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 peacock'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 analogy

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 passing out of

sunlight; and though his method was incapable of accuracy, and his

results consequently untrustworthy, it served to demonstrate the immense

altitude of these circumvallations, 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 Cassini,

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 substituted those of philosophers, mathematicians,

and other celebrities; and Cassini determined by actual measurement the

relative position of many of the principal 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, classed

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 massive 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 altitudes,

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 circumstances I have frequently seen

the surface, in many places covered with minute glittering points of

light, shining with a silvery lustre, 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 Oceanus Procellarum,

extending from a high northern latitude to beyond latitude 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 classed

with the Maria. This distinction, speaking generally, prevails among the

Maria,--those of larger size, such as the Oceanus 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 altitude 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

circumstances, 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 diminishing in altitude 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 class, 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 massed 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

class. The Oceanus 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 association

with craters of every size. Illustrations 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, passing through gaps in

their walls as, for example, in the cases of Madler, Lassell, &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 assume 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 assuming 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 class,

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 analogue, 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 massed 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

classification 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

circumvallation, the dimensions of their cavernous opening as compared

with that of the more or less truncated conical mass of matter

surrounding it, all afford a basis for grouping them under distinctive

titles, that are not only convenient to the selenographer, but which

undoubtedly represent, as a rule, actual diversities in their origin and

physical character.

These distinguishing titles, 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 passes, and broken by more recent craters and depressions. As

a rule, the area within the circumvallation (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 class. Here we have

a vast plain, 115 miles from side to side, encircled by a massive 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 latitude, with its noble 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 associated with other types of ringed enclosures, is by no

means invariably found within the walled-plains; though, as in the case

of _Petavius_, _Langrenus_, _Gassendi_, 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 coarse, rough grained, and covered with

minute elevations, although an hour or so afterwards it appears as smooth

as glass.

Although it is a distinguishing 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 _Longomontanus_. The first, which may be taken as

representative of the class (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 _Gassendi_, _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 mass of rock interposed; in others, of low

curvilinear ridges, forming a more or less complete circumvallation. 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 class, 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 massive, 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 masses, 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 altitude above

the interior, is perfectly continuous (like Copernicus), or only

interrupted by narrow passes. It is a suggestive circumstance 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 buttress, or

long-extending spur, is a feature frequently associated 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

diminishing 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 class 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 shine with a glistening lustre, 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 class, 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 associated 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 classed 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 accumulation 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 altitude.

MOUNTAIN RANGES, ISOLATED MOUNTAINS, &c.--The more massive 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 altitude; 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 associated with this range is the so-called

great Alpine valley, which cuts through it west of Plato. The _Caucasus_

consist of a massive 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, surpasses 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 mass 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 altitude, 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 altitude 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.

On the north side of the Mare Imbrium, east of Plato, there is a

beautiful narrow range of bright outlying heights, called the _Teneriffe

Mountains_, which include many isolated objects of considerable altitude,

one of the loftiest rising about 8000 feet. Farther towards the east lies

another group of a very similar character, called the _Straight Range_,

from its linear regularity. It extends from west to east for a distance

of about 60 miles, being a few miles shorter than the last, and includes

a peak of 6000 feet.

_The Harbinger Mountains_.--A remarkable group, north-west of

Aristarchus, including some peaks as high as 7000 feet, and other details

noticed in the catalogue.

The above comprise all the mountain ranges in the northern hemisphere of

any prominence, or which have received distinctive names, except the

_Hercynian Mountains_, on the north-east limb, east of the walled plain

Otto Struve. These are too near the edge to be well observed, but, from

what can be seen of them, they appear to abound in lofty peaks, and to

bear more resemblance to a terrestrial chain than any which have yet been

referred to.

The mountain systems of the southern hemisphere, except the ranges

visible on the limb, are far less imposing and remarkable than those just

described. The _Pyrenees_, on the western side of the Mare Nectaris,

extend in a meridional direction for nearly 190 miles, and include a peak

east of Guttemberg of nearly 12,000 feet, and are traversed in many

places by fine valleys.

_The Altai Mountains_ form a magnificent chain, 275 miles in length,

commencing on the outer eastern slope of Piccolomini, and following a

tolerably direct north-east course, with a few minor bendings, to the

west side of Fermat, where they turn more towards the north, ultimately

terminating about midway between Tacitus and Catherina. The region

situated on the south-east is a great table-land, without any prominent

features, rising gently towards the mountains, which shelve steeply down

to an equally barren expanse on the north-west, to which they present a

lofty face, having an average altitude of about 6000 feet. The loftiest

peak, over 13,000 feet, rises west of Fermat.

_The Riphaean Mountains_, a remarkably bright group, occupying an

isolated position in the Mare Procellarum south of Landsberg, and

extending for more than 100 miles in a meridional direction. They are

most closely aggregated at a point nearly due west of Euclides, from

which they throw off long-branching arms to the north and south, those on

the north bifurcating and gradually sinking to the level of the plain.

The loftiest peaks are near the extremity of this section, one of them

rising to 3000 feet. Two bright craters are associated with these

mountains, one nearly central, and the other south of it.

_The Percy Mountains_.--This name is given to the bright highlands

extending east of Gassendi towards Mersenius, forming the north-eastern

border of the Mare Humorum. They abound in minute detail--bright little

mountains and ridges--and include some clefts pertaining to the Mersenius

rill-system; but their most noteworthy feature is the long bright

mountain-arm, branching out from the eastern wall of Gassendi, and

extending for more than 50 miles towards the south-east.

