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Full text of "The moon considered as a planet, a world, and a satellite"

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THE MOON 



COXSIDERED AS 



A PLANET, A AVORLD, AND A SATELLITE. 



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THE MOON 



CONSIDERED AS 



A PLANET, A WOELD, AND A SATELLITE. 



By JAMES NASMYTH, C.E. 



AND 



JAMES CAEPENTER, F.E.A.S. 

LATE OF THE ROYAL OBSERVATORY, GREENWICH. 



WITH TWENTY-SIX ILLUSTRATIVE PLATES OF LUNAR OBJECTS, PHENOMENA, 
AND SCENERY: NUMEROUS WOODCUTS, &c. 




LONDON: 
JOHN MUKEAY, ALBEMARLE STREET. 

1885. 



V\3 



LONDOX : 
BRADBURY, AOXEW, & CO,, PRINTERS, WHITEFRURS. 



3/J^4^ 



Cic" 



TO 

HIS GEACE THE DUKE OF AKGYLL, 

m RECOGNITION OF HIS LONG CONTINUED INTEEEST IN THE SUBJECT 
OF WHICH IT TREATS, 



^fti^ Volume 



IS MOST RESPECTFULLY DEDICATED 
BY 



THE AUTHORS. 



PEEFACE, 



The reason for this book's appearance may be set forth 
in a few words. A long course of careful scrutiny of 
the lunar surface with the aid of telescopes of considerable 
power, and a consequent familiarity with the wonderful 
details there presented, convinced us that there was yet 
something to be said about the moon, that existing works 
on Astronomy did not contain. Much valuable labour has 
been bestowed upon the topography of the moon, and this 
subject we do not pretend to advance. Enough has also 
been written for the benefit of those who desire an acquaint- 
ance with the intricate movements of the moon in space ; 
and accordingly we pass this subject without notice. But 
very little has been written respecting the moon's 
physiography, or the causative phenomena of the features, 
broad and detailed, that the surface of our satellite presents 
for study. Our observations had led us to some con- 
clusions, respecting the cause of volcanic energy and the 
mode of its action as manifested in the characteristic 



viii PREFACE. 

craters and other eruptive phenomena that abound upon 
the moon's surface. We have endeavoured to explain these 
phenomena by reference to a few natural laws, and to 
connect them with the general hypothesis of planet forma- 
tion which is now widely accepted by cosmologists. The 
principal aim of our work is to lay these proffered explana- 
tions before the students and admirers of astronomy and 
science in general ; and we trust that what we have 
deduced concerning the moon may be taken as referring to 
a certain extent to other planets. 

Some reflections upon the moon considered as a world, 
in reference to questions of habitability, and to the peculiar 
conditions which would attend a sojourn on the lunar 
surface, have appeared to us not inappropriate. These, 
though instructive, are rather curious than important. 
More worthy of respectful consideration are the few 
remarks we have offered upon the moon as a satellite and 
a benefactor to the inhabitants of this Earth. 



In reference to the Illustrations accompanying this work, 
more especially those which represent certain portions of 
the lunar surface as they are revealed by the aid of powerful 
telescopes, such as those which we employed in our scrutiny, 
it is proper that we should say a few words here on the 
means by which they have been produced. 



PREFACE. ix 

During upwards of thirty years of assiduous observation, 
every favourable opportunity has been seized to educate the 
eye, not only in respect to comprehending the general 
character of the moon's 'surface, but also to examining 
minutely its marvellous details under every variety of 
phase, in the hope of rightly understanding their true 
nature, as well as the causes which had produced them. 
This object was aided by making careful drawings of each 
portion or object when it was most favourably presented 
in the telescope. These drawings were again and again 
repeated, revised, and compared with the actual objects, 
the eye thus advancing in correctness and power of 
appreciating minute details, while the hand was acquiring, 
by assiduous practice, the art of rendering correct repre- 
sentations of the objects in view. In order to present these 
Illustrations with as near an approach as possible to the 
absolute integrity of the original objects, the idea occurred 
to us that by translating the drawings into models which, 
when placed in the sun's rays, would faithfully reproduce 
the lunar effects of light and shadow, and then photo- 
graphing the models so treated, we should produce most 
faithful representations of the original. The result was in 
every way highly satisfactory, and has yielded pictures 
of the details of the lunar surface such as we feel every 
confidence in submitting to those of our readers who have 
made a special study of the subject. It is hoped that those 
also who have not had opportunity to become intimately 



X PREFACE. 

acquainted with the details of the lunar surface, will be 
enabled to become so by aid of these Illustrations. 

In conclusion, we think it desirable to add that the 
photographic Illustrations above referred to are printed by- 
well-established pigment processes which ensure their entire 
permanency. 



PEEFACE TO THE THIED EDITION. 



The first and second editions of this work, which has 
been so well received by those who are specially qualified 
to judge of its value, have been out of print for several 
years, and as enquiries for copies continue to be made we 
have been induced to bring out a new edition, in a more 
compact size and at a reduced price. It is hoped that these 
qualifications may bring the book within the reach of many 
who have hitherto been unable to obtain it. 



CONTENTS. 



CHAPTER I. 

ON THE COSMICAL OKIGIN OF THE PLANETS OF THE SOLAE 

SYSTEM. 



PAGE 



Origination of Material Things — Celestial Vapours — Nebulae — Their vast 
Numbers — Sir W. Herschel's Observations and Classification — Buffon's 
Cosmogony — Laplace's Nebular Hypothesis — Doubts upon its 
Validity — Support from Spectrum Analysis 1 



CHAPTER IT. 

THE GENERATION OF COSMICAL HEAT. 

Conservation of Force — Indestructibility of Force — Its Convertibility into 
Heat — Dawn of the Doctrine — Mayer's Deductions — Joule's Experi- 
ments — Mechanical Equivalent of Heat — Gravitation the Source of , 
Cosmical Heat— Calculations of Mayer and Helmholtz — The Moon as 
an Incandescent Sphere — Not necessarily Burning — ^Loss of Heat by 
Radiation — Cooling of External Crust — Commencement of Selenor 
logical History 13 



CHAPTER III. 

THE SUBSEQUENT COOLING OF THE IGNEOUS BODY. 

Cooling commenced from Outer Surface — Contraction by Cooling— Expan- 
sion of Molten Matter upon Solidification — Water not exceptional — 
Similar Behaviour of Molten Iron — Floating of Solid on Molten Metal 
— Currents in a Pot of Molten Metal — Bursting of Iron Bottle by 
Congelation of Bismuth within — Evidence from Furnace Slag — From 
the Crater of Vesuvius — Effects of Contraction of Moon's Crust and 
Expansion of Interior — Production of Ridges and Wrinkles — Theory of 
Wrinkles — Examples from Shrivelled Apple and Hand ... 21 



xu CONTENTS. 



CHAPTER TV. 

THE FORM, MAGNITUDE, WEIGHT, AND DENSITY OF THE 
LUNAR GLOBE. 

PAGE 

Form of Moon — Not perfectly Spherical — Bulged towards Earth — Diameter 
— Angular Measure — Linear Measure — Parallax of Moon — Distance — 
Area of Lunar Sphere — Solid Contents — Mass of Moon — Law of Gravi- 
tation — Mass determined by Tides and other Means — Density — How 
obtained — Specific Gravity of Lunar Matter — Force of Gravity at 
Surface — How determined — Weights of similar Bodies on Earth and 
Moon — Effects of like Forces acting against Gravity on Earth and 
Moon . 35 



CHAPTER V. 

ON THE EXISTENCE OR NON-EXISTENCE OF A LUNAR 
ATMOSPHERE. 

Subject of Controversy — Phenomena of Terrestrial Atmosphere — No Counter- 
parts on Moon — Negative Evidence from Solar Eclipses — No Twilight 
on Moon — Evidence from Spectrum Analysis — From Occultations of 
Stars — Absence of Water or Moisture — Cryophorus — No Reddening of 
Sun's Rays by Vapours on Moon — No Air or Water to complicate 
Discussions of Lunar Volcanic Phenomena 44 



CHAPTER VI. 

THE GENERAL ASPECT OF THE LUNAR SURFACE. 

Pre-Telescopic Ideas — Human Countenance — Other supposed Resemblances — 
Portrait of Full Moon — Permanence of Features — Rotation of Moon — 
Solar Period and Solar Day on Moon — Libration — Diurnal — In 
Latitude — In Longitude — Visible and Invisible Hemispheres — Teles- 
copic Scrutiny — Galileo's Views — Features Visible with Low Power — 
Low Powers on small and large Telescopes — Salient Features — Craters 
— Plains — Bright Streaks— Mountains — Higher Telescopic Powers — 
Detail Scrutiny of Features therewith — Discussion of High Powers — 
Education of Eye — Highest practicable Power — Size of smallest 
Visible Objects 58 

CHAPTER VII. 

TOPOGRAPHY OF THE MOON. 

Eeasons for Mapping the Moon — Early Maps — ^Labours of Langreen — 
Hevelius — Riccioli — Cassini — Schroeter — Modem Maps — Lohrman's — 
Beer and Maedler's — Excellence of the last — Measurement of 



CONTENTS: 



Mountain Heights — ^Need of a Picture Map — Formation of our own — 
Skeleton Map — Table of conspicuous Objects — Descriptions of special 
Objects — Copernicus — Gassendi — Eudoxus and Aristotle — Triesnecker 
— Theophilus, Cyrillus, and Catharina — Ptolemy, Alphons, and Arza- 
chael — Thebit — Plato — Valley of the Alps — Pico — Tycho — Wargentin 
— Aristarchus and Herodotus — Walter — Archimedes and the Apennines 74 

CHAPTER YIII. 

ON LUNAR CEATEES. 

Use of term Crater for Terrestrial and Lunar Formations — Truly Volcanic 
Nature of Lunar Craters — Terrestrial and Lunar Volcanic Areas 
compared — Similarity — Difference only in Magnitude — Central Cone 
— Found in great ndd small Lunar Craters — Formative Process 
of Terrestrial Volcanoes — Example from Vesuvius — Vast Size of 
Lunar Craters — Eeasons assigned — Origin of Moon's Volcanic Force 
— Aqueous Vapour Theory untenable — Expansion upon Solidifi- 
cation Theory —Formative Process of a Lunar Crater — Volcanic Vent 
— Commencement of Eruption — Erection of Eampart — Hollowing of 
Crater — Formation of Central Cone — Of Plateau — ^Various Heights of 
Plateaux — Coneless Craters — Filled-up Craters — Multiple Cones — 
Craters on Plateau — Double Eamparts — Landslip Terraces — Eutted 
Eamparts — Overlapping and Superposition of Craters — Source-Connec- 
tion of such — Frothlike Aggregations of Craters — Majestic Dimensions 
of Larger Craters 74 

CHAPTER IX. 
ON THE GEEAT EING-FOEMATIONS NOT MANIFESTLY VOLCANIC. 

Absence of Central Cones — Vast Diameters — Difficult of Explanation — 
Hooke''s Idea — Suggested Cause of True Circularity — Scrope's Hypo- 
thesis of Terrestrial Tumescences — Eozet's Tourbillonic Theory — 
Dana's Ebullition Theory 133 

CHAPTER X. 

PEAKS AND MOUNTAIN EANGES. 
Paucity of extensive Mountain Systems on Moon — Contrast with Earth — • 
Lunar Mountains found in less disturbed Eegions — Lunar Apennines, 
Caucasus, and Alps — Valley of Alps — " Crag and Tail " Contour — 
Isolated Peaks — How produced — Analogy from Freezing Fountain — 
Terrestrial Counterparts and their Explanation by Scrope — Blowing 
Cone on Teneriffe — Comparative Gentleness of Mountain-forming 
Action — Eelation between Mountain Systems and Crater Systems — 
Wrinkle Eidges 140 



xiv CONTENTS. 

CHAPTER XI. 

CEACKS AND RADIATING STREAKS. 

PAGE 

Description — Divergence from Focal Craters — Experimental Explanation of 
their Cause — Radial Cracking of Crust — Outflow of Matter there- 
from — Analogy from " Starred " Ice — No Shadows cast by Streaks — 
Their probable Slight Elevation— Open Cracks — Great Numbers — 
Length — Depth — In-fallen Fragments — Shrinkage a Cause of Cracks 
— ^Lateness of their Production 150 



CHAPTER XII. 

COLOUR AND BRIGHTNESS OF LUNAR DETAILS : CHRONOLOGY 
OF FORMATIONS, AND FINALITY OF EXISTING FEATURES. 

Absence of Conspicuous Colour — Slight tints of " Seas " — Cause — Probable 
Variety of Tints in small Patches — Diversity of Brightness of Details 
— Most Conspicuous at Full Moon— Classification of Shades — Exag- 
gerated Contrasts in Photographs — Brightest Portions probably the 
latest formed — Chronology of Formations — Large Craters older than 
Small — Mountains older than Craters — Bright Streaks comparatively 
recent — Cracks most recent of all Features — Question of existing 
Change — Evidence from Observation — Paucity of such Evidence — 
Supposed Case of Linne — Theoretical Discussion — Relative Cooling 
Tendencies of Earth and Moon — Earth nearly assumed its Final 
Condition — Moon probably cooled Ages upon Ages ago — Possible slight 
Changes from Solar Heating — Disintegrating Action . . .161 



CHAPTER XIII. 

THE MOON AS A WORLD : DAY AND NIGHT UPON ITS SURFACE. 
Existence of Habitants on other Planets — Interest of the Question — Con- 
ditions of Life — Absence of these from Moon — No Air or Water and 
intense Heat and Cold — Possible Existence of Protogerms of Life — A 
Day on the Moon imagined — Instructiveness of the Realization — 
Length of Lunar Day — No Dawn or Twilight — Sudden Appearance of 
Light— Slowness of Sun in Rising— No Atmospheric Tints— Blackness 
• of Sky and Visibility of Stars and Fainter Luminosities at Noon-Day — 
Appearance of the Earth as a Stationary Moon — Its Phases — Eclipse of 
Sun by Earth — Attendant Phenomena — Lunar Landscape — Height 
Essential to secure a Point of View — Sunrise on a Crater — Desolation 
of Scene— No Vestige of Life— Colour of Volcanic Products— No At- 
mospheric Perspective— Blackness of Shadows— Impressions on other 
Senses than Sight— Heat of Sun untempered— Intense Cold in Shade 
— Dead Silence— No Medium to conduct Sound — Lunar Afternoon 



CONTENTS. XV 

PAGE 

and Sunset — Night — The Earth a Moon— Its Size, Eotation, and 
Features — Shadow of Moon upon it — Lunar Night-Sky — Constellations 
— Comets and Planets — No Visible Meteors — Bombardment by Dark 
Meteoric Masses — Lunar Landscape by Night — Intensity of Cold . 175 



CHAPTER XIV. 

THE MOON AS A SATELLITE : ITS RELATION TO THE EARTH 

AND MAN. 

The Moon as a Luminary— Segoadary Nature of Light-giving Function- 
Primary Office as a Sanitary Agent — Cleansing Effects of the Tides — 
Tidal Rivers and Transport thereby — The Moon a " Tug " — Available 
Power of Tides— Tide-Mills— Transfer of Tidal Power Inland— The 
Moon as a Navigator's Guide— Longitude found by the Moon — Moon's 
Motions — Discovered by Observations — Grouped into Theories — Repre- 
sented by Tables — The Nautical Almanac — The Moon as a Long- 
Period Timekeeper — Reckoning by " Moons " — Eclipses the Starting- 
Points of Chronologies — Furnish indisputable Dates — Solar Surround- 
ings revealed by Eclipses' when Moon screens the Sun — Solar Corona — 
Moon as a Medal of Creation, a Half -formed "World — Abuses of the 
Moon — Superstitions — Erroneous Ideas regarding Moonlight betrayed 
by Artists and Authors — The Moon and the Weather — Errors and Facts 
— ^Atmospheric Tides — ^Warmth from Moon — Paradoxical Effect in 
cooling the Earth 193 



CHAPTER XV. 
CONCLUDING SUMMARY 208 



LIST OF PLATES, 



( To face each other 33 



\ . To face each otlicr 101 



♦ 

PLATES. PAGE 

Gassendi Frontispiece. 

I. Ceatee op Vesuvius, 1864 Tofacexmge 29 

II. Back of Hand 

III. Sheivelled Apple 

IV. Full Moon Tofaceimge 59 

V. Pictuee Map of the Moon . 79 

VI. Vesuvius and Neighbouehood of Naples ) 

VII. POETION OF THE MoON'S SUEFACE 

VIII. COPEENICUS Tofaceimge 110 

IX. The Lunae Apennines, Aechimedes, etc 114 

X. Aeistotle and Eudoxus To face page 120 

XI. Teiesneckee 124 

XII. Theophilus, Cyeillus, and Cathaeina 128 

XIII. Ptolemy,. Alphons, Aezachael, etc 132 

XIV. Plato, the Valley of the Alps, Pico, etc 136 

XV. Meecatoe and Campanus 140 

XVI. Tycho and its Sueeoundings 144 

XVII. Ideal Sketch of Pico 148 

XVIII. Glass Globe, Ceacked by Inteenal Peessuee \ 

\ To face each other 151 
XIX. Full Moon ) 

XX. WAEGENTIN Tofaceimge 154 

XXI. Aeistaechus and Heeodotus 160 

XXII. OVEELAPPING CEATEES 166 

XXIII. NOEMAL LUNAE CEATEE 176 

XXIV. Aspect op an Eclipse of the Sun as it would appeae as 

SEEN FEOM THE MOON 184 

XXV. Geoup OP Lunae Mountains At end. 



THE MOON. 



CHAPTER I. 



ON THE COSMICAL ORIGIN OF THE PLANETS OF THE 
SOLAR SYSTEM. 

In this chapter we propose to treat briefly of the probable forma- 
tion of the various members of the solar system from matter which 
previously existed in space in a condition different from that in 
which we at present find it — i.e., in the form of planets and 
satellites. 

It is almost impossible to conceive that our world with its 
satellite, and its fellow worlds with their satellites, and also the 
great centre of them all, have always, from the commencement of 
time, possessed their present form : all our experiences of the 
working of natural laws rebel against such a supposition. In 
every phenomenon of nature upon this earth — the great field from 
which we must glean our experiences and form our analogies — we 
see a constant succession of changes going on, a constant pro- 
gression from one stage of development to another taking place, a 
perpetual mutation of form and nature of the same material 
substance occurring : we see the seed transformed into the plant, 
the flower into the fruit, and the ovum into the animal. In the 
inorganic world we witness the operation of the same principle ; 
but, by reason of their slower rate of progression, the changes 

9 



2 THE MOON. [chap. i. 

there are manifested to us rather by their resulting effects than 
by their visible course of operation. And when we consider, as we 
are obliged to do, that the same laws work in the greatest as well 
as the smallest processes of nature, we are compelled to believe in 
an antecedent state of existence of the matter that composes the 
host of heavenly bodies, and amongst them the earth and its 
attendant moon. 

In the pursuit of this course of argument we are led to inquire 
whether there exists in the universe any matter from which 
planetary bodies could be formed, and how far their formation 
from such matter can be explained by the operation of known 
material laws. 

Before the telescope revealed the hidden wonders of the skies, 
and brought its rich fruits into our garner of knowledge concerning 
the nature of the universe, the philosophic minds of some early 
astronomers, Kepler and Tycho Brahe to wit, entertained the idea 
that the sun and the stars — the suns of distant systems — were 
formed by the condensation of celestial vapours into spherical 
bodies ; Kepler basing his opinion on the phenomena of the 
sudden shining forth of new stars on the margin of the Milky 
Way. But it was when the telescope pierced into the depths of 
celestial space, and brought to light the host of those marvellous 
objects, the nebulae, that the strongest evidence was afforded of the 
probable validity of these suppositions. The mention of " nebulous 
stars" made by the earlier astronomers refers only to clusters of 
telescopic stars which the naked eye perceives as small patches of 
nebulous light ; and it does not appear that even the nebula in 
Andromeda, although so plainly discernible as to be often now-a- 
days mistaken by the uninitiated for a comet, was known, until it 
was discovered by means of a telescope, in 1612, by Simon Marius, 
who described it as resembling a candle shining through semi-trans- 
parent horn, as in a lantern, and without any appearance of stars. 
Forty years after this date Huygens discovered the splendid nebula 
iu the sword handle of Orion, and in 1665 another was detected 



CHAP. I.] C08MICAL ORIGIN OF PLANETARY SYSTEM. 3 

by Hevelius. In 1667 Halley (afterwards Astronomer Royal), 
discovered a fourth; a fifth was found by Kirsch in 1681, and a 
sixth by Halley again in 1714. Half a century after this the 
labours of Messier expanded the list of known nebulae and clusters 
to 103, a catalogue of which appeared in the '* Connaissance du 
Temps" (the French ** Nautical Almanac") for the years 1783 — 
1784. But this branch of celestial discovery achieved its most 
brilliant results when the rare penetration, the indomitable per- 
severance, and the powerful instruments of the elder Herschel 
were brought to bear upon it. In the year 1779 this great 
astronomer began to search after nebulse with a seven-inch reflector, 
which he subsequently superseded by the great one of forty feet 
focus and four feet aperture. In 1786 he published his first cata- 
logue of 1000 nebulsB ; three years later he astonished the learned 
world by a second catalogue containing 1000 more, and in 1802 a 
third came forth comprising other 500, making 2500 in all ! This 
number has been so far increased by the labours of more recent 
astronomers that the last complete catalogue, that of Sir John 
Herschel, published a few years ago, contains the places of 5063 
nebulae and clusters. 

At the earlier periods of Herschel's observations, that illustrious 
observer appears to have inclined to the belief that all nebulae were 
but remote clusters of stars, so distant, so faint, and so thickly 
agglomerated as to afiectthe eye only by their combined luminosity, 
and at this period of the nebular history it was supposed that 
increased telescopic power would resolve them into their component 
stars. But the familiarity which Herschel gained with the phases 
of the multitudinous nebulae that passed in review before his eyes, 
led him ultimately to adopt the opinion, advanced by previous 
philosophers, that they were composed of some vapoury or elemen- 
tary matter out of which, by the process of condensation, the 
heavenly bodies were formed ; and this led him to attempt a 
classification of the known nebulae into a cosmical arrangement, in 
which, regarding a chaotic mass of vapoury matter as the primordial 



4 THE MOON. [chap. i. 

state of existence, he arranged them into a series of stages of 
progressive development, the individuals of one class heing so 
nearly allied to those in the next that, to use his own expression, 
not so much difference existed between them " as there would be 
in an annual description of the human figure were it given from 
the birth of a child till he comes to be a man in his prime." 
{Philosophical Transactions, Vol. CI., pp. 271, et seq.) 

His category comprises upwards of thirty classes or stages of 
progression, the titles of a few of which we insert here to illustrate 
the completeness of his scheme. 

Class 1. Of extensive diffused nebulosity. (A table of 52 patches 
of such nebulosity actually observed is given, some of 
which extend over an area of five or six square degrees, 
and one of which occupies nine square degrees.) 
6. Of milky nebulosity with condensation. 
15. Of nebulae that are of an irregular figure. 
17. Of round nebulae. 

20. Of nebulae that are gradually brighter in the middle. 
25. Of nebulae that have a nucleus. 

29. Of nebulae that draw progressively towards a period of 
final condensation. 

30. Of planetary nebulae. 
33. Of stellar nebulae nearly approaching the appearance of 

stars. 

In a walk through a forest we see trees in every stage of growth, 
from the tiny sapling to the giant of the woods, and no doubt can 
exist in our minds that the latter has sprung from the former. 
We cannot at a passing glance discern the process of development 
actually going on ; to satisfy ourselves of this, we must record the 
appearance of some single tree from time to time through a long 
series of years. And what a walk through a forest is to an 
observer of the growth of a tree, a lifetime is to the observer of 



CHAP. I.] C08MICAL ORIGIN OF PLANETARY SYSTEM, 5 

changes in such objects as the nebulae. The transition from one 
state to another of the nebulous development is so slow that a life- 
time is hardly sufficient to detect it. Nor can any precise evidence 
of change be obtained by the comparison of drawings or descriptions 
of nebulae at various epochs, with whatever care or skill such 
drawings be made, for it will be admitted that no two draughtsmen 
will produce each a drawing of the most simple object from the 
same point of view, in which every detail in the one will coincide 
exactly with every detail in the other. There is abundant evidence 
of this in the existing representations of the great nebula in Orion ; 
a comparison of the drawings that have been lately made of this 
object, with the most perfect instruments and by the most skilful 
of astronomical draughtsmen, reveals varieties of detail and even of 
general appearance such as could hardly be imagined to occur in 
similar delineations of one and the same subject ; and any one who 
himself makes a perfectly unbiassed drawing at the telescope will 
find upon comparison of it with others that it will offer many points 
of difference. The fact is that the drawing of a man, like his pen- 
manship, is a personal characteristic, peculiar to himself, and the 
drawings of two persons cannot be expected to coincide any more 
than their handwritings. The appearance of a nebula varies also 
to a great extent with the power of the telescope used to observe it 
and the conditions under which it is observed ; the drawings of 
nebulae made with the inferior telescopes of a century or two 
centuries ago, the only ones that, by comparison with those made 
in modern times, could give satisfactory evidence of changes of 
form or detail, are so rude and imperfect as to be useless for the 
purpose, and it is reasonable to suppose that those made in the 
present day will be similarly useless a century or two hence. Since 
then we can obtain no evidence of the changes we must assume 
these mysterious objects to be undergoing, ipso facto, by observa- 
tion of one nebula at various periods, we must for the present 
accept the prima facie evidence offered (as in the case of the trees 
in a forest) by the observation of various nehulce at one period. 



6 THE MOON. [chap. i. 

" The total dissimilitude," says Herschel at the close of the 
observations we have alluded to, ** between the appearance of a 
diffusion of the nebulous matter and of a star, is so striking, that 
an idea of the conversion of the one into the other can hardly occur 
to any one who has not before him the result of the critical 
examination of the nebulous system which has been displayed in 
this [his] paper. The end I have had in view, by arranging my 
observations in the order in which they have been placed, has been 
to show that the above-mentioned extremes may be connected by 
such nearly allied intermediate steps, as will make it highly pro- 
bable that every succeeding state of the nebulous matter is the 
result of the action of gravitation upon it while in a foregoing one, 
and by such steps the successive condensation of it has been 
brought up to the planetary condition. From this the transit to 
the stellar form, it has been shown, requires but a very small addi- 
tional compression of the nebulous matter." 

Where the researches of Herschel terminated those of Laplace 
commenced. Herschel showed how a mass of nebulous matter so 
diffused as to be scarcely discernible might be and probably was, by 
the mere action of gravitation, condensed into a mass of compara- 
tively small dimensions when viewed in relation to the immensity 
of its primordial condition. Laplace demonstrated how the known 
laws of gravitation could and probably did from such a partially 
condensed mass of matter produce an entire planetary system with 
all its subordinate satellites. 

The first physicist who ventured to account for the formation of 
the various bodies of our solar system was Buffon, the celebrated 
French naturalist. His theory, which is fully detailed in his 
renowned work on natural history, supposed that at some period 
of remote antiquity the sun existed without any attendant planets, 
and that a comet having dashed obliquely against it, ploughed up 
and drove off a portion of its body sufficient in bulk to form the 
various planets of our system. He suggests that the matter thus 
carried off " at first formed a torrent the grosser and less dense 



CHAP. I.] C08MICAL ORIGIN OF PLANETARY SYSTEM. 1 

parts of which were driven the farthest, and the densest parts, 
having received only the like impulsion, were not so remotely 
removed, the force of the sun's attraction having retained them : " 
that " the earth and planets therefore at the time of their quitting 
the sun were burning and in a state of liquefaction; " that '* by 
degrees they cooled, and in this state of fluidity they took their 
form." He goes on to say that the obliquity of the stroke of the 
comet might have been such as to separate from the bodies of the 
principal planets small portions of matter, which would preserve 
the same direction of motion as the principal planets, and thus 
would form their attendant satellites. 

The hypothesis of BufFon, however, is not sufficient to explain all 
the phenomena of the planetary system ; and it is imperfect, inas- 
much as it begins by assuming the sun to be already existing, 
whereas any theory accounting for the primary formation of the 
solar system ought necessarily to include the origination of the 
most important body thereof, the sun itself. Nevertheless, it is 
but due to Buffon to mention his ideas, for the errors of one 
philosophy serve a most useful end by opening out fields of inquiry 
for subsequent and more fortunate speculators. 

Laplace, dissatisfied with Bufi'on's theory, sought one more pro- 
bable, and thus was led to the proposition of the celebrated nebular 
hypothesis which bears his name, and which, in spite of its dis- 
believers, has never been overthrown, but remains the only pro- 
bable, and, with our present knowledge, the only possible explana- 
tion of the cosmical origin of the planets of our system. Although 
Laplace puts forth his conjectures, to use his own words, *' with 
the deference which ought to inspire everything that is not a result 
of observation and calculation," yet the striking coincidence of all 
the planetary phenomena with the conditions of his system gives 
to those conjectures, again to use his modest language, " a pro- 
bability strongly approaching certitude." 

Laplace conceived the sun to have been at one period the nucleus 
of a vast nebula, the attenuated surrounding matter of which 



8 THE MOON. [chap. i. 

extended beyond what is now the orbit of the remotest planet of the 
system. He supposed that this mass of matter in process of con- 
densation possessed a rotatory motion round its centre of gravity, 
and that the parts of it that were situated at the limits where 
centrifugal force exactly counterbalanced the attractive force of the 
nucleus were abandoned by the contracting mass, and thus were 
formed successively a number of rings of matter concentric with 
and circulating around the central nucleus. As it would be impro- 
bable that all the conditions necessary to preserve the stability of 
such rings of matter in their annular form could in all cases exist, 
they would break up into masses which would be endued with a 
motion of rotation, and would in consequence assume a spheroidal 
form. These masses, which hence constituted the various planets, 
in their turn condensing, after the manner of the parent mass, and 
abandoning their outlying matter, would become surrounded by 
similarly concentric rings, which would break up and form the 
satellites surrounding the various planetary masses ; and, as a 
remarkable exception to the rule of the instability of the rings and 
their consequent breakage, Laplace cited the case of Saturn sur- 
rounded by his rings as the only instances of unbroken rings that 
the whole system offers us ; unless indeed we include the zodiacal 
light, that cone of hazy luminosity that is frequently seen stream- 
ing from our luminary shortly before and after sunset, and which 
Laplace supposed to be formed of molecules of matter, too volatile 
to unite either with themselves or with the planets, and which 
must hence circulate about the sun in the form of a nebulous 
ring, and with such an appearance as the zodiacal actually 
presents. 

