing qualification, for under these conditions they often appear to be covered with wrinkles, flexures, and little asperities, which, to be visible at all, must be of considerable size. In fact, were it possible to examine them from a distance of a few miles, instead of from a standpoint which, under the most favourable circ.u.mstances, cannot be reckoned at less than 300, and this through an interposed aerial medium always more or less perturbed, they would probably be described as rugged and uneven, as some modern lava sheets.

RIDGES.--Among the Maria which exhibit the most remarkable arrangement of ridges is the Mare Humorum, in the south-eastern quadrant. Here, if it be observed under a rising sun, a number of these objects will be seen extending from the region north of the ring-mountain Vitello in long undulating lines, roughly concentric with the western border of the "sea," and gradually diminis.h.i.+ng in alt.i.tude as they spread out, with many ramifications, to a distance of 200 miles or more towards the north.

At this stage of illumination they are strikingly beautiful in a good telescope, reminding one of the ripple-marks left by the tide on a soft sandy beach. Like most other objects of their cla.s.s, they are very evanescent, gradually disappearing as the sun rises higher in the lunar firmament, and ultimately leaving nothing to indicate their presence beyond here and there a ghostly streak or vein of a somewhat lighter hue than that of the neighbouring surface. The Mare Nectaris, again, in the south-western quadrant, presents some fine examples of concentric ridges, which are seen to the best advantage when the morning sun is rising on Rosse, a prominent crater north of Fracastorius. This "sea" is evidently concave in cross-section, the central portion being considerably lower than the margin, and these ridges appear to mark the successive stages of the change of level from the coast-line to the centre. They suggest the "caving in" of the surface, similar to that observed on a frozen pond or river, where the "cat's ice" at the edge, through the sinking of the water beneath, is rent and tilted to a greater or less degree. The Mare Serenitatis and the Mare Imbrium, in the northern hemisphere, are also remarkable for the number of these peculiar features. They are very plentifully distributed round the margin and in other parts of the former, which includes besides one of the longest and loftiest on the moon's visible surface--the great serpentine ridge, first drawn and described nearly a hundred years ago by the famous selenographer, Schroter of Lilienthal. Originating at a little crater under the north- east wall of great ring-plain Posidonius, it follows a winding course across the Mare toward the south, throwing out many minor branches, and ultimately dies out under a great rocky promontory--the Promontory Acherusia, at the western termination of the Haemus range. A comparatively low power serves to show the curious structural character of this immense ridge, which appears to consist of a number of corrugations and folds ma.s.sed together, rising in places, according to Neison, to a height of 700 feet and more. The Mare Imbrium also affords an example of a ridge, which, though shorter, is nearly as prominent, in that which runs from the bright little ring-plain Piazzi Smyth towards the west side of Plato. The region round Timocharis and other quarters of the Mare are likewise traversed by very noteworthy features of a similar cla.s.s. The Ocea.n.u.s Procellarum also presents good instances of ridges in the marvellous ramifications round Encke, Kepler, and Marius, and in the region north of Aristarchus and Herodotus. Perhaps the most perfect examples of surface swellings are those in the Mare Tranquilitatis, a little east of the ring-plain Arago, where there are two nearly equal circular mounds, at least ten miles in diameter, resembling tumuli seen from above. Similar, but more irregular, objects of a like kind are very plentiful in many other quarters.

It is a suggestive peculiarity of many of the lunar ridges, both on the Maria and elsewhere, that they are very generally found in a.s.sociation with craters of every size. Ill.u.s.trations of this fact occur almost everywhere. Frequently small craters are found on the summits of these elevations, but more often on their flanks and near their base. Where a ridge suddenly changes its direction, a crater of some prominence generally marks the point, often forming a node, or crossing-place of other ridges, which thus appear to radiate from it as a centre. Sometimes they intrude within the smaller ring-mountains, pa.s.sing through gaps in their walls as, for example, in the cases of Madler, La.s.sell, &c. Various hypotheses have been advanced to account for them. The late Professor Phillips, the geologist, who devoted much attention to the telescopic examination of the physical features of the moon, compared the lunar ridges to long, low, undulating mounds, of somewhat doubtful origin, called "kames" in Scotland, and "eskers" in Ireland, where on the low central plain they are commonly found in the form of extended banks (mainly of gravel), with more or less steep sides, rising to heights of from 20 to 70 feet. They are sometimes only a few yards wide at the top, while in other places they spread out into large humps, having circular or oval cavities on their summits, 50 or 60 yards across, and as much as 40 feet deep. Like the lunar ridges, they throw out branches and exhibit many breaches of continuity. By some geologists they are supposed to represent old submarine banks formed by tidal currents, like harbour bars, and by others to be glacial deposits; in either case, to be either directly or indirectly due to alluvial action. Their outward resemblance to some of the ridges on the moon is unquestionable; and if we could believe that the Maria, as we now see them, are dried-up sea-beds, it might be concluded that these ridges had a similar origin; but their close connection with centres of volcanic disturbance, and the numbers of little craters on or near their track, point to the supposition that they consist rather of material exuded from long-extending fissures in the crust of the "seas," and in other surfaces where they are superimposed.