The principal ranges on the limb are the _Leibnitz Mountains_, extending

from S. lat. 70 deg. on the west to S. lat. 80 deg. on the east limb.

They include some giant peaks and plateaus, noteworthy objects in

profile, some of which, according to Schroter and Madler, rise to 26,000

feet. The _Dorfel Mountains_, between S. lat. 80 deg. and 57 deg. on the

eastern limb, include, if Schroter's estimate is correct, three peaks

which exceed 26,000 feet. On the eastern limb, between S. lat. 35 deg.

and 18 deg., extend the _Rook Mountains_, which have peaks, according to

Schroter, as high as 25,000 feet. Next in order come the _Cordilleras_,

which extend to S. lat. 8 deg., and the _D'Alembert Mountains_, lying

east of Rocca and Grimaldi, closely associated with them, and probably

part of the same system. Some of the peaks approach 20,000 feet. In

addition to these mountain ranges there are others less prominent on the

limb in the northern hemisphere, which have not been named.

ISOLATED MOUNTAINS are very numerous in different parts of the moon, the

most remarkable are referred to in the appendix. Many remain unnamed.

CLEFTS OR RILLS.--Though Fontenelle, in his _Entretiens sur la Pluralite

des Mondes_, informs his pupil, the Marchioness, that "M. Cassini

discovered in the moon something which separates, then reunites, and

finally loses itself in a cavity, which from its appearance seemed to be

a river," it can hardly be supposed that what the French astronomer saw,

or fancied he saw, with the imperfect telescopes of that day, was one of

the remarkable and enigmatical furrows termed clefts or rills, first

detected by the Hanoverian selenographer Schroter; who, on October 7,

1787, discovered the very curious serpentine cleft near Herodotus, having

a few nights before noted for the first time the great Alpine valley west

of Plato, once classed with the clefts, though it is an object of a very

different kind. Between 1787 and 1797 Schroter found ten rills; but

twenty years elapsed before an addition was made to this number by the

discoveries of Gruithuisen, and, a short time after, by those of

Lohrmann, who in twelve months (1823-24) detected seventy. Kinau, Madler,

and finally Schmidt, followed, till, in 1866, when the latter published

his work, _Ueber Rillen auf dem Monde_, the list was thus summarised:--

In the 1st or N.W. quadrant 127 rills

In the 2nd or N.E. quadrant 75 rills

In the 3rd or S.E. quadrant 141 rills

In the 4th or S.W. quadrant 82 rills

or 425 in all. Since the date of this book the number of known rills has

been more than doubled; in fact, scarcely a lunation passes without new

discoveries being made.

The significance of the word _rille_ in German, a groove or furrow,

describes with considerable accuracy the usual appearance of the objects

to which it is applied, consisting as they do of long narrow channels,

with sides more or less steep, and sometimes vertical. They often extend

for hundreds of miles in approximately straight lines over portions of

the moon's surface, frequently traversing in their course ridges,

craters, and even more formidable obstacles, without any apparent check

or interruption, though their ends are sometimes marked by a mound or

crater. Their length ranges from ten or twelve to three hundred miles or

more (as in the great Sirsalis rill), their breadth, which is very

variable within certain limits, from less than half a mile to more than

two, and their depth (which must necessarily remain to a great extent

problematical) from 100 to 400 yards. They exhibit in the telescope a

gradation from somewhat coarse grooves, easily visible at suitable times

in very moderately sized instruments, to striae so delicate as to require

the largest and most perfect optical means and the best atmospheric

conditions to be glimpsed at all. Viewed under moderate amplification,

the majority of rills resemble deep canal-like channels with roughly

parallel sides, displaying occasionally local irregularities, and fining

off to invisibility at one or both ends. But, if critically scrutinised

in the best observing weather with high powers, the apparent evenness of

their edges entirely disappears, and we find that the latter exhibit

indentations, projections, and little flexures, like the banks of an

ordinary stream or rivulet, or, to use a very homely simile, the serrated

edges and little jagged irregularities of a biscuit broken across. In

some cases we remark crateriform hollows or sudden expansions in their

course, and deep sinuous ravines, which render them still more

unsymmetrical and variable in breadth. With regard to their distribution

on the lunar surface; they are found in almost every region, but perhaps

not so frequently on the surface of the Maria as elsewhere, though, as in

the case of the Triesnecker and other systems, they often abound in the

neighbourhood of disturbed regions in these plains, and in many cases

along their margins, as, for example, the Gassendi-Mersenius and the

Sabine-Ritter groups. The interior of walled plains are frequently

intersected by them, as in Gassendi, where nearly forty, more or less

delicate examples, have been seen; in Hevel, where there is a very

interesting system of crossed clefts, and within Posidonius. If we study

any good modern lunar map, it is evident how constantly they appear near

the borders of mountain ranges, walled-plains, and ring-plains; as, for

instance, at the foot of the Apennines; near Archimedes, Aristarchus,

Ramsden, and in many other similar positions. Rugged highlands also are

often traversed by them, as in the case of those lying west of Le Monnier

and Chacornac, and in the region west of the Mare Humorum. It may be here

remarked, however, as a notable fact, that the neighbourhood of the

grandest ring-mountain on the moon, Copernicus, is, strange to say,

devoid of any features which can be classed as true clefts, though it

abounds in crater-rows. The intricate network of rills on the west of

Triesnecker, when observed with a low power, reminds one of the wrinkles

on the rind of an orange or on the skin of a withered apple. Gruithuisen,

describing the rill-traversed region between Agrippa and Hyginus, says

that "it has quite the look of a Dutch canal map." In the subjoined

catalogue many detailed examples will be given relating to the course of

these mysterious furrows; how they occasionally traverse mountain arms,

cut through, either completely or partially (as in Ramsden), the borders

of ring-plains and other enclosures, while not unfrequently a small mound

or similar feature appears to have caused them to swerve suddenly from

their path, as in the case of the Ariadaeus cleft, and in that of one

member of the Mercator-Campanus system.