This hypothesis, although it could not well be refuted, has been 
by many hesitatingly received, and for a reason which was at one 
time cogent. In the earlier stages of nebular research it was 
clearly seen, as we have previously remarked, that many of the 
so-called nebulae, which appeared at first to consist of masses of 
vapouiy matter, became, when scrutinised with telescopes of 



CHAP. I.] COSMIC AL ORIGIN OF PLANETARY SYSTEM. 9 

higher power, resolved into clusters containing countless numbers 
of stars, so small and so closely agglomerated, that their united 
lustre only impressed the more feeble eye as a faint nebulosity ; 
and as it was found that each accession of telescopic power 
increased the numbers of nebulae that were thus resolved, it was 
thought that every nebula would at some period succumb to the 
greater penetration of more powerful instruments ; and if this were 
the case, and if no real nebulae were hence found to exist, how, it 
was argued, could the nebular hypothesis be maintained ? One of 
the most important nebulae bearing upon this question was the 
great one in the sword handle of Orion, one of the grandest and 
most conspicuous in the whole heavens. On account of the bright- 
ness of some portions of this object, it seemed as though it ought 
to be readily resolvable, supposing all nebulae to consist of stars, 
but all attempts to resolve it were in vain, even with the powerful 
telescopes of Sir John Herschel and the clear zenethal sky of the 
Cape of Good Hope. At length the question was thought to be 
settled, for upon the completion of Lord Kosse's giant reflector, 
and upon examination of the nebula with it, his lordship stated 
that there could be little, if any, doubt as to its resolvability, and 
then it was maintained, by the disbelievers in the nebular theory, 
that the last stronghold of that theory had been broken 
down. 

But the truths of nature are for ever playing at hide and seek 
with those who follow them : — the dogmas of one era are the 
exploded errors of the next. Within the past few years a new 
science has arisen that furnishes us with fresh powers of penetra- 
tion into the vast and secret laboratories of the universe ; a new 
eye, so to speak, has been given us by which we may discern, by 
the mere light that emanates from a celestial body, something of 
the chemical elements of which it is composed. When Newton 
two hundred years ago toyed with the prism he bought at Stour- 
bridge fair, and projected its pretty rainbow tints upon the wall, 
his great mind little suspected that that phantom riband of 



10 THE MOON. [chap. i. 

gorgeous colours would one day be called upon to give evidence 
upon the probable cosmical origin of worlds. Yet such in truth 
has been the case. Every substance when rendered luminous 
gives off light of some colour or degree of refrangibility peculiar 
to itself, and although the eye cannot detect any difference between 
one character of light and another, the prism gives the means of 
ascertaining the quality and degree of refrangibility of the light 
emanating from any source however distant, and hence of gaining 
some knowledge of the nature of that source. If, for instance, a 
ray of light from a solid body in combustion is passed through a 
prism, a spectrum is produced which exhibits light of all colours or 
all degrees of refrangibility ; if the light from such a body, before 
passing through the prism, be made to pass through gases or 
certain metallic vapours, the resulting spectrum is found to be 
crossed transversely by numbers of fine dark lines, apparently 
separating the various colours, or cutting the spectrum into bands. 
The solar spectrum is of this class ; the once mysterious lines first 
observed by WoUaston, and subsequently by Fraunhofer, and known 
as ** Fraunhofer 's lines," have now been interpreted, chiefly by the 
sagacious German chemist Kirchhoff, and identified as the effects 
of absorption of certain of the sun's rays by chemical vapours con- 
tained in his atmosphere. The fixed stars yield spectra of the 
same character, but varying considerably in feature, the lines 
crossing the stella spectra differing in position and number from 
those of the sun, and one star from another, proving the stars to 
possess varied chemical constitutions. But there is another class 
of spectra, exhibited when light from other sources is passed 
through the prism. These consist, not of a luminous riband of 
light like the solar spectrum, but of bright isolated lines of coloured 
light with comparatively wide dark spaces separating them. Such 
spectra are yielded only by the light emitted from luminous gases 
and metals or chemical elements in the condition of incandescent 
vapour. Every gas or element in the state of luminous vapour 
yields a spectrum peculiar to itself, and no two elements when 



CHAP. I.] C08MICAL ORIGIN OF PLANETARY SYSTEM. 11 

vaporized before the prism show the same combinations of luminous 
lines. 

Now iu the course of some observations upon the spectra of the 
fixed stars by Dr. Huggins, it occurred to that gentleman to turn 
his telescope, armed with a spectroscope, upon some of the brighter 
of the nebulae, and great was his surprise to find that instead of 
yielding continuous spectra, as they must have done had their light 
been made up of that of a multitude of stars, they gave spectra 
containing only two or three isolated bright lines ; such a spectrum 
could only be produced by some luminous gas or vapour, and of 
this form of matter we are now justified in declaring, upon the 
strength of numerous modern observations, these remarkable bodies 
are composed ; and it is a curious and interesting fact that some of 
the nebulae styled resolvable, from the fact of their exhibiting 
points of light like stars, yield these gaseous spectra, whence Dr. 
Huggins concludes that the brighter points taken for stars are in 
reality nuclei of greater condensation of the nebular matter : and so 
the fact of the apparent resolvability of a nebula affords no positive 
proof of its non- nebulous character. 

These observations — which have been fully confirmed by Father 
Secchi of the Roman College — by destroying the evidence in favour 
of nebulae being remote clusters, add another attestation to the 
probability of the truth of the nebular hypothesis, and we have now 
the confutation of the luminologist to add to that of the astronomers 
who, in the person of the illustrious Arago, asserted that the ideas 
of the great author of the '* Mecanique Celeste " ** were those only 
which by their grandeur, their coherence, and their mathematical 
character could be truly considered as forming a physical cos- 
mogony." 

Confining, then, our attention to the single object of the universe 
it is our task to treat of — the Moon — and without asserting as an 
indisputable fact that which we can never hope to know otherwise 
than by inference and analogy, we may assume that that body once 
existed in the form of a vast mass of diffused or attenuated matter, 



12 THE MOON. [chap. i. 

and that, by the action of gravitation upon the particles of that 
matter, it was condensed into a comparatively small and compact 
planetary body. 

But while the process of condensation or compaction was going 
on, another important law of nature — but recently unfolded to our 
knowledge — was in powerful operation, the discussion of which law 
we reserve for a separate Chapter. 



CHAPTER II. 

THE GENERATION OF COSMICAL HEAT. 

In the preceding Chapter we endeavoured to show how the 
action of gravitation upon the particles of diffused primordial matter 
would result in the formation, by condensation and aggregation, of 
a spherical planetary body. We have now to consider another 
result of the gravitating action, and for this we must call to our aid 
a branch of scientific enquiry and investigation unrecogi^ized as 
such at the period of Laplace's speculations, and which has 
been developed almost entirely within the past quarter of a 
century. 

The " great philosophical doctrine of the present era of science," 
as the subject about to engage our attention has been justly termed, 
bears the title of the " Conservation of Force," or — as some 
ambiguity is likely to attend the definition of the term " Force " — 
the " Conservation of Energy." The basis of the doctrine is the 
broad and comprehensive natural law which teaches us that the 
quantity of force comprised by the universe, like the quantity of 
matter contained in it, is a fixed and invariable amount, which can 
be neither added to nor taken from, but which is for ever under- 
going change and transformation from one form to another. That 
we cannot create force ought to be as obvious a fact as that we 
cannot create matter ; and what we cannot create we cannot destroy. 
As in the universe we see no new matter created, but the same 
matter constantly disappearing from one form and reappearing in 



14 THE MOON. [chap. ii. 

another, so we can find no new force ever coming into action — no 
description of force that is not to be referred to some previous 
manner of existence. 

Without entering upon a metaphysical discussion of the term 
*' force," it will be sufficient for our purpose to consider it as some- 
thing which produces or resists motion, and hence we may argue 
that the ultimate effect of force is motion. The force of gravity on 
the earth results in the motion or tendency of all bodies towards its 
centre, and similarly, the action of gravitation upon the atoms or 
particles of a primeval planet resulted in the motion of those 
particles towards each other. We cannot conceive force otherwise 
than by its effects, or the motion it produces. 

And force we are taught is indestructible ; therefore motion 
must be indestructible also. But when a falling body strikes the 
earth, or a gun-shot strikes its target, or a hammer delivers a blow 
upon an anvil, or a brake is pressed against a rotating wheel, 
motion is arrested, and it would seem natural to infer that it is 
destroyed. But if we say it is indestructible, what becomes of it ? 
The philosophical answer to the question is this — that the motion 
of the mass becomes transferred to the particles or molecules 
composing it, and transformed to molecular motion, and this 
molecular motion manifests itself to us as heat. The particles or 
atoms of matter are held together by cohesion, or, in other words, 
by the action of molecular attraction. When heat is applied to 
these particles, motion is set up among them, they are set in 
vibration, and thus, requiring and making wider room, they urge 
each other apart, and the well-known expansion by heat is the 
result. If the heat be further continued a more violent molecular 
motion ensues, every increase of heat tending to urge the atoms 
further apart, till at length they overcome their cohesive attraction 
and move about each other, and a liquid or molten condition results. 
If the heat be still further increased, the atoms break away from 
their cohesive fetters altogether and leap off the mass in the form 
of vapour, and the matter thus assumes the gaseous or vaporous 



CHAP. II.] THE GENERATION OF C08MICAL HEAT. 15 

form. Thus we see that the phenomena of heat are phenomena of 
motion, and of motion only. 

This mutual relation between heat and work presented itself as 
an embryo idea to the minds of several of the earlier philosophers, 
by whom it was maintained in opposition to the material theory 
which held heat to be a kind of matter or subtle fluid stored up in 
the inter-atomic spaces of all bodies, capable of being separated 
and procured from them by rubbing them together, but not 
generated thereby. Bacon, in his ** Novum Organum," says that 
*' heat itself, its essence and quiddity, is motion and nothing else." 
Locke defines heat as ''a very brisk agitation of the insensible 
parts of an object, which produces in us that sensation from whence 
we denominate the object hot ; so what in our sensation is heat, in 
the object is nothing but motion.'' Descartes and his followers 
upheld a similar opinion. Eichard Boyle, two hundred years ago, 
actually wrote a treatise entitled *' The Mechanical Theory of Heat 
and Cold," and the ingenious Count Eumford made some highly 
interesting and significant experiments on the subject, which are 
described in a paper read before the Koyal Society in 1798, entitled 
" An Inquiry concerning the Source of Heat excited by Friction." 
But the conceptions of these authors remained isolated and un- 
fruitful for more than a century, and might have passed, meantime, 
into the oblivion of barren speculation, but for the impulse which 
this branch of inquiry has lately received. Now, however, they 
stand forth as notable instances of truth trying to force itself into 
recognition while yet men's minds were unprepared or disinclined 
to receive it. The key to the beautiful mechanical theory of heat 
was found by these searching minds, but the unclasping of the lock 
that should disclose its beauty and value was reserved for the 
philosophers of the present age. 

Simultaneously and independently, and without even the know- 
ledge of each other, three men, far removed from probable inter- 
course, conceived the same ideas and worked out nearly similar 
results concerning the mechanical theory of heat. Seeing that 



16 THE MOON. [chap. ii. 

motion was convertible into heat, and heat into motion, it became 
of the utmost importance to determine the exact relation that 
existed between the two elements. The first who raised the idea 
to philosophic clearness was Dr. Julius Robert Mayer, a physician 
of Heilbronn in Germany. In certain observations connected with 
his medical practice it occurred to him that there must be a 
necessary equivalent between work and heat, a necessary numerical 
relation between them. " The variations of the diiBference of colour 
of arterial and venous blood directed his attention to the theory of 
respiration. He soon saw in the respiration of animals the origin 
of their motive powers, and the comparison of animals to thermic 
machines afterwards suggested to him the important principle with 
which his name will remain for ever connected." 

Next in order of publication of his results stands the name of 
Colding, a Danish engineer, who about the year 1843 presented a 
series of memoirs on the steam-engine to the Royal Society of 
Copenhagen, in which he put forth views almost identical with 
those of Mayer. 

Last in publication order, but foremost in the importance of his 
experimental treatment of the subject, was our own countryman. 
Dr. Joule of Manchester. '' Entirely independent of Mayer, with 
his mind firmly fixed upon a principle, and undismayed by the 
coolness with which his first labours appear to have been received, 
he persisted for years in his attempts to prove the invariability of 
the relation which subsists between heat and ordinary mechanical 
power." (We are quoting from Professor TyndalFs valuable work 
on " Heat considered as a Mode of Motion.") " He placed water 
in a suitable vessel, agitated the water by paddles, and determined 
both the amoimt of heat developed by the stirring of the liquid and 
the amount of labour expended in its production. He did the same 
with mercury and sperm oil. He also caused discs of cast iron to 
rub against each other, and measured the heat produced by their 
friction, and the force expended in overcoming it. He urged water 
through capillary tubes, and determined the amount of heat 



CHAP, n.] THE GENERATION OF COSMICAL HEAT. 17 

generated by the friction of the liquid against the sides of the 
tubes. And the results of his experiments leave no shadow of 
doubt upon the mind that, under all circumstances, the quantity of 
heat generated by the same amount of force is fixed and invariable. 
A given amount of force, in causing the iron discs to rotate against 
each other, produced precisely the same amount of heat as when it 
was applied to agitate water, mercury, or sperm oil. * * * * 
The absolute amount of heat generated by the same expenditure of 
power, was in all cases the same." 

" In this way it was found that the quantity of heat which would 
raise one pound of water one degree Fahrenheit in temperature, is 
exactly equal to what would be generated if a pound weight, after 
having fallen through a height of 772 feet, had its moving force 
destroyed by collision with the earth. Conversely, the amount of 
heat necessary to raise a pound of water one degree in temperature, 
would, if all applied mechanically, be competent to raise a pound 
weight 772 feet high, or it would raise 772 pounds one foot high. 
The term * foot-pounds ' has been introduced to express in a con- 
venient way the lifting of one pound to the height of a foot. Thus 
the quantity of heat necessary to raise the temperature of a pound 
of water one degree Fahrenheit being taken as a standard, 772 
foot-pounds constitute what is called the mechanical equivalent of 
heat." 

By a process entirely different, and by an independent course of 
reasoning, Mayer had, a few months previous to Joule, determined 
this equivalent to be 771*4 foot-pounds. Such a remarkable coin- 
cidence arrived at by pursuing different routes gives this value a 
strong claim to accuracy, and raises the Mechanical Theory of Heat 
to the dignity of an exact science, and its enunciators to the 
foremost place in the ranks of physical philosophers. 

In linking together the labours of the two remarkable men above 
alluded to, Prof. Tyndall remarks, that " Mayer's labours have in 
some measure the stamp of profound intuition, which rose however 
to the energy of undoubting conviction in the authors mind. 



18 THE MOON. [chap. ii. 

Joule's labours, on the contrary, are an experimental demonstration. 
Mayer thought his theory out, and rose to its grandest applications. 
Joule worked his theory out, and gave it the solidity of natural 
truth. True to the speculative instinct of his country, Mayer drew 
large and mighty conclusions from slender premises; while the 
Englishman aimed above all things at the firm establishment of 

facts To each belongs a reputation which will 

not quickly fade, for the share he has had, not only in establishing 
the dynamical theory of heat, but also in leading the way towards a 
right appreciation of the general energies of the universe." 

But from these generalities we must pass to the application of 
the mechanical theory of heat to our special subject. We have 
learnt that every form of motion is convertible into heat. We 
know that the falling meteor or shooting star, whose motion is 
impeded by friction against the earth's atmosphere, is heated 
thereby to a temperature of incandescence. Let us then suppose 
that myriads of such cosmical particles come into collision from the 
effect of their mutual attraction, or that the component atoms of a 
vast nebulous mass violently converged under the like influence. 
What would follow ? Obviously the generation of an intense heat 
by the arrest of converging motion, such a heat as would result in 
the fusion of the whole into one mass. Mayer, in one of his most 
remarkable papers (" Celestial Dynamics ") remarks that the 
" Newtonian theory of gravitation, whilst it enables us to determine 
from its present form, the earth's state of aggregation in ages past, 
at the same time points out to us a source of heat powerful enough 
to produce such a state of aggregation — powerful enough to melt 
worlds : it teaches us to consider the molten state of a planet as the 
result of the mechanical union of cosmical masses, and to derive 
the radiation of the sun and the heat in the bowels of the earth 
from a common origin." 

And the same laws that governed the formation of the earth, gov- 
erned also the formation of the moon : the variations of Nature's oper- 
ations are quantitative only and not qualitative. The Divine Will that 



CHAP. II.] THE GENERATION OF C08MICAL HEAT. 19 

made the earth made the moon also, and the means and mode of work- 
ing were the same for both. The geological phenomena of the earth 
afford unmistakeable evidence of its original fluid or molten condi- 
tion, and the appearance of the moon is as unmistakeahly that of a 
body once in an igneous or molten state. The enigma of the 
earth's primary formation is solved by the application of the 
dynamical theory of heat. By this theory the generation of cos- 
mical heat is removed from the quicksands of conjecture and 
established upon the firm ground of direct calculation : for the 
absolute amount of heat generated by the collision of a given 
amount of matter is (of course, with some little uncertainty) 
deducible from a mathematical formula. Mayer has computed the 
amount of heat that the matter of the earth would have generated, 
if it had been formed originally of only two parts drawn into 
collision by their mutual attraction, and has found that it would be 
from to 32,000 or 47,000* Centigrade degrees, according as one 
part was infinitely small as compared with the other, or as the 
two parts were of equal size. Professor Helmholtz, another 
labourer in the same field of science, has computed the amount of 
heat generated by the condensation of the whole of the matter com- 
posing the solar system : this he finds would be equivalent to the 
hes-t that would be required to raise the temperature of a mass of 
water equal to the sum of the masses of all the bodies of the 
system to 28,000,000 (twenty-eight million) degrees of the 
Centigrade scale. 

These examples afford abundant evidence of sufficient heat having 
been generated by the aggregation of the matter of the moon to re- 
duce it to a state of fusion, and so to produce, from a nebulous chaos 
of diffused cosmical matter, a molten body of definite outline and size. 

It is requisite here to remark that fusion does not necessarily 
imply combustion. It has been frequently asked. How can a 
volcanic theory of the lunar phenomena be upheld consistently with 
the condition that it possesses no atmosphere to support fire ? To 

* The melting temperature of iron is 1500° Centigrade. 

c 2 



20 THE MOON, [chap. ii. 

this we would reply that to produce a state of incandescence or a 
molten condition it is not necessary that the hody he surrounded hy 
an atmosphere. The intensely rapid motion of the particles of 
matter of hodies, which the dynamical theory shows to he the 
origin of the molten state, exists quite independently of such 
external matter as an atmosphere. The complex mixture of gases 
and vapours which we term ** air," has nothing whatever to do with 
the fusion of suhstances, whatever it may have to do with their 
combustion. Combustion is a chemical phenomenon, due to the 
combination of the oxygen of that air with the heated particles of 
the combustible matter : oxygen is the sole supporter of combus- 
tion, and hence combustion is to be regarded rather as a phenome- 
non of oxygen than as a phenomenon of the matter with which that 
oxygen combines. The greatest intensity of heat may exist without 
oxygen, and consequently without combustion. In support of this 
argument it will be sufficient to adduce, upon the authority of Dr. 
Tyndall, the fact that a platinum wire can be raised to a luminous 
temperature and SLctn&Wy fused in a perfect vacuum. 

But while the mass of condensing cosmical matter was thus 
accumulating and forming the globe of the moon, the heat conse- 
quent upon^the aggregation of its particles was suffering some 
diminution from the effect of radiation. So long as the radiated 
heat lost fell short of the dynamical heat generated, no effect of 
cooling would be manifest ; but when the vis viva of the condensing 
matter was all converted into its equivalent of heat, or when the 
accession of heat fell short of that radiated, a necessary cooling must 
ensue, and this cooling would be accompanied by a soUdification of 
that part of the mass which was most free to radiate its heat into 
surrounding space : that part would obviously be the outer surface. 

With the solidification of this external crust began the " year 
one " of selenological history. 

The phenomena attendant upon the cooling of the mass we will 
consider in the next Chapter. 



CHAPTER III. 

THE SUBSEQUENT COOLING OF THE IGNEOUS BODY. 

In the foregoing chapters we have endeavoured to show, by 
the light of modern science, first, how diffused cosmical matter 
was probably condensed into a planetary mass by the mutual gravita- 
tion of its particles, and secondly, how, the after destruction of the 
gravitative force, by the collision of the converging particles of 
matter, resulted in the generation of such sufficient heat as to 
reduce the whole mass to a molten condition. Our present task 
is to consider the subsequent cooling of the mass, and the 
phenomena attendant upon or resulting therefrom. This brief 
chapter is important to our subject, as we shall have frequent 
occasion to refer to the leading principle we shall endeavour to 
illustrate in it, in subsequently treating of the causes to which the 
special selenological features are to be attributed. 

First, then, as regards the cooling of the igneous mass that con- 
stituted the moon at the inconceivably remote period when possibly 
that body was really ** a lesser light " shining with a luminosity of 
its own, due to its then incandescent state, and not simply a 
reflector, as it is now, of light which it receives from the sun. 
If we could conceive it possible that the igneous mass in the act of 
cooling parted with its heat from the central part first and so 
began to solidify from its centre, or if it had been possible for the 
mass to have cooled uniformly and simultaneously throughout its 
whole depth, or that each substratum had cooled before its super- 
stratum, we should have had a moon whose surface would have 



22 THE MOON. [chap. hi. 

been smooth and without any such remarkable asperities and 
excrescences as are now presented to our view. But these sup- 
positions are inadmissible : on the contrary we are compelled to 
consider that the portion of the igneous or molten body that first 
cooled was its exterior surface, which, radiating its heat into 
surrounding space, became solid and comparatively cool while the 
interior retained its hot and molten condition. So that at this 
early stage of the moon's history it existed in the form of a solid 
shell inclosing a molten interior. 

Now at this period of its formation, the moon's mass, partly 
cooled and solidified and partly molten, would be subject ta the 
influence of two powerful molecular forces : the first of these 
would consist in the diminution of bulk or contraction of volume 
which accompanies the cooling of solidified masses of previously 
molten substances; the second would arise from a phenomenon 
which we may here observe is by no means so generally known as 
from its importance it deserves to be : and as we shall have fre- 
quent occasion to refer to it as one of the chief agencies in produc- 
ing the peculiar structural characteristics of the moon's surface, it 
may be well here to give a few examples of its action, that our 
reference to it hereafter may be more clearly understood. 

The broad general principle of the phenomenon here referred to 
is this : — that fusible substances are (with a few exceptions) spe- 
cifically heavier while in their molten condition than in the 
solidified state, or in other words that molten matter occupies less 
space, weight for weight, than the same matter after it has passed 
from the melted to the solid condition. It follows as an obvious 
corollary that such substances contract in bulk in fusing or melt- 
ing, and expand in becoming solid. It is this expansion upon 
solidification that now concerns us. 

Water, as is well known, increases in density as it cools, till it 
reaches the temperature of SQ'' Fahrenheit, after which, upon a 
further decrease of temperature, its density begins to decrease, or 
in other words its bulk expands, and hence the well-known fact of 



CHAP. III.] THE SUBSEQUENT COOLING OF THE IGNEOUS BODY. 23 

ice floating in water, and the inconvenient fact of water-pipes bursting 
in a frost. This action in water is of the utmost importance in 
the grand economy of nature, and it has been accepted as a mar- 
vellous exception to the general law of substances increasing in 
density (or shrinking) as they decrease in temperature. Water is, 
however, by no means the exceptional substance that it has been so 
generally considered. It is a fact perfectly familiar to iron- 
founders, that when a mass of solid cast-iron is dropped into a pot 
of molten iron of identical quality, the solid is found to float 
persistently upon the molten metal — so persistently that when it 
is intentionally thrust to the bottom of the pot, it rises again the 
moment the submerging agency is withdrawn. As regards the 
amount of buoyancy we believe it may be stated in round numbers 
to be at least two or three per cent. It has been suggested by 
some who are familiar with this phenomenon that the solid mass 
may be kept up by a spurious buoyancy imparted to it by a film of 
adhering air, or that surface impurities upon the solid metal may 
tend to reduce the specific gravity of the mass and thereby prevent 
it sinking, and that the fact of floatation is not absolutely a proof of 
greater specific lightness. But in controversion of the suggestions, 
we can state as the result of experiment that pieces of cast-iron 
which have had their surface roughness entirely removed, leaving 
the bright metal exposed, still float on the molten metal, and 
further that when, under the influence of the great heat of the 
molten mass, the solid is gradually melted away, and consequently 
any possible surface impurities or adhering air must necessarily 
have been removed, the remaining portion continues to float to the 
last. The inevitable inference from this is that in the case of cast- 
iron the solid is specifically lighter than the molten, and, therefore, 
that in passing from the molten to the solid condition this substance 
undergoes expansion in bulk. 

We are able to offer a confirmation of this inference in the case 
of cast-iron by a remarkable phenomenon well known to iron- 
founders, but of which we have never met with special notice. 



24 



THE MOON. 



[chap. in. 



When a ladle or pot of molten iron is drawn from the melting 
furnace and allowed to stand at rest, the surface presents a most 
remarkable and suggestive appearance. Instead of remaining calm 
and smooth it is a scene of a lively commotion : the thin coat of 
scoria or molten oxide which forms on the otherwise bright surface 
of the metal is seen, as fast as it forms at the circumference of the 
pot, to be swept by active convergent currents towards the centre, 




Fig. 1. 



where it accumulates in a patch. "While this action is proceeding, 
the entire upper surface of the metal appears as if it were covered 
with animated vermicules of scoria, springing into existence at the 
circumference of the pot, and from thence rapidly streaming and 
wriggling themselves towards the centre. 

Our illustration (Fig. 1) is intended, so far as such means can do 
so, to convey some idea of this remarkable appearance at one instant 
of its continued occurrence. To interpret our illustration rightly it 
is necessary to imagine this vermicular freckling to be constantly 



CHAP. III.] THE SUBSEQUENT COOLING OP THE IGNEOUS BODY. 25 

and rapidly streaming from all points of the periphery of the pot 
towards the centre, where, as we have said, it accumulates in the 
form of a floating island. We may observe that the motion is most 
rapid when the hot metal is first put into the cool ladle : as the 
fluid metal parts with some of its heat and the ladle gets hot by 
absorbing it, this remarkable surface disturbance becomes less 
energetic. 




Fig. 2. 



Now if we carefully consider this peculiar action and seek a cause 
for the phenomenon, we shall be led to the conclusion that it arises 
from the expansion of that portion of the molten mass which is in 
contact with or close proximity to the comparatively cool sides of 
the ladle, which sides act as the chief agent in dispersing the heat of 
the melted metal. The motion of the scoriae betrays that of the 
fluid metal beneath, and careful observation will show that the 
motion in question is the result of an upward current of the metal 
around the circumference of the ladle, as indicated by the arrows A, 
B, c in the accompanying sectional drawing of the ladle (Fig. 2). 



?6 ; THE MOON. [chap. hi. 

The upward current of the metal can actually be seen when specially 
looked for, at the rim of the pot, where it is deflected into the con- 
vergent horizontal direction and where it presents an elevatory 
appearance as shown in the figure. It is difficult to assign to this 
effect any other cause than that of an expansion and consequent 
reduction of the specific gravity of the fluid metal in contact with 
or in close proximity to the cooler sides of the pot, as, according 
to the generally entertained idea that contraction universally 
accompanies cooling, it would be impossible for the cooler to float 
on the hotter metal, and the curious surface-currents above referred 
to would be in contrary direction to that which they invariably take, 
i.e., they would diverge from the centre instead of converging to it. 
The external arrows in the figure represent the radiation of the heat 
from the outer sides of the pot, which is the chief cause of cooling. 

Turning from cast-iron to other metals we find further manifesta- 
tions of this expansive solidification. Bismuth is a notable example. 
In his lectures on Heat, Dr. Tyndall exhibited an experiment in 
which a stout iron bottle was filled with molten bismuth, and the 
stopper tightly closed. The whole was set aside to cool, and as 
the metal within approached consolidation the bottle was rent open 
by its expansion, just as would have been the case had the bottle 
been filled with water and exposed to freezing temperature. Mercury 
affords another example. Thermometers which have to be exposed 
to Arctic temperatures are generally filled with spirit instead of 
quicksilver, because the latter has been found to burst the bulbs 
when the cold reached the congealing point of the metal, the burst- 
ing being a consequence of the expansion which accompanies the 
act of congelation. Silver also expands in passing from the fluid to 
the solid state, for we are informed by a practical refiner that solid 
floats on molten silver as ice floats on water ; it also, as likewise do 
gold and copper, exhibits surface converging currents in the melting- 
pot like those depicted above for molten iron. 

It may, however, be objected that metals are too distantly 
related to volcanic substances to justify inferences being drawn 



CHAP. III.] THE SUBSEQUENT COOLING OF THE IGNEOUS BODY. 27 

from their behaviour in explanation of volcanic phenomena. "With 
a view therefore of testing the question at issue with a substance 
admitted as closely allied to volcanic material, we appealed to the 
furnace slag of iron-works. The following are extracts from the 
letters of an iron manufacturer of great experience * to whom we 
referred the question : — 

"I beg to inform you that cold slag floats in molten slag in the 
same way cold iron floats in molten iron. 

"I filled a box with hot molten felag run quickly from a blast 
furnace ; the box was about 5 J feet square by 2 feet deep, and I 
dropped into the slag a piece of cold slag weighing 16 lbs., when it 
came to the top in a second. I pushed it down to the bottom 
several times and it always made its appearance at the top : indeed 
a small portion of it remained above the molten slag." 

Here then we have a substance closely allied to volcanic material 
which manifests the expansile principle in question ; but we may go 
still further and give evidence from the very fountain-head by 
instancing what appears to be a most cogent example of its opera- 
ation which we observed on the occasion of a visit to the crater of 
Vesuvius in 1865 while a modified eruption was in progress. On 
this occasion we observed white-hot lava streaming down from 
apertures in the sides of a central cone within the crater and form- 
ing a lake of molten lava on the plateau or bottom of the crater ; 
on the surface of this molten lake vast cakes of the same lava which 
had become solidified were floating, exactly in the same manner as 
ice floats in water. The solidified lava had cracked, and divided 

* Mr. T. Heunter, Manager of the Iron-works of James Murray, Esq., of Dal- 
mellington, Ayrshire. Another authority (Mr. Snelus, of the West Cumberland 
Iron Company), writes as follows : " I had a hole dug on the ' cinder-fall,' and 
allowed the running slag to flow through it so as to form a tolerably large pool and 
yet keep fluid. Any crust that formed was skimmed off. A portion of the same 
slag was cooled, and the solid lump thrown into the pool. It floated just at the 
surface." Mr. Snelus adds, by the way, that he tried " Bessemer- Pig " in the same 
way, and that the solid pig sunk in the molten for a minute and then rose and 
floated just at the surface, with about one-twentieth of its bulk above the level of 
the fluid. 



THE MOON. 



[chap. ni. 




x 




O 03 



cr 



CHAP. III.] THE SUBSEQUENT COOLING OF THE IGNEOUS BODY. 29 

into cakes, in consequence of its contraction and also of the 
uprising of the. accumulating fluid lava on which it floated, more 
and more space being thus afforded for it to separate, on account 
of the crater widening upwards, while through the joints or fissures 
the fluid lava could be seen beneath. But for the decrease in 
density and consequent expansion in volume which accompanied 
solidification, this floating of the solidified lava on the molten could 
not have occurred. Reference to Fig. 3, which represents a section 




Fig. 4. 

of the crater of Vesuvius on the occasion above referred to, will 
perhaps assist the reader to a more clear idea of what we have 
endeavoured to describe. A a are the streams of white-hot lava 
issuing from openings in the sides of the central cone, and 
accumulating beneath the solidified crust b b in the lake of molten 
lava at c c ; the solidified crust b b as it was floated upwards 
dividing into separate cakes as represented in Fig. 4. (See also 
Plate I.) 