This conjecture is rendered still more probable by the fact that we sometimes find the direction of clefts (which are undoubted surface cracks) prolonged in the form of long narrow ridges or of rows of little hillocks. We are, however, not bound to a.s.sume that all the manifold corrugations observed on the lunar plains are due to one and the same cause; indeed, it is clear that some are merely the outward indications of sudden drops in the surface, as in the case of the ridges round the western margin of the Mare Nectaris, and in other situations, where subsidence is manifested by features a.s.suming the outward aspect of ordinary ridges, but which are in reality of a very different structural character.

The Maria, like almost every other part of the visible surface, abound in craters of a minute type, which are scattered here and there without any apparent law or ascertained principle of arrangement. Seeing how imperfect is our acquaintance with even the larger objects of this cla.s.s, it is rash to insist on the antiquity or permanence of such diminutive objects, or to dogmatise about the cessation of lunar activity in connection with features where the volcanic history of our globe, if it is of any value as an a.n.a.logue, teaches us it is most likely to prevail.

Most observers will agree with Schmidt, that observations and drawings of objects on the sombre depressed plains of the moon are easier and pleasanter to make than on the dazzling highlands, and that the lunar "sea" is to the working selenographer like an oasis in the desert to the traveller--a relief in this case, however, not to an exhausted body, but to a weary eye.

RING-MOUNTAINS, CRATERS, &C.--It is these objects, in their almost endless variety and bewildering number, which, more than any others, give to our satellite that marvellous appearance in the telescope which since the days of Galileo has never failed to evoke the astonishment of the beholder. However familiar we may be with the lunar surface, we can never gaze on these extraordinary formations, whether ma.s.sed together apparently in inextricable confusion, or standing in isolated grandeur, like Copernicus, on the grey surface of the plains, without experiencing, in a scarcely diminished degree, the same sensation of wonder and admiration with which they were beheld for the first time. Although the attempt to bring all these _bizarre_ forms under a rigid scheme of cla.s.sification has not been wholly successful, their structural peculiarities, the hypsometrical relation between their interior and the surrounding district, their size, and the character of their circ.u.mvallation, the dimensions of their cavernous opening as compared with that of the more or less truncated conical ma.s.s of matter surrounding it, all afford a basis for grouping them under distinctive t.i.tles, that are not only convenient to the selenographer, but which undoubtedly represent, as a rule, actual diversities in their origin and physical character.

These distinguis.h.i.+ng t.i.tles, as adopted by Schroter, Lohrmann, and Madler, and accepted by subsequent observers, are WALLED-PLAINS, MOUNTAIN RINGS, RING-PLAINS, CRATERS, CRATER-CONES, CRATERLETS, CRATER-PITS, DEPRESSIONS.

WALLED-PLAINS.--These formations, approximating more or less to the circular form, though frequently deviating considerably from it, are among the largest enclosures on the moon. They vary from upwards of 150 to 60 miles or under in diameter, and are often encircled by a complex rampart of considerable breadth, rising in some instances to a height of 12,000 feet or more above the enclosed plain. This rampart is rarely continuous, but is generally interrupted by gaps, crossed by transverse valleys and pa.s.ses, and broken by more recent craters and depressions. As a rule, the area within the circ.u.mvallation (usually termed "the floor") is only slightly, if at all, lower than the region outside: it is very generally of a dusky hue, similar to that of the grey plains or Maria, and, like them, is usually variegated by the presence of hills, ridges, and craters, and is sometimes traversed by delicate furrows, termed clefts or rills.