Of the actual nature of the lunar rills we are, it must be confessed,

supremely ignorant. With some of the early observers it was a very

favourite notion that they are artificial works, constructed presumably

by Kepler's _sub-volvani_, or by other intelligences. There is perhaps

some excuse to be made for the freaks of an exuberant fancy in regard to

objects which, if we ignore for a moment their enormous dimensions,

judged by a terrestrial standard, certainly have, in their apparent

absence of any physical relation to neighbouring objects, all the

appearance of being works of art rather than of nature. The keen-sighted

and very imaginative Gruithuisen believed that in some instances they

represent roads cut through interminable forests, and in others the

dried-up beds of once mighty rivers. His description of the Triesnecker

rill-system reads like a page from a geographical primer. A portion of it

is compared to the river Po, and he traces its course mile by mile up to

the "delta" at its place of disemboguement into the Mare Vaporum. From

the position of some rills with respect to the contour of the surrounding

country, it is evident that if water were now present on the moon, they,

being situated at the lowest level, would form natural channels for its

reception; but the exceptions to this arrangement are so numerous and

obvious, that the idea may be at once dismissed that there is any analogy

between them and our rivers. The eminent selenographer, the late W.K.

Birt, compared many of them to "inverted river-beds" from the fact that,

as often as not, they become broader and deeper as they attain a higher

level. The branches resemble rivers more frequently than the main

channels; for they generally commence as very fine grooves, and, becoming

broader and broader, join them at an acute angle. An attempt again has

been made to compare the lunar clefts with those vast gorges, the

marvellous results of aqueous action, called canyons, which attain their

greatest dimensions in North America; such as the Great Canyon of the

Colorado, which is at least 300 miles in length, and in places 2000 yards

in depth, with perpendicular or even overhanging sides; but the analogy,

at first sight specious, utterly breaks down under closer examination.

Some selenographers consider them to consist of long-extending rows of

confluent craters, too minute to be separately distinguished, and to be

thus due to some kind of volcanic action. This is undoubtedly true in

many instances, for almost every lunar region affords examples of crater-

rows merging by almost imperceptible gradations into cleft-like features,

and crater-rows of considerable size resemble clefts under low powers.

Still it seems probable that the greater number of these features are

immense furrows or cracks in the surface and nothing more; for the higher

the magnifying power employed in their examination, the less reason there

is to object to this description. Dr. Klein of Cologne believes that

rills of this class are due to the shrinkage of parts of the moon's

crust, and that they are not as a rule the result of volcanic causes,

though he admits that there may be some which have a seismic origin. No

good reason has as yet been given for the fact that they so frequently

cross small craters and other objects in their course, though it has been

suggested that the route followed by a rill from crater to crater in

these instances may be a line of least surface resistance, an explanation

far from being satisfactory.

Whether variations in the visibility of lunar details, when observed

under apparently similar conditions, actually occur from time to time

from some unknown cause, is one of those vexed questions which will only

be determined when the moon is systematically studied by experienced

observers using the finest instruments at exceptionally good stations;

but no one who examines existing records of observations of rills by

Gruithuisen, Lohrmann, Madler, Schmidt, and other observers, can well

avoid the conclusion that the anomalies brought to light therein point

strongly to the probability of the existence of some agency which

occasionally modifies their appearance or entirely conceals them from

view.

The following is one illustration out of many which might be quoted. At a

point in its course, nearly due north of the ring-plain Agrippa, the

great Ariadaeus cleft sends out a branch which runs into the well-known

Hyginus cleft, reminding one, as Dr. Klein remarks, of two rivers

connected in the shortest way by a canal. This uniting furrow was

detected by Gruithuisen, who observed it several times. On some occasions

it appeared perfectly straight, at others very irregular; but, what is

very remarkable, although two such accurate observers as Lohrmann and

Madler frequently scrutinised the region, neither of them saw a trace of

this object; and but for its rediscovery by Schmidt in 1862, its

existence would certainly have been ignored by selenographers as a mere

figment of Gruithuisen's too lively imagination. Dr. Klein has frequently

seen this rill with great distinctness, and at other times sought for it

in vain; though on each occasion the conditions of illumination,

libration, and definition were practically similar. I have sometimes

found this cleft an easy object with a 4 inch achromatic. Again, many

rills described by Madler as very delicate and difficult to trace, may

now be easily followed in "common telescopes." In short, the more direct

telescopic observations accumulate, and the more the study of minute

detail is extended, the stronger becomes the conviction, that in spite of

the absence of an appreciable atmosphere, there may be something

resembling low-lying exhalations from some parts of the surface which

from time to time are sufficiently dense to obscure, or even obliterate,

the region beneath them.