Let us now consider what would be the effect produced upon a 
spherical mass of molten matter in progress of cooling, first under 
the action of the above described expansion which precedes solidifi- 
cation, and then by the contraction which accompanies the cooling 



30 . THE MOON. [chap. in. 

of a solidified body. The first portion of sucli a mass to part witli 
its heat being its external surface, this portion would expand, but 
there being no obstacle to resist the expansion there would be no 
other result than a temporary slight enlargement of the sphere. 
;This external portion would on cooling form a solid shell encompas- 
sing a more or less fluid molten nucleus, but as this interior has in 
its turn, on approaching the point of solidification, to expand also, 
and there being, so to speak, no room for its expansion, by reason 
of its confinement within its solid casing, what would be the 
consequence ? — the shell would be rent or burst open, and a portion 
of the molten interior ejected with more or less violence according 
to circumstances, and many of the characteristic features of volcanic 
action would be thus produced : the thickness of the outer shell, 
the size of the vent made by the expanding matter for its escape, 
and other conditions conspiring to modify the nature and extent of 
the eruption. Thus there would result vast floodings of the 
exterior surface of the shell by the so extruded molten matter, 
volcanoes, extruded mountains, and other manifestations of eruptive 
phenomena. The sectional diagram (Fig. 5) will help to convey a 
clear idea of this action. Basing our reasoning on the principle we 
have thus enunciated, namely, that molten telluric matter expands 
on nearing the point of solidification, and which we have en- 
deavoured to illustrate by reference to actual examples of its 
operation, we consider we are justified in assuming that such a. 
course of volcanic phenomena has very probably occurred again and 
again upon the moon ; that this expansion of volume which 
accompanies the solidification of molten matter furnishes a key to 
the solution of the enigma of volcanic action; and that such 
theories as depend upon the agency of gases, vapour, or water are 
at all events untenable with regard to the moon, where no gases, 
vapour, or water, appear to exist. 

That an upheaving and ejective force has been in action with 
varying intensity beneath the whole of the lunar surface is manifest 
from the aspect of its structural details, and we are impressed with 



CHAP, ni.] THE SUBSEQUENT COOLING OF THE IGNEOUS BODY. 31 

the conviction that the principle we have set forth, namely the 







11 

I ^ 









o o 

rd IS 



a-^ 



.9 a 



2 O Ki 

a -^ ^ 

g « fl 

a ^ « 

g ° o 

^ § a 

'ti "zi ■*^ 

O c8 o 



-g a i 
ft £ ^ 

S S5 fl 






T3 "^ 

o o 



- bo 53 









ag 



-s i 



paroxysms of expansion which successively occurred as portions of 
its molten interior approached solidification, supply us with a 



32 



THE MOON. 



[chap. III. 



rational cause to wliich such vast ejective and upheaving phenomena 
may be assigned. Many features of terrestrial geology likewise 
req^uire such an expansive force whereby to explain them ; we 
therefore venture to recommend this source and cause of ejective 
action to the careful consideration of geologists. 

When the molten substratum had burst its confines, ejected its 
superfluous matter, and produced the resulting volcanic features, it 
would, after final solidification, resume the normal process of con- 




FiG. 6. 




Fig. 7. 



traction upon cooling, and so retreat or shrink away from the 
external shell. Let us now consider what would be the result of 
this. Evidently the external shell or crust would become relatively 
too large to remain at all points in close contact with the subjacent 
matter. The consequence of too large a solid shell having to 
accommodate itself to a shrunken body underneath, is that the 
skin, so to term the outer stratum of solid matter, becomes 
shrivelled up into alternate ridges and depressions, or wrinkles. 
In its attempt to crush down and follow the contracting substratum 



t^ ; t ''" r I 




.'Nasrryth, 



""WooAburytype" 



K OF H A N D TO ILLUSTRATE THE ORIGIN OF CERTAIN MOUNTAIN RANGES 
RESU lTING FROM S H R I N K AGE F THE i NT E Ri OR 



PLATE ill, 




J.Nasmyth. 



' Wo o dbucrytyp e " 



SH RIVELLED APPLE. 

TO ILLUSTRATE THE ORIGIN OF CERTAIN MOUNTAIN RANGES 

RESULTING FROM SHRINKAGE OF THE INTERIOR 

OF THE GLOB E. 



UNINKUSlTl 



CHAP. III.] THE SUBSEQUENT COOLING OF THE IGNEOUS BODY. 33 

it would have to displace the superabundant or superfluous material 
of its former larger surface by thrusting it (by the action of tan- 
gential force) into undulating ridges as in Fig. 6, or broken 
elevated ridges as in Fig. 7, or overlappings of the outer crust as in 
Fig. 8, or ridges capped by more or less fluid molten matter 
extruded from beneath, as indicated in Fig. 9, a class of action 
which might occur contemporaneously with the elevation of the 
ridge or subsequently to its formation. 

A long-kept shrivelled apple affords an apt illustration of this 
wrinkle theory ; another example may be observed in the human 




t \ J ;i__' \_ 



c 

Fig. 



L / ^ 




face and hand, when age has caused the flesh to shrink and so 
leave the comparatively unshrinking skin relatively too large as a 
covering for it. We illustrate both of these examples by actual 
photographs of the respective objects, which are reproduced on 
Plates II. and III. Whenever an outer covering has to accommodate 
and apply itself to an interior body that has become too small for it, 



34 THE MOON. [chap. hi. 

wrinkles are inevitably produced. The same action that shrivels 
the human skin into creases and wrinkles, has also shrivelled 
certain regions of the igneous crust of the earth. A map of a 
mountainous part of our globe affords abundant evidence of such a 
cause having been in action ; such maps are pictures of wrinkles. 
Several parts of the lunar surface, as we shall by-and-by see, present 
us with the same appearances in a modified degree. 

To the few primary causes we have set forth in this chapter — to 
the alternate expansion and contraction of successive strata of the 
lunar sphere, when in a state of transition from an igneous and 
molten to a cooled and solidified condition, we believe we shall be 
able to refer well-nigh all the remarkable and characteristic features 
of the lunar surface which will come under our notice in the course 
of our survey. 



CHAPTER IV. 

THE FORM, MAGNITUDE, WEIGHT, AND DENSITY OF THE 
LUNAR GLOBE. 

We have not hitherto had occasion to refer to what we may 
term the physical elements of the moon : by which we mean 
the various data concerning form, size, weight, density, &c. 
of that body, derived from observation and calculation. To 
this purpose, therefore, we will now devote a few pages, con- 
fining ourselves to such matters as specially bear upon the 
requirements of our subject, omitting such as are irrelevant to 
our purpose, and touching but lightly upon such as are com- 
monly known, or are explained in ordinary elementary treatises 
on astronomy. 

First, then, as regards the form of the moon. The form of the 
lunar disc, when fully illuminated, we perceive to be a perfect 
circle ; that is to say, the measured diameters in all directions are 
equal ; and we are therefore led to infer that the real form of the 
moon is that of a perfect sphere. We know that the earth and the 
rest of the planets of our system are spheroidal, or more or less 
flattened at the poles, and we also know that this flattening is a 
consequence of axial rotation ; the extent of the flattening, or the 
oblateness of the spheroid, depending upon the speed of that rota- 
tion. But in the case of the moon the axial rotation is so slow that 
the flattening produced thereby although it must exist, is so 
slight as to be imperceptible to our observation. We might there- 
fore conclude that the moon is a perfectly spherical body, did not 

D 2 



36 THE MOON. [chap. iv. 

theory step in to show us that there is another cause by which its 
form is disturbed. Assuming the moon to have been once in a 
fluid state, it is demonstrable that the attraction of the earth would 
accumulate a mass of matter, like a tidal elevation, in the direction 
of a line joining the centres of the two bodies : and as a conse- 
quence, the real shape of the moon must be an ellipsoid, or some- 
what egg-shaped body, the major axis of which is directed towards 
the earth. That some such phenomenon has obtained is evident 
from the coincidence of the times of orbital revolution and axial 
rotation of the lunar sphere. " It would be against all probability," 
says Laplace, "to suppose that these two motions had been at their 
origin perfectly equal ; " but it is sufficient that their primitive 
difference was but small, in which case the constant attraction by 
the earth of the protuberant part of the moon would establish the 
equality which at present exists. 

It is, however, sufficient for all purposes with which we are con- 
cerned to regard the moon as a sphere, and the next point to be 
considered is its size. To determine this, two data are necessary 
— its apparent or angular diameter, and its distance from the 
earth. The first of these is obtained by measuring the angle com- 
prised between two lines directed from the eye to two opposite 
** limbs " or edges of the moon. If, for instance, we were to take a 
pair of compasses and, placing the joint at the eye, open out the 
legs till the two points appear to touch two opposite edges of the 
moon, the two legs would be inclined at an angle which would 
represent the diameter of the moon, and this angle we could 
measure by applying a divided arc or protractor to the compasses. 
In practice this measurement is made by means of telescopes 
attached to accurately divided circles ; the difference between the 
readings of the circle when the telescope is directed to opposite 
limbs of the moon giving its angular diameter at the time of the 
observation. But from the fact that the orbit of the moon is an 
ellipse, it is evident that she is at some times much nearer to us 
than at others, and, as a consequence, her apparent magnitude is 



CHAP. IV.] FORM, MAGNITUDE, WEIGHT, AND DENSITY. 37 

variable : there is also a slight variation depending upon the 
altitude of the moon at the time of the measurement ; the mean 
diameter, however, or the diameter at mean distance from the 
centre of the earth has, from long course of observation, been 
found to be Sr 9''. 

To convert this apparent angular diameter into real linear 
measurement, it is necessary to know either the distance of the 
moon from the earth, or in astronomical language as leading to a 
knowledge of that distance, what is the amount of the moon's 
parallax. Parallax, generally, is an apparent change of position of 
an object arising from change of the point of view. The parallax 
of a heavenly body is the angle which the earth would subtend if it 
were seen from that body. Supposing an observer on the moon 
could measure the earth's angular diameter, just as we measure 
that of the moon, his measurement would represent what is called 
the parallax of the moon. But we cannot go to the moon to make 
such a measurement ; nevertheless there is a simple method, 
explained in most treatises on astronomy, which consists in observ- 
ing the moon from stations on the earth widely separated, and by 
which we can obtain a precisely similar result. Without detailing 
the process, it is sufficient for us to know that the angle which 
would be subtended by the earth if seen from the moon, or the 
moon's parallax, is according to the latest determination, equal to 
1° 54' 5". This value, however, varies considerably with the varia- 
tions of distance due to the elliptic orbit of the moon : the number 
we have given represents the mean parallax, or the parallax at mean 
distance. 

But we have to turn these angular measurements into miles. To 
effect this we have only to work a simple rule-of-three sum. . It 
will easily be understood that, as the angular diameter of the earth 
seen from the moon is to the angular diameter of the moon seen 
from the earth, so is the diameter of the earth in miles to the dia- 
meter of the moon in miles. The diameter of the earth we know 
to be 7912 nxiles : putting this therefore in its proper place in 



88 THE MOON. [chap. iv. 

the proportion sum, and duly working it out by the schoolboy's 
rule, we get : — 

MILES. MILES. 

V 54' . 5" : 31' .9" : : 7912 : 2160 

And 2160 miles is therefore the diameter of the lunar globe. 

Knowing the diameter, we can easily obtain the other elements 
of magnitude. According to the well-known relation of the dia- 
meter of a sphere to its area, we find the area of the moon to be f 
14,657,000 square miles : or half that number, 7,328,500 miles, as 
the area of the hemisphere at any one time presented to our view. 
And similarly, from the relation of the solidity of a sphere to its 
diameter, we find the solid contents of the moon to be 5276 millions 
of cubic miles of matter. 

Comparing these data with corresponding dimensions of the 
earth, we find that the diameter of the moon is g^j ; the area 
^^ ; and the volume ^g.^, of the respective elements of the earth. 
Those who prefer a graphical to a numerical comparison, may 
judge of the sizes of the two bodies by the accompanying illustra- 
tion (Fig. 10). To gain an idea of their distance from each other 
it is necessary to suppose the two discs in the diagram to be five 
feet apart ; the real distance of the moon from the earth being 
about 238,790 miles at its mean position. 

Next, we come to what is technically termed the mass, but what 
in common language we may call the weight of the moon. It is 
important to know this, because the weight of a body taken in con- 
nection with its size furnishes us with a knowledge of its density, 
or the specific gravity of the material of which it is composed. But 
it is not quite so easy to determine the mass as the dimensions of 
the moon : to measure it, we have seen is easy enough ; to weigh it 
is a comparatively difficult matter. To solve the problem we have 
to appeal to Newton's law of universal gravitation. This law 
teaches us that every particle of matter in the universe attracts 
every other particle with a force which is directly proportional to 
the mass, and inversely proportional to the square of the distance 



CHAP. IV.] FORM, MAGNITUDE, WEIGHT, AND DENSITY. 



of the attracting par- 
ticles. There are 
several methods hy 
which this law is 
applied to the mea- 
surement of the 
mass of the moon. 
One of the simplest 
is by the agency of 
the Tides. We know 
that the moon, at- 
tracting the waters, 
produces a certain 
amount of elevation 
of the aqueous cover- 
ing of the earth ; 
and we know that 
the sun produces 
also a like elevation, 
but to a much 
smaller extent, by 
reason of its much 
greater distance. 
Now measuring ac- 
curately the heights 
of the solar and 
lunar tides, and 
making allowance 
for the difference of 
distance of the sun 
and moon from the 
earth, we can com- 
pare directly the 
effect that is due to 




40 THE MOON. [chap. iv. 

the sun with the effect that is due to the moon : and since the 
masses of the two bodies are just in proportion to the effects they 
produce, it is evident that we have a comparison between the mass 
of the sun and that of the moon ; and knowing what is the sun's 
mass we can, by simple proportion, find that of the moon. Another 
method is as follows : — The moon is retained in her orbital path 
by the attraction of the earth ; if it were not for this attraction she 
would fly off from her course in a tangential line. She has thus a 
constant tendency to quit her orbit, which the earth's attraction as 
constantly overcomes. It is evident from this that the earth pulls 
the moon towards itself by a definite amount in every second of 
time. But while the earth is pulling the moon, the moon is also 
pulling the earth : they are pulling each other together ; and 
moreover each is exerting a pull which is jproportional to its mass. 
Knowing, then, the mass of the earth, which we do with consider- 
able accuracy, we can find what share of the whole pulling force 
is due to it, the residue being the moon's share : the proportion 
which this residue bears to the earth's share gives us the pro- 
portion of the moon's mass to that of the earth, and hence the 
mass of the moon. 

There are yet two other methods : one depending upon the phe- 
nomena of nutation, or the attraction of the sun and moon upon the 
protuberant matter of the terrestrial spheroid ; and the other upon 
a displacement of the centre of gravity of the earth and moon, 
which shows itself in observations of the sun. By each and all of 
these methods has the lunar mass been at various times determined, 
and it has been found, as the latest and best accepted value, that 
the mass of the moon is one-eightieth that of the earth. 

From the known diameter of the earth we ascertain that its 
volume is 259,360 millions of cubic miles : and from the various 
experiments that have been made to determine the mean density of 
the earth, it has been found that that mean density it about 5 J 
times that of water ; that is to say, the earth weighs 5 J times 
heavier than would a sphere of water of ecjual size. Now a cubic 



CHAP. IV.] FORM, MAGNITUDE, WEIGHT, AND DENSITY. 41 

foot of water weighs 62*3211 pounds, and from this we can find by 
simple multiplication what is the weight of a cubic mile of water, 
and, similarly, what would be the weight of 259*360 cubic miles of 
water, and the last result multiplied by 5 J will give the weight of 
the earth in tons : The calculation, although extremely simple, in- 
volves a confusing heap of figures ; but the result, which is all that 
concerns us, is, that the weight of the earth is 5842 trillions of 
tons : and since, as we have above stated, the mass of the earth is 
80 times that of the moon, it follows that the weight of the moon 
is 73 trillions of tons. 

The cubical contents of a body compared with its weight gives 
us its density. In the moon we have 5276 millions of cubic miles 
of matter, the total weight of which is 73 trillions of tons. Now, 
5276 millions of cubic miles of water would weigh about 21J- 
trillions of tons ; and as this number is to 73 as 1 is to 3*4, it is clear 
that the density of the lunar matter is 3*4 greater than water: and 
inasmuch as the earth is 5j times denser than water, we see that 
the moon is about 0*62 as dense as the earth, or that the material 
of the moon is lighter, bulk for bulk, than the mean material of the 
terraqueous globe in the proportion of 62 to 100, or, nearly 6 to 10. 
This specific gravity of the lunar material (3*4) we may remark is 
about the same as that of flint glass or the diamond : and curiously 
enough it nearly coincides with that of some of the aerolites that 
have from time to time fallen to the earth ; hence support has been 
claimed for the theory that these bodies were originally fragments 
of lunar matter, probably ejected at some time from the lunar 
volcanoes with such force as to propel them so far within the sphere 
of the earth's attraction that they have ultimately been drawn to its 
surface. 

Beverting, now, to the mass of the moon : we must bear in mind 
that the mass or weight of a planetary body determines the weight 
of all objects on its surface. What we call a pound on the earth, 
would not be a pound on the moon ; for the following reason : — - 
When we say that such and such an object weighs so much, we 



42 THE MOON. [chap. iv. 

really mean that it is attracted towards the earth with a certain 
force depending upon its own weight. This attraction we call 
gravity ; and the falling of a weight to the earth is an example of 
the action of the law of universal gravitation. The earth and the 
weight fall together — or are held together if the weight is in con- 
tact with the earth — with a force which depends directly upon the 
mass of the two, and upon the distance hetween them. Newton 
proved that the attraction of a sphere upon external objects is pre- 
cisely as if the whole of its matter were contained at its centre. So 
that the attractive force of the earth upon a ton weight at its surface 
is the attraction which 5842 trillions of tons exert upon one ton 
situated 3956 miles (the radius of the earth) distant. If the weight 
of the earth were only half the above quantity, it is clear that the 
attraction would be only half what it is ; and hence the ton weight, 
being pulled by only half the force, would only be equal to half a 
ton ; that is to say, only half as much muscular force (or any other 
force but gravity) would be required to lift it. It is plain, there- 
fore, that what weighs a pound on the earth could not weigh a 
pound on the moon, which is only -^ of the weight of the earth. 
What, then, is the relation between a pound on the earth and the 
same mass of matter on the moon ? It would seem, since the 
moon's mass is -gV of the earth, that the pound transported to the 
moon ought to weigh the eightieth part of a pound there ; and so 
it would if the distance from the centre of the moon to its surface 
were the same as the distance of the centre of the earth from its 
surface. But the radius of the moon is only ^^ that of the earth ; 
and the force of gravity varies inversely as the square of the distance 
between the centres of the gravitating masses. So that the attrac- 
tion by the moon of a body at its surface, as compared with that of 
the earth, is Vo divided by the square of ~ ; and this, worked out, 
is equal to J. The force of gravity upon the moon is, therefore, J of 
that on the earth ; and hence a pound upon the earth would be 
little more than 2| ounces on the moon ; and it follows as a conse- 
quence that any force, such as muscular exertion, or the energy of 



CHAP. IV.] FORM, MAGNITUDE, WEIGHT, AND DENSITY. 43 

chemical, plutonic or explosive forces, would be six times more 
effective upon the moon than upon the earth. A man who could 
jump six feet from the earth, could with the same muscular effort 
jump thirty-six feet from the moon; the explosive energy that 
would project a body a mile above the earth would project a like 
body six miles above the surface of the moon. 

It is the practice, in elementary and popular treatises on 
astronomy, to state merely the numerical results in giving data 
such as those embodied in the foregoing pages ; and uninitiated 
readers, not knowing the means by which the figures are arrived 
at, are sometimes disposed to regard them with a certain amount 
of doubt or uncertainty. On this account we have thought it 
advisable to give, in as brief and concise a form as possible, the 
various steps by which these seemingly unattainable results are 
obtained. 

The data explained in the foregoing text are here collected to 
facilitate reference. 

Diameter of Moon . . .2160 miles . . . . 5:^ that of earth. 

Area 14,657,000 square miles . . j^^ „ „ 

Area of the visible hemisphere 7,328,500 square miles 

Solid contents . ... 5276 millions of cubic miles . ^^ „ „ 

Mass 73 trillions of tons > . . ^ „ „ 

Density 3-39 (water = 1) . . . 0.62 „ „ 

Force of gravity at surface i ?, » 

Mean distance from earth . . . 238,790 miles. 



CHAPTER V. 

ON THE EXISTENCE OR NON-EXISTENCE OF A LUNAR 
ATMOSPHERE. 

At the close of the preceding chapter we stated that any force 
acting in opposition to that of gravity would be six times more 
effective on the moon than on the earth. But, in fact, it would in 
many cases be still more so; at all events, so far as projectile 
forces are concerned ; for the reason that " the powerful coercer of 
projectile range," as the earth's atmosphere has been termed, has 
no counterpart, or at most a very disproportionate one, upon the 
moon. 

The existence of an atmosphere surrounding the moon has been 
the subject of considerable controversy, and a great deal of evidence 
on both sides of the question has been offered from time to time, 
and is to be found scattered through the records of various classes 
of observations. Some of the more important items of this 
evidence it is our purpose to set forth in the course of the present 
chapter. 

With the phenomena of the terrestrial atmosphere, with the 
effects that are attributable to it, we are all well familiar, and our 
best course therefore is to examine, as far as we are able, whether 
counterparts of any of these effects are manifested upon the moon. 
For instance, the clouds that are generated in and float through 
our air would, to an observer on the moon, appear as ever-changing 
bright or dusky spots, obliterating certain of the permanent details 
of the earth's surface, and probably skirting the terrestrial disc, 



CHAP, v.] NON-EXISTENCE OF A LUNAR ATMOSPHERE. 45 

like the changing belts we perceive on the planet Jupiter, or 
diversifying its features with less regularity, after the manner 
exhibited by the planet Mars. If such clouds existed on the moon 
it is evident that the details of its surface must be, from time to 
time, similarly obscured ; but no trace of such obscuration has 
ever been detected. When the moon is observed with high 
telescopic powers, all its details come out sharp and clear, without 
the least appearance of change or the slightest symptoms of 
cloudiness other than the occasional want of general definition, 
which may be proved to be the result of unsteadiness or want 
of homogeneity in our own atmosphere ; for we must tell the 
uninitiated that nights of pure, good definition, such as give the 
astronomer opportunity of examining with high powers the minute 
details of planetary features, are very few and far between. Out of 
the three hundred and sixty-five nights of a year there are probably 
not a dozen that an astronomer can call really fine : usually, even 
on nights that are to all common appearance superbly brilliant, 
some strata of air of dift'erent densities or temperatures, or in rapid 
motion, intervene between the observer and the object of his 
observation, and through these, owing to the ever-changing 
refractions which the rays of light coming from the object suffer in 
their course, observation of the delicate markings of a planet is 
impossible : all is blurred and confused, and nothing but bolder 
features can be recognized. It has in consequence sometimes 
happened that a slight indistinctness of some minute detail of the 
moon has been attributed to clouds or mists at the lunar surface, 
whereas the real cause has been only a bad condition of our own 
atmosphere. It may be confidently asserted that when all indis- 
tinctness due to terrestrial causes is taken account of or eliminated, 
there remain no traces whatever of any clouds or mists upon the 
surface of the moon. 

This is but one proof against the existence of a lunar 
atmosphere, and, it may be argued, not a very conclusive one ; 
because there may still be an atmosphere, though it be not 



4B THE MOON. [chap. v. 

sufficiently aqueous to condense into clouds and not sufficiently 
dense to obscure the lunar details. The probable existence of an 
atmosphere of such a character used to be inferred from a 
phenomenon seen during total eclipses of the sun. On these 
occasions the black body of the moon is invariably surrounded by a 
luminous halo, or glory, to which the name "corona" has been 
applied ; and, further, besides this corona, apparently floating in 
it and sometimes seemingly attached to the black edge of the 
moon, are seen masses of cloud-like matter of a bright red colour, 
which, from the form in which they were first seen and from their 
flame-like tinge, have become universally known as the ''red- 
flames.*' It used to be said that this corona could only be the 
consequence of a lunar atmosphere lit up as it were by the sun's 
rays shining through it, after the manner of a sunbeam lighting up 
the atmosphere of a dusty chamber ; and the red flames were held 
by those who first observed them to be clouds of denser matter 
floating in the said atmosphere, and refracting the red rays of solar 
light as our own clouds are seen to do at sunrise and sunset. But 
the evidence obtained, both by simple telescopic observation and 
by the spectroscope, from recent extensively observed eclipses of 
the sun has set this question quite at rest ; for it has been settled 
finally and indisputably that both the above appearances pertain 
to the sun, and have nothing whatever to do with the moon. 

The occurrence of a solar eclipse offers other means in addition 
to the foregoing whereby a lunar atmosphere would be detected. 
We know that all gases and vapours absorb some portion of any 
light which may shine through them. If then our satellite had an 
atmosphere, its black nucleus when seen projected against the 
bright sun in an eclipse would be surrounded by a sort of penumbra, 
or zone of shadow, in contact with its edge, somewhat like that we 
have shown in an exaggerated degree in the annexed cut (Fig. 11), 
and the passage of this penumbra over solar spots and other 
features of the solar photosphere would to some extent obscure the 
more minute details of such features. No such dusky band has 




CHAP, v.] NON-EXISTENCE OF A LUNAR ATMOSPHERE. 47 

however been at any time observed. On the contrary, a band 
somewhat brighter than the general surface of the sun has 
frequently been seen in contact with the black edge of the moon : 
this in its turn was held to indicate an atmosphere about the moon; 
but Sir George Airy has shown that a lunar atmosphere, if it really 
did exist, could not produce such an appearance, and that the cause 
of it must be sought in other directions. If this effect were really 
due to the passage of the solar rays through a lunar atmosphere a 




Fig. 11. 

similar effect ought to be produced by the passage of the sun's rays 
through the terrestrial atmosphere : and we might hence expect to / 
see the shadow of the earth projected on the moon during a lunar \ 
eclipse surrounded by a sort of bright zone or halo : we need hardly 
say such an appearance has never manifested itself. Similarly as 
we stated that the delicate details of solar spots would be obscured 
by a lunar atmosphere, small stars passing behind the moon would 
suffer some diminution in brightness as they approached apparent 
contact with the moon's edge : this fading has been watched for on 
many occasions, and in a few cases such an appearance has been 
suspected, but in by far the majority of instances nothing like a 
diminution of brightness or change of colour of the stars has been 
seen ; stars of the smallest magnitude visible under such circum- 



48 



THE MOON. 



[chap. v. 



stances retain their feeble lustre unimpaired up to the moment of 
their disappearance behind the moon's limb. 

Again, in a solar eclipse, even if there were an atmosphere about 
the moon not sufficiently dense to form a hazy outline or impair 
the distinctness of the details of a solar spot, it would still manifest 
its existence in another way. As the moon advances upon the 
sun's disc the latter assumes, of course, a crescent form. Now if 
air or vapour enveloped the moon, the exceedingly delicate cusps of 
this crescent would be distorted or turned out of shape. Instead of 




Fig. 12. 



remaining symmetrical, like the lower one in the annexed drawing 
(Fig. 12), they would be bent or deformed after the manner we have 
shown in the upper one. The slightest symptom of a distortion 
like this could not fail to obtrude itself upon an observer's eye ; but 
in no instance has anything of the kind been seen. 

Reverfing to the consequences of the terrestrial atmosphere : one 
of the most striking of these is the phenomenon of diffused day- 
light, which we need hardly remind the reader is produced by the 
scattering or diffusion of the sun's rays among the minute particles 
of vapour composing or contained in that atmosphere. Were it not 
for this reflexion and diffusion of the sun's light, those parts of our 
earth not exposed to direct sunshine would be hidden in darkness, 



CHAP, v.] NON-EXISTENCE OF A LUNAR ATMOSPHERE. 49 

receiving no illumination beyond the feeble amount that might be 
reflected from proximate terrestrial objects actually illuminated by 
direct sunlight. Twilight is a consequence of this reflexion of 
light by the atmosphere when the sun is below the horizon. If, 
then, an atmosphere enveloped the moon, we should see by diffused 
light those parts of the lunar details that are not receiving the 
direct solar beams ; and before the sun rose and after it had set 
upon any region of the moon, that region would still be partially 
illuminated by a twilight. But, on the contrary, the shadowed 
portions of a lunar landscape are pitchy black, without a trace of 
diffused-light illumination, and the effects that a twilight would 
produce are entirely absent from the moon. Once, indeed, one 
observer, Schroeter, noticed something which he suspected was due 
to an effect of this kind : when the moon exhibited itself as a very 
slender crescent, he discovered a faint crepuscular light, extending 
from each of the cusps along the circumference of the unenlight- 
ened part of the disc, and he inferred from estimates of the length 
and breadth of the line of light that there was an atmosphere about 
the moon of 5376 feet in height. This is the only instance on 
record, we believe, of such an appearance being seen. 

Spectrum analysis would also betray the existence of a lunar 
atmosphere. The solar rays falling on the moon are reflected from 
its surface to the earth. If, then, an atmosphere existed, it is 
plain that the solar rays must first pass through such atmosphere 
to reach the reflecting surface, and returning from thence, again 
pass through it on their way to the earth ; so that they must in 
reality pass through virtually twice the thickness of any atmosphere 
that may cover the moon. And if there be any such atmosphere, 
the spectrum formed by the moon's light, that is, by the sun's light 
reflected from the moon, would be modified in such a manner as to 
exhibit absorption-lines different from those found in the spectrum 
of the direct solar rays, just as the absorption-lines vary according 
as the sun's rays have to pass through a thinner or a denser 
stratum of the terrestrial atmosphere. Guided by this reasoning, 



60 THE MOON. [chap. v. 

Drs. Huggins and Miller made numerous observations upon the 
spectrum of the moon's light, which are detailed in the *' Philo- 
sophical Transactions " for the year 1864; and their result, quoting 
the words of the report, was " that the spectrum analysis of the 
light reflected from the moon is wholly negative as to the existence 
of any considerable lunar atmosphere." 

Upon another occasion, Dr. Huggins made an analogous observa- 
tion of the spectrum of a star at the moment of its occultation, 
which observation he records in the following words : — ** When an 
observation is made of the spectrum of a star a little before, or at 
the moment of its occultation by the dark limb of the moon, several 
phenomena characteristic of the passage of the star's light through 
an atmosphere might possibly present themselves to the observer. 
If a lunar atmosphere exist, which either by the substances of 
which it is composed, or by the vapours diffused through it, can 
exert a selective absorption upon the star's light, this absorption 
would be indicated to us by the appearance in the spectrum of new 
dark lines immediately before the star is occulted by the moon." 

*' If finely divided matter, aqueous or otherwise, were present 
about the moon, the red rays of the star's light would be enfeebled 
in a smaller degree than the rays of higher refrangibilities." 