_Ptolemaeus_, in the third quadrant, and not far removed from the centre of the disc, may be taken as a typical example of the cla.s.s. Here we have a vast plain, 115 miles from side to side, encircled by a ma.s.sive but much broken wall, which at one peak towers more than 9000 feet above a level floor, which includes details of a very remarkable character. The adjoining _Alphonsus_ is another, but somewhat smaller, object of the same type, as are also _Albategnius_, and _Arzachel_; and _Plato_, in a high northern lat.i.tude, with its n.o.ble many-peaked rampart and its variable steel-grey interior. _Grimaldi_, near the eastern limb (perhaps the darkest area on the moon), _Schickard_, nearly as big, on the south- eastern limb, and _Bailly_, larger than either (still farther south in the same quadrant), although they approach some of the smaller "seas" in size, are placed in the same category. The conspicuous central mountain, so frequently a.s.sociated with other types of ringed enclosures, is by no means invariably found within the walled-plains; though, as in the case of _Petavius_, _Langrenus_, _Ga.s.sendi_, and several other noteworthy examples, it is very prominently displayed. The progress of sunrise on all these objects affords a magnificent spectacle. Very often when the rays impinge on their apparently level floor at an angle of from 1 deg.

to 2 deg., it is seen to be coa.r.s.e, rough grained, and covered with minute elevations, although an hour or so afterwards it appears as smooth as gla.s.s.

Although it is a distinguis.h.i.+ng characteristic that there is no great difference in level between the outside and the inside of a walled-plain, there are some very interesting exceptions to this rule, which are termed by Schmidt "Transitional forms." Among these he places some of the most colossal formations, such as _Clavius_, _Maurolycus_, _Stofler_, _Janssen_, and _Longomonta.n.u.s_. The first, which may be taken as representative of the cla.s.s (well known to observers as one of the grandest of lunar objects), has a deeply sunken floor, fringed with mountains rising some 12,000 feet above it, though they scarcely stand a fourth of this height above the plain on the west, which ascends with a very gentle gradient to the summit of the wall. Hence the contrast between the shadows of the peaks of the western wall on the floor at sunrise, and of the same peaks on the region west of the border at sunset is very marked. In _Ga.s.sendi_, _Phocylides_, and _Wargentin_ we have similar notable departures from the normal type. The floor of the former on the north stands 2000 feet _above_ the Mare Humorum. In _Phocylides_, probably through "faulting," one portion of the interior suddenly sinks to a considerable depth below the remainder; while the very abnormal _Wargentin_ has such an elevated floor, that, when viewed under favourable conditions, it reminds one of a shallow oval tray or dish filled with fluid to the point of overflowing. These examples, very far from being exhaustive, will be sufficient to show that the walled-plains exhibit noteworthy differences in other respects than size, height of rampart, or included detail. Still another peculiarity, confined, it is believed, to a very few, may be mentioned, viz., convexity of floor, prominently displayed in Petavius, Mersenius, and Hevel.

MOUNTAIN RINGS.--These objects, usually encircled by a low and broken border, seldom more than a few hundred feet in height, are closely allied to the walled-plains. They are more frequently found on the Maria than elsewhere. In some cases the ring consists of isolated dark sections, with here and there a bright ma.s.s of rock interposed; in others, of low curvilinear ridges, forming a more or less complete circ.u.mvallation. They vary in size from 60 or 70 miles to 15 miles and less. The great ring north of Flamsteed, 60 miles across, is a notable example; another lies west of it on the north of Wichmann; while a third will be found south- east of Encke;--indeed, the Mare Procellarum abounds in objects of this type. The curious formation on the Mare Imbrium immediately south of Plato (called "Newton" by Schroter), may be placed in this category, as may also many of the low dusky rings of much smaller dimensions found in many quarters of the Maria. As has been stated elsewhere, these features have the appearance of having once been formations of a much more prominent and important character, which have suffered destruction, more or less complete, through being partially overwhelmed by the material of the "seas."