If, as seems most probable, these gigantic cracks are due to contractions

of the moon's surface, it is not impossible, in spite of the assertions

of the text-books to the effect that our satellite is now "a changeless

world," that emanations may proceed from these fissures, even if, under

the monthly alternations of extreme temperatures, surface changes do not

now occasionally take place from this cause also. Should this be so, the

appearance of new rills and the extension and modification of those

already existing may reasonably be looked for. Many instances might be

adduced tending to confirm this supposition, to one of which, as coming

under my notice, I will briefly refer. On the evening of November 11,

1883, when examining the interior of the great ring-plain Mersenius with

a power of 350 on an 8 1/2 inch reflector; in addition to the two closely

parallel clefts discovered by Schmidt, running from the inner foot of the

north-eastern rampart towards the centre, I remarked another distinct

cleft crossing the northern part of the floor from side to side. Shortly

afterwards, M. Gaudibert, one of our most experienced selenographers, who

has discovered many hitherto unrecorded clefts, having seen my drawing,

searched for this object, and, though the night was far from favourable,

had distinct though brief glimpses of it with the moderate magnifying

power of 100. Mersenius is a formation about 40 miles in diameter, with a

prominently convex interior, containing much detail which has received

more than ordinary attention from observers. It has, moreover, been

specially mapped by Schmidt and others, yet no trace of this rill was

noted, though objects much more minute and difficult have not been

overlooked. Does not an instance of this kind raise a well-grounded

suspicion of recent change which it is difficult to explain away?

To see the lunar clefts to the best advantage, they must be looked for

when not very far removed from the terminator, as when so situated the

black shadow of one side, contrasted with the usually brightly-

illuminated opposite flank, renders them more conspicuous than when they

are viewed under a higher sun. Though, as a rule, invisible at full moon,

some of the coarser clefts--as, for example, a portion of the Hyginus

furrow, and that north of Birt--may be traced as delicate white lines

under a nearly vertical light.

For properly observing these objects, a power of not less than 300 on

telescopes of large aperture is needed; and in studying their minute and

delicate details, we are perhaps more dependent on atmospheric conditions

than in following up any other branch of observational astronomy. Few

indeed are the nights, in our climate at any rate, when the rough,

irregular character of the steep interior of even the coarser examples of

these immense chasms can be steadily seen. We can only hope to obtain a

more perfect insight into their actual structural peculiarities when they

are scrutinised under more perfect climatic circumstances than they have

been hitherto. When observing the Hyginus cleft, Dr. Klein noticed that

at one place the declivities of the interior displayed decided

differences of tint. At many points the reflected sunlight was of a

distinctly yellow hue, while in other places it was white, as if the

cliffs were covered with snow. He compares this portion of the rill to

the Rhine valley between Bingen and Coblentz, but adds that the latter,

if viewed from the moon, would probably not present so fresh an

appearance, and would, of course, be frequently obscured by clouds.

Since the erection of the great Lick telescope on Mount Hamilton, our

knowledge of the details of some of the lunar clefts has been greatly

increased, as in the case of the Ariadaeus cleft, and many others.

Professor W.H. Pickering, also, at Arequipa, has made at that ideal

astronomical site many observations which, when published, will throw

more light upon their peculiar characteristics.

A few years ago M.E.L. Trouvelot of Meudon drew attention to a curious

appearance which he noted in connection with certain rills when near the

terminator, viz., extremely attenuated threads of light on their sites

and their apparent prolongations. He observed it in the ring-plain

Eudoxus, crossing the southern side of the floor from wall to wall; and

also in connection with the prominent cleft running from the north side

of Burg to the west of Alexander, and in some other situations. He terms

these phenomena _Murs enigmatiques_. Apparent prolongations of clefts in

the form of rows of hillocks or small mounds are very common.

FAULTS.--These sudden drops in the surface, representing local

dislocations, are far from unusual: the best examples being the straight

wall, or "railroad," west of Birt; that which strikes obliquely across

Plato; another which traverses Phocylides; and a fourth that has

manifestly modified the mountain arm north of Cichus. They differ from

the terrestrial phenomena so designated in the fact that the surface

indications of these are destroyed by denudation or masked by deposits of

subsequent date. In many cases on the moon, though its course cannot be

traced in its entirety by its shadow, yet the existence of a fault may be

inferred by the displacement and fracture of neighbouring objects.

VALLEYS.--Features thus designated, differing greatly both in size and

character, are met with in almost every part of the surface, except on

the grey plains. While the smallest examples, from their delicacy,

tenuity, and superficial resemblance to rills, are termed rill-valleys,

the larger and more conspicuous assume the appearance of coarse chasms,

gorges, or trough-like depressions. Between these two extremes, are many

objects of moderate dimensions--winding or straight ravines and defiles

bounded by steep mountains, and shallow dales flanked by low rounded

heights. The rill valleys are very numerous, only differing in many

instances from the true rills in size, and are probably due to the same

cause. Among the most noteworthy valleys of the largest class must, of

course, be placed the great valley of the Alps, one of the most striking

objects in the northern hemisphere, which also includes the great valley

south-east of Ukert. The Rheita valley, the very similar chasm west of

Reichenbach, and the gorge west of Herschel, are also notable examples in

the southern hemisphere. The borders of some of the Maria (especially

that of the Mare Crisium) and of many of the depressed rimless

formations, furnish instances of winding valleys intersecting their

borders: the hilly regions likewise often abound in long branching

defiles.

BRIGHT RAY-SYSTEMS.--Reference has already been made to the faint light

streaks and markings often found on the floors of the ring-mountains and

in other situations, and to the brilliant _nimbi_ surrounding some of the

smaller craters; but, in addition to these, many objects on the moon's

visible surface are associated with a much more remarkable and

conspicuous phenomenon--the bright rays which, under a high sun, are seen

either to radiate from them as apparent centres to great distances, or,

in the form of irregular light areas, to environ them, and to throw out

wide-spreading lucid beams, extending occasionally many hundreds of miles

from their origin. The more striking of these systems were recognised and

drawn at a very early stage of telescopic observation, as may be seen if

we consult the quaint old charts of Hevel, Riccioli, Fontana, and other

observers of the seventeenth century, where they are always prominently,

though very inaccurately, portrayed. The principal ray-systems are those

of Tycho, Copernicus, Kepler, Anaxagoras, Aristarchus, Olbers, Byrgius A,

and Zuchius; while Autolycus, Aristillus, Proclus, Timocharis, Furnerius

A, and Menelaus are grouped as constituting minor systems. Many

additional centres exist, a list of which will be found in the appendix.