** If there be about the moon an atmosphere free from vapour, 
and possessing no absorptive power, but of some density, then the 
spectrum would not be extinguished by the moon's limb at the same 
instant throughout its length. The violet and blue rays would lie 
behind the red rays." 

" I carefully observed the disappearance of the spectrum of 
€ Piscium at its occultation of January 4, 1865, for these pheno- 
mena ; but no signs of a lunar atmosphere were detected." 

But perhaps the strongest evidence of the non-existence of any 

r Appreciable lunar atmosphere is afforded by the non-refraction of 
the light of a star passing behind the edge of the lunar disc. 
Refraction, we know, is a bending of the rays of light coming from 
any object, caused by their passage through strata of transparent 



CHAP, v.] NON-EXISTENCE OF A LUNAR ATMOSPHERE. 51 

matter of different densities ; we have a familiar example in the 
apparent bending of a stick when half plunged into water. There 
is a simple schoolboy's experiment which illustrates refraction in a 
very cogent manner, but which we should, from its very simplicity, 
hesitate to recall to the reader's mind did it not very aptly represent 
the actual case we wish to exemplify. A coin is placed on the 
bottom of an empty basin, and the eye is brought into such a 
position that the coin is just hidden behind the basin's rim. 
Water is then poured into the basin and, without the eye being 
moved from its former place, as the depth of water increases, the 
coin is brought by degrees fully into view ; the water refracting or 
turning out of their course the rays of light coming from the coin, 
and lifting them, as it were, over the edge of the basin. Now a 
perfectly similar phenomenon takes place at every sunrise and 
sunset on the earth. When the sun is really below the horizon, 
it is nevertheless still visible to us because it is brought up by the 
refraction of its light by the dense stratum of atmosphere through 
which the rays have to pass. The sun is, therefore, exactly 
represented by the coin at the bottom of the basin in the boy's 
experiment, the atmosphere answers to the water, and the horizon 
to the rim or edge of the basin. If there were no atmosphere 
about the earth, the sun would not be so brought up above the 
horizon, and, as a consequence, it would set earlier and rise later 
by about a minute than it really does. This, of course, applies 
not merely to the sun, but to all celestial bodies that rise and set. 
Every planet and every star remains a shorter time below the 
horizon than it would if there were no atmosphere surrounding 
the earth. 

To apply this to the point we are discussing. The moon in her 
orbital course across the heavens is continually passing before, or 
occulting, some of the stars that so thickly stud her apparent path. 
And when we see a star thus pass behind the lunar disc on one 
side and come out again on the other side, we are virtually 
observing the setting and rising of that star upon the moon. If, 

B 3 



52 THE MOON. [chap. v. 

then, the moon had an atmosphere, it is clear, from analogy to the 
case of the earth, that the star must disappear later and reappear 
sooner than if it has no atmosphere : just as a star remains too 
short a time below the earth's horizon, or behind the earth, in 
consequence of the terrestrial atmosphere, so would a star remain 
too short a time behind the moon if an atmosphere surrounded 
that body. The point is settled in this way : — The moon's 
apparent diameter has been measured over and over again and is 
known with great accuracy ; the rate of her motion across the sky 
is also known with perfect accuracy : hence it is easy to calculate 
how long the moon will take to travel across a part of the sky exactly 
equal in length to her own diameter. Supposing, then, that we 
observe a star pass behind the moon and out again, it is clear that, 
if there be no atmosphere, the interval of time during which it 
remains occulted ought to be exactly equal to the computed time 
which the moon would take to pass over the star. If, however, from 
the existence of a lunar atmosphere, the star disappears too late and 
reappears too soon, as we have seen it would, these two intervals 
will not agree ; the computed time will be greater than the 
observed time, and the difiference, if any there be, will represent 
the amount of refraction the star's light has sustained or 
suffered, and hence the extent of atmosphere it has had to pass 
through. 

Comparisons of these two intervals of time have been repeatedly 
made, the most recent and most extensive was executed under the 
direction of the Astronomer-Royal several years ago, and it was 
based upon no less than 296 occupation observations. In this 
determination the measured or telescopic semidiameter of the 
moon was compared with the semidiameter deduced from the 
occultations, upon the above principle, and it was found that the 
telescopic semidiameter was greater than the occultation semi- 
diameter by two seconds of angular measurement or by about a 
thousandth part of the whole diameter of the moon. Sir George 
Airy, commenting on this result, says that it appears to him that 



CHAP, v.] NON-EXISTENCE OF A LUNAR ATMOSPHERE 53 

the origin of this difference is to he sought in one of two causes. 
" Either it is due to irradiation * of the telescopic semi diameter, 
and I do not douht that a part at least of the two seconds is to be 
ascribed to that cause ; or it may be due to refraction by the 
moon's atmosphere. If the whole two seconds were caused by 
atmospheric refraction this would imply a horizontal refraction of 
one second, which is only ^oVo part of the earth's horizontal 
refraction. It is possible that an atmosphere competent to pro- 
duce this refraction would not make itself visible in any other 
way." This result accords well, considering the relative accuracy 
of the means employed, with that obtained a century ago by the 
French astronomer Du Sejour, who made a rigorous examination 
of the subject founded on observations of the solar eclipse of 1764. 
He concluded that the horizontal refraction produced by a possible 
lunar atmosphere amounted to 1"'5 — a second and a half — or 
about 14*0 of that produced by the earth's atmosphere. The 
greater weight is of course to be allowed to the more recent deter- 
mination in consideration of the large number of accurate obser- 
vations upon which it was based. 

But an atmosphere 2,000 times rarer than our air can scarcely 
be regarded as an atmosphere at all. The contents of an air-pump 
receiver can seldom be rarefied to a greater extent than to about 
T^oc of the density of air at the earth's surface, with the best of 
pneumatic machines; and the lunar atmosphere, if it exist at all, 
is thus proved to be twice as attenuated as what we are accus- 
tomed to recognise as a vacuum. In discussing the physical 
phenomena of the lunar surface, we are, therefore, perfectly justified 
in omitting all considerations of an atmosphere, and adapting our 
arguments to the non-existence of such an appendage. 

* Irradiation is an ocular phenomenon in virtue of iwhich all strongly illu- 
minated objects appear to the eye to be larger than they really are. The 
impression produced by light upon the retina appears to extend itself around the 
focal image formed by the lenses of the eye. It is from the effect of irradiation 
that a white disc on a black ground looks larger than a black disc of the same size 
on a white ground. 



54 THE MOON. [chap. v. 

And if there be no air upon the moon, vre are almost forced to 
conclude that there can be no water ; for if water covered any part 
of the lunar globe it must be vaporised under the influence of the 
long period of uninterrupted sunshine (upwards of 300 hours) that 
constitutes the lunar day, and would manifest itself in the form of 
clouds or mists obscuring certain parts of the surface. But, as we 
have already said, no such obliteration of details ever takes place ; 
and, as we have further seen, no evidence of aqueous vapour is 
manifested upon the occasion of spectrum observations. Since, 
then, the effects of watery vapour are absent, we are forced to 
conclude that the cause is absent also. 

Those parts of the moon which the ancient astronomers assumed, 
from their comparatively smooth and dusky appearance, to be seas, 
have long since been discovered to be merely extensive regions of 
less reflective surface material ; for the telescope reveals to us 
irregularities and asperities covering well-nigh the whole of them, 
which asperities could not be seen if they were covered with water ; 
unless, indeed, we admit the possibility of seeing to the bottom of 
the water, not only perpendicularly, but obliquely. Some observers 
have noticed features that have led them to suppose that water 
was at one time present upon the moon, and has left its traces in 
the form of appearances of erosive action in some parts. But if 
water ever existed, where is it now ? One writer, it is true, has 
suggested as possible, that whatever air, and we presume he would 
include whatever water also, the moon may possess, is hidden 
away in sublunarean caves and hollows ; but even if water existed 
in these places it must sometimes assume the vapoury form, and 
thus make its presence known. 

Sir John Herschel pointed out that if any moisture exists upon 
the moon, it must be in a continual state of migration from the 
illuminated or hot^ to the unilluminated or cold side of the lunar 
globe. The alternations of temperature, from the heat produced 
by the unmitigated sunshine of 14 days' duration, to the intensity 
of cold resulting from the absence of any sunshine whatever for an 



CHAP, v.] NON-EXISTENQE OF A LUNAR ATMOSPHERE. 



55 



equal period, must, he argued, produce an action similar to that of 
the cryophorus in transporting the lunar moisture from one hemi- 
sphere to the other. The cryophorus is a little instrument 
invented by the late Dr. WoUaston ; it consists of two bulbs of 
glass connected by a bent tube, in the manner shown in the 
annexed illustration. Fig. 13. One of the bulbs. A, is half-filled 





Fig. 13. 



with water, and, all air being exhausted, the instrument is her- 
metically sealed, leaving nothing within but the water and the 
aqueous vapour which rises therefrom in the absence of atmos- 




Fio. 14. 



pheric pressure. When the empty bulb, B, is placed in a freezing 
mixture, a rapid condensation of this vapour takes place within it, 
and as a consequence the water in the bulb A gives off more 
vapour. The abstraction of heat from the water, which is a 
natural consequence of this evaporation, causes it to freeze into a 
solid mass of ice. Now upon the moon the same phenomenon 



56 THE MOON. [chap. v. 

would occur did the material exist there to supply it. In the 
accompanying diagram let A represent the illuminated or heated 
hemisphere of the moon, and B the dark or cold hemisphere ; the 
former being probably at a temperature of 300° above, and the 
latter 200° below Fahrenheit's zero. Upon the above principle, if 
moisture existed upon A it would become vaporised, and the 
vapour would migrate over to B, and deposit itself there as hoar- 
frost ; it would, therefore, manifest itself to us while in the act of 
migrating by clouding or dimming the details about the boundary 
of the illuminated hemisphere. The sun, rising upon any point 
upon the margin of the dark hemisphere, would have to shine 
through a bed of moisture, and we may justly suppose, if this 
were the case, that the tops of mountains catching the first beams 
of sunlight would be tinged with colour, or be lit up at first with 
but a faint illumination, just as we see in the case of terrestrial 
mountains whose summits catch the first, or receive the last beams 
of the rising or setting sun. Nothing of this kind is, however, 
perceptible : when the solar rays tip the lofty peaks of lunar 
mountains, these shine at once with brilliant light, quite as vivid 
as any of those parts that receive less horizontal illumination, or 
upon which the sun is almost perpendicularly shining. 

All the evidence, then, that we have the means of obtaining, 
goes to prove that neither air nor water exists upon the moon. 
Two complicating elements affecting all questions relating to the 
geology of the terraqueous globe we inhabit may thus be dismissed 
from our minds while considering the physical features of the 
lunar surface. Fire on the one hand and water on the other, are ? 
the agents to which the configurations of the earth's surface are 
referable : the first of these produced the igneous rocks that form 
the veritable foundations of the earth, the second has given rise to 
the superstructure of deposits that constitute the secondary and 
tertiary formations : were these last removed from the surface of 
our planet, so as to lay bare its original igneous crust, that crust, 
so far as reasoning can picture it to us, would probably not differ 



CHAP, v.] NON-EXISTENCE OF A LUNAR ATMOSPHERE. 57 

essentially from the visible surface of the moon. In considering 
the causes that have given birth to the diversified features of that 
surface, we may, therefore, ignore the influence of air and water 
action and confine our reasoning to igneous phenomena alone : our 
task in this matter, it is hardly necessary to remark, is materially 
simplified thereby. 



CHAPTER VI. 

THE GENERAL ASPECT OF THE LUNAR SURFACE. 

We have now reached that stage of our subject at which it 
behoves us to repair to the telescope for the purpose of examining 
and familiarising ourselves with the various classes of detail that 
the lunar surface presents to our view. 

That the moon is not a smooth sphere of matter is a fact that 
manifested itself to the earliest observers. The naked eye 
perceives on her face spots exhibiting marked differences of 
illumination. These variations of light and shade, long before the 
invention of the telescope, induced the belief that she possessed 
surface irregularities like those that diversify the face of the earth, 
and from analogy it was inferred that seas and continents alter- 
nated upon the lunar globe. It was evident, from the persistence 
and invariability of the dusky markings, that they were not due to 
atmospheric peculiarities, but were veritable variations in the 
character or disposition of the surface material. Fancy made 
pictures of these unchangeable spots : untutored gazers detected in 
them the indications of a human countenance, and perhaps the 
earliest map of the moon was a rough reproduction of a man's face, 
the eyes, nose and mouth representing the more salient spots 
discernible upon the lunar disc. Others recognised in these spots 
the configuration of a human form, head, arms and legs complete, 
which a French superstition that lingers to the present day held to 
be the image of Judas Iscariot transported to the moon in punish- 
ment for his treason. Again, an Indian notion connects the lunar 



PLATE IV. 




■Vroodburytype" 



FULL MOON 



CHAP. VI.] GENERAL ASPECT OF THE LUNAR SURFACE. 59 

spots with a representation of a roebuck or a hare, and hence the 
Sanskrit names for the moon, mrigadhara, a roebuck-bearer, and 
^sa'sabhrit, a hare-bearer. Of these similitudes the one which has 
the best pretensions to a rude accuracy is that first mentioned ; 
for the resemblance of the full moon to a human countenance, 
wearing a painful or lugubrious expression, is very striking. Our 
illustration of the full moon (Plate IV.) is derived from an actual 
photograph ; * the relative intensities of light and shade are hence 
somewhat exaggerated ; otherwise it represents the full moon very 
nearly as the naked eye sees it, and by gazing at the plate from a 
short distance, t the well-known features will manifest themselves, 
while they who choose may amuse themselves by arranging the 
markings in their imagination till they conform to the other 
appearances alluded to. 

We may remark in passing that by one sect of ancient writers 
the moon was supposed to be a kind of mirror, receiving the image 
of the earth and reflecting it back to terrestrial spectators. 
Humboldt affirmed that this opinion had been preserved to his 
day as a popular belief among Ihe people of Asia Minor. He says, 
*'I was once very much astonished to hear a very well educated 
Persian from Ispahan, who certainly had never read a Greek book, 
mention when I showed him the moon's spots in a large telescope 
in Paris, this hypothesis as a widely diflused belief in his country : 
V * What we see in the moon,' said the Persian, * is ourselves ; it is 
the map of our earth.' " *' Quite as extravagant an idea, though 
perhaps a more excusable one, was that held by some ancient 
philosophers, to the effect that the spots on the moon were the 
shadows of opaque bodies floating in space between it and the sun. 

* For the original photograph from which this plate was produced, and for 
permission to reproduce it, we owe our acknowledgments to Warren De la Kue 
and Joseph Beck, Esquires. 

f The proper distance for realising the conditions under which the moon itself 
is seen will be that at which our disc is just covered by a wafer about a quarter of 
an inch in diameter, held at arm's length. This will subtend an angle of about 
half a degree, which is nearly the angular diameter of the moon. 



60 THE MOON. [chap. vi. 

An observer watching the forms and positions of the lunar face- 
marks, from night to night and from lunation to lunation, cannot 
fail to notice the circumstance that they undergo no easily 
perceptible change of position with respect to the circular outline 
of the disc ; that in fact the face of .the moon presented to our 
view is always the same, or very nearly so. If the moon had no 
orbital motion we should be led from the above phenomenon to 
conclude that she had no axial motion, no movement of rotation ; 
but when we consider the orbital motion in connection with the 
permanence of aspect, we are driven to the conclusion — one, how- 
ever, which superficial observers have some difficulty in recognising 
— ^that the moon has an axial rotation equal in period to her orbital 
revolution. Since the moon makes the circuit of her orbit in 
twenty-seven days and one -third (more exactly 27d. 7h. 43m. lis.), 
it follows that this is the time of her axial rotation, as referred 
to the stars, or as it would be made out by an observer located at 
a fixed position in space outside the lunar orbit. But if referred 
to the sun this period appears different ; because the moon while 
revolving round the earth is, with the earth, circulating around the 
sun. Suppose the three bodies, moon, earth, and sun, to be in a 
line at a certain period of a lunation, as they are at full moon : by 
the time the moon has completed her twenty-seven days' journey 
around the earth, the latter will have moved along twenty-seven 
days' march of its orbit, which is about twenty- seven degrees of 
celestial longitude : the sun will apparently be that much distant 
from a straight line passing through earth and moon, and the 
moon must therefore move forward to overtake the sun before she 
can assume the full phase again. She will take something over 
two days to do this ; hence the solar period of her revolution 
becomes more than twenty-nine days (to be exact, 29d. 12h. 44m. 
2s. '87). This is the length of a solar day upon the moon — the 
interval from one sunrise to another at any spot upon the equator 
of our satellite, and the interval between successive reappearances 
of the same phase to observers on the earth. The physical cause 



CHAP. VI.] GENERAL ASPECT OF THE LUNAR SURFACE. 61 

of the coincidence of times of rotation and revolution was touched 
upon in a previous chapter. 

We have said that the moon continuously presents to us the same 
hemisphere. This is generally true, but not entirely so. Galileo, 
by long scrutiny, familiarised himself with every detail of the lunar 
disc that came within the limited grasp of his telescopes, and he 
recognised the fact that according as the position of the moon varied 
in the sky, so the aspect of her face altered to a slight degree ; that 
certain regions at the edge of her disc alternately came in sight and 
receded from his view. He perceived, in fact, an appa7'e7it rocking 
to and fro of the globe of the moon ; assort of balancing or libratory 
motion. "When the moon was near the horizon he could see spots 
upon her uppermost edge, which disappeared as she approached the 
zenith, or highest point of her nightly path ; and as she neared this 
point, other spots, before invisible, came into view, near to what 
had been her lower edge. Galileo was not long in referring this 
phenomenon to its true cause. The centre of motion of the moon 
being the centre of the earth, it is clear that an observer on the 
surface of the latter, looks down upon the rising moon as from an 
eminence, and thus he is enabled to see more or less over or around 
her. As the moon increases in altitude, the line of sight gradually 
becomes parallel to the line joining the observer and the centre of 
the earth, and at length he looks her full in the face : he loses the 
full view and catches another side face view as she nears the horizon 
in setting. This phenomenon, occurring as it does, with a daily 
period, is known as the diurnal lihration. 

But a kindred phenomenon presents itself in another period, and 
from another cause. The moon rotates upon her axis at a speed 
that is rigorously uniform. But her orbital motion is not uniform, 
sometimes it is faster, and at other times slower than its average 
rate. Hence, the angle through which she moves along her orbit 
in a given time, now exceeds, and now falls short of the angle 
through which she turns upon her axis. Her visible hemisphere 
thus changes to an extent depending upon the difference between 



62 THE MOON. [chap. vi. 

these orbital and axial angles, and the apparent balancing thus 
produced is called the lihration in longitude. Then there is a libra- 
tion in latitude due to the circumstance that the axis of the moon 
is not exactly perpendicular to the plane of her orbit ; the effect of 
this inclination being, that we sometimes see a little more of the 
north than of the south polar regions of our satellite, and vice 
versa.* 

The extent of the moon's librations, taking them all and in com- 
bination into account, amounts to about seven degrees of arc of 
latitude or longitude upon the moon, both in the north- south and 
east- west directions. And taking into account the whole effect of 
them, we may conclude that our view of the moon's surface, instead 
of being confined to one half, is extended really to about four- 
sevenths of the whole area of the lunar globe. The remaining 
three- sevenths must for ever remain a terra incognita to the 

* The libratory movement has been taken advantage of, at the suggestion of Sir 
Chas. Wheatstone, for producing stereoscopic photographs of the moon. In the 
early days of stereoscopic photography the object to be photographed was placed 
upon a kind of turn-table, and, after a picture had been taken of it in one position, 
the table was turned through a small angle for the taking of the second picture ; 
the two placed side by side then represented the object as it would have been seen 
by two eyes widely separated, or whose visual rays inclined at an angle equal to 
that through which the table was turned ; and when the pictures were viewed 
through a stereoscope, they combined to produce the wonderful effect of solidity 
now familiar to every one. The moon, by its librations, imitates the turn-table 
movement; and, from a large number of photographs of her, taken at different 
points of her orbit and at different seasons of the year, it is possible to select two 
which, while they exhibit the same phase of illumination, at the same time present 
the requisite difference in the points of view from which they are taken to give the 
effect of stereoscopicity when viewed binocularly. Mr. De la Rue, the father of 
celestial photography, has been enabled to produce several such pairs of pictures 
from the vast collection of lunar photographs that he has accumulated. Any one 
of these pairs of portraits, when stereoscopically combined, reproduces, to quote 
the words of Sir John Herschel, " the spTierical form just as a giant might see it 
whose stature were such that the interval between his eyes should equal the dis- 
tance between the place where the earth stood when one view was taken, and that 
to which it would have to be removed (our moon being fixed) to get. the other. 
Nothing can surpass the impression of real corporeal form thus conveyed by some 
of these pictures as taken by Mr. De la Rue with his powerful reflector, the 
production of which (as a step in some sort taken by man outside of the planet he 
inhabits) is one of the most remarkable and unexpected triumphs of scientific art." 



CHAP. VI.] GENERAL ASPECT OF THE LUNAR SURFACE. 68 

habitants of this earth, unless, indeed, from some catastrophe which 
it would be wild fancy to anticipate, a period of rotation should be 
given to the moon different from that which it at present possesses. 
Some highly fanciful theorists have speculated upon the possible 
condition of the invisible hemisphere, and have propounded the 
absurd notion that the opposite side of the moon is hollow, or that 
the moon is a mere shell ; others again have urged that the hidden 
half is more or less covered with water, and others again, 
that it is peopled with inhabitants. There is, however, no good 
reason for supposing that what we may call the back of the moon 
has a physical structure essentially diiferent from the face presented 
towards us. So far as can be judged from the peeps that libration 
enables us to obtain, the same characteristic features (though of 
course with different details) prevail over the whole lunar surface. 

The speculative ideas held by the philosophers of the pre-tele- 
scopic age, touching the causes which produced the inequalities of 
light and shade upon the moon, received their coup de grace from 
the revelations of Galileo's glasses. Our satellite was one of the 
earliest objects, if not actually the first, upon which the Florentine 
turned his telescope ; and he found that the inequalities upon her 
surface were due to differences in its configuration analogous to the 
continents and islands, and (as might then have been thought) the 
seas of our globe. He could trace, even with his moderate means, 
the semblance of mountain-tops upon which the sun shone while 
their lower parts were in shadow, of hills that were brightly 
illuminated upon their sides towards the sun, of brightly shining 
elevations, and deeply shadowed depressions, of smooth plains, and 
regions of mountainous ruggedness. He saw that the boundary of 
sunlight upon the moon was not a clearly defined line, as it would 
be if the lunar globe were a smooth sphere, as the Aristotelians had 
asserted, but that the terminator was uneven and broken into 
an irregular outline. From these observations the Florentine 
astronomer concluded that the lunar world was covered not only 
with mountains like our globe, but with mountains whose heights 



64 THE MOON. [chap. vi. 

far surpassed those existing upon the earth, and whose forms were 
strangely limited to circularity. 

Galileo's best telescopes magnified only some thirty times, and 
the views which he thus obtained, must have been similar to those 
exhibited by the smaller photographs of the moon produced in late 
years by Mr. De la Rue and now familiar to the scientific public. 
Of course there is in the natural moon as viewed with a small tele- 
scope a vivid brilliancy which no art can imitate, and in photographs 
especially there is a tendency to exaggeration of the depths of 
shade in a lunar picture. This arises from the circumstance that 
various regions of the moon do not impress a chemically sensitized 
plate as they impress the retina of the eye. Some portions, 
notably the so-called *' seas " of the moon, which to the eye appear 
but slightly duller than the brighter parts, give off so little actinic 
light that they appear as nearly black patches upon a photograph, 
and thus give an undue impression of the relative brightness of 
various parts of the lunar surface. Doubtless by sufficient exposure 
of the plate in the camera-telescope the dark patches might be ren- 
dered lighter, but in that case the more strongly illuminated por- 
tions, which after all are those most desirable to be preserved, 
would be lost by the effect which photographers understand as 
** solarization." 

In speaking of a view of the moon with a magnifying power of 
thirty, it is necessary to bear in mind that the visible features will 
differ considerably with the diameter of the object-glass of the 
telescope to which this power is applied. The same details would 
not be seen alike with the same power upon an object-glass of 10 
inches diameter and one of 2 inches. The superior illumination of 
the image in the former case would bring into view minute details' 
that could not be perceived with the smaller aperture. He who 
would for curiosity wish to see the moon, or any other object, as 
Galileo saw it, must use a telescope of the same size and character 
in all respects as Galileo's : it will not do to put his magnifying 
power upon a larger telescope. With large telescopes, and low 



CHAP. VI.] GENERAL ASPECT OF THE LUNAR SURFACE. 65 

powers used upon bright objects like the moon, there is a blinding 
flood of light which tends to contract the pupil of the eye and pre- 
vent the passage of the whole of the pencil of rays coming through 
the eye-piece. Although this last result may be productive of no 
inconvenience, it is clearly a waste of light, and it points to a rule 
that the lowest power that a telescope should bear is that which 
gives a pencil of light equal in diameter to the pupil of the eye 
under the circumstances of brightness attendant upon the object 
viewed. In observing faint objects this point assumes more 
importance, since it is then necessary that all available light should 
enter the pupil. The thought suggests itself that an artificial 
enlargement of the pupil, as by a dose of belladonna, might be of 
assistance in searching for faint objects, such as nebulae and 
comets : but we prefer to leave the experiment for those to try 
who pursue that branch of astronomical observation. 

A merely cursory examination of the moon with the low power to 
which we have alluded is sufficient to show us the more salient fea- 
tures. In the first place we cannot help being struck with the 
immense preponderance of circular or craterform asperities, and 
with the general tendency to circular shape which is apparent in 
nearly all the lunar surface markings ; for even the larger regions 
known as the " seas " and the smaller patches of the same character 
seem to repeat in their outlines the round form of the craters. It 
is at the boundary of sunlight on the lunar globe that we see these 
craterform spots to the best advantage, as it is there that the rising 
or setting sun casts long shadows over the lunar landscape, and 
brings elevations and asperities into bold relief. They vary greatly 
in size, some are so large as to bear an estimable proportion to the 
moon's diameter, and the smallest are so minute as to need the 
most powerful telescopes and the finest conditions of atmosphere to 
perceive them. It is doubtful whether the smallest of them have 
ever been" seen, for there is no reason to doubt that there exist 
countless numbers that are beyond the revealing powers of our 
finest telescopes. 



66 THE MOON. [chap. vi. 

From tlie great number and persistent character of tliese circum- 
vallations, Kepler was led to think that they were of artificial con- 
struction. He regarded them as pits excavated by the supposed 
habitants of the moon to shelter themselves from the long and 
intense action of the sun. Had he known their real dimensions, of 
which we shall have to speak when we come to describe them more 
in detail, he would have hesitated in propounding such a hypothesis ; 
nevertheless it was, to a certain extent, justified by the regular and 
seemingly unnatural recurrence of one particular form of structure, 
the like of which is, too, so seldom met with as a structural feature 
of the surface of our own globe. 

The next most striking features, revealed by a low telescopic 
power upon the moon, are the seemingly smooth plains that have 
the appearance of dusky spots, and that collectively cover a con- 
siderable portion — about two-thirds — of the entire disc. The 
larger of these spots retain the name of seas, the term having been 
given when they were supposed to be watery expanses, and having 
been retained, possibly to avoid the confusion inevitable from a 
change of name, after the existence of water upon the moon was 
disproved. Following the same order of nomenclature, the smaller 
spots have received the appellations of lakes, hays, and fens. We 
see that many of these "seas" are partially surrounded by ramparts 
or bulwarks which, under closer examination, and having regard to 
their real magnitude, resolve themselves into immense mountain 
chains. The general resemblance in form which the bulwarked 
plains thus exhibit to the circular craters of large size, would lead 
us to suppose that the two classes of objects had the same formative 
origin, but when we take into account the immense size of the 
former, and the process by which we infer the latter to have been 
developed, the supposition becomes untenable. 

Another of the prominent features which we notice as highly 
curious, and in some phases of the moon — at about the time of full 
— the most remarkable of all, are certain bright lines that appear 
to radiate from some of the more conspicuous craters, and extend 



CHAP. VI.] GENERAL ASPECT OF THE LUNAR SURFACE. 67 

for hundreds of miles around. No selenological formations have 
so sorely puzzled observers as these peculiar streaks, and a great 
deal of fanciful theorizing has been bestowed upon them. As we 
are now only glancing at the moon, we do not enter upon explana- 
tions concerning them or any other class of details ; all such will 
receive due consideration in their proper order in succeeding 
chapters. 

We thus see that the classes of features observable upon the 
moon are not great in number : they may be summed up as craters 
and their central cones, mountain chains^ with occasional isolated 
peaks, smooth plains^ with more or less of irregularity of surface, 
and bright radiating streaks. But when we come to study with 
higher powers the individual examples of each class we meet with 
considerable diversity. This is especially the case with the craters, 
which appear under very numerous variations of the one order of 
structure, viz., the ring-form. A higher telescopic power shows us 
that not only do these craters exist of all magnitudes within a limit 
of largeness, but seemingly with no limit of smallness, but that in 
their structure and arrangement they present a great variety of 
points of difference. Some are seen to be considerably elevated 
above the surrounding surface, others are basins hollowed out of 
that surface and with low surrounding ramparts ; some are merely 
like walled plains or amphitheatres with flat plateaux, while the 
majority have their lowest point of hollowness considerably below 
the general level of the surrounding surface ; some are isolated 
upon the plains, others are aggregated into a thick crowd, and 
overlapping and intruding upon each other; some have elevated 
peaks or cones in their centres, and some are without these central 
cones, while the plateaux of others again contain several minute 
craters instead ; some have their ramparts whole and perfect, others 
have them breached or malformed, and many have them divided 
into terraces, especially on their inner sides. 

In the plains, what with a low power appeared smooth as a water 
surface becomes, under greater magnification, a rough and furrowed 

F 2 



68 THE MOON. [chap. vi. 

area, here gently undulated and there broken into ridges and 
declivities, with now and then deep rents or cracks extending for 
miles and spreading like river-beds into numerous ramifications. 
Craters of all sizes and classes are scattered over the plains ; these 
appear generally of a different tint to the surrounding surface, for 
the light reflected from the plains has been observed to be slightly 
tinged with colour. The tint is not the same in all cases : one 
large sea has a dingy greenish tinge, others are merely grey, and 
some others present a pale reddish hue. The cause of this diver- 
sity of colour is mysterious ; it has been supposed to indicate the 
existence of vegetation of some sort ; but this involves conditions 
that we know do not exist. 