RING-PLAINS.--These are by far the most numerous of the ramparted enclosures of the moon, and though it is occasionally difficult to decide in which cla.s.s, walled-plain or ring-plain, some objects should be placed, yet, as a rule, the difference between the structural character of the two is abundantly obvious. The ring-plains vary in diameter from sixty to less than ten miles, and are far more regular in outline than the walled-plains. Their ramparts, often very ma.s.sive, are more continuous, and fall with a steep declivity to a floor almost always greatly depressed below the outside region. The inner slopes generally display subordinate heights, called terraces, arranged more or less concentrically, and often extending in successive stages nearly down to the interior foot of the wall. With the intervening valleys, these features are very striking objects when viewed under good conditions with high powers. In some cases they may possibly represent the effects of the slipping of the upper portions of the wall, from a want of cohesiveness in the material of which it is composed; but this hardly explains why the highest terrace often stands nearly as high as the rampart. Nasmyth, in his eruption hypothesis, suggests that in such a case there may have been two eruptions from the same vent; one powerful, which formed the exterior circle, and a second, rather less powerful, which has formed the interior circle. Ultimately, however, coming to the conclusion that terraces, as a rule, are not due to any such freaks of the eruption, he ascribes them to landslips. In any case, we can hardly imagine that material standing at such a high angle of inclination as that forming the summit ridge of many of the ring-plains would not frequently slide down in great ma.s.ses, and thus form irregular plateaus on the lower and flatter portions of the slope; but this fails to explain the symmetrical arrangement of the concentric terraces and intermediate valleys. The inner declivity of the north-eastern wall of Plato exhibits what to all appearance is an undoubted landslip, as does also that of Hercules on the northern side, and numerous other cases might be adduced; but in all of them the appearance is very different from that of the true terrace.

The _glacis_, or outer slope of a ring-plain, is invariably of a much gentler inclination than that which characterises the inner declivity: while the latter very frequently descends at an angle varying from 60 deg. to 50 deg. at the crest of the wall, to from 10 deg. to 2 deg. at the bottom, where it meets the floor; the former extends for a great distance at a very flat gradient before it sinks to the general level of the surrounding country. It differs likewise from the inner descent, in the fact that, though often traversed by valleys, intersected by deep gullies and irregular depressions, and covered with humpy excrescences and craters, it is only rarely that any features comparable to the terraces, usually present on the inner escarpment, can be traced upon it.

Elongated depressions of irregular outline, and very variable in size and depth, are frequently found on the outer slopes of the border. Some of them consist of great elliptical or sub-circular cavities, displaying many expansions and contractions, called "pockets," and suggesting the idea that they were originally distinct cup-shaped hollows, which from some cause or other have coalesced like rows of inosculating craters.

While many of these features are so deep that they remain visible for a considerable time under a low sun, others, though perhaps of greater extent, vanish in an hour or so.

As in the case of the walled-plains, the ramparts of the ring-plains exhibit gaps and are broken by craters and depressions, but to a much less extent. Often the lofty crest, surmounted by _aiguilles_ or by blunter peaks, towering in some cases to nearly double its alt.i.tude above the interior, is perfectly continuous (like Copernicus), or only interrupted by narrow pa.s.ses. It is a suggestive circ.u.mstance that gaps, other than valleys, are almost invariably found either in the north or south walls, or in both, and seldom in other positions. The b.u.t.tress, or long-extending spur, is a feature frequently a.s.sociated with the ring- plain rampart, as are also numbers of what, for the lack of a better name, must be termed little hillocks, which generally radiate in long rows from the outer foot of the slope. The spurs usually abut on the wall, and, either spreading out like the sticks of a fan or running roughly parallel to each other, extend for long distances, gradually diminis.h.i.+ng in height and width till they die out on the surrounding surface. They have been compared to lava streams, which those round Aristillus, Aristoteles, and on the flank of Clavius _a_, certainly somewhat resemble, though, in the two former instances, they are rather comparable to immense ridges. In addition to the above, the spurs radiating from the south-eastern rampart of Condamine and the long undulating ridges and rows of hillocks running from Cyrillus over the eastern _glacis_ of Theophilus, may be named as very interesting examples.

Neison and some other selenographers place in a distinct cla.s.s certain of the smaller ring-plains which usually have a steeper outer slope, and are supposed to present clearer indications of a volcanic origin than the ring-plains, terming them "Crater-plains."