The rays emanating from Tycho surpass in extent and interest any of the

others. Hundreds of distinct light streaks originate round the grey

margin of this magnificent object, some of them extending over a greater

part of the moon's visible superficies, and "radiating," in the words of

Professor Phillips, "like false meridians, or like meridians true to an

earlier pole of rotation." No systematic attempt has yet been made to map

them accurately as a whole on a large scale, for their extreme intricacy

and delicacy would render the task a very difficult one, and, moreover,

would demand a long course of observation with a powerful telescope in an

ideal situation; but Professor W.H. Pickering, observing under these

conditions at Arequipa, has recently devoted considerable attention both

to the Tycho and other rays, with especially suggestive and important

results, which may be briefly summarised as follows:--

(1.) That the Tycho streaks do not radiate from the apparent centre of

this formation, but point towards a multitude of minute craterlets on its

south-eastern or northern rims. Similar craterlets occur on the rims of

other great craters, forming ray-centres.

(2.) Speaking generally, a very minute and brilliant crater is located at

the end of the streak nearest the radiant point, the streak spreading out

and becoming fainter towards the other end. The majority of the streaks

appear to issue from one or more of these minute craters, which rarely

exceed a mile in diameter.

(3.) The streaks which do not issue from minute craters, usually lie upon

or across ridges, or in other similar exposed situations, sometimes

apparently coming through notches in the mountain walls.

(4.) Many of the Copernicus streaks start from craterlets within the rim,

flow up the inside and down the outside of the walls. Kepler includes two

such craterlets, but here the flow seems to have been more uniform over

the edges of the whole crater, and is not distinctly divided up into

separate streams.

(5.) Though there are similar craters within Tycho, the streaks from them

do not extend far beyond the walls. All the conspicuous Tycho streaks

originate outside the rim.

(6.) The streaks of Copernicus, Kepler, and Aristarchus are greyish in

colour, and much less white than those associated with Tycho: some white

lines extending south-east from Aristarchus do not apparently belong to

the system. In the case of craterlets lying between Aristarchus and

Copernicus the streaks point away from the latter.

(7.) There are no very long streaks; they vary from ten to fifty miles in

length, and are rarely more than a quarter of a mile broad at the crater.

From this point they gradually widen out and become fainter. Their width,

however, at the end farthest from the crater, seldom exceeds five miles.

These statements, especially those relating to the length of the streaks,

are utterly opposed to prevailing notions, but Professor Pickering

specifies the case of the two familiar parallel rays extending from the

north-east of Tycho to the region east of Bullialdus. His observations

show that these streaks, originating at a number of little craters

situated from thirty to sixty miles beyond the confines of Tycho, "enter

a couple of broad slightly depressed valleys. In these valleys are found

numerous minute craters of the kind above described, with intensely

brilliant interiors. When the streaks issuing from those craters near

Tycho are nearly exhausted, they are reinforced by streaks from other

craters which they encounter upon the way, the streaks becoming more

pronounced at these points. These streaks are again reinforced farther

out. These parallel rays must therefore not be considered as two streaks,

but as two series of streaks, the components of which are placed end to

end."

Thus, according to Professor Pickering, we must no longer regard the rays

emanating from the Tycho region and other centres as continuous, but as

consisting of a succession of short lengths, diminishing in brilliancy

but increasing in width, till they reach the next crater lying in their

direction, when they are reinforced; and the same process of gradual

diminution in brightness and reinforcement goes on from one end to the

other.

The following explanation is suggested to account for the origin of the

rays:--"The earth and her satellite may differ not so much as regards

volcanic action as in the densities of their atmospheres. Thus if the

craterlets on the rim of Tycho were constantly giving out large

quantities of gas or steam, which in other regions was being constantly

absorbed or condensed, we should have a wind uniformly blowing away from

that summit in all directions. Should other summits in its vicinity

occasionally give out gases, mixed with any fine white powder, such as

pumice, this powder would be carried away from Tycho, forming streaks."

The difficulty surrounding this very ingenious hypothesis is, that

though, assuming the existence of pumice-emitting craters and regions of

condensation, there might be a more or less lineal and streaky deposition

of this white material over large areas of the moon, why should this

deposit be so definitely arranged, and why should these active little

craters happen to lie on these particular lines?

The confused network of streaks round Copernicus seem to respond more

happily to the requirements of Professor Pickering's hypothesis, for here

there is an absence of that definiteness of direction so manifestly

displayed in the case of the Tycho rays, and we can well imagine that

with an area of condensation surrounding this magnificent object beyond

the limits of the streaks, and a number of active little craters on and

about its rim, the white material ejected might be drawn outwards in

every direction by wind currents, which possibly once existed, and,

settling down, assume forms such as we see.

Nasmyth's well-known hypothesis attributes the radiating streaks to

cracks in the lunar globe caused by the action of an upheaving force, and

accounts for their whiteness by the outwelling of lava from them which

has spread to a greater or less distance on either side. If the moon has

been fractured in this way, we can easily suppose that the craters formed

on these fissures, being in communication with the interior, might eject

some pulverulent white matter long after the rest of the surface with its

other types of craters had attained a quiescent stage.