The mountains, under higher magnification, do not present such 
diversity of formation as the craters, or at least the points of 
difference are not so apparent ; but they exhibit a plentiful variety 
of combinations. There are a few perfectly isolated examples that 
cast long shadows over the plains on which they stand like those of 
a towering cathedral in the rising or setting sun. Sometimes they 
are collected into groups, but mostly they are connected into 
stupendous chains. In one of the grandest of these chains, it has 
been estimated that a good telescope will show 3000 mountains 
clustered together, without approach to symmetrical order. The 
scenery which they would present, could we get any other than the 
" bird's eye view " to which we are confined, must be imposing in 
the extreme, far exceeding in sublime grandeur anything that the 
Alps or the Himalayas offer ; for while on the one hand the lunar 
mountains equal those of the earth in altitude, the absence of an 
atmosphere, and consequently of the effects produced thereby, must 
give rise to alternations of dazzling light and black depths of shade 
combining to form panoramas of wild scenery that, for want of a 
parallel on earth, we may well call unearthly. But we are debarred 
the pleasure of actually contemplating such pictures by the circum- 
stance that we look down upon the mountain tops and into the 
valleys, so that the great height and close aggregation of the peaks 



CHAP. VI.] GENERAL ASPECT OF TEE LUNAR SURFACE. 69 

and hills are not so apparent. To compare the lunar and terrestrial 
inountain scenery would be ** to compare the different views of a 
town seen from the car of a balloon with the more interesting 
prospects by a progress through the streets." Some of the pecu- 
liarities of the lunar scenery we have, however, endeavoured to 
realize in a subsequent chapter. 

A high power gives us little more evidence than a low one upon 
the nature of the long bright streaks that radiate from some of the 
more conspicuous craters, but it enables us to see that those streaks 
do not arise from any perceptible difference of level of the surface 
— that they have no very definite outline, and that they do not 
present any sloping sides to catch more sunlight, and thus shine 
brighter, than the general surface. Indeed, one great peculiarity 
of them is that they come out most forcibly where the sun is 
shining perpendicularly upon them ; hence they are best seen 
where the moon is at full, and they are not visible at all at those 
regions upon which the sun is rising or setting. We also see that 
they are not diverted by elevations in their path, as they traverse in 
their course craters, mountains, and plains alike, giving a slight 
additional brightness to all objects over which they pass, but 
producing no other effect upon them. To employ a commonplace 
simile, they look as though, after the whole surface of the moon 
had assumed its final configuration, a vast brush charged with a 
whitish pigment had been drawn over the globe in straight lines 
radiating from a central point, leaving its trail upon everything it 
touched, but obscuring nothing. 

Whatever may be the cause that produces this brightness of 
certain parts of the moon without reference to configuration of 
surface, this cause has not been confined to the formation of the 
radiating lines, for we meet with many isolated spots, streaks, and 
patches of the same bright character. Upon some of the plains 
there are small areas and lines of luminous matter possessing 
peculiarities similar to those of the radiating streaks, as regards 
visibility with the high sun, and invisibility when the solar rays 



70 THE MOON. [chap. vi. 

fall upon tliem liorizontally. Some of the craters also are sur- 
rounded by a kind of aureole of this highly reflective matter. A 
notable specimen is that called Linne, concerning which a great 
hue and cry about change of appearance and inferred continuance 
of volcanic action on the moon was raised some years ago. This 
object is an insignificant little crater of about a mile or two in 
diameter, in the centre of an ill- defined spot of the character 
referred to, and about eight or ten miles in diameter. With a low 
sun the crater alone is visible by its shadow ; but as the luminary 
rises the shadow shortens and becomes all but invisible, and then 
the white spot shines forth. These alternations, complicated by 
variations of atmospheric condition, and by the interpretations of 
difi'erent observers, gave rise to statements of somewhat exagge- 
rated character to the effect that considerable changes, of the nature 
of volcanic eruptions, were in progress in that particular region of 
the moon. 

In the foregoing remarks we have alluded somewhat indefinitely 
to high powers ; and an enquiring but unastronomical reader may 
reasonably demand some information upon this point. It might 
have been instructive to have cited the various details that may be 
said to come into view with progressive increases of magnification. 
But this would be an all but impossible task, on account of the 
varying conditions under which all astronomical observations must 
necessarily be made. When we come to delicate tests, there are 
no standards of telescopic power and definition. Assuming the 
instrument to be of good size and high optical character, there is yet 
a powerful influencer of astronomical definition in the atmosphere 
and its variable state. Upon two-thirds of the clear nights of a 
year the finest telescopes cannot be used to their full advantage, 
because the minute flutterings resulting from the passage of the 
rays of light through moving strata of air of different densities are 
magnified just as the image in the telescope is magnified, till all 
minute details are blurred and confused, and only the grosser 
features are left visible. And supposing the telescope and atmo- 



CHAP. VI.] GENERAL ASPECT OF THE LUNAR SURFACE. 71 

sphere in good state, there is still an important point, the state of 
the observer's eye, to be considered. After all it is the eye that 
sees, and the best telescopic assistance to an untrained eye is of 
small avail. The eye is as susceptible of education and develop- 
ment as any other organ ; a skilful and acute observer is to a mere 
casual gazer what a watchmaker would be to a ploughman, a 
miniature painter to a whitewasher. This fact is not generally 
recognized ; no man would think of taking in hand an engraver's 
burin, and expecting on the instant to use it like an adept, or of 
going to a smithy and without previous preparation trying to forge 
a horse-shoe. Yet do folks enter observatories with uneducated 
eyes, and expect at once to realise all the wonderful things that 
their minds have pictured to themselves from the perusal of astro- 
nomical books. We have over and over again remarked the 
dissatisfaction which attends the first looks of novices through a 
powerful telescope. They anticipate immediately beholding 
wonders, and they are disappointed at finding how little they can 
see, and how far short the sight falls of what they had expected. 
Courtesy at times leads them to express wonder and surprise, 
which it is easy to see is not really felt, but sometimes honesty 
compels them to give expression to their disappointment. This 
arises from the simple fact that their eyes are not fit for the work 
which is for the moment imposed upon them ; they know not what 
to look for, or how to look for it. The first essay at telescopic 
gazing, like first essays generally, serves but to teach us our 
incapability. 

To a tutored eye a great deal is visible with a comparatively low 
power, and practised observers strive to use magnifying powers as 
low as possible, so as to diminish, as far as may be, the evils 
arising from an untranquil atmosphere. With a power so small as 
30 or 40, many exceedingly delicate details on the moon are visible 
to an eye that is familiar with them under higher powers. With 
200 we may say that every ordinary detail will come out under 
favourable conditions ; but when minute points of structure, mere 



72 THE MOON. [chap. vi. 

nooks and corners as it were, are to be scrutinised, 300 may be 
used with advantage. Another hundred diameters almost passes 
the practical limit. Unless the air be not merely fine, but super- 
fine, the details become " clothy " and tremulous ; the extra points 
brought out by the increased power are then only caught by 
momentary glimpses, of which but a very few are obtained during 
a lengthy period of persistent scrutiny. We may set down 250 as 
the most useful, and 350 the utmost effective power that can be 
employed upon the particular work of which we are treating. 
Could every detail on the moon be thoroughly and reliably repre- 
sented as this amount of magnification shows it, the result would 
leave little to be wished for. 

. But it may be asked by some, what is the absolute effect of such 
powers as those we have spoken of, in bringing the moon apparently 
nearer to our eyes? and what is the actual size of the smallest 
object visible under the most favourable circumstances ? A linear 
mile upon the moon corresponds to an angular interval of 0*87 of a 
second ; this refers to regions about the centre of the disc ; near 
the circumference the foreshortening makes a difference, very great 
as the edge is approached. Perhaps the smallest angle that the 
eye can without assistance appreciate is half a minute ; that is to 
say, an object that subtends to the eye an arc of less than half a 
minute can scarcely be seen.* Since there are 60 seconds in a 
minute, it follows that we must magnify a spot a second in diameter 
upon the moon thirty times before we can see it ; and since a 
second represents rather more than a mile, really about 2000 
yards, on the moon, as seen from the earth, the smallest object 
visible with a power of 30 will be this number of yards in diameter 
or breadth. To see an object 200 yards across, we should require 
to magnify it 300 times, and this would only bring it into view as a 

* This is a point of some uncertainty. Dr. Young stated (Lectures Vol. II. 
p. 575) that " a minute is perhaps nearly the smallest interval at which two objects 
can be distinguished, although a line subtending only a tenth of a minute in breadth 
may sometimes be perceived as a single object." 



CHAP. VI.] GENERAL ASPECT OF THE LUNAR SURFACE. 73 

point ; 20 yards would require a power of 3000, and 1 yard 60,000 
to effect the same thing. Since, as we have said, the highest 
practicable power with our present telescopes, and at ordinary 
terrestrial elevations, is 350, or for an extreme say 400, it is 
evident that the minutest lunar object or detail of which we can 
perceive as a point must measure about 150 yards : to see the form 
of an object, so as to discriminate whether it be round or square, it 
would require to be probably twice this size ; for it may be safely 
assumed that we cannot perceive the outline of an object whose 
average breadth subtends a less angle than a minute. 

Arago put this question into another shape : — The moon is 
distant from us 237,000 miles (mean). A magnifying power of a 
thousand would show us the moon as if she were distant 237 miles 
from the naked eye. 

2000 would bring her within 118 miles. 
4000 „ „ „ 59 „ 

6000 „ „ „ 39 „ 

Mont Blanc is visible to the naked eye from Lyons, at the distance 
of about 100 miles ; so that to see the mountains of the moon as 
Mont Blanc is seen from Lyons would require the impracticable 
power of 2500. 



CHAPTER VII. 

TOPOGRAPHY OF THE MOON. 

It is scarcely necessary to seek the reasons which prompted 
astronomers, soon after the invention of the telescope, to map the 
surface features of the moon. They may have considered it desir- 
able to record the positions of the spots upon her disc, for the pur- 
pose of facilitating observations of the passage of the earth's 
shadow over them in lunar eclipses ; or they may have been actu- 
ated by a desire to register appearances then existing, in order that 
if changes took place in after years these might be readily detected. 
Scheiner was one of the earliest of lunar cartographers ; he worked 
about the middle of the seventeenth century ; but his delineations 
were very rough and exaggerated. Better maps — ^the best of the 
time, according to an old authority — were engraved by one Mellan, 
about the years 1634 or 1635. At about the same epoch Langreen 
and Hevelius were working upon the same subject. Langreen 
executed some thirty maps of portions of the moon, and introduced 
the practice of naming the spots after philosophers and eminent 
men. Hevelius spent several years upon his task, the results of 
which he published in a bulky volume containing some 50 maps of 
the moon in various phases, and accompanied by 500 pages of 
letter-press. He rejected Langreen's system of nomenclature, and 
called the spots after the seas and continents of the earth to which 
he conceived they bore resemblance. Riccioli, another seleno- 
grapher, whose map was compiled from observations made by 
Grimaldi, restored Langreen's nomenclature, but he confined him- 



CHAP. VII.] TOPOGRAPHY OF THE MOON. 75 

self to the names of eminent astronomers, and his system has 
gained the adhesion of the map-makers of later times. Cassini 
prepared a large map from his own observations, and it was 
engraved about the year 1692. It appears to have been regarded 
as a standard work, for a reduced copy of it was repeatedly issued 
with the yearly volumes of the Connaissance des Temps (the 
*' Nautical Almanack " of France) some time after its publication. 
These small copies have no great merit : the large copper plate of 
the original was, we are told by Arago, who received the statement 
from Bouvard, sold to a brazier by a director of the French Govern- 
ment Printing- Office, who thought proper to disembarrass the 
stores of that establishment, by ridding them of what he considered 
lumber ! La Hire, Mayer, and Lambert followed, during the 
succeeding century, in this branch of astronomical delineation. At 
the commencement of the present century, the subject was very 
earnestly taken up by the indefatigable Schroeter, who, although 
he does not appear to have produced a complete map, produced a 
topograph of the moon in a large series of partial maps and draw- 
ings of special features. Schroeter was a fine observer, but his 
delineations show him to have been an indifferent draughtsman. 
Some of his drawings are but the rudest representations of the 
objects he intended to depict; many of the bolder features of con- 
spicuous objects are scarcely recognizable in them. A bad artist is 
as likely to mislead posterity as a bad historian, and it cannot be 
surprising if observers of this or future generations, accepting 
Schroeter's drawings as faithful representations, should infer from 
them remarkable changes in the lunar details. It is much to be 
regretted that Schroeter's work should be thus depreciated. Lohr- 
man of Dresden, was the next cartographer of the moon ; in 1824 
he put forth a small but very excellent map of 15 inches diameter, 
and published a book of descriptive text, accompanied by sectional 
charts of particular areas. His work, however, was eclipsed by the 
great one which we owe to the joint energy of MM. Beer and 
Maedler, and which represents a stupendous amount of observing 



76 THE MOON. [chap. vii. 

work carried on during several years prior to 1836, the date of 
their publication. The long and patient labour bestowed upon 
their map and upon the measures on which it depends, deserve the 
highest praise which those conversant with the subject can bestow, 
and it must be very long before their efforts can be superseded. 

Beer and Maedler's map has a diameter of 37 inches : it repre- 
sents the phase of the moon visible in the condition of mean libra- 
tion. The details were charted by a careful process of triangula- 
tion. The disc was first divided into " triangles of the first order,*' 
the points of which (conspicuous craters) were accurately laid down 
by reference to the edges of the disc : one hundred and seventy-six 
of these triangles, plotted accurately upon an orthographic projec- 
tion of the hemisphere, formed the reliable basis for their charting 
work. From these a great number of " points of the second order " 
were laid down, by measuring their distance and angle of position 
with regard to points first established. The skeleton map thus 
obtained was filled up by drawings made at the telescope : the 
diameters of the measurable craters being determined by the 
micrometer. 

Beer and Maedler also measured the heights of one thousand 
and ninety-five lunar mountains and crater- summits : the resulting 
measures are given in a table contained in the comprehensive text- 
book which accompanies their map. These heights are found by 
one of two methods, either by measuring the length of the shadow 
which the object casts under a known elevation of the sun above its 
horizon, or by measuring the distance between the illuminated 
point of the mountain and the "terminator" in the following 
manner. In the annexed figure (Fig. 15) let the circle represent 
the moon and m a mountain upon it : let s a be the line of direction 
of the sun's rays, passing the normal surface of the moon at a and 
just tipping the mountain top. a will be the terminator, and there 
will be darkness between it and the star-like mountain summit m. 
The distance between a and m is measured : the distance a b is 
known, for it is the moon's radius. And since the line s m is a 



CHAP. VII.] 



TOPOGRAPHY OF THE MOON, 



77 



tangent to the circle the angle b a m is a right angle. We know 
the length of its two sides ab, am, and we can therefore hy the 
known properties of the right-angled triangle find the length of the 




Fm. 15. 

hypothenuse bm : and since bm is made up of the radius ba plus the 
mountain height, we have only to subtract the moon's radius from 
the ascertained whole length of the hypothenuse and we have the 
height of the mountain. MM. Beer and Maedler exhibited their 
measures in French toises : in the heights we shall have occasion 
to quote, these have been turned into English feet, upon the assump- 
tion that the toise is equal to 6*39 English feet. The nomencla- 
ture of lunar features adopted by Beer and Maedler is that intro- 
duced by Riccioli : mountains and teatures hitherto undistinguished 
were named by them after ancient and modern philosophers, in 



78 THE MOON. [chap. vii. 

Biccioli's system, and occasionally after terrestial features. Some 
minute objects in the neighbourliood of large and named ones were 
included under the name of the large one and distinguished by 
Greek or Roman letters. 

The excellent map resulting from the arduous labours of these 
astronomers is simply a map : it does not pretend to be a picture. 
The asperities and depressions are symbolized by a conventional 
system of shading and no attempt is made to exhibit objects as 
they actually appear in the telescope. A casual observer comparing 
details on the map with the same details on the moon itself would 
fail to identify or recognize them except where the features are very 
conspicuous. Such an observer would be struck by the shadows 
by which the lunar objects reveal themselves : he would get to 
know them mostly by their shadows, since it is mainly by those 
that their forms are revealed to a terrestial observer. But such a 
map as that under notice indicates no shadows, and objects have to 
be identified upon it rather by their positions with regard to one 
another or to the borders of the moon than by any notable features 
they actually present to view. This inconvenience occurred to us 
in our early use of Beer and Maedler's chart, and we were induced 
to prepare for ourselves a map in which every object is shown some- 
what, if imperfectly, as it actually appears at some period of a 
lunation. This was done by copying Beer and Maedler's outlines 
and filling them up by appropriate shading. To do justice to our 
task we enlarged our map to a diameter of six feet. Upon a circle 
of this diameter the positions and dimensions of all objects were 
laid down from the German original. Then from our own observa- 
tions we depicted the general aspect of each object : and we so 
adjusted the shading that all objects should be shown under about 
the same angle of illumination — a condition which is never fulfilled 
upon the moon itself, but which we consider ourselves justified in 
exhibiting for the purpose of conveying a fair impression of how the 
various lunar objects actually appear at some one or other part of a 
lunation. 



SM.iFom'^b^ 




o 



CHAP. VII.] 



TOPOGRAPRT OF TUil MOON, 



79 




SKELETON MAP OF THE MOON. 
TO ACCOMPANY PICTURE MAP (Plate V.). 



80 



THE MOON. 



[chap. VII. 



The picture-map thus produced has been photographed to a 
size convenient for this work : and in order to make it available for 
the identification of such objects^-chiefly lunar craters — as we 
may have occasion to refer to, we have prepared a skeleton map 
(p. 79) which includes the more conspicuous objects of that 
nature. The progressive numbers in the annexed list refer to the 
skeleton map on page 79, and the description of the object to which 
they are annexed will be found on pp. 82 — 100. 



No. Name. 

1. Newton. 

2. Short. 

3. Simpelius. 

4. Manzinus. 

5. Moretus. 

6. Gruemberger. 

7. Casatus. 

8. Klaproth. 

9. Wilson. 

10. Kircher. 

11. Bettinus. 

12. Blancanus. 

13. Clavius. 

14. Scheiner. 

15. Zuchius. 

16. Segner. 

17. Bacon. 

18. Nearchus. 

19. Vlacq. 

20. Hommel, 

21. Licetus. 

22. Maginus. 

23. Longomontanus. 

24. Schiller. 

25. Phocylides. 

26. Wargentin. 

27. Inghirami. 

28. Schickard. 

29. Wilhelm I. 

30. Tycho. 

31. Saussure. 

32. Stoefler. 

33. Maurolycus. 

34. Barocius. 

35. Fabricius. 



No. Name, 

36. Metius. 

37. Fernelius. 

38. Heinsius, 

39. Hainzel. 

40. Bouvard. 

41. Piazzi. 

42. Ramsden. 

43. Capuanus. 

44. Cichus. 

45. Wurzelbauer. 

46. Gauricus. 

47. Hell. 

48. Walter. 

49. Nonius. 

50. Eiccius. 

51. Rheita. 

52. Furnerius. 

53. Stevinus. 

54. Hase. 

55. Snell. 

56. Borda. 

57. Neander. 

58. Piccolomini. 

59. Pontanus. 

60. Poisson. 

61. Aliacensis. 

62. Werner. 

63. Pitatus. 

64. Hesiodus. 

65. Mercator. 

66. Vitello. 

67. Fourier. 

68. Lagrange. 

69. Vieta. 

70. Doppelmajer, 



No. Name. 

71. Campanus. 

72. Kies. 

73. Purbach. 

74. La Caille. 

75. Playfair. 

76. Azophi. 

77. Sacrobosco. 

78. Fracastorius. 

79. Santbech. 

80. Petavius. 

8 1 . Wilhelm Humboldt. 

82. Polybius. 

83. Geber. 

84. Arzachael. 

85. Thebit. 

86. Bullialdus. 

87. Hippalus. 

88. Cavendish. 

89. Mersenius. 

90. Gassendi. 

91. Lubiniezky. 

92. Alpetragius, 

93. Airy. 

94. Almanon. 

95. Catharina. 

96. Cyrillus. 

97. Theophilus. 

98. Colombo. 

99. Vendelinus. 

100. Langreen. 

101. Goclenius. 

102. Guttemberg. 

103. Isidorus. 

104. Capella. 

105. Kant. 



CHAP. VII.] 



TOPOGRAPHY OF THE MOON. 



81 



No. Name. 

106. Descartes. 

107. Abulfeda. 

108. Parrot. 

109. Albategnius. 

110. Alphons. 

111. Ptolemy. 

112. Herschel. 

113. Davy. 

114. Guerik^. 

116. Bonpland. 

117. Lalande. 

118. Reaumur. 

120. Letronne. 

121. Billy. 

122. Fontana. 

123. Hansteen. 

124. Damoiseau. 

125. Grimaldi. 

126. Flamsteed. 

127. Landsberg. 

128. Moesting. 

129. Deambrel. 

130. Taylor. 

131. Messier. 

132. Maskelyne. 

133. Sabine. 

134. Ritter. 

135. Godin. 

136. Soemmering. 

137. Schroeter. 

138. Gambart.. 

139. Reinhold. 

140. Encke. 

141. Hevelius. 

142. Riccioli. 

143. Lohrman. 

144. Cavalerius. 

145. Eeiner. 

146. Kepler. 

147. Copernicus. 



No. 

148. Stadius. 

149. Pallas. 

150. Triesnecker. 

151. Agrippa. 

152. Arago. 

153. Taruntius. 

154. Apollonius. 

155. Schubert. 

156. Firmicus. 

157. Silberschlag. 

158. Hyginus. 

159. Ukert. 

160. Boscovicli. 

161. Ross. 

162. Proclus. 

163. Picard. 

164. Condorcet. 

165. Pliny or Menelaus. 

167. Manilius. 

168. Erastothenes. 

169. Gay Lussac. 

170. Tobias Mayer. 

171. Marius. 

172. Gibers. 

173. Vasco de Gama. 

174. Seleucus. 

175. Herodotus. 

176. Aristarchus. 

177. La Hire. 

178. Pytheas. 

179. Bessel. 

180. Vitruvius. 

181. Maraldi. 

182. Macrobius. 

183. Cleomides. 

184. Eoemer. 

185. Littrow. 

186. Posidonius. 

187. Geminus. 

188. Linnseus. 



No. Name. 

189. Autolycus. 

190. Aristillus. 

191. Archimedes. 

192. Timocharis. 

193. Lambert. 

194. Diophantus. 

195. Delisle. 

196. Briggs. 

197. Lichtenberg. 

199. Calippus. 

200. Cassini. 

201. Gauss. 

202. Messala. 

203. Struve. 

204. Mason. 

205. Plana. 

206. Burg. 

207. Baily. 

208. Eudoxus. 

209. Aristotle. 

210. Plato. 

211. Pico. 

212. Helicon. 

213. Maupertuis. 

214. Condamine. 

215. Bianchini. 

216. Sharp. 

217. Mairan. 

218. Gerard. 

219. Repsold. 

220. Pythagoras. 

221. Fontenelle. 

222. Timaeus. 

223. Epigenes. 

224. Gartner. 

225. Thalee. 

226. Strabo. 

227. Endymion. 

228. Atlas. 

229. Hercules. 



The strong family likeness pervading the craters of the moon 
renders it unnecessary that we should attempt a description of each 
one of them or even of one in twenty. We have, however, thought 
that a few remarks upon the salient features of a few of the most 



82 THE MOON. [chap. vii. 

important may be acceptable in explanation of our illustrative 
plates ; and what we have to say of the few may be taken as repre- 
sentative of the many. 



COPEKNICUS, 147. Plate VIII. 

This may deservedly be considered as one of the grandest and 
most instructive of lunar craters. Although its vast diameter (46 
miles) is exceeded by others, yet, taken as a whole, it forms one of 
the most impressive and interesting objects of its class. Its situa- 
tion, near the centre of the lunar disc, renders all its wonderful 
details, as well as those of its immediately surrounding objects, so 
conspicuous as to establish it as. a very favourite object. Its vast 
rampart rises to upwards of 12,000 feet above the level of the 
plateau, nearly in the centre of which stands a magnificent group 
of cones, three of them attaining the height of upwards of 2400 
feet. 

The rampart is divided by concentric segmental terraced ridges, 
which present every appearance of being enormous landslips, result- 
ing from the crushing of their over-loaded summits, which have 
slid down in vast segments and scattered their debris on to the 
plateau. Corresponding vacancies in the rampart may be observed 
from whence these prodigious masses have broken away. The same 
may be noticed, although in a somewhat modified degree, around 
the exterior of the rampart. In order to approach a realization of 
the sublimity and grandeur of this magnificent example of a lunar 
volcanic crater, our reader will do well to endeavour to fix his atten- 
tion on its enormous magnitude and attempt to establish in his 
mind's eye a correct conception of the scale of its details as well 
as its general dimensions, which, as they so prodigiously transcend 
those of the largest terrestrial volcanic craters, require that our ideas 
as to magnitude of such objects should be, so to speak, educated 
upon a special standard. It is for this reason we are anxious our 
reader, when examining our illustrations, should constantly refer 



CHAP. VII.] TOPOGRAPHY OF THE MOON, 83 

the objects represented in them to the scale of miles appended to 
each plate, otherwise a just and true conception of the grandeur of 
the objects will escape him. 

Copernicus is specially interesting, as being evidently the result 
of a vast discharge of molten matter which has been ejected at the 
focus or centre of disruption of an extensively upheaved portion of 
the lunar crust. A careful examination of the crater and the dis- 
trict around it, even to the distance of more than 100 miles on every 
side, will supply unmistakable evidence of the vast extent and force 
of the original disruption, manifested by a wonderfully complex 
reticulation of bright streaks which diverge in every direction from 
the crater as their common centre. These streaks do not appear 
on our plate, nor are they seen upon the moon except at and near 
the full phase. They show conspicuously, however, by their united 
lustre on the full moon, Plate IV. Every one of those bright 
streaks, we conceive, is a record of what was originally a crack or 
chasm in the solid crust of the moon, resulting from some vastly 
powerful upheaving agency over the site of whose focus of energy 
Copernicus stands. The cracking of the crust must have been 
followed by the ejection of subjacent molten matter up through the 
reticulated cracks ; this, spreading somewhat on either side of them, 
has left these bright streaks as a visible record of the force and 
extent of the upheaval; while at the focus of disruption from 
whence the cracks diverge, the grand outburst appears to have 
taken place, leaving Copernicus as its record and result. 

Many somewhat radial ridges or spurs may be observed leading 
away from the exterior banks of the great rampart. These appear 
to be due to the more free egress which the extruded matter would 
find near the focus of disruption. The spur-ridges may be traced 
fining away for fully 100 miles on all sides, until they become such 
delicate objects as to approach invisibility. Several vast open 
chasms or cracks may be observed around the exterior of the ram- 
part. They appear to be due to some action subsequent to the 
formation of the great crater — probably the result of contraction on 

G 2 



84 THE MOON. [chap. vii. 

the cooling of the crust, or of a deep-seated upheaval long subse- 
quent to that which resulted in the formation of Copernicus itself, 
as they intersect objects of evidently prior formation. 

Under circumstances specially favourable for " fine vision," for 
upwards of 70 miles on all sides around Copernicus, myriads of 
comparatively minute but perfectly-formed craters may be observed. 
The district on the south-east side is specially rich in these 
wonderfully thickly scattered craters, which we have reason to 
suppose stand over or upon the reticulated bright streaks; but, 
as the circumstances of illumination which are requisite to enable 
us to detect the minute craters are widely adverse to those which 
render the bright streaks visible, namely, nearly full moon for the 
one and gibbous for the other, it is next to impossible to establish 
the fact of coincidence of the sites of the two by actual simultaneous 
observation. 

At the east side of the rampart, multitudes of these compara- 
tively minute craters may also be detected, although not so closely 
crowded together as those on the west side ; but among those on 
the east may be seen myriads of minute prominences roughening 
the surface ; on close scrutiny these are seen to be small mounds of 
extruded matter which, not having been ejected with sufficient 
energy to cause the erupted material to assume the crater form 
around the vent of ejection, have simply assumed the mound form 
so well known to be the result of volcanic ejection of moderate force. 

Were we to select a comparatively limited portion of the lunar 
surface abounding in the most unmistakable evidence of volcanic 
action in every variety that can characterize its several phases, we 
could not choose one yielding in all respects such instructive 
examples as Copernicus and its immediate surroundings. 

GASSENDI, 90. Frontispiece. 

An interesting crater about 54 miles in diameter ; the height of 
the most elevated portion of the surrounding wall from the plateau 



CHAP. VII.] TOPOGRAPHY OF THE MOON. 85 

being about 9600 feet. The centre is occupied by a group pf 
conical mountains, three of which are most conspicuous objects and 
rise to nearly 7000 feet above the level of the plateau. As in other 
similar cases, these central mountains are doubtless the result of 
the expiring effort of the eruption which had formed the great 
circular wall of the crater. The plateau is traversed by several 
deep cracks or chasms nearly one mile wide. 

Both the interior and exterior of the wall of the crater are 
terraced with the usual segmental ridges or landslips. A remark- 
able detached portion of the interior bank is to be seen on the east 
side, while on the west exterior of the wall may be seen an equally 
remarkable example of an outburst of lava subsequent to the 
formation of the wall or bank of the crater ; it is of conical form 
and cannot fail to secure the attention of a careful observer. 

Interpolated on the north wall of the crater may be seen a crater 
of about 18 miles diameter which has burst its bank in towards the 
great crater, upon whose plateau the lava appears to have discharged 
itself. 

The neighbourhood of Gassendi is diversified by a vast number of 
mounds and long ridges of exudated matter, and also traversed by 
enormous chasms and cracks, several of which exceed one mile wide 
and are fully 100 miles in length, and, as is usual with such cracks, 
traverse plain and mountain alike, disregarding all inequalities. 

Numbers of small craters are scattered around ; the whole form- 
ing an interesting and instructive portion of the lunar surface. 

EUDOXUS, 208, and ARISTOTLE, 209. Plate X. 

Two gigantic craters, Eudoxus being nearly 35 miles in diameter 
and upwards of 11,000 feet deep, while Aristotle is about 48 miles 
in diameter, and about 10,000 feet deep (measuring from the 
summit of the rampart to the plateau). These two magnificent 
craters present all the true volcanic characteristics in a remarkable 
degree. The outsides, as well as the insides of their vast surround' 



86 THE MOON. [chap. vii. 

ing walls or banks display on the grandest scale the landslip feature, 
the result of the over-piling of the ejected material, and the conse- 
quent crushing down and crumbling of the substructure. The 
true eruptive character of the action which formed the craters is 
well evinced by the existence of the groups of conical mountains 
which occupy the centres of their circular plateaux, since these 
conical mountains, there can be little doubt, stand over what 
were once the vents from whence the ejected matter of the craters 
was discharged. 

On the west side of these grand craters may be seen myriads of 
comparatively minute ones (we use the expression *' comparatively 
minute," although most of them are fully a mile in diameter). So 
thickly are these small craters crowded together, that counting 
them is totally out of the question ; in our original notes we have 
termed them " Froth craters " as the most characteristic description 
of their aspect. 

The exterior banks of Aristotle are characterized by radial ridges 
or spurs ; these are most probably the result of the flowing down 
of great currents of very fluid lava. To the east of the craters some 
very lofty mountains of exudation may be seen, and immediately 
beyond them an extensive district of smaller mountains of the same 
class, so thickly crowded together as under favourable illumination 
to present a multitude of brilliant points of light contrasted by 
intervening deep shade. On the west bank of Aristotle a very 
perfect crater may be seen, 27 miles in diameter, having all the 
usual characteristic features. 

About 40 miles to the east of Eudoxus there is a fine example of 
a crack or fissure extending fully 50 miles — 30 miles through a 
plain, and the remaining 20 miles cutting through a group of very 
lofty mountains. This great crack is worthy of attention, as giving 
evidence of the deep-seated nature of the force which occasioned it 
inasmuch as it disregards all surface impediments, traversing plain 
and group of mountains alike. 