CRATERS.--Under this generic name is placed a vast number of formations exhibiting a great difference in size and outward characteristics, though generally (under moderate magnification) of a circular or sub-circular shape. Their diameter varies from 15 miles or more to 3, and even less, and their flanks rise much more steeply to the summit, which is seldom very lofty, than those of the ring-plains, and fall more gradually to the floor. There is no portion of the moon in which they do not abound, whether it be on the ramparts, floors, and outer slopes of walled and ring plains, the summits and escarpments of mountain ranges, amid the intricacies of the highlands, or on the grey surface of the Maria. In many instances they have a brighter and newer aspect than the larger formations, often being the most brilliant points on their walls, when they are found in this position. Very frequently too they are not only very bright themselves, but stand on bright areas, whose borders are generally concentric with them, which s.h.i.+ne with a glistening l.u.s.tre, and form a kind of halo of light around them. Euclides and Bessarion A, and the craters east of Landsberg, are especially interesting examples. It seems not improbable that these areas may represent deposits formed by some kind of matter ejected from the craters, but whether of ancient or modern date, it is, of course, impossible to determine. Future observers will perhaps be in a better position to decide the question without cavil, if such eruptions should again take place. Like the larger enclosures, these smaller objects frequently encroach upon each other-- crater-ring overlapping crater-ring, as in the case of Thebit, where a large crater, which has interfered with the continuity of the east wall, has, in its turn, been disturbed by a smaller crater on its own east wall. The craters in many cases, possibly in the majority if we could detect them, have central mountains, some of them being excellent tests for telescopic definition--as, for example, the central peaks of Hortensius, Bessarion, and that of the small crater just mentioned on the east wall of Thebit A. A tendency to a linear arrangement is often displayed, especially among the smaller cla.s.s, as is also their occurrence in pairs.

CRATER-CONES.--These objects, plentifully distributed on the lunar surface, are especially interesting from their outward resemblance to the parasitic cones found on the flanks of terrestrial volcanoes (Etna, for instance). In the larger examples it is occasionally possible to see that the interiors are either inverted cones without a floor, or cup-shaped depressions on the summit of the object. Frequently, however, they are so small that the orifice can only be detected under oblique illumination.

Under a high sun they generally appear as white spots, more or less ill- defined, as on the floors of Archimedes, Fracastorius, Plato, and many other formations, which include a great number, all of which are probably crater cones, although only a few have been seen as such. It is a significant fact that in these situations they are always found to be closely a.s.sociated with the light streaks which traverse the interior of the formations, standing either on their surface or close to their edges.

The instrumental and meteorological requirements necessary for a successful scrutiny of the smallest type of these features, are beyond the reach of the ordinary observer in this country, as they demand direct observation in large telescopes under the best atmospheric conditions.

Some years ago Dr. Klein of Cologne called attention to some very interesting types of crater-cones, which may be found on certain dark or smoky-grey areas on the moon. These, he considers, may probably represent active volcanic vents, and urges that they should be diligently examined and watched by observers who possess telescopes adequate to the task. The most noteworthy examples of these objects are in the following positions:--(1) West of a prominent ridge running from Beaumont to the west side of Theophilus, and about midway between these formations; (2) in the Mare Vaporum, south of Hyginus; (3) on the floor of Werner, near the foot of the north wall; (4) under the east wall of Alphonsus, on the dusky patch in the interior; (5) on the south side of the floor of Atlas.

I have frequently described elsewhere with considerable detail the telescopic appearance of these features under various phases, and have pointed out that though large apertures and high powers are needed to see these cones to advantage, the dusky areas, easily traced on photograms, might be usefully studied by observers with smaller instruments, as if they represent the _ejecta_ from the crater-cones which stand upon them, changes in their form and extent could very possibly be detected. In addition to those already referred to, a number of mysterious dark spots were discovered by Schmidt in the dusky region about midway between Copernicus and Gambart, which Klein describes as perforated like a sieve with minute craters. A short distance south-west of Copernicus stands a bright crater-cone surrounded by a grey nimbus, which may be cla.s.sed with these objects. It is well seen under a high light, as indeed is the case with most of these features.