The Tycho rays, when viewed under ordinary conditions, appear to extend

in unbroken bands to immense distances. One of the most remarkable,

strikes along the eastern side of Fracastorius, across the Mare Nectaris

to Guttemberg, while another, more central, extends, with local

variations in brightness, through Menelaus, over the Mare Serenitatis

nearly to the north-west limb. This is the ray that figures so

prominently in rude woodcuts of the moon, in which the Mare Serenitatis

traversed by it is made to resemble the Greek letter PHI. The Kepler,

Aristarchus, and Copernicus systems, though of much smaller extent, are

very noteworthy from the crossing and apparent interference of the rays;

while those near Byrgius, round Aristarchus, and the rays from Proclus,

are equally remarkable.

[Nichol found that the rays from Kepler cut through rays from Copernicus

and Aristarchus, while rays from the latter cut through rays from the

former. He therefore inferred that their relative ages stand in the

order,--Copernicus, Aristarchus, Kepler.]

As no branch of selenography has been more neglected than the observation

of these interesting but enigmatical features, one may hope that, in

spite of the exacting conditions as to situation and instrumental

requirements necessary for their successful scrutiny, the fairly equipped

amateur in this less favoured country will not be deterred from

attempting to clear up some of the doubts and difficulties which at

present exist as to their actual nature.

THE MOON'S ALBEDO, SURFACE BRIGHTNESS, &c.--Sir John Herschel maintained

that "the actual illumination of the lunar surface is not much superior

to that of weathered sandstone rock in full sunshine." "I have," he says,

"frequently compared the moon setting behind the grey perpendicular

facade of the Table Mountain, illuminated by the sun just risen in the

opposite quarter of the horizon, when it has been scarcely

distinguishable in brightness from the rock in contact with it. The sun

and moon being at nearly equal altitudes, and the atmosphere perfectly

free from cloud or vapour, its effect is alike on both luminaries."

Zollner's elaborate researches on this question are closely in accord

with the above observational result. Though he considers that the

brightest parts of the surface are as white as the whitest objects with

which we are acquainted, yet, taking the reflected light as a whole, he

finds that the moon is more nearly black than white. The most brilliant

object on the surface is the central peak of the ring-plain Aristarchus,

the darkest the floor of Grimaldi, or perhaps a portion of that of the

neighbouring Riccioli. Between these extremes, there is every gradation

of tone. Proctor, discussing this question on the basis of Zollner's

experiments respecting the light reflected by various substances,

concludes that the dark area just mentioned must be notably darker than

the dark grey syenite which figures in his tables, while the floor of

Aristarchus is as white as newly fallen snow.

The estimation of lunar tints in the usual way, by eye observations at

the telescope, involving as it does physiological errors which cannot be

eliminated, is a method far too crude and ambiguous to form the basis of

a scientific scale or for the detection of slight variations. An

instrument on the principle of Dawes' solar eyepiece has been suggested;

this, if used with an invariable and absolute scale of tints, would

remove many difficulties attending these investigations. The scale which

was adopted by Schroter, and which has been used by selenographers up to

the present time, is as follows:--

0 deg. = Black.

1 deg. = Greyish black.

2 deg. = Dark grey.

3 deg. = Medium grey.

4 deg. = Yellowish grey.

5 deg. = Pure light grey.

6 deg. = Light whitish grey.

7 deg. = Greyish white.

8 deg. = Pure white.

9 deg. = Glittering white.

10 deg. = Dazzling white.

The following is a list of lunar objects published in the

_Selenographical Journal_, classed in accordance with this scale:--

0 deg. Black shadows.

1 deg. Darkest portions of the floors of Grimaldi and Riccioli.

1 1/2 deg. Interiors of Boscovich, Billy, and Zupus.

2 deg. Floors of Endymion, Le Monnier, Julius Caesar, Cruger, and

Fourier _a_.

2 1/2 deg. Interiors of Azout, Vitruvius, Pitatus, Hippalus, and Marius.

3 deg. Interiors of Taruntius, Plinius, Theophilus, Parrot,

Flamsteed, and Mercator.

3 1/2 deg. Interiors of Hansen, Archimedes, and Mersenius.

4 deg. Interiors of Manilius, Ptolemaeus, and Guerike.

4 1/2 deg. Surface round Aristillus, Sinus Medii.

5 deg. Walls of Arago, Landsberg, and Bullialdus. Surface round

Kepler and Archimedes.

5 1/2 deg. Walls of Picard and Timocharis. Rays from Copernicus.

6 deg. Walls of Macrobius, Kant, Bessel, Mosting, and Flamsteed.

6 1/2 deg. Walls of Langrenus, Theaetetus, and Lahire.

7 deg. Theon, Ariadaeus, Bode B, Wichmann, and Kepler.

7 1/2 deg. Ukert, Hortensius, Euclides.

8 deg. Walls of Godin, Bode, and Copernicus.

8 1/2 deg. Walls of Proclus, Bode A, and Hipparchus c.

9 deg. Censorinus, Dionysius, Mosting A, and Mersenius B and c.

9 1/2 deg. Interior of Aristarchus, La Peyrouse DELTA.

10 deg. Central peak of Aristarchus.

TEMPERATURE OF THE MOON'S SURFACE.--Till the subject was undertaken some

years ago by Lord Rosse, no approach was made to a satisfactory

determination of the surface temperature of the moon. From his

experiments he inferred that the maximum temperature attained, at or near

the equator, about three days after full moon, does not exceed 200 deg.