There are several other features in and around these two mag- 



CHAP. VII.] TOPOGRAPHY OF THE MOON. 87 

nificent craters well worthy of careful observation and scrutiny, all 
of them excellent types of their respective classes. 



TKIESNECKER, 150. Plate XI. 

A fine example of a normal lunar volcanic crater, having all the 
usual characteristic features in great perfection. Its diameter is 
about 20 miles, and it possesses a good example of the central cone 
and also of interior terracing. 

The most notable feature, however, in connection with this 
crater, and on account of which we have chosen it as a subject for 
one of our illustrations, is the very remarkable display of chasms 
or cracks which may be seen to the west side of it. Several of 
these great cracks obviously diverge from a small crater near the 
west external bank of the great one, and they subdivide or branch 
out, as they extend from the apparent point of divergence, while 
they are crossed or intersected by others. These cracks or chasms 
(for their width merits the latter appellation) are nearly one mile 
broad at the widest part, and after extending to fully 100 miles, 
taper away till they become invisible. Although they are not test 
objects of the highest order of difficulty, yet to see them with 
perfect distinctness requires an instrument of some perfection and 
all the conditions of good vision. When such are present, a keen 
and practised eye will find many details to rivet its attention, among 
which are certain portions of the edges of these cracks or chasms 
which have fallen in and caused interruptions to their continuity. 

THEOPHILUS, 97 ; (5yRILLUS, 96 ; CATHARINA, 95. Plate XII. 

These three magnificent craters form a very conspicuous group 
near the middle of the south-east quarter of the lunar disc. 
Their respective diameters and depths are as follows : — 
Theophilus, 64 miles diameter; depth of plateau from summit 
of crater wall, 16,000 feet ; central cone 5200 feet high. 



88 THE MOON. [chap. vii. 

Cyrillus, 60 miles diameter ; depth of plateau from summit of 
crater wall, 15,000 feet ; central cone, 5800 feet high. 

Catharina, 65 miles diameter ; depth of plateau from summit of 
crater wall, 13,000 feet ; centre of plateau occupied by a confused 
group of minor craters and debris. 

Each of these three grand craters is full of interesting details, 
presenting in every variety the characteristic features which so 
fascinate the attention of the careful observer of the moon's 
wonderful surface, and affording unmistakable evidence of the 
tremendous energy of the volcanic forces which at some incon- 
ceivably remote period piled up such gigantic formations. 

Theophilus by its intrusion within the area of Cyrillus shows in 
a very striking manner that it is of comparatively more recent 
formation than the latter crater. There are many such examples 
in other parts of the lunar disc, but few of so very distinct and 
marked a character. 

The flanks or exterior banks of Theophilus, especially those on 
the west side, are studded with apparently minute craters, all of 
which when carefully scrutinized are found to be of the true volcanic 
type of structure ; and minute as they are, by comparison, they 
would to a beholder close to them appear as very imposing objects ; 
but so gigantic are the more notable craters in the neighbourhood, 
that we are apt to overlook what are in themselves really large 
objects. It is only by duly training the mind, as we have pre- 
viously urged, so as ever to keep before us the vast scale on which 
the volcanic formations of the lunar surface are displayed, that we 
can do them the justice which their intrinsic grandeur demands. 
We trust that our illustrations may in some measure tend to 
educate the mind's eye, so as to derive to the full the tranquil 
enjoyment which results from the study of the manifestation of 
one of the Creator's most potent agencies in dealing with the 
materials of his worlds, namely, volcanic force. So rich in 
wonderful features and characteristic details is this magnificent 
group and its neighbourhood, that a volume might be filled 



CHAP. VII.] TOPOGRAPHY OF THE MOON. 89 

in the attempt to do justice, by description, to objects so full 
of suggestive subject for study. 



PTO]LEMY, 111 ; ALPHONS, 110; ARZACHAEL, 84, ETC.— Plate XIII. 

The portion of tbe lunar surface comprised within the limits of 
this illustration being situated nearly in the centre of the moon's 
disc, is very favourably placed for revealing the multitude of 
interesting features and details therein represented. They consist 
of every variety of volcanic crater from " Ptolemy," whose vast 
rampart is eighty- six miles diameter, down to those whose dimen- 
sions are, comparatively, so minute as to render them at the extreme 
limits of visibility. 

Alphons and Arzachael, two of the next largest craters in our 
illustration, situated immediately above Ptolemy, are sixty and 
fifty-five miles in diameter respectively, and are possessed, in a 
remarkable degree, of all the distinctive characteristic features 
of lunar craters, having magnificent central cones, lofty ragged 
ramparts, together with very striking manifestations of landslip 
formations as appear in the great segmental terraces within their 
ramparts, together with several minor craters interpolated on their 
plateau. *' Alphons," the middle crater of this fine group, has its 
plateau specially distinguished by several cracks or chasms fully 
one mile wide, the direction or ** strike " of which coincide in a very 
remarkable manner with several other similar cracks which form 
conspicuous features among the multitude of interesting details 
comprised within the limits of our illustration, — the most notable 
of these is an enormous straight cliff traversing the diameter of a 
low-ridged circular formation, seen in the upper right-hand corner 
of our plate. This great clifi" is sixty miles long and from 1000 to 
2000 feet high ; it is a well-known object to lunar observers, and 
has been termed " The Eailway," on account of its straightness as 
revealed by the distinct shadow projected by it on the plateau when 
seen under its sunrise aspect. The face of this vast cliff", although 



90 THE MOON. [chap. vii. 

generally straight, is seen, when minutely scrutinized, to be some- 
what serrated in its outline, while on its upper edge may be 
detected some very minute but perfectly formed craters. The 
existence of this remarkable cliflf appears to be due either to an 
upheaval or a down-sinking of portion of the surface of the circular 
area across whose diameter it extends. 

To the right-hand side of the cliff are two small craters, from 
the side of which a fine example of a crack may be detected passing 
through in its course a low dome-formed hill; this crack is parallel 
to the cliff, having in that respect the same general strike or 
parallel direction which so remarkably distinguishes the other 
cracks observable in this portion of the moon's surface. 

On the left hand of this great cliff is situated a coneless crater, 
named *' Thebit," on the right-hand rampart of which may be 
observed two small craters, the lesser of which is 2*75 miles 
diameter and has a central cone. We specially remark this fact, 
as it is the smallest lunar crater but one, in which we have, with 
perfect distinctness, detected a central cone. Not but that many 
smaller lunar craters exist possessed of this unmistakable evidence 
of their volcanic origin ; but so minute are the specks of light which 
the central cones of such small craters reflect, that they, for that 
reason, most probably fail to reveal themselves. 

PLATO, 210. Plate XIV. 

This crater, besides being a conspicuous object on account of its 
great diameter, has many interesting details in and around it requir- 
ing a fine instrument and favourable circumstances to render them 
distinctly visible. The diameter of the crater is 70 miles ; the 
surrounding wall or rampart varies in height from 4000 to upwards 
of 8000 feet, and is serrated with noble peaks which cast their 
black shadows across the plateau in a most picturesque manner, 
like the towers and spires of a great cathedral. Reference to our 
illustration will convey a very fair idea of this interesting appearance. 



CHAP. VII.] TOPOGRAPHY OF THE MOON. 91 

On the north-east inside of the circular wall or rampart may be 
observed a fine example of landslip, or sliding down of a considerable 
mass of the interior side of the crater's wall. The landslip nature 
of this remarkable detail is clearly established by the fact of the 
bottom edge of the downslipped mass projecting in towards the 
centre of the plateau to a considerable extent. Other smaller land- 
slip features may be seen, but none on so grand and striking a 
scale as the one referred to. A number of exceedingly minute 
craters may be detected on the surface of the plateau. The plateau 
itself is remarkable for its low reflective power, which causes it to 
look like a dingy spot when Plato is viewed with a small mag- 
nifying power. The exterior of the crater wall is remarkable for 
the rugged character of its formation, and forms a great contrast in 
that respect to the comparatively smooth unbroken surface of the 
plateau, which by the way is devoid of a central cone. The sur- 
rounding features and objects indicated in our illustration are of 
the highest interest, and a few of them demand special description. 

THE VALLEY OF THE ALPS. Plate XIV. 

This remarkable object lies somewhat diagonally to the west of 
Plato ; when seen with a low magnifying power (80 to 100), it 
appears as a rut or groove tapering towards each extremity. It 
measures upwards of 75 miles long by about six miles wide at the 
broadest part. When examined under favourable circumstances, 
with a magnifying power of from 200 to 300, it is seen to be a vast 
flat-bottomed valley bordered by gigantic mountains, some of which 
attain heights upwards of 10,000 feet ; towards the south-east of 
this remarkable valley, and on both sides of it, are groups of iso- 
lated mountains, several of which are fully 8000 feet high. This 
flat-bottomed valley, which has retained the integrity of its form 
amid such disturbing forces as its immediate surroundings indicate, 
is one of the many structural enigmas with which the lunar surface 
abounds. To the north-west of the valley a vast number of isolated 



92 THE MOON. [chap. vii. 

mounds or small mountains of exudation may be seen ; so 
numerous are they as to defy all attempts to count them with any- 
thing like exactness ; and among them, a power of 200 to 300 will 
enable an observer, under favourable circumstances, to detect vast 
numbers of small but perfectly-formed craters. 

PICO, 211. Plate XIV. 

This is one of the most interesting examples of an isolated vol- 
canic "mountain of exudation," and it forms a very striking object 
when seen under favourable circumstances. Its height is upwards 
of 8000 feet, and it is about three times as long at the base as it is 
broad. The summit is cleft into three peaks, as may be ascertained 
by the three-peaked shadow it casts on the plain. Five or six 
minute craters of very perfect form may be detected close to the 
base of this magnificent mountain. There are several other iso- 
lated peaks or mountains of the same class within 30 or 40 miles 
of it which are well worthy of careful scrutiny, but Pico is the 
master of the situation, and offers a glorious subject for realizing 
a lunar day-dream in the mind's eye, if we can only by an effort of 
imagination conceive its aspect under the fiercely brilliant sunshine 
by which it is illuminated, contrasted with the intensely black lunar 
heavens studded with stars shining with a steady brightness of 
which, by reason of our atmosphere intervening, we can have no 
adequate conception save by the aid of a well-directed imagin ation. 

TYCHO, 30. PLATE XVI. 

This magnificent crater, which occupies the centre of the crowded 
group in our Plate, is 54 miles in diameter, and upwards of 16,000 
feet deep, from the highest ridge of the rampart to the surface of 
the plateau, whence rises a grand central cone 5000 feet high. It 
is one of the most conspicuous of all the lunar craters, not so much 
on account of its dimensions as from its occupying the great focus 
of disruption from whence diverge those remarkable bright streaks, 



CHAP. VII.] TOPOGRAPHY OF THE MOON. 93 

many of which may he traced over 1000 miles of the moon's surface, 
disregarding in their course all interposing ohstacles. There is 
every reason to conclude that Tycho is an instance of a vast disrup- 
tive action which rent the solid crust of the moon into radiating 
fissures, which were subsequently occupied by extruded molten 
matter, whose superior luminosity marks the course of the cracks in 
all directions from the crater as their common centre of divergence. 
So numerous are these bright streaks when examined by the aid of 
the telescope, and they give to this region of the moon's surface 
such an extra degree of luminosity, that, when viewed as a whole, 
their locality can be distinctly seen at full moon by the unassisted 
eye as a bright patch of light on the southern portion of the disc. 
(See Plate IV.) The causative origin of the streaks is discussed 
and illustrated in Chapter XI. 

The interior of this fine crater presents striking examples of the 
concentric terrace-like formations that we have elsewhere assigned 
to vast landslip actions. Somewhat similar concentric terraces may 
be observed in other lunar craters ; some of these, however, appear to 
be the results of some temporary modification of the ejective force, 
which has caused the formation of more or less perfect inner ram- 
parts : what we conceive to be true landslip terraces are always dis- 
tinguished from these by their more or less fragmentary character. 

On reference to Plate IV., showing the full moon, a very remark- 
able and special appearance will be observed in a dingy district or 
zone immediately surrounding the exterior of the rampart of Tycho, 
and of which we venture to hazard what appears to us a rational 
explanation : namely, that as Tycho may be considered to have 
acted as a sort of safety-valve to the rending and ejective force which 
caused, in the first instance, the cracking of this vast portion of the 
moon's crust — the molten matter that appears to have been forced 
up through these cracks, on finding a comparatively free exit by the 
vent of Tycho, so relieved the district immediately around him as 
to have thereby reduced, in amount, the exit of the molten matter, 
and so left a zone comparatively free from the extruded lava which, 



94 THE MOON. [chap. vii. 

according to our view of the subject, came up simultaneously 
through the innumerable fissures, and, spreading sideways along 
their courses, left everlasting records of the original positions of the 
radiating cracks in the form of the bright streaks which we now 
behold. 

" WAKGENTIN," 26. Plate 5¥i»s 

This object is quite unique of its kind — a crater about 53 miles 
across, that to all appearance has been filled to the brim with lava 
that has been left to consolidate. There are evidences of the 
remains of a rampart, especially on the south-west portion of the 
rim. The general aspect of this extraordinary object has been not 
unaptly compared to a ** thin cheese." The terraced and rutted 
exterior of the rampart has all the usual characteristic details of 
the true crater. The surface of the high plateau is marked by a 
few ridges branching from a point nearly in its centre, together with 
some other slight elevations and depressions ; these, however, can 
only be detected when the sun's rays fall nearly parallel to the sur- 
face of the plateau. 

To the north of this interesting object is the magnificent ring 
formation Schickard, whose vast diameter of 123 miles contrasts 
strikingly with that of the sixteen small craters within his rampart, 
and equally so with a multitude of small craters scattered around. 
There are many objects of interest on the portion of the lunar 
surface included within our illustration, but as they are all of the 
usual type, we shall not fatigue the attention of our readers by 
special descriptions of them. 



AKISTARCHUS, 176, and HERODOTUS, 175. Plate 



^ 



These two fine examples of lunar volcanic craters are conspicu- 
ously situated in the north-east quarter of the moon's disc. 
Aristarchus has a circular rampart nearly 28 miles diameter, the 
summit of which is about 7500 feet above the surface of the plateau, 



CHAP. VII.] TOPOGRAPHY OF THE MOON. 95 

while its height ahove the general surface of the moon is 2600 feet. 
A central cone having several subordinate peaks completes the true 
volcanic character of this crater : its rampart banks, both outside 
and inside, have fine examples of the segmental crescent- shaped 
ridges or landslips, which form so constant and characteristic a 
feature in the structure of lunar craters. Several very notable 
cracks or chasms may be seen to the north of these two craters. 
They are contorted in a very unusual and remarkable manner, the 
result probably of the force which formed them having to encounter 
very varying resistance near the surface. 

Some parts of these chasms gape to the width of two to three 
miles, and when closely scrutinized are seen to be here and there 
partly filled by masses which have fallen inward from their sides. 
Several smaller craters are scattered around, which, together with 
the great chasms and neighbouring ridges, give evidence of varied 
volcanic activity in this locality. We must not omit to draw 
attention to the parallelism or general similarity of "strike" 
in the ridges of extruded matter ; this appearance has special 
interest in the eyes of geologists, and is well represented in our 
illustration. 

Aristarchus is specially remarkable for the extraordinary capa- 
bility which the material forming its interior and rampart banks 
has of reflecting light. Although there are many portions of the 
lunar surface which possess the same property, yet few so remark- 
ably as in the case of Aristarchus, which shines with such bright- 
ness, as compared with its immediate surroundings, as to attract 
the attention of the most unpractised observer. Some have 
supposed this appearance to be due to active volcanic discharge still 
lingering on the lunar surface, an idea in which, for reasons to be 
duly adduced, we have no faith. Copernicus, in the remarkable 
bright streaks which radiate from it, and Tycho also, as well as 
several other spots, are apparently composed of material very 
nearly as highly reflective as that of Aristarchus. But the 
comparative isolation of Aristarchus, as well as the extraordinary 



96 THE MOON. [chap. vii. 

light-reflecting property of its material, renders it especially 
noticeable, so much so as to make it quite a conspicuous object 
when illuminated only by earth-light, when but a slender crescent 
of the lunar disc is illuminated, or when, as during a lunar eclipse, 
the disc of the moon is within the shadow of the earth and is 
lighted only by the rays refracted through the earth's atmosphere. 

There are no features about Herodotus of any such speciality as 
to call for remark, except it be the breach of the north side of its 
rampart by the southern extremity of a very remarkable contorted 
crack or chasm, which to all appearance owes its existence to some 
great disruptive action subsequent to the formation of the crater. 

WALTER, 48, and adjacent Intrusfv^e Craters. Plate XXII. 

This plate represents a southern portion of the moon's surface, 
measuring 170 by 230 miles. It includes upwards of 200 craters 
of all dimensions, from Walter, whose rampart measures nearly 70 
miles across, down to those of such small apparent diameter as to 
require a well-practised eye to detect them. In the interior of the 
great crater, Walter, a remarkable group of small craters may be 
observed surrounding his central cone, which in this instance is 
not so perfectly in the centre of the rampart as is usually the case. 
The number of small craters which we have observed within the 
rampart is 20, exclusive of those on the rampart itself. The entire 
group represented in the Plate suggests in a striking manner the 
wild scenery which must characterize many portions of the lunar 
surface ; the more so if we keep in mind the vast proportions of 
the objects which they comprise, upon which point we may remark 
that the smallest crater represented in this Plate is considerably 
larger than that of Vesuvius. 



ARCHIMEDES, 191 ; AUTOLYCUS, 189 ; ARISTILLUS, 190, AND the 
APENNINES. Plate IX. 

This group of three magnificent craters, together with their 



CHAP. VII.] TOPOGRAPHY OF THE MOON. 97 

remarkable surroundings, especially including the noble range of 
mountains termed the Apennines, forms on the whole one of the 
most striking and interesting portions of the lunar surface. If the 
reader is not acquainted with what the telescope can reveal as to 
the grandeur of the effect of sunrise on this very remarkable 
portion of the moon's surface, he should carefully inspect and 
study our illustration of it ; and if he will pay due regard to our 
previously repeated suggestion concerning the attached scale of 
miles, he will, should he have the good fortune to study the actual 
objects by the aid of a telescope, be well prepared to realize and 
duly appreciate the magnificence of the scene which will be 
presented to his sight. 

Were we to attempt an adequate detailed description of all the 
interesting features comprised within our illustration, it would, of 
itself, fill a goodly volume ; as there is included within the space 
represented every variety of feature which so interestingly charac- 
terizes the lunar surface. All the more prominent details are types 
of their class ; and are so favourably situated in respect to almost 
direct vision, as to render their nature, forms, and altitudes above 
and depths below the average surface of the moon most distinctly 
and impressively cognizable. 

Archimedes is the largest crater in the group ; it has a diameter 
of upwards of 52 miles, measuring from summit to summit of its 
vast circular rampart or crater wall, the average height of which, 
above the plateau, is about 4300 feet ; but some parts of it rise 
considerably higher, and, in consequence, cast steeple-like shadows 
across the plateau when the sun's rays are intercepted by them at 
a low angle. The plateau of this grand crater is devoid of the 
usual central cone. Two comparatively minute but beautifully- 
formed craters may be detected close to the north-east interior side 
of the surrounding wall of the great crater. Both outside and in- 
side of the crater wall may be seen magnificent examples of the 
landslip subsidence of its overloaded banks ; these landslips form 
vast concentric segments of the outer and inner circumference of 



98 TEE MOON. [chap. vii. 

the great circular rampart, and doubtless belong to its era of 
formation. Two very fine examples of cracks, or chasms, may be 
observed proceeding from the opposite external sides of the crater, 
and extending upwards of 100 miles in each direction ; these cracks, 
or chasms, are fully a mile wide at their commencement next the 
crater, and narrow away to invisibility at their further extremity. 
Their course is considerably crooked, and in some parts they are 
partially filled by masses of the material of their sides, which have 
fallen inward and partially choked them. The depths of these 
enormous chasms must be very great, as they probably owe their 
existence to some mighty upheaving action, which there is every 
reason to suppose originated at a profound depth, since the general 
surface on each side of the crater does not appear to be disturbed 
as to altitude, which would have been the case had the upheaving 
action been at a moderate depth beneath. We would venture to 
ascribe a depth of not less than ten miles as the most moderate esti- 
mate of the profundity of these terrible chasms. If the reader would 
realize the scale of them, let him for a moment imagine himself a 
traveller on the surface of the moon coming upon one of them, and 
finding his onward progress arrested by the sudden appearance of 
its vast black yawning depths ; for by reason of the angle of his 
vision being almost parallel to the surface, no appearance of so 
profound a chasm would break upon his sight until he came com- 
paratively close to its fearful edge. Our imaginary lunar traveller 
would have to make a very long detour, ere he circumvented this 
terrible interruption to his progress. If the reader will only 
endeavour to realize in his mind's eye the terrific grandeur of a 
chasm a mile wide and of such dark profundity as to be, to all 
appearance, fathomless — portions of its rugged sides fallen in wild 
confusion into the jaws of the tortuous abyss, and catching here 
and there a ray of the sun sufficient only to render the darkness of 
the chasm more impressive as to its profundity — he will, by so 
doing, learn to appreciate the romantic grandeur of this, one of the 
many features which the study of the lunar surface presents to the 



CHAP. VII.] TOPOaBAPHY OF THE MOON. 99 

careful observer, and which exceed in sublimity the wildest efforts 
of poetic and romantic imagination. The contemplation of these 
views of the lunar world are, moreover, vastly enhanced by especial 
circumstances which add greatly to the impressiveness of lunar 
scenery, such as the unchanging pitchy-black aspect of the heavens 
and the death-like silence which reigns unbroken there. 

These digressions are, in some respects, a forestalment of what 
we have to say by-and-by, and so far they are out of place ; but 
with the illustration to which the above remarks refer placed before 
the reader, they may, in some respects, enhance the interest of its 
examination. 

The upper portion of our illustration is occupied by the magnifi- 
cent range of volcanic mountains named after our Apennines, 
extending to a length of upwards of 450 miles. This mountain 
group rises gradually from a comparatively level surface towards the 
south-west, in the form of innumerable comparatively small moun- 
tains of exudation, which increase in number and altitude towards 
the north-east, where they culminate and suddenly terminate in a 
sublime range of peaks, whose altitude and rugged aspect must 
form one of the most terribly grand and romantic scenes which 
imagination can conceive. The north-east face of the range 
terminates abruptly in an almost vertical precipitous face, and over 
the plain beneath intense black steeple or spire-like shadows are 
cast, some of which at sunrise extend fully 90 miles, till they lose 
themselves in the general shading due to the curvature of the lunar 
surface. Nothing can exceed the sublimity of such a range of 
mountains, many of which rise to heights of 18,000 to 20,000 feet 
at one bound from the plane at their north-east base. The most 
favourable time to examine the details of this magnificent range is 
from about a day before first quarter to a day after, as it is then 
that the general structure of the range as well as the character of 
the contour of each member of the group can, from the circum- 
stances of illumination then obtaining, be most distinctly inferred. 

Several comparatively small perfectly-formed craters are seen 

H 2 



100 THE MOON. [chap. vii. 

interspersed among the mountains, giving evidence of the truly 
volcanic character of the surrounding region, which, as hefore said, 
comprises in a comparatively limited space the most perfect and 
striking examples of nearly every class of lunar volcanic phe- 
nomena. 

We have endeavoured on Plate XXV. to give some idea of a 
landscape view of a small portion of this mountain range. 



Plate v 




"'." 'c o db urytyp e 



VESUVIUS. 

AND NEIGHBOURHOOD OF i\APL.ES. 




PORTION OF THE MOCNS SURFACE 

OF THE SAME AREA AS THAT 

GIVEN IN THE ILLUSTRATION 

OF VESUVIUS AND NEIGHBOURHOOD 

OF N A P I, E S. 



^ 



CHAPTER VIII. 

ON LUNAR CRATERS. 

As we stated in our brief general description of the visible 
hemisphere of the moon, and as a cursory glance at our map and 
plates will have shown, the predominant features of the lunar 
surface are the circular or amphitheatrical formations that, by their 
number, and from their almost unnatural uniformity of design, 
induced the belief among early observers that they must have been 
of artificial origin. In proceeding now to examine the details of 
our subject with more minuteness than before, these annular 
formations claim the first share of our attention. 
■ By general acceptation the term " crater " has been used to 
represent nearly all the circular hollows that we observe upon the 
moon; and without doubt the word in its literal sense, as indicat- 
ing a cup or circular cavity, is so far aptly applied. But among 
geologists it has been employed in a more special sense to define 
the hollowing out that is found at the summit of some extinct, and 
the majority of active, volcanoes. In this special sense it may be 
used by the student of the lunar surface, though in some, and 
indeed in the majority of cases, the lunar crater difi'ers materially 
in its form with respect to its surroundings from those on the 
earth ; for while, as we have said, the terrestrial crater is generally 
a hollow on a mountain top with its flat bottom high above the 
level of the surrounding country, those upon the moon have their 
lowest points depressed more or less deeply below the general 
surface of the moon, the external height being frequently only a 



102 THE MOON. [chap. viii. 

half or one-third of the internal depth. Yet are the lunar craters 
truly volcanic ; as Sir John Herschel has said, they offer the true 
volcanic character in its highest perfection. We have upon the 
earth some few instances in which the geological conditions which 
have determined the surface-formation have been identical with 
those that have obtained upon the moon ; and as a result we have 
some terrestrial volcanic districts that, could we view them under 
the same circumstances, would be identical in character with what 
we see by telescopic aid upon our satellite. The most remarkable 
case of this similarity is offered by a certain tract of the volcanic 
area about Naples, known from classic times as the Campi 
Phlegrceij or burning fields, a name given to them in early days, 
either because they showed traces of ancient earth-fire, or because 
there were attached to the localities traditions concerning hot- 
springs and sulphurous exhalations, if not of actual fiery eruptions. 
The resemblance of which we are speaking is here so close that 
Professor Phillips, in his work on Vesuvius, which by the way con- 
tains a historical description of the district in question, calls the 
moon a grand Phlegreian field. How closely the ancient craters of 
this famous spot resemble the generality of those upon the moon may 
be judged from Plates VI. and VII., in which representations of two 
areas, terrestrial and lunar, of the same extent, are exhibited side 
by side, the terrestrial region being the volcanic neighbourhood of 
Naples, and the lunar a portion of the surface about the crater 
Theophilus. 

In comparing these volcanic circles together, we are however 
brought face to face with a striking difference that exists between 
the lunar and terrestrial craters. This is the difference of 
magnitude. None of those Plutonian amphitheatres included in 
the terrestrial area depicted exceed a mile in diameter, and few 
larger volcanic vents than these are known upon the earth. Yet 
when we turn to the moon, and measure some of the larger craters 
there, we are astonished to find them ranging from an almost 
invisible minuteness to 74 miles in diameter. The same dispro- 



CHAP. VIII.] LUNAR CRATERS. 108 

portion exists between the depths of the two classes of craters. 
To give an idea of relative dimensions, we would refer to our 
illustration of Copernicus* and its hundreds of comparatively 
minute surrounding craters. Our terrestrial Vesuvius would be 
represented by one of these last, which upon the plate measures 
about the twentieth of an inch in diameter ! And this dispro- 
portion strikes us the more forcibly when we consider that the 
lunar globe has an area only one- thirteenth of that of the earth. 
In view of this great apparent discrepancy it is not surprising that 
many should have been incredulous as to the true volcanic 
character of the lunar mountains, and have preferred to designate 
them by some *' non-committal " term, as an American geologist 
(Professor Dana) has expressed it. But there is a feature in the 
majority of the ring-mountains that, as we conceive, demonstrates 
completely the fact of volcanic force having been in full action, and 
that seems to stamp the volcanic character upon the crater-forms. 
This special feature is the central cone, so well Imown as a 
characteristic of terrestrial volcanoes, accepted as the result of the 
last expiring effort of the eruptive force, and formed by the 
deposit, immediately around the volcanic orifice, of matter which 
there was not force enough to project to a greater distance. Upon 
the moon we have the central cone in small craters comparable to 
those on the earth, and we have it in progressively larger examples, 
upon all scales, up to craters of 74 miles in diameter, as we have 
shown on p. 106. Where, then, can we draw the line ? Where 
can we say the parallel action to that which placed Vesuvius in or 
near the centre of the arc of Somma, or the cone figured in our 
sectional drawing of Vesuvius (Fig. 3) in the middle of its present 
crater — where can we say that the action in question ceased to 
manifest itself on the moon, seeing that there is no break in the 
continuity of the crater-and-cone system upon the moon anywhere 
between craters of If miles and 74 miles in diameter ? We have, 

* Plate VIII. 



104 THE MOON. [chap. viii. 

it is true, many examples of coneless craters, but these are of all 
sizes, down to tlie smallest, and up to a magnitude that would 
almost seem to render untenable the ejective explanation : of these 
we shall specially speak in turn, but for the present we will confine 
ourselves to the normal class of lunar craters, those that have 
central cones, and that are in all reasonable probability truly 
volcanic. 

And in the first place let us take a passing glance at the 
probable formative process of a terrestrial volcano. Rejecting the 
hypothesis of Von Buch, which geologists have on the whole found 
to be untenable, and which ascribes the formation of all mountains 
to the elevation of the earth's crust by some thrusting power 
beneath, we are led to regard a volcano as a pyramid of ejected 
matter, thrown out of and around an orifice in the external solid 
shell of the earth by commotions engendered in its molten nucleus. 
What is the precise nature and source of the ejective force 
geologists have not perfectly agreed upon, but we may conceive 
that highly expanded vapour, in all probability steam, is its 
primary cause. The escaping aperture may have been a weak 
place since the foundations of the earth were laid, or it may have 
been formed by a local expansion of the nucleus in the act of 
cooling, upon the principle enunciated in our third chapter; or, 
again, the expansile vapour may have forced its own way through 
that point of the confining shell that ofi'ered it the least resistance. 
The vent once formed, the building of the volcanic mountain 
commenced by the out-belching of the lava, ashes, and scoriae, and 
the dispersion of these around the vent at distances depending 
upon the energy with which they were projected. As the action 
continued, the ejected matter would accumulate in the form of a 
mound, through the centre of which communication would be 
maintained with the source of the ejected materials and the seat of 
the explosive agency. The height to which the pile would rise 
must depend upon several conditions : upon the steady sustenance 
of the matter, and upon the form and weight of the component 



CHAP. VIII.] 



LUNAR CRATERS. 