CRATERLETS, CRATER-PITS.--To a great extent the former term is needless and misleading, as the so-called craters merge by imperceptible gradations into very minute objects, as small as half a mile in diameter, and most probably, if we could more accurately estimate their size, still less. The crater-pit, however, has well-marked peculiarities which distinguish it from all other types, such as the absence of a distinguishable rim and extreme shallowness. They appear to be most numerous on the high-level plains and plateaus in the south-western quadrant, and may be counted by hundreds under good atmospheric conditions on the outer slopes of Walter, Clavius, and other large enclosures. In these positions they are often so closely aggregated that, as Nasmyth remarks, they remind one of an acc.u.mulation of froth. Even in an 8 1/2 inch reflector I have frequently seen the outer slope of the large ring-plain on the north-western side of Vendelinus, so perforated with these objects that it resembled pumice or vesicular lava, many of the little holes being evidently not circular, but square shaped and very irregular. The interior of Stadius and the region outside abounds in these minute features, but the well-known crater-row between this formation and Copernicus seems rather to consist of a number of inosculating crater-cones, as they stand very evidently on a raised bank of some alt.i.tude.

MOUNTAIN RANGES, ISOLATED MOUNTAINS, &c.--The more ma.s.sive and extended mountain ranges of the moon are found in the northern hemisphere, and (what is significant) in that portion of it which exhibits few indications of other superficial disturbances. The most prominently developed systems, the _Alps_, the _Caucasus_, and the _Apennines_, forming a mighty western rampart to the Mare Imbrium and giving it all the appearance of a vast walled plain, present few points of resemblance to any terrestrial chain. The former include many hundred peaks, among which, Mont Blanc rises to a height of 12,000 feet, and a second, some distance west of Plato, to nearly as great an alt.i.tude; while others, ranging from 5000 to 8000 feet, are common. They extend in a south-west direction from Plato to the Caucasus, terminating somewhat abruptly, a little west of the central meridian, in about N. lat. 42 deg. One of the most interesting features a.s.sociated with this range is the so-called great Alpine valley, which cuts through it west of Plato. The _Caucasus_ consist of a ma.s.sive wedge-shaped mountain land, projecting southwards, and partially dividing the Mare Imbrium from the Mare Serenitatis, both of which they flank. Though without peaks so lofty as those pertaining to the Alps, there is one, immediately east of the ring-plain Calippus, which, towering to 19,000 feet, surpa.s.ses any of which the latter system can boast. The _Apennines_, however, are by far the most magnificent range on the visible surface, including as they do some 3000 peaks, and extending in an almost continuous curve of more than 400 miles in length from Mount Hadley, on the north, to the fine ring-plain Eratosthenes, which forms a fitting termination, on the south. The great headland Mount Hadley rises more than 15,000 feet, while a neighbouring promontory on the south-east of it is fully 14,000 feet, and another, close by, is still higher above the Mare. Mount Huygens, again, in N. lat. 20 deg., and the square-shaped ma.s.s Mount Wolf, near the southern end of the chain, include peaks standing 18,000 and 12,000 feet respectively above the plain, to which their flanks descend with a steep declivity. The counterscarp of the Apennines, in places 160 miles in width from east to west, runs down to the Mare Vaporum with a comparatively gentle inclination. It is everywhere traversed by winding valleys of a very intricate type, all trending towards the south-west, and includes some bright craters and mountain-rings. The _Carpathians_, forming in part the southern border of the Mare Imbrium, extend for a length of more than 180 miles eastward of E., long. 16 deg., and, embracing the ring-plain Gay- Lussac, terminate west of Mayer. They present a less definite front to the Mare than the Apennines, and are broken up and divided by irregular valleys and gaps; their loftiest peak, situated on a very projecting promontory north-west of Mayer, rising to a height of 7000 feet.

Notwithstanding their comparatively low alt.i.tude, the region they occupy forms a fine telescopic picture at lunar sunrise. The _Sinus Iridum highlands_, bordering the beautiful bay on the north-east side of the Mare Imbrium, rank among the loftiest and most intricate systems on the moon, and, like the Apennines, present a steep face to the grey plain from which they rise, though differing from them in other respects. They include many high peaks, the loftiest, in the neighbourhood of the ring- plain Sharp, rising 15,000 feet. There are probably some still higher mountains in the vicinity, but the difficulties attending their measurement render it impossible to determine their alt.i.tude with any approach to accuracy.

_The Taurus Mountains_ extend from the west side of the Mare Serenitatis, near Le Monnier and Littrow, in a north-westerly direction towards Geminus and Berselius, bordering the west side of the Lacus Somniorum.

They are a far less remarkable system than any of the preceding, and consist rather of a wild irregular mountain region than a range. In the neighbourhood of Berselius are some peaks which, according to Neison, cannot be less than 10,000 feet in height.