C., while the minimum is not much under zero C. Subsequent experiments,

however, both by himself and Professor Langley, render these results more

than doubtful, without it is admitted that the moon has an atmospheric

covering. Langley's results make it probable that the temperature never

rises above the freezing-point of water, and that at the end of the

prolonged lunar night of fourteen days it must sink to at least 200 deg.

below zero. Mr. F.W. Verey of the Alleghany Observatory has recently

conducted, by means of the bolometer, similar researches as to the

distribution of the moon's heat and its variation with the phase, by

which he has deduced the varying radiation from the surface in different

localities of the moon under various solar altitudes.

LUNAR OBSERVATION.--In observing the moon, we enjoy an advantage of which

we cannot boast when most other planetary bodies are scrutinised; for we

see the actual surface of another world undimmed by palpable clouds or

exhalations, except such as exist in the air above us; and can gaze on

the marvellous variety of objects it presents much as we contemplate a

relief map of our own globe. But inasmuch as the manifold details of the

relief map require to be placed in a certain light to be seen to the best

advantage, so the ring-mountains, rugged highlands, and wide-extending

plains of our satellite, as they pass in review under the sun, must be

observed when suitable conditions of illumination prevail, if we wish to

appreciate their true character and significance.

As a general rule, lunar objects are best seen when they are at no great

distance from "the terminator," or the line dividing the illumined from

the unillumined portion of the spherical surface. This line is constantly

changing its position with the sun, advancing slowly onwards towards the

east at a rate which, roughly speaking, amounts to about 30.5 min. in an

hour, or passing over 10 deg. of lunar longitude in about 19 hrs. 40

mins. When an object is situated on this line, the sun is either rising

or setting on the neighbouring region, and every inequality of the

surface is rendered prominent by its shadow; so that trifling variations

in level and minor asperities assume for the time being an importance to

which they have no claim. If we are observing an object at lunar sunrise,

a very short time, often only a few minutes, elapses before the confusion

caused by the presence of the shadows of these generally unimportant

features ceases to interfere with the observation, and we can distinguish

between those details which are really noteworthy and others which are

trivial and evanescent. Every formation we are studying should be

observed, and drawn if possible, under many different conditions of

illumination. It ought, in fact, to be examined from the time when its

loftiest heights are first illumined by the rising sun till they

disappear at sunset. This is, of course, practically impossible in the

course of one lunation, but by utilising available opportunities, a

number of observations may be obtained under various phases which will be

more or less exhaustive. It cannot be said that much is known about any

object until an attempt has been made to carry out this plan. Features

which assume a certain appearance at one phase frequently turn out to be

altogether different when viewed under another; important details

obscured by shadows, craters masked by those of neighbouring objects, or

by the shadows of their own rims, are often only revealed when the sun

has attained an altitude of ten degrees or more. In short, there is

scarcely a formation on the moon which does not exemplify the necessity

of noting its aspect from sunrise to sunset. Regard must also be had to

libration, which affects to a greater or less degree every object;

carrying out of the range of observation regions near the limb at one

time, and at another bringing into view others beyond the limits of the

maps, which represent the moon in the mean state of libration. The area,

in fact, thus brought into view, or taken out of it, is between 1/12th

and 1/13th of the entire area of the moon, or about the 1/6th part of the

hemisphere turned away from the earth. It is convenient to bear in mind

that we see an object under nearly the same conditions every 59 d. 1 h.

28 m., or still more accurately, after the lapse of fifteen lunations, or

442 d. 23 h. Many observers avoid the observation of objects under a high

light. This, however, should never be neglected when practicable, though

in some cases it is not easy to carry out, owing to the difficulty in

tracing details under these circumstances.

Although to observe successfully the minuter features, such as the rills

and the smaller craterlets, requires instruments of large aperture

located in favourable situations, yet work of permanent value may be

accomplished with comparatively humble telescopic means. A 4 inch

achromatic, or a silver-on-glass reflector of 6 or 6 1/2 inches aperture,

will reveal on a good night many details which have not yet been

recorded, and the possessor of instruments of this size will not be long

in discovering that the moon, despite of what is often said, has not been

so exhaustively surveyed that nothing remains for him to do.

Only experience and actual trial will teach the observer to choose the

particular eyepiece suitable for a given night or a given object. It will

be found that it is only on very rare occasions that he can accomplish

much with powers which, perhaps only on two or three nights in a year in

this climate, tell to great advantage; though it sometimes happens that

the employment of an eyepiece, otherwise unsuitable for the night, will,

during a short spell of good definition, afford a fleeting glimpse of

some difficult feature, and thus solve a doubtful point. It has often

been said that the efficiency of a telescope depends to a great extent on

"the man at the eye end." This is as true in the case of the moon as it

is in other branches of observational astronomy.

Observers, especially beginners, frequently fall into great error in

failing to appreciate the true character of what they see. In this way a

shallow surface depression, possibly only a few feet below the general

level of the neighbouring country, is often described as a "vast gorge,"

because, under very oblique light, it is filled with black shadow; or an

insignificant hillock is magnified into a mountain when similarly viewed.

Hence the importance, just insisted on, of studying lunar features under

as many conditions as possible before finally attempting to describe

them.

However indifferent a draughtsman an observer may be, if he endeavours to

portray what he sees to the best of his ability, he will ultimately

attain sufficient skill to make his work useful for future reference: in

any case, it will be of more value than a mere verbal description without

a sketch. Doubt and uncertainty invariably attend to a greater or less

extent written notes unaccompanied by drawings, as some recent

controversies, respecting changes in Linne and elsewhere, testify. Now

that photographs are generally available to form the basis of a more

complete sketch, much of the difficulty formerly attending the correct

representation of the outline and grosser features of a formation has

been removed, and the observer can devote his time and attention to the

insertion and description of less obvious objects.