105 



masses, which will determine the slope of the mountain's sides. 
Supposing the action to subside gradually, the tapering form will 
be continued upwards by the comparatively gentle deposition of 
material around the orifice, and a perfect cone will result of some 
such form as that represented below, which is the outline ascribed 




Fig. 16. 

by Professor Phillips to Vesuvius in pre-historic, or even pre- 
traditional times, and which may be seen in its full integrity in 
the cases of Etna, Teneriffe, Fusi - Yama, the great volcanic 
mountain of Japan, and many others. The earliest recorded form 
of Vesuvius is that of a truncated cone represented in Fig. 17, 




Fig. 17. 



which shows its condition, according to Strabo, in the century 
preceding the Christian Era. Now this form may have been 
assumed under two conditions. If, as Phillips has surmised, the 
mountain originally had a peaked summit with but a small crater- 
orifice, at the point, then we must ascribe its decapitation to a 
subsequent eruption which in its violence carried away the upper 
portion, either suddenly, or through a comparatively slow process of 
grinding away or widening out of the sides of the orifice by the 
chafing or fluxing action of the out-going materials. But it is 
probable that the mountain never had the perfect summit indicated 
in our first outline. The violent outburst that caused the great 



106 



THE MOON. 



[chap. VIII. 



COMPAHIDHTO HtU.. 

IViMUesWaaaT 



CoMe«MiaMToTll£BlT 



Small CiuTefc 
1ii»ibi*Waijej»" 



#• 



t^HPANIOH TO PABpr 

llJ«Iile3PfamT 



Heriscuel 





,<<*• 'i<'-/,,. 




,v.^-> 




ERATOSTHNES. 

Sa Mies Item? 




A6RIPPA. 

SOMJles'lJlan? 





THE0P1<ILUS. 

ClilfilcallSain'' 








Petavius. 



DIAGRAM OP 
78 



LUNAR CRATERS, FORMING A SERIES RANGING FROM If MILES TO 
MILKS IN DIAMETER, ALL CONTAINING CENTRAL CONES. 



CHAP. VIII.] LUNAR CRATERS. 107 

crater-opening of our second figure may have been but one 
paroxysmal phase of the eruption that built the mountain : a 
sudden cessation of the eruptive force when at its greatest 
intensity, and when the orifice was at its widest, would leave 
matters in an opposite condition to that suggested as the result 
of a slow dying out of the action : instead of the peak we should 
have a wide crater-mouth. It is of small consequence for our 
present purpose whether the crater was contemporaneous with the 
primitive formation of the mountain, or whether it was formed 
centuries afterwards by the blowing away of the mountain's head ; 




Fia. 18. 



for upon the vast scale of geological time, intervals such as those 
between successive paroxysms of the same eruption, and those 
between successive eruptions, are scarcely to be discriminated, even 
though the first be days and the second centuries. We may 
remark that the widening of a crater by a subsequent and probably 
more powerful eruption than that which originally produced it is 
well established. We have only to glance at the sketch. Fig. 18, 
of the outline of Vesuvius as it appeared between the years a.d. 79 
and 1631 to see how the old crater was enlarged by the terrible 
Pompeian eruption of the first-mentioned year. Here we have a 
crater ground and blown away till its original diameter of a mile 
and three-quarters has been increased to nearly three miles. 
Scrope had no hesitation in expressing his conviction that the 
external rings, such as those of Santorin, St. Jago, St. Helena, the 
Cirque of Teneriffe, the Curral of Madeira, the cliff range that 
surrounds the island of Bourbon, and others of similar form and 
structure, however wide the area they enclose, are truly the " basal 
wrecks " of volcanic mountains that have been blown into the air 



108 THE MOON. [chap. viii. 

each by some eruption of peculiar paroxysmal violence and 
persistence ; and that the circular or elliptical basins which they 
wholly or in part surround are in all cases true craters of eruption. 
When the violent outburst that produces a great crater in a 
volcanic mountain-top more or less completely subsides, the funnel 
or escaping orifice becomes choked with debris. Still the vent 
strives to keep itself open, and now and then gives out a small 
delivery of cindery matter, which, being piled around the vent, 
after the manner of its great prototype, forms the inner cone. 




Fig. 19. 

This last may in its turn bear an open crater upon its summit, 
and a still smaller cone may form within it. As the action further 
dies away, the molten lava, no longer seething and boiling, and 
spirting forth with the rest of the ejected matter, wells upwards 
slowly, and cooling rapidly as it comes in contact with the atmo- 
sphere, solidifies and forms a flat bottom or floor to the crater. 

It may happen that a subsequent eruption from the original vent 
will be comparable in violence to the original one, and then the 
inner cone assumes a magnitude that renders it the principal 
feature of the mountain, and reduces the old crater to a secondary 
object. This has been the case with Vesuvius. During the erup- 
tion of 1631 the great cone which we now call Vesuvius was thrown 
up, and the ancient crater now distinguished as Monte Somma 
became a subsidiary portion of the whole mountain. Then the 
appearance was that shown in Fig. 19, and which does not difi'er 
greatly from that presented in the present day. The summit of 
the Vesuvian cone, however, has been variously altered ; it has 
been blown away, leaving a large crateral hollow, and it has rebuilt 
itself nearly upon its former model. 



CHAP. VIII.] LUNAR CRATERS. 109 

When we transfer our attention to the volcanoes of the moon, we 
find ourselves not quite so well favoured with means for studying 
the process of their formation ; for the sight of the building up of 
a volcanic mountain such as man has been permitted to behold 
upon the earth has not been allowed to an observer of the moon. 
The volcanic activity, enfeebled though it now be, of which we are 
witnesses from time to time on the earth, has altogether ceased 
upon our satellite, and left us only its effects as a clue to the means 
by which they were produced. If we in our time could have seen 
the actual throwing up of a lunar crater, our task of description 
would have been simple ; as it is we are compelled to infer the con- 
structive action from scrutiny of the finished structure. 

We can scarcely doubt that where a lunar crater bears general 
resemblance to a terrestrial crater, the process of formation has 
been nearly the same in the one case as in the other. Where 
variations present themselves they may reasonably be ascribed to 
the difference of conditions pertaining to the two spheres. The 
greatest dissimilarity is in the point of dimensions ; the projection 
of materials to 20 or more miles distance from a volcanic vent 
appears almost incredible, until we realize the full effect of the 
conditions which upon the moon are so favourable to the dispersive 
action of an eruptive force. In the first place, the force of gravity 
upon our satellite is only one- sixth of that to which bodies are 
subject upon the earth. Secondly, by reason of the small mag- 
nitude of the moon and its proportionally much larger surface in 
ratio to its magnitude, the rate at which it parted with its cosmical 
heat must have been much more rapid than in the case of the earth, 
especially when enhanced by the absence of the heat-conserving 
power of an atmosphere of air or water vapour ; and the disruptive 
and eruptive action and energy may be assumed to be greater in 
proportion to the more rapid rate of cooling ; operating, too, as 
eruptive action would on matter so much reduced in weight as it is 
on the surface of the moon, we thus find in combination conditions 
most favourable to the display of volcanic action in the highest 



110 THE MOON. [chap. VIII. 

degree of violence. Moreover, as the ejected material in its passage 
from the centre of discharge had not to encounter any atmospheric 
resistance, it was left free to continue the primary impulse of its 
ejection without other than gravitative diminution, and thus to 
deposit itself at distances from its source vastly greater than those 
of which we have examples on the earth. 

We can of course only conjecture the source or nature of the 
moon's volcanic force. If geologists have had difficulty in assign- 
ing an origin to the power that threw up our earthly volcanoes, 
into whose craters they can penetrate, whose processes they can 
watch, and w^hose material they can analyze, how vastly more 
difficult must be the inquiry into the primary source of the power 
that has been at work upon the moon, which cannot be virtually 
approached by the eye within a distance of six or eight hundred 
miles, and the material of which we cannot handle to see if it be 
compacted by heat, or distended by vapours. Steam is the agent 
to which geologists have been accustomed to look for explanation 
of terrestrial volcanoes ; the contact of water with the molten '^ 
nucleus of our globe is accepted as a probable means whereby 
volcanic commotions are set up and ejective action is generated. 
But we are debarred from referring to steam as an element of 
lunary geology, by reason of the absence of water from the lunar 
globe. We might suppose that a small proportion of water once 
existed ; but a small proportion would not account for the immense 
display of volcanic action which the whole surface exhibits. If we 
admitted a Neptunian origin to the disturbances of the moon's 
crust, we should be compelled to suppose that water had existed 
nearly in as great quantity, area for area, there as upon our globe ; 
but this we cannot reasonably do. 

Aqueous vapour being denied us, we must look in other direc- 
tions for an ejective force. Of the nature of the lunar materials 
we can know nothing, and we might therefore assume anything ; 
some have had recourse to the supposition of expansive vapours 
given off by some volatile component of the said material while in 



PLATIY \^'! 






■/-• ■ . ■ .\ ' y/fer-?^: fV,v :v\^\v ■:Ov^:^■.- 



J.Kai-.myt.K. 



C P E Pv N ! C U S. 

/p5 rC 2p 30 f-0 50 Cp 7,0 <3p 

^^''■^^ ' Scale 



] LVWAM CEATEBS. Ul 

EBtateof fiMBOii,oggciifigmtedby fthfimiflilcnfmhm PrafieflBor 

Dioft refes to so^lnir as proboifafy an important dement in the 
moon's geology, soggesting this solnAanee becaose of the part 
idiidi it appeals to play in the Tokanie or igneoos opeations of 
onr ^he, and on aoeoont of its preaence in eosmical meteors that 
have ecme ivithin range of onr anatysia. Ai^ matter soUimated 
hy heat in the sabstnita of the moon would he condensed i^on 
reaching the edd smroonding spaee, and woold he deposited in a 
state of fine powder, or otherwise in a soGd fionn. Maedler has 
attiibnted the highly leflectiTe portions of some parts <tftiie smfiMe^ 
snch as the hright streams that radiate from some of the ciaten^ 
Gopemieas and Tydio fiir instance, to the Titzificatum of tiie 
sorfiuie matter hy gaseous cnncnts. But in suppoi^taoaDS fiks 
these we must remember that the proibahility of truth diminishes 
as the free ground for peculation widois. It does not appear 
dear how ei^anslTe Ts^urs could hsTe lain dormant till the moon 
assumed a solid crust, as aQ such would douhtiess make tiidr 
escape before asj shell was fimned, and at an epodi idien there 
was ample fiidlity far tbdr expansion. 

While we are not insensible of ibe Talne of an eipansJve Tiponr 
eai^kiiation, if it could be based on anything beyond mere eonjee- 
ture, we are disposed to attadi greater weig^ to that afforded hy 
the princ^le sketched in our third chapter, Tix., of eipansion i^on 
solidification. We ga^e, as we think, ample proof tiiat moiten 
matter of Tdcanie nature, when about passing to the solid states 
increases its bulk to a considerable degree, and we suggested that 
the lunar globe at one period of its histoiy must hafe been, wbat 
our earth is now, a sotid shell encon^assing a mdten nucleus ; 
and further, that this last, in i^proaching its solid condition, 
expanded and burst open or rent its confining crust. At first 
si^t it may seem that we are ascribing too great a degree of 
energy to the eipansiro fiiroe which molten substances exhibit in 
passing to the solid condition, seeing that in generd such forces 
are slow and giadud in thdr action ; but this anomaly disiqppears 



112 THE MOON. [chap. viii. 

when we consider the vast bulk of the so expanding matter, and 
the comparatively small amount that in its expansion it had to 
displace. It is true that there are individual mountains on the 
moon covering many square miles of surface, that as much as a 
thousand cubic miles of material may have been thrown up at a 
single eruption ; but what is this compared to the entire bulk of 
the moon itself? A grain of mustard-seed upon a globe three 
feet in diameter represents the scale of the loftiest of terrestrial 
mountains ; a similar grain upon a globe one foot in diameter, 
would indicate the proportion of the largest upon the moon. A 
model of our satellite with the elevations to scale would show 
nothing more than a little roughness, or superficial blistering. 
Turn for a moment to our map (Plate V.), upon which the 
shadows give information as to the heights of the various 
irregularities, and suppose it to represent the actual size of some 
sphere whose surface has been broken up by reactions of some 
kind of the interior upon the exterior — suppose it to have been a 
globe of fragile material filled with some viscous substance, and 
that this has expanded, cracked its shell, oozed out in the process 
of solidification, and solidified : the irregularity of surface which 
the small sphere, roughened by the out-leaking matter, would 
present, would not be less than that exhibited in the map under 
notice. "When we say that a lunar crater has a diameter of 30 
miles, we raise astonishment that such a structure could result 
from an eruption by the expansive force of solidifying matter ; but 
when we reflect that this diameter is less than the two-hundreth 
part of the circumference of the moon, we need have no difficulty 
in regarding the upheaval as the result of a force slight in 
comparison to the bulk of the material giving rise to it. We have 
upon the moon evidence of volcanic eruptions being the final result 
of most extensive dislocations of surface, such as could only be 
produced by some widely difi'used uplifting force. We allude to 
the frequent occurrence of chains of craters lying in a nearly 
straight line, and of craters situated at the converging point of 



CHAP. VIII.] LUNAR CRATERS. 113 

visible lines of surface disturbance. Our map will exhibit many 
examples of both cases. An examination of the upper portion 
(the southern hemisphere of the moon) will reveal abundant 
instances of the linear arrangement, three, four, five or even more 
crateral circles will be found to lie with their centres upon the same 
great-circle track, proving almost undoubtedly a connexion between 
them so far as the original disturbing force which produced them 
is concerned. Again, in the craters Tycho (30), Copernicus (147), 
Kepler (146), and Proclus (162), we see instances of the situation 
of a volcanic outburst at an obvious focus of disturbance. These 
manifest an up-thrusting force covering a large sub- surface area, 
and escaping at the point of least resistance. Such an extent of 
action almost precludes the gaseous explanation, but it is compatible 
with the expansion on consolidation theory, since it is reasonable 
to suppose that in the process of consolidation the viscous nucleus 
would manifest its increase of bulk over considerable areas, dis- 
turbing the superimposed crust either in one long crack, out of the 
wider opening parts of which the expanded material would find its 
escape, or " starring " it with numerous cracks, from the con- 
verging point of which the confined matter would be ejected in 
greatest abundance and, if ejected there with great energy and 
violence, would result in the formation of a volcanic crater. 

The actual process by which a lunar crater would be formed 
would differ from that pertaining to a terrestrial crater only to the 
extent of the different conditions of the two globes. We can 
scarcely accept Scrope's term ''basal wrecks" (of volcanic moun- 
tains that have had the summits blown away) as applicable to the 
craters of the moon, for the reason that the lunar globe does not 
offer us any instance of a mountain comparable in extent to the 
great craters and whose summit has not been blown away. 
Scrope's definition implies a double, or divided process of forma- 
tion : first the building up of a vast conical hill and then the 
decapitation and " evisceration " of it at some later period. There 
are grounds for this inferred double action among the terrestrial 



114 



THE MOON. 



[chap. VIII. 



volcanoes, since both tlie perfect cone and its summitless counter- 
part are numerously exemplified. But upon the moon we have 
no perfect cone of great size, we have no exception whereby the 
rule can be proved. It is against probability, supposing every 
lunar crater to have once been a mountain, that in every case the 
mountain's summit should have been blown away ; and we are 
therefore compelled to consider that the moon's volcanic craters 
were formed by one continuous outburst, and that their " eviscera- 




Fia. 20. 



tion " was a part of the original formative process. We do not, 
however, include the central cone in this consideration : that may 
be reasonably ascribed to a secondary action or perhaps, better, to a 
weaker or modified phase of the original and only eruption. 

Under these circumstances we conceive the upcasting and 
excavating of a normal lunar crater to have been primarily caused 
by a local manifestation of the force of expansion upon solidification 
of the sub-surface matter of the moon, resulting in the creation of 
a mere " star " or crack in and through the outermost and solid 
crust. As we shall have to rely upon diagrams to explain the 
more complicated features, we give one of this elementary stage 
also as a commencement of the series; and Fig. 20 therefore 










- ^^>/i:^ 



THE LUMA^ APFNN'N^S. AP CH ! \^ E D E S, &c, &c. 



CHAP. VIII.] 



LUNAR CRATERS. 



115 



represents a probable section of the lunar surface at a point which 
was subsequently the location of a crater. From the vent thus 
formed we conceive the pent-up matter to have found its escape, 
not necessarily at a single outburst, but in all probability in a 
paroxysmal manner, as volcanic action manifests itself on our globe. 
The first outflow of molten material would probably produce no more 
than a mere hill or tumescence as shewn sectionally in Fig. 21; and 
if the ejective force were small this might increase to the magnitude 




Fig. 21. 

of a mountain by an exudative process to be alluded to hereafter. 
But if the ejective force were violent, either at the moment of the 
first outburst or at any subsequent paroxysm, an action repre- 
sented in Fig. 22 would result : the unsupported edges or lips of 
the vent-hole would be blown and ground or fluxed away, and a 
funnel-formed cavity would be produced, the ejected matter (so 
much of it as in falling was not caught by the funnel) being 
deposited around the hollow and forming an embryo circular 
mountain. The continuance of this action would be accompanied 
by an enlargement of the conical cavity or crater, not only by the 
outward rush of the violently discharged material, but also by the 
" sweating " or grinding action of such of it as in descending fell 

I 2 



116 



THE MOON. 



[chap. VIII. 



within the hollow. And at the same time that the crater en- 
larged the rampart would extend its circumference, for it would 
he formed of such material as did not fall hack again into the 
crater. Upon this view of the crater- forming process we hase the 




Fio. 22. 



sketch, Fig. 23, of the probable section of a lunar crater at one 
period of its development. 

So long as each succeeding paroxysm was greater than its prede- 
cessor, this excavating of the hollow and widening of its mouth and 
mound would he extended. But when a weaker outburst came, or 
when the energy of the last eruption died away, a process of slow 
piling up of matter close around the vent would ensue. It is 
obvious that when the ejective force could no longer exert itself to a 
great distance it must merely have lifted its burden to the relieving 
vent and dropped it in the immediate neighbourhood. Even if the 
force were considerable, the eifect, so long as it was insufficient to 



CHAP. VIII.] 



LUNAR CRATERS. 



117 



throw the ejecta beyond the rim of the crater, would be to pile 
material in the lowermost part of the cavity ; for what was not cast 
over the edge would roll or flow down the inner slope and accumu- 
late at the bottom. And as the eruption died away, it would add 
little by little to the heap, each expiring effort leaving the out-giving 




Fio. 23. 



matter nearer the orifice, and thus building up the central cone that 
is so conspicuous a feature in terrestrial volcanoes, and which is also 
a marked one in a very large proportion of the craters of the moon. 
This formation of the cone is pictorially described by Fig. 24. 

In the volcanoes of the earth we observe another action either 
concurrent with or immediately subsequent to the erection or forma- 
tion of the cone : this is the outflow or the welling forth of fluid 
lava, which in cooling forms the well-known plateau. We have 
this feature copiously represented upon the moon, and it is 
presumable that it has in general been produced in a manner 



118 



THE MOON, 



[chap. VIII. 



analogous to its counterparts upon the earth. We may conceive 
that the fluid matter was either spirted forth with the solid or semi- 
solid constituents of the cone, in which case it would drain down 
and fill the bottom of the crater ; or we may suppose that it issued 
from the summit of the cone and ran down its sides, or that, as we 
see upon the earth, it found its escape before reaching the apex, by 




FiQ. 24. 



forcing its way through the basal parts. These actions are indi- 
cated hypothetically for the moon in Fig. 25 ; and the parallel 
phenomena for the earth are shewn by the actual case (represented 
in Fig. 26 and on Plate I.) of Vesuvius as it was seen by one of the 
authors in 1864, when the principal cone was vomiting forth ashes, 
stones, and red-hot lava, while a vent at the side emitted very 
fluid lava which was settling down and forming the plateau. 

Although we cannot, obviously, see upon the moon evidence of 
a cone actually overtopped by the rising lake of lava, yet it is not 



CHAP. VIII.] 



LUNAB CRATERS. 



119 



unreasonable to suppose that such a condition of things actually 
occurred in many of those instances in which we observe craters 
without central cones, but with plateaux so smooth as to indicate 
previous fluidity or viscosity. From the state of things exhibited 
in Fig. 25 the transition to that shewn in Fig. 27 is easily, and to 
our view reasonably, conceivable. We are in a manner led up to 




Fig. 25. 

this idea by a review of the various heights of central cones above 
their surrounding plateaux. For instance, in such examples as 
Tycho or Theophilus, we have cones high above the lava floor ; in 
Copernicus, Arzachael and Alphons they are comparatively lower ; 
the lava in these and some other craters does not appear to have 
risen so high ; while in Aristotle and Eudoxus among others, we 
have only traces of cones, and it is supposable that in these cases 
the lava rose so high as nearly to overtop the central cones. Why 
should it not have risen so far as to overtop and therefore conceal 
some cones entirely ? We oifer this as at least a feasible explana- 



120 



THE MOON. 



[chap. VIII. 



tion of some coneless craters : it is not necessary to suppose that it 
applies to all such, however : there may have been many craters, 
the formation of which ceased so abruptly that no cone was pro- 



.- ^ ^N'> 






1^ /)V^^ 




duced, though the welling forth of lava occurred from the vent, 
which may have been left fully open, as in Fig. 28, or so far choked 
as to stay the egress of solid ejecta and yet allow the fluid material 
to ooze upwards through it, and so form a lake of molten lava which 
on consolidation became the plateau. As most of the examples of 
coneless craters exhibit on careful examination minute craters on 
the surface of the otherwise smooth plateaux, we may suppose that 
such minute craters are evidences of the upflow of lava which 
resulted in the plateaux. 



P L A T E X. 










Mmmm 




JKasirlyth. 



"Vvbodiiurjrtype ' 



ARISTOTLE & EUDOXUS 



ro SO 7p 20 30 40 £0 W 

MILES ^ ~ 

Scale. 



CHAP. VIII.] 



LUNAR CRATERS. 



121 



We have strong evidence in support of this upflow of lava 
offered by the case of the crater Wargentin (No. 29), situated 
near the south-east border of the disc, and of which we give 
a special plate. (Plate XVIII.) It appears to be really a crater 
in which the lava has risen almost to the point of overflowing, 
for the plateau is nearly level with the edge of the rampart. This 
edge appears to have been higher on one side than the other, for on 
the portion nearest the centre of the visible disc we may, under 
favourable circumstances, detect a segment of the basin's brim 
rising above the smooth plateau as indicated in our illustration. 




Fig. 27. 



Upon the opposite side there is no such feature visible, the plateau 
forms a sharp table-like edge. It is just possible that an actual 
overflow of lava took place at this part of the crater, but from the 
unfavourable situation of this remarkable object it is impossible to 
decide the point by observation. There is no other crater upon the 
visible hemisphere of the moon that exhibits this filled- up con- 
dition ; but, unique as it is, it is sufficient to justify our conclusion 
that the plateau-forming action upon the moon has been a flowing- 
up of fluid matter from below subsequent to the formation of the 
crater-rampart, and not, as a casual glance at the great smooth- 
bottom craters might lead us to suspect, a result of some sort of 
diluvial deposit which has filled hollows and cavities and so brought 



122 



THE MOON. 



[chap. VIII. 



up an even surface. The elevated basin of Wargentin could not 
have been filled thus while the surrounding craters with ramparts 
equally or less high remained empty : its contained matter must 
have been supplied from within, we must conjecture by the upflow 
of lava from the orifice which gave forth the material to form the 



Fig. 28. 

crater al rampart in the first instance. We are free to conjecture 
that at some period of this table-mountain's formation it was a 
crater with a central cone, and that the rising lava over-topped this 
last feature in the manner shewn by Fig. 29. 

The question occurs whether other craters may not have been 
similarly filled and have emptied themselves by the bursting of the 
wall under the pressure of the accumulated lake of lava within. 
We know that this breaching is a common phenomenon in the 
volcanoes of our globe ; the district of Auvergne furnishing us with 
many examples ; and there are some suspicious instances upon the 



CHAP. VIII.] 



LUNAR CRATERS. 



123 



moon. Copernicus exhibits signs of such disruption, as also does 
the smaller crater intruding upon the great circle of Gassendi. 
(See Frontispiece.) But the existence of such discharging breaches 
implies the outpouring of a body of fluid or semi-fluid material, 
comparable in cubical content to the capacity of the crater, and of 
this we ought to see traces or evidence in the form of a bulky or 




Fia. 29. 

extensive lava stream issuing from the breach. But although there 
are faint indications of once viscous material lying in the direction 
that escaping fluid would take, we do not find anything of the 
extent that we should expect from the mass of matter that would 
constitute a craterfull. It is true that if the escaping fluid had 
been very limpid it might have spread over a large area and have 
formed a stratum too thin to be detected. Such a degree of 
limpidity as would be required to fulfil this condition we are hardly, 
however, justified in assuming. 



124 



THE MOON. 



[chap. Vtll. 



To return to the subject of central cones. Although there are 
cases in which the simple condition of a single cone exists, yet in 
the majority we see that the cone-forming process has been divided 
or interrupted, the consequence being the production of a group of 
conical hills instead of a single one. Copernicus offers an example 
of this character, six, some observers say seven, separate points of 




light, indicating as many peaks tipped with sunshine, having been 
seen when the greater part of the crater has been buried in shadow. 
Eratosthenes, Bulialdus, Maurolicus, Petavius, Langreen, and 
Gassendi, are a few among many instances of craters possessing 
more than a central single cone. This multiplication of peaks upon 
the moon doubtless arose from similar causes to those which produce 
the same feature in terrestrial volcanoes. Our sketch of Vesuvius in 
1864 (Plate I. and Fig. 26) shews the double cone and the probable 
source of the secondary one in the diverted channel of the out- 
coming material. A very slight interruption in the first instance 



PLATE X 




J.Nasmyth 



""Woodtiirytype' 



TRIESNECKER. 



fO s o 



CHAP. VIII.] 



LUNAR GRATMRS. 



125 



would suffice to divert the stream and form another centre of action, 
or a choking of the original vent would compel the issuing matter 
to find a less resisting thoroughfare into open space, and the process 
of cone-building would be continued from the new orifice, perhaps 
to be again interrupted after a time and again driven in another 
direction. In this manner, by repeated arrests and diversions of 




Fig. 31. 

the ejecta, cone has grown upon the side of cone, till, ere the force 
has entirely spent itself, a cluster of peaks has been produced. It 
may have been that this action has taken place after the formation 
of the plateau, in the manner indicated by Fig. 30 ; a spasmodic 
outburst of comparatively slight violence having sought relief from 
the original vent, and the flowing matter, finding the one orifice 
not sufficiently open to let it pass, having forced other exit through 
the plateau. 

In frequent instances we observe the state of things represented 
in Fig. 31, in which the plateau is studded with few or many small 



126 THE MOON. [chap. viii. 

craters. This is the case with Plato, with Arzachael, Hipparchus, 
Clavius (which contains about 15 small internal craters), and many 
others. It is probable that these subsidiary craters were produced 
by an after-action like that which has produced duplicated cones, 
but in which the secondary eruption has been of somewhat violent 
character, for it may almost be regarded as an axiom that violent 
eruptions excavate craters and weak ones pile up cones. In the 
cases referred to it seems reasonable to suppose that the main vent 
has been the channel for an up-cast of material, but that at some 
depth below the surface this material met with some obstruction or 
cause of diversion, and that it took a course which brought it out 
far away from the cone upon the floor of the plateau. It might 
even be carried so far as to be upon the rampart, and it is no 
uncommon thing to see small craters in such a situation, though 
when they appear at such a distance from the primary vent, it 
seems more reasonable to suppose that they do not belong to it, 
but have arisen from a subsequent and an independent action. 

We find scarcely an instance of a small crater occurring just in 
the centre of a large one, or taking the place of the cone. This is 
a curious circumstance. Whenever we have any central feature in 
a great crater that feature is a cone. The tendency of this fact is 
to prove that cones were produced by very weak efforts of this 
expiring force, for had there been any strength in the last paroxysm 
it is presumable that it would have blown out and left a crater. 
No very violent eruptions have therefore taken place from the vents 
that were connected with the great craters of the moon, nothing 
more powerful than could produce a cone of exudation or a cinder- 
heap. And with regard to cones, it is noteworthy that whether 
they be single or multiple, they never rise so high as the circular 
ramparts of their respective craters. This supports the inferred 
connexion between the crater origin and the cone origin, for 
supposing the two to have been independent, a supposition 
untenable in view of the universality of the central position of the 
cone, it is scarcely conceivable that the mountains should have 



CHAP. VIII.] LUNAR CBATEB8. 127 

always been located within ramparts higher than themselves. The 
less height argues less power in the upcasting agency, and the 
diminished force may well be considered as that which would 
almost of necessity precede the expiration of the eruption. 

Occasionally a crater is met with that has a double rampart, and 
the concentricity suggests that there 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. 
It is not, however, evident that this duplication of the ring has 
always been due to a double eruption. In many cases there is 
duplication of only a portion : a terrace exhibits itself around a 
part of the circular range, sometimes upon the outside and some- 
times upon the inside. These terraces are not likely to have been 
formed by any freak of the eruption, and we are led to ascribe them 
in general to landslip phenomena. When, in the course of a 
volcano's formation, the piling-up of material about the vent has 
continued till the lower portions have been unable to support the 
upper, or when from any cause the material composing the pile 
has lost its cohesiveness, the natural consequence has been a 
breaking away of a portion of the structure and its precipitation 
down the inclined sides of the crater. Vast segments of many of 
the lunar mountain-rings appear to have been thus dislodged from 
their original sites and cast down the flanks to form crescent ranges 
of volcanic rocks either within or without the circle. Nearly every 
one of our plates contains craters exhibiting this feature in more 
or less extensive degree. Sometimes the separated portion has 
been very small in proportion to the circumference of the crater : 
Plato is an instance in which a comparatively small mass has been 
detached. In other cases very large segments have slid down and 
lie in segmental masses on the plateaux or form terraces around 
the rampart. Aristarchus, Triesnecker, and Copernicus exhibit 
this larger extent of dislocation. 

It is possible that these landslips occurred long after the forma- 
tion of the craters that have been subject to them. They are 



128 TH:E moon. [chap. viii. 

probably attributable to recent disintegration of the lunar rocks, 
and we have a powerful cause for this in the alternations of tem- 
perature to which the lunar crust is exposed. We shall have occa- 
sion to revert to this subject by-and-by ; at present it must suffice 
to point out that the extremes of cold and heat, between which 
the lunar soil varies, are, with reasonable probability, assumed 
to be on the one hand the temperature of space (which is supposed 
to be between 200° and 250'' below zero), and, on the other hand, a 
degree of heat equal to about twice that of boiling water. A range 
of at least 500° must work great changes in such heterogeneous 
materials as we may conjecture those of the lunar crust to be, by 
the alternate contractions and expansions which it must engender, 
and which must tend to enlarge existing fissures and create new 
ones, to grind contiguous surfaces and to dislodge unstable masses. 
This cause of change, it is to be remarked, is one which is still 
exerting itself. 