PROGRESS OF SELENOGRAPHY.--Till within recent years, the systematic study

of the lunar surface may be said to have been confined, in this country

at any rate, to a very limited number of observers, and, except in rare

instances, those who possessed astronomical telescopes only directed them

to the moon as a show object to excite the wonder of casual visitors. The

publication of Webb's "Celestial Objects" in 1859, the supposed physical

change in the crater Linne, announced in 1866, and the appearance of an

unrecorded black spot near Hyginus some ten years later, had the effect

of awakening a more lively interest in selenography, and undoubtedly

combined to bring about a change in this respect, which ultimately

resulted in the number of amateurs devoting much of their time to this

branch of observational astronomy being notably increased. Still, large

telescopes, as a rule, held aloof for some unexplained reason, or were

only employed in a desultory and spasmodic fashion, without any very

definite object. When the Council of the British Association for the

Advancement of Science, stimulated by the Linne controversy, deemed the

moon to be worthy of passing attention, observations, directed to objects

suspected of change (the phenomena on the floor of Plato) were left to

three or four observers, under the able direction of Mr. Birt, the

largest instruments available being an 8 1/4 inch reflector and the

Crossley refractor of 9 inches aperture! During the last decade, however,

all this has been changed, and we not only have societies, such as the

British Astronomical Association, setting apart a distinct section for

the systematic investigation of lunar detail, but some of the largest and

most perfect instruments in the world, among them the noble refractor on

Mount Hamilton, employed in photographing the moon or in scrutinising her

manifold features by direct observation. Hence, it may be said that

selenography has taken a new and more promising departure, which, among

other results, must lead to a more accurate knowledge of lunar

topography, and settle possibly, ere long, the vexed question of change,

without any residuum of doubt.

Lunar photography as exemplified by the marvellous and beautiful pictures

produced at the Lick Observatory under the auspices of Dr. Holden, and

the exquisite enlargements of them by Dr. Weinek of Prague; at Paris by

the brothers Henry; and at Brussels by M. Prinz; point to the not far

distant time when we shall possess complete photographic maps on a large

scale of the whole visible disc under various phases of illumination,

which will be of inestimable value as topographical charts. When this is

accomplished, the observer will have at his command faithful

representations of any formation, or of any given region he may require,

to utilise for the study of the smaller details by direct observation.

Desultory and objectless drawings and notes have hitherto been more or

less characteristic of the work done, even by those who have given more

than ordinary attention to the moon. Though these, if duly recorded, are

valuable as illustrating the physical structure, the estimated brightness

under various phases, and other peculiarities of lunar features, they do

not materially forward investigations relating to the discovery of

present lunar activity or to the detection of actual change. It is

reiterated _ad nauseam_ in many popular books that the moon is a

changeless world, and it is implied that, having attained a state when no

further manifestations of internal or external forces are possible, it

revolves round the earth in the condition, for the most part, of a

globular mass of vesicular lava or slag, possessing no interest except as

a notable example of a "burnt-out planet." In answer to these dogmatic

assertions, it may be said that, notwithstanding the multiplication of

monographs and photographs, the knowledge we possess, even of the larger

and more prominent objects, is far too slight to justify us in

maintaining that changes, which on earth we should use a strong adjective

to describe, have not taken place in connection with some of them in

recent years. Would the most assiduous observer assert that his knowledge

of any one of the great formations, in the south-west quadrant, for

example, is so complete that, if a chasm as big as the Val del Bove was

blown out from its flanks, or formed by a landslip, he would detect the

change in the appearance of an area (some three miles by four) thus

brought about, unless he had previously made a very prolonged and

exhaustive study of the object? Or, again, among formations of a

different class, the craters and crater-cones; might not objects as large

as Monte Nuovo or Jorullo come into existence in many regions without any

one being the wiser? It would certainly have needed a persistent lunar

astronomer, and one furnished with a very perfect telescope, to have

noted the changes that have occurred within the old crater-ring of Somma

or among the Santorin group during the past thirty years, or even to have

detected the effects resulting from the great catastrophe in A.D. 79, at

Vesuvius; yet these objects are no larger than many which, if they were

situated on our satellite, would be termed comparatively small, if not

insignificant.

One of the principal aims of lunar research is to learn as much as

possible as to the present condition of the surface. Every one qualified

to give an opinion will admit that this cannot be accomplished by roaming

at large over the whole visible superficies, but only by confining

attention to selected areas of limited extent, and recording and

describing every object visible thereon, under various conditions of

illumination, with the greatest accuracy attainable. This plan was

suggested and inaugurated nearly thirty years ago by Mr. Birt, under the

patronage of the British Association; but as he proposed to deal with the

entire disc in this way, the magnitude and ambitious character of the

scheme soon damped the ardour of those who at first supported it, and it

was ultimately abandoned. It was, however, based on the only feasible

principle which, as it seems to the writer, will not result in doubt and

confusion. Now that photography has come to the assistance of the

observer, Mr. Birt's proposal, if confined within narrower limits, would

be far less arduous an undertaking than before, and might be easily

carried out. A complete photographic survey of a few selected regions, as

a basis for an equally thorough and exhaustive scrutiny by direct

observation, would, it is believed, lead to a much more satisfactory and

hopeful method for ultimately furnishing irrefragable testimony as to

permanency or change than any that has yet been undertaken.