In a few cases we have an entirely opposite interruption of the 
uniformity of a crater's contour. Instead of the breaking away of 
the ring in segments, we see the entire circuit marked with deep 
ruts that run down the flanks in a radial direction, giving us 
evidence of a downward streaming of semi-fluid matter, instead of a 
disruption of solid masses. "We cannot doubt that these ruts have 
been formed by lava currents, and they indicate a condition of 
ejected material different from that which existed in the cases 
where the landslip character is found. In these last the ejecta 
appears to have been in the form of masses of solidified or rapidly 
solidifying matter, which remained where deposited for a time and 
then gave way from overloading or loss of cohesiveness, whereas 
the substances thrown out in the case of the rutted banks were 
probably mixed solid and fluid, the former remaining upon the 
flanks while the latter trickled away. Nothing so well represents, 
upon a small scale, this radial channelling as a heap of wetted 
sand left for a while for the water to drain off from it. The solid 
grains in such a heap sustain its general mass-form, but the liquid 



p i A T I" y 1 1 







J.ITafiTT/tb 



" WooAburytype" 



THEOPHiLUS,CYRiLLUS,& CATHARINA 



6 W 20 30 'K} SO 60 TO 80 



CHAP. VIII.] LUNAR CRATERS. 129 

in passing away cuts the surface into fissures running from the sum- 
mit to the base, and forms it into a model of a volcanic mountain 
with every feature of peak, crag, and chasm reproduced. This simi- 
larity of efi'ect leads us to suspect a parallelism of cause, and thus to 
the inference that the material which originally formed such a crater- 
mountain as Aristillus (which is a most prominent example of this 
rutted character, and appears in Plate IX., side by side with a crater 
that has its banks segmentally broken), must have been of the com- 
pound nature indicated ; and that an action analogous to that which 
ruts a damp sand-heap, rutted also the banks of the lunar crater. 

Before passing from the subject of craters it behoves us to say a 
few words upon the curious manner in which these formations are 
complicated by intermingling and superposition. Yet, upon this 
point, we may be brief, for in the way of description our plates 
speak more forcibly than is possible by words. In particular we 
would refer to Plate XII., which represents the conspicuous group 
of craters of which the three largest members have been respectively 
named Theophilus, Cyrillus, and Catharina. But the area included 
in this plate is by no means an extraordinary one ; there are regions 
about Tycho wherein the craters so crowd and elbow each other that, 
in their intricate combinations, they almost defy accurate depiction. 
Our map and Plate XVI. will serve to give some idea of them. This 
intermingling of craters obviously show^s that all the lunar volcanoes 
were not simultaneously produced, but that after one had been formed, 
an eruption occurred in its immediate neighbourhood and blew a 
portion of it away ; or it may have been that the same deep-seated 
vent at different times gave forth discharges of material the courses 
of which were more or less diverted on their way to the surface. 

We have before alluded to the frequent occurrence of lines of 
craters upon the moon. In these lines the overlapping is frequently 
visible ; it is seen in Plate XII. before referred to, w^here the ring 
mountains are linked into a chain slightly curved, and upon the 
map, Plate V., the nearly central craters Ptolemy and Alphons, 
the latter of which overlaps the former, are seen to form part of a 



130 THE MOON. [chap. viii. 

line of craters marking a connection of primary disturbance. An 
extensive crack suggests itself as a favourable cause for the produc- 
tion of this overlaying of craters, for it would serve as a sort of 
** line of fire " from various points at which eruptions would burst 
forth, sometimes weak or far apart, when the result would be lines 
of isolated craters, and sometimes near together, or powerful, when 
the consequence would be the intrusion of one upon the other, and 
the perfect production of the latest formed at the expense or to the 
detriment of those that had been formed previously. The linear 
grouping of volcanoes upon the earth long ago struck observant 
minds. The fable of the Typhon lying under Sicily and the 
Phlegreian fields and disturbing the earth by its writhings, is a 
mythological attempt to explain the particular case in that region. 

The capricious manner in which these intrusions occur is very 
curious. Very commonly a small crater appears upon the very 
rampart of a greater one, and a more diminutive one still will 
appear upon the rampart of the parasite. Stoefiler presents us 
with one example of this character, Hipparchus with another, 
Maurolycus with a third, and these are but a few cases of many. 
Here and there we observe several craters ranged in a line with 
their rims in one direction all perfect, and the whole appearing like 
a row of coins that have fallen from a heap. There is an example 
near to Tycho which we reproduce in Plate XXEE. In this case one 
is led to conjecture that the ejective agency, after exerting itself 
in one spot, travelled onward and renewed itself for a time ; that it 
ceased after forming crater number two, and again journeyed for- 
ward in the same line, recommencing action some miles further, 
and again subsiding ; yet again pushing forward and repeating its 
outburst, till it produced the fourth crater, when its power became 
expended. In each of these successive eruptions the centre of dis- 
charge has been just outside the crater last formed ; and the close 
connexion of the members of the group, together with the fact of 
their nearly similar size, appears to indicate a community of origin. 
For it seems feasible that as a general rule the size of a crater may 



CHAP. VIII.] LUNAR CRATERS. 131 

be taken as a measure of the depth of force that gave rise to the 
eruption producing it. This may not be true for particular cases, 
but it will hold where a great number are collectively considered ; 
for if we assume the existence of an average disturbing force, it is 
apparently clear that it will manifest itself in disturbing greater or 
less surface-areas in proportion as it acts from greater or less 
depths. Or, mutatis mutandis, if we assume an uniform depth 
for the source of action, the greater or less surface disturbance will 
be a measure of greater or less eruptive intensity. 

Perhaps the most remarkable case of a vast number of craters, 
which, from their uniform dimensions, suggest the idea of com- 
munity of source-power or source-depth, is that offered by the 
region surrounding Copernicus, which, as will be seen by our plate 
of that object, is a vast Phlegreian field of diminutive craters. So 
countless are the minute craters that a high magnifying power 
brings into view when atmospheric circumstances are favourable, 
and so closely are they crowded together, that the resulting 
appearance suggests the idea of froth, and we should be disposed 
to christen this the " frothy region " of the moon, did not a danger 
exist in the tendency to connect a name with a cause. The craters 
that are here so abundant are doubtless the remains of true 
volcanoes analogous to the parasitical cones that are to be found on 
several terrestrial mountains, and not such accidental formations as 
the Hornitos described by Humboldt as abounding in the neigh- 
bourhood of the Mexican volcano, Jurillo, but which the traveller 
did not consider to be true cones of eruption.* Although upon our 
plate, and in comparison with the great crater that is its chief 
feature, these countless hollows appear so small as at first sight to 
appear insignificant, we must remember that the minutest of them 
must be grand objects, each probably equal in dimensions to 
Vesuvius. For since, as we have shown in an early chapter, the 
smallest discernible telescopic object must subtend an angle to our 

* « Cosmos," Bohn's Edition, Vol. V. p. 322. 

K 2 



132 



THE MOON. 



[chap. VIII. 



eye of about a second, and since this angle extended to the moon 
represents a mile of its surface, it follows that these tiny specks 
of shadow that besprinkle our picture, are in the reality craters of 
a mile diameter. This comparison may help the conception of the 
stupendous magnitude of the moon's volcanic features ; for it is a 
conception most difficult to realize. It is hard to bring the mind 
to grasp the fact that that hollow of Copernicus is fifty miles in 
diameter. We read of an army having encamped in the once 
peaceful crater of Vesuvius, and of one of the extinct volcanoes of 
the Camj)i Phlegrcei being used as a hunting preserve by an Itahan 
king. These facts give an idea of vastness to those who have not 
the good fortune to see the actual dimensions of a volcanic orifice 
themselves. But it is almost impossible to conjure up a vision of 
what that fifty-mile crater would look like upon the moon itself ; 
and for want of a terrestrial object as a standard of comparison, our 
picture, and even the telescopic view of the moon itself, fails to 
render the imagination any help. We may try to realize the 
vastness by considering that one of our average English counties 
could be contained within its ramparts, or by conceiving a moun- 
tainous amphitheatre whose opposite sides are as far apart as the 
cathedrals of London and Canterbury, but even these comparisons 
leave us unimpressed with the true majesty which the object would 
present to a spectator upon the surface of our satellite. 




TUB FORMATION OF THB CENTRAL CONK, FINAL ACTION OF A LUNAR VOLCANO. 



PLATE XIII. 




" "V/bo dburytype" 

PTOLEMY, ALPHONS,ARZACHAEL&c 



1 so 'p 'iO jp w so 6p 7p sp 9f > 

SCALF. 



TJNIVEiiSITj:)) 



CHAPTER IX. 

ON THE GREAT RING-FORMATIONS NOT MANIFESTLY VOLCANIC. 

In our previous chapter we have given a reason for regarding as 
true volcanic craters all those circular formations, of whatever size, 
that exhibit that distinctive feature the central cone. Between the 
smallest crater with a cone that we can detect under the best tele- 
scopic conditions, namely, the companion to Hell, If mile diameter, 
and the great one called Petavius, 78 miles in diameter, we find no 
break in the continuity of the crater- cum- cone system that would 
justify us in saying that on the one side the volcanic or eruptive 
cause ceased, and on the other side some other causative action 
began. But there are numerous circular formations that surpass 
the magnitude of Petavius and its peers, but that have no circular 
cone, and are, therefore, not so manifestly volcanic as those which 
possess this feature. Our map will show many striking examples 
of this class at a glance. We may in particular refer tnier alia to 
Ptolemy near the centre of the moon, to Grimaldi (No. 125), 
Schickard (No. 28), Schiller (No. 24), and Clavius (No. 13), ail of 
which exceed 100 miles in diameter. Even the great Mare 
Crisium^ nearly 300 miles in diameter, appears to be a formation 
not distinct from those which we have just named. These present 
little of the generic crater character in their appearance ; and they 
have been distinguished therefrom by the name of Walled or JRam- 
parted Plains, Their actual origin is beyond our explanation, and 
in attempting to account for them we must perforce allow consider- 
able freedom to conjecture. They certainly, as Hooke suggested, 



134 



THE MOON, 



[chap. IX. 



present a " broken bubble "-like aspect ; but one cannot reasonably 
imagine the existence of any form of mineral matter that would 
sustain itself in bubble form over areas of many hundreds of square 
miles. And if it were reasonable to suppose the great rings to be 
the foundations of such vast volcanic domes, we must conclude 
these to have broken when they could no longer sustain themselves, 
and in that case the surface beneath should be strewed with debrisy 










^-5^ 
-/-- 



X 



Fig. 32. 

of which, however, we can find no trace. Moreover, we might fairly 
expect that some of the smaller domes would have remained stand- 
ing : we need hardly say that nothing of the kind exists. 

The true circularity of these objects appears at first view a 
remarkable feature. But it ceases to be so if we suppose them to 
have been produced by some very concentrated sublunar force of an 
upheaving nature, and if only we admit the homogeneity of the 
moon's crust. For if the crust be homogeneous, then any up- 
heaving force, deeply seated beneath it, will exert itself \cith equal 
effects at equal distances from the source : the lines of equal effect 
will obviously be radii of a sphere with the source of the disturb- 



CHAP. IX.] RING-FORMATIONS NOT MANIFESTLY VOLCANIQ. 



135 



ance for its centre, and they will meet a surface over the source in 
a circle. This will be evident from Fig. 32, in which a force is 
supposed to act at F below the surface s s s s. The matter com- 
posing s s being homogeneous, the action of F will be equal at 
equal distances in all directions. The lines of equal force, F/, F/, 
will be of equal length, and they will form, so to speak, radii of a 
sphere of force. This sphere is cut by the plane at s s s s, and as 
the intersection necessarily takes place everywhere at the extremity 
of these radii, the figure of intersection is demonstrably a circle 



Fia. 33. 



Fig. 34. 




V^:\-;t-f-r/.= /"^ 




(shown in perspective as an ellipse in the figure). Thus we see 
that an intense but extremely confined explosion, for instance, 
beneath the moon's crust must disturb a circular area of its surface, 
if the intervening material be homogeneous. If this be not homo- 
geneous there would be, where it offered less than the average 
resistance to the disturbance, an outward distortion of the circle ; 
and an opposite interruption to circularity if it offers more than the 
average resistance. This assumed homogeneity may possibly be the 
explanation of the general circularity of the lunar surface features, 
small and great. 

We confess to a difficulty in accounting for such a very local 
generation of a deep-seated force ; and, granting its occurrence, we 
are unprepared with a satisfactory theory to explain the resultant 
effect of such a force in producing a raised ring at the limit of the 
circular disturbance. We may, indeed, suppose that a vast circular 



136 THE MOON. [chap. ix. 

cake or conical frustra would be temporarily upraised as in Fig. 33, 
and that upon its subsidence a certain extrusion of subsurface 
matter would occur around the line or zone of rupture as in Fig. 34. 
This supposition, however, implies such a peculiarly cohesive con- 
dition of the matter of the uplifted cake, that it is doubtful whether 
it can be considered tenable. We should expect any ordinary form 
of rocky matter subjected to such an upheaval to be fractured and 
distorted, especially when the original disturbing force is greater in 
the centre than at the edge, as, according to the above hypothesis, 
it would be ; and in subsiding, the rocky plateau would thus retain 
some traces of its disturbance ; but in the circular areas upon the 
moon there is nothing to indicate that they have been subjected to 
such dislocations. 

Mr. Scrope in his work on volcanoes has given a hypothetical 
section of a portion of the earth's crust, which presents a bulging 
or tumescent surface in some measure resembling the effect which 
such a cause as we have been considering would produce. We give 
a slightly modified version of his sketch in Fig. 35, showing what 
would be the probable phenomena attending such an upheaval as 
regards the behaviour of the disturbed portion of the crust, and also 
that of the lava or semifluid matter beneath: and, as will be seen 
by the sketch, a possible phase of the phenomena is the production 
of an elevated ridge or rampart at the points of disruption c c ; and 
where there is a ring of disruption, as by our hypothesis there 
would be, the ridge or rampart c c would be a circle. In this draw- 
ing we see the cracking and distortion to which the elevated area 
would be subjected, but of which, as previously remarked, the cir- 
cular areas of the moon present no trace of residual appearance. 

Those who have offered other explanations of these vast ring- 
formed mountain ranges, have been no more happy m their conjec- 
tures. M. Bozet, who communicated a paper on selenology to the 
French Academy in 1846, put forth the following theory. He 
argued that during the formation of the solid scoriaceous pelicules 
of the moon, circular or tourbillonic movements were set up ; and 











<|s^ 



CHAP. IX,] RING-FORMATIONS NOT MANIFESTLY VOLCANIC. 



137 



these, by throwing the scoriae from the centre to the circumference, 
caused an accumulation thereof at the limit of the circulation. He 
considered that this phenomenon continued during the whole pro- 
cess of solidification, but that the amplitude of the whirlpool 
diminished with the decreasing fluidity of the surface material. 
Further, he suggested that when many vortices were formed, and 




Fig. 35. " " 

A A. Fissures gaping downwards and injected by intumescent lava 
beneath, b b b. Fissures gaping upwards and allowing wedges of rock 
to drop below the level of the intervening masses, g c. Wedges forced 
upwards by horizontal compression, e p. Neutral plane or pivot axis, 
above and below which the directions of the tearing strain and horizontal 
compression are severally indicated by the smaller arrows ; the larger 
arrows beneath represent the direction of the primary expansive force. 



the distances of their centres, taken two and two, were less than 
the sums of their radii, there resulted close spaces terminated by 
arcs of circles ; and when for any two centres the distance was 
greater than the sum of their radii of action, two separate and com- 
plete rings were formed. We have only to remark on this, that we 
are at a loss to account for the origination of such vorticose move- 
ments,, and M. Kozet is silent on the point. If the great circles 
are to be referred to an original sea of molten matter, it appears to 



138 THE MOON. [chap. ix. 

US more feasible to consider that wherever we see one of them there 
has been, at the centre of the ring, a great outflow of lava that has 
flooded the surrounding surface. Then, if from any cause, and it 
is not difficult to assign one, the outflow became intermittent, or 
spasmodic, or subject to sudden impulses, concentric waves would 
be propagated over the pool and would throw up the scoria or the 
solidifying lava in a circular bank at the limit of the fluid area. 

This hypothesis does not difi'er greatly from the ebullition theory 
proposed by Professor Dana, the American geologist, to explain 
these formations. He considered that the lunar ring-mountains 
were formed by an action analogous to that which is exemplified on 
the earth in the crater of Kilauea, in the Hawaiian islands. This 
crater is a large open pit exceeding three miles in its longer 
diameter, and nearly a thousand feet deep. It has clear bluff walls 
round a greater part of its circuit, with an inner ledge or plain at 
their base, raised 340 feet above the bottom. This bottom is a 
plain of solid lavas, entirely open to-day, which may be traversed 
with safety (we are quoting Professor Dana's own statement 
written in 1846, and therefore not correctly applying to the present 
time) : over it there are pools of boiling lava in active ebulHtion, 
and one is more than a thousand feet in diameter. There are also 
cones at times, from a few yards to two or three thousand feet in 
diameter, and varying greatly in angle of inclination. The largest 
of these cones have a circular pit or crater at the summit. The 
great pit itself is oblong, owing to its situation on a fissure, but 
the lakes upon its bottom are round, and in them, says Professor 
Dana, " the circular or slightly elliptical form of the moon's 
craters is exemplified to perfection." 

Now Dana refers this great pit crater and its contained lava- 
lakes to " the fact that the action at Kilauea is simply boiling, 
owing to the extreme fluidity of the lavas. The gases or vapours 
which produce the state of active ebullition escape freely in small 
bubbles, with little commotion, like jets over boiling water ; while 
at Vesuvius and other like cones they collect in immense bubbles 



CHAP. IX.] RING-FORMATIONS NOT MANIFESTLY VOLCANIC. 139 

before they accumulate force enough to make their way through ; 
and consequently the lavas in the latter case are ejected with so 
much violence that they rise to a height often of many thousand 
feet and fall around in cinders. This action builds up the pointed 
mountain, while the simple boiling of Kilauea makes no cinders 
and no cinder cones." 

Professor Dana continues, *'If the fluidity of lavas, then, is 
sufficient for this active ebullition, we may have boiling going on 
over an area of an indefinite extent ; for the size of a boiling lake 
can have no limits except such as may arise from a deficiency of 
heat. The size of the lunar craters is therefore no mystery. 
Neither is their circular form difficult of explanation ; for a boiling 
pool necessarily, by its own action, extends itself circularly around 
its centre. The combination of many circles, and the large sea- 
like areas, are as readily understood." * 

In justice to Professor Dana it should be stated that he included 
in this theory of formation all lunar craters, even those of small 
size and possessing central cones ; and he put forth his views in 
opposition to the eruptive theory which we have set forth, and which 
was briefly given to the world more than twenty-five years ago. As 
regards the smallest craters with cones, we believe few geologists will 
refuse their compliance with the supposition that they were formed 
as our crater-bearing volcanoes were formed : and we have pointed 
out the logical impossibility of assigning any limit of size beyond 
which the eruptive action could not be said to hold good, so long as 
the central cone is present. But when we come to ring-mountains 
having no cones, and of such enormous size that we are compelled 
to hesitate in ascribing them to ejective action, we are obliged to 
face the possibility of some other causation. And, failing an 
explanation of our own that satisfied us, we have alluded to the 
few hypotheses proffered by others, and of these Professor Dana's 
appears the most rational, since it is based upon a parallel found on 
the earth. In citing it, however, we do necessarily not indorse it. 

* American Journal of Science, Second Series, Vol. IT. 



CHAPTER X. 

PEAKS AND MOUNTAIN EANGES. 

The lunar features next in order of conspicuity are the mountain 
ranges, peaks, and hill- chains, a class of eminences more in 
common with terrestrial formations than the craters and circular 
structures that have engaged our notice in the preceding' chapters.- 

In turning our attention to these features, we are at the outset 
struck with the paucity on the lunar surface of extensive mountain 
systems as compared with its richness in respect of crateral 
formations ; and a field of speculation is opened by the recognition 
of the remarkable contrast which the moon thus presents to the 
earth, where mountain ranges are the rule, and craters like the 
lunar ones are decidedly exceptional. Another conspicuous but 
inexplicable fact is that the most important ranges upon the moon 
occur in the northern half of the visible hemisphere, where the 
craters are fewest and the comparatively featureless districts 
termed *' seas " are found. The finest range is that named after 
our Apennines and which is included in our illustrative Plate, 
No. IX. It extends for about 450 miles, and has been estimated 
to contain upwards of 3000 peaks, one of which — Mount Huyghens 
— attains the altitude of 18,000 feet. The Caucasus is another 
lunar range which appears like a diverted northward extension of 
the Apennines, and, although a far less imposing group than the 
last named, contains many lofty peaks, one of which approaches 
the altitude assigned to Mount Huyghens while several others 
range between 11,000 and 14,000 feet high. Another consider- 



PLATE XV 




J.Naarayth. 



'"WbocLbTirytype" 



MERCATOR & CAMPANUS 

'lO S m 20 30 fC 50 60 70 
^ ' 1 1 1 J ' 1 > 

Scale 



CHAP. X.] PEAKS AND MOUNTAIN RANGES. 141 

able range is the Alps, situated between the Caucasus and the 
crater Plato, and reproduced on Plate XIV. It contains some 
700 peaked mountains and. is remarkable for the immense valley, 
80 miles long and about five broad, that cuts it with seemingly 
artificial straightness ; and that, were it not for the flatness of its 
bottom, might set one speculating upon the probability of some 
extraneous body having rushed by the moon at an enormous 
velocity, gouging the surface tangentially at this point and cutting 
a channel through the impeding mass of mountains. There are 
other mountain ranges of less magnitude than the foregoing ; but 
those we have specified will sufiice to illustrate our suggestions 
concerning this class of features. 

We remark, too, that there is a prevailing tendency of the ranges 
just mentioned to present their loftiest constituents in abrupt 
terminal lines, facing nearly the same direction, the reverse of that 
towards w^hich they are carried by the moon's rotation ; and as 
they recede from the high terminal line, the mountains gradually 
fall off in height, so that in bulk the ranges present the " crag and 
tail " contour which individual hills upon the earth so frequently 
exhibit. 

Isolated peaks are found in small numbers upon the moon ; there 
are a few striking examples of them nevertheless, and these are 
chiefly situated in the mountainous region just alluded to. Several 
are seen to the east (right hand) of the Alpine range depicted on 
Plate XIV. The best known of these is Pico, which rises abruptly 
from a generally smooth plain to a height of 7000 feet. It may be 
recognized as the lower of the two long shadowing spots located 
almost centrally above the crater Plato in the illustration just 
mentioned. Above it, at an actual distance of 40 miles, there is 
another peak (unnamed), about 4000 feet high ; and away to the 
west, beyond the small crater joined by a hill-ridge to Plato, is a 
third pyramidal mountain nearly as high as Pico. 

It seems natural to regard the great mountain chains as agglo- 
merations of those peaks of which we have isolated examples in 



142 



THE MOON. 



[chap. X. 



Pico and its compeers, and thus to consider that the formation of a 
mountain chain has been a multiplication of the process that formed 
the single pyramid- shaped eminences. At first thought it might 
appear that the great mountain ranges were produced by bodily 
upthrustings of the crust of the moon by some subsurface convul- 
sions. But such an explanation could hardly hold in relation to 
the isolated peaks, for it is difficult, if not impossible, to conceive 




Fio. 36. 



that these abrupt mountains, almost resembling a sugarloaf in 
steepness, could have been protruded 6?i masse through a smooth 
region of the crust. On the contrary, it is quite consistent with 
probability to suppose that they were built up by a slow process 
somewhat analogous to that to which we have ascribed the piling 
of the central cones of the greater craters. We believe they may 
be regarded as true mountains of exudation, produced by the com- 
paratively gentle oozing of lava from a small orifice and its solidifi- 
cation around it ; the vent however remaining open and the summit 
or discharging orifice continually rising with the growth of the 



CHAP. X.] 



PEAKS AND MOUNTAIN RANGES. 



143 



mountain, as indicated in the annexed cut, Fig. 36. This process 
is well exemplified in the case of a water fountain playing during 
a severe frost ; the water as it falls around the lips of the orifice 
freezes into a hillock of ice, through the centre of which, however, 
a vent for the fluid is preserved. As the water trickles over the 
mound it is piled higher and higher hy accumulating layers of ice, 




Fia. 37. 



till at length a massive cone is formed whose height will be deter- 
mined by the force or " head " of the water. Substitute lava for 
water, and we have at once a formative process which may very 
fairly be considered as that which has given rise to the isolated 
mountains of the moon. 

There are upon the earth mountainous forms resembling the 
isolated peaks of the moon, and which have been explained by a 
similar theory to the above. We reproduce a figure of one observed 
by Dana at Hawaii (Fig. 37), and a sketch of another observed on 
the summit of the volcano of Bourbon (Fig. 38) ; we also repro- 
duce (Fig. 39) an ideal section of the latter, given by Mr. Scrope, 



144 THE MOON. [chap. x. 

and showing the successive layers of lava which would be disposed 




Fig. 38. 



by just such an action as that manifested in the case of the freezing 




Fig. 39. 



fountain ; and we quote that author's words in reference to this 
explanation of the formation of Etna and other volcanic mountains. 



PLATE XV i 




J.¥asrayth 



"Wbodbiirytype.' 



TYCHO AND ITS SURROUNDINGS 

TO 5 JO -20 30 iW SO 60 70 SO 



Scale 



CHAP. X.] PEAKS AND MOUNTAIN RANGES. 145 

" On examining," says Mr. Scrope,* " the structure of the moun- 
tain (Etna) we find its entire mass, so far as it is exposed to view 
by denudation or other causes (and one enormous cavity, the Yal de 
Bove penetrates deeply into its very heart), to be composed of beds 
of lava-rock alternating more or less irregularly with layers of scoriae, 
lapillo and ashes, almost precisely identical in mineral character, as 
well as in general disposition, with those erupted by the volcano at 
known dates within the historical period. Hence we are fully 
justified in believing the whole mountain to have been built up in 
the course of ages in a similar manner by repeated intermittent 
eruptions. And the argument applies by the rules of analogy to 
all other volcanic mountains, though the history of their recent 
eruptions may not be so well recorded, provided that their structure 
corresponds with, and can be fairly explained by, this mode of pro- 
duction. It is also further applicable, under the same reservation, 
to all mountains composed entirely, or for the most part, of volcanic 
rocks, even though they may not have been in eruption within our 
time." 

To these illustrations furnished from Scrope's work we add 
another, copied from a photograph by Professor Piazzi Smyth, of 
a " blowing cone " at the base of Teneriffe (Fig. 40), which is but 
one of many that are to be found on that mountain, and which has 
been formed by a process similar to that we have been considering, 
but acting upon a comparatively small scale. Professor Smyth 
describes this cone as about 70 feet high and of parabolic figure, 
composed of hard lava and with an upper aperture still yawning, 
*' whence the burning breath of fires beneath once issued in fury 
and with destruction." 

Reverting now to the moon, we remark that, if the foregoing 
explanation of the isolated lunar peaks be tenable, it should hold 
equally for the groups of them which we see in the lunar Apennines, 
Alps, Caucasus, and other ranges of like character. There occur 
in some places intermediate groups which link the one to the other. 

* " Volcanoes," page 155. 



146 



THE MOON. 



[chap. X. 



Just above the crater Archimedes, on Plate IX., for instance, we 
see several single peaks and small clumps of them leading by suc- 
cessive multiple-peak examples to what may be called chains of 
mountains like many that are included in the contiguous Apennine 




Fig. 40. 

SMALL VOLCANIC MOUNTAIN AT THE END OF A STREET AT TENERIFFE. 



system. And, in view of this connexion between the single peaks 
and the mountain ranges formed of aggregations of such peaks, it 
seems to us reasonable to conclude that the latter were formed by 
the comparatively slow escape of lava through multitudinous open- 
ings in a weak part of the moon's crust, rather than to suppose 
that the crust itself has been bodily upheaved and retained in its 
disturbed position. The high peaks that many mountains in such 



CHAP. X.] 



PEAKS AND MOUNTAIN RANGES. 



147 



a chain exhibit accord better with the former than the latter 
explanation ; for it is difficult to imagine how such lofty eminences 
could be erected by an upheaval, and we must remember that the 
moon has none of the denuding elements which are at work upon 
the earth, weather wearing its mountain forms into sharpness and 
steepness.* 

And we have ground for believing the mountain-forming process 
on the moon to have been a comparatively gentle one, in the fact 
that the mountain systems appear in regions otherwise little 
disturbed, and where craters, which have all the appearances of 
violent origin, are few and far between. Evidently the mountain 
and crater-forming processes, although both due to extrusive 
action, were in some measure difierent, and it is reasonable to 
suppose that the difference was in degree of intensity ; so that 
while a violent ejection of volcanic material would give rise to a 
crater, a more gradual discharge would pile up a mountain. In 

* In reference to such prominences on the lunar surface as cast steeple-like 
shadows, it is well to remark that we must not in all cases infer, from the acute 
spire-like form of the shadow, that the object which casts the shadow is of a 
similar sharp or spire-like form, which the first impression would naturally lead us 
to suppose. A comparatively blunt or rounded eminence will project a long and 
pointed shadow when the rays of light fall on the object at a low angle, and 
especially so when the shadow is projected on a convex surface. We illustrate this 
with a copy of an actual photograph of the shadow cast by half a pea, Fig. 41. 




Fia. 41. 



L 2 



148 THE MOON. [chap. x. 

this view craters are evidences of erujptive, and mountains of com- 
paratively gentle exudative action. 

We can hardly speculate with any degree of safety upon the 
cause of this varying intensity of volcanic discharge. We may 
ascribe it to variation of depth of the initial disturbing force, or to 
suddenness of its action ; or it may be that different degrees of 
fluidity of the lava have had modifying effects ; or on the other 
hand different qualities of the crust-material ; or yet again 
differences of period — the quieter extrusions having occurred at a 
time when the volcanic forces were dying down. There is an 
alliance between lunar craters and mountains that goes far to show 
that there has been no radical difference in their origins. 
For instance, as we have previously pointed out, craters in some 
cases run in linear groups, as if in those cases they had been 
formed along a line of disruption or of least resistance of the 
crust ; and the mountain chains have a corresponding linear 
arrangement. Then we see craters and mountain chains disposed 
in what seem obviously the same arcs of disturbance. Thus Coper- 
nicus (No. 147), Erastothenes (No. 168), and the Apennines appear 
to belong to one continuous line of eruption ; and it requires no 
great stretch of imagination to suppose that the Caucasus, Eudoxus 
(No. 208), and Aristotle (No. 209) form a continuation of the same 
line. Then around the Mare Serenetatis we see mountainous ridges 
and craters alternating one with the other as though the exuding 
action there, normally sufficient to produce the ridges, had at some 
points become forcible enough to produce a crater. Again, upon 
the very mountain ranges themselves, as for instance among the 
Apennines, we find small craters occurring. We see, too, that the 
great craters are in many cases surrounded by radiating systems of 
ridges which almost assume mountainous proportions, and which 
are doubtless exuded matter from " starred '* cracks, the centres of 
which are occupied by the craters. The same kind of ridges here 
and there occur apart from craters (see for instance Plate XX., 
below Aristarchus and Herodotus) and sometimes they occur in the 



CHAP.x.] PEAKS AND MOUNTAIN RANGES. 149 

neighbourhood of extensive cracks, to which they also seem allied. 
We must indeed regard a linear crack as the origin either of a 
ridge (if the exudation is slight) or of a mountain chain (if the 
exudation is more copious) or a string of craters (if the extrusion 
rises to eruptive violence). But the subject of cracks is important 
enough to be treated in a separate chapter. 

We alluded in Chap. III. to the phenomena of wrinkhng or 
puckering as productive of certain mountainous formations; and 
we pointed out the striking similarity in character of configuration 
between a shrivelled skin and a terrestrial mountain region. We 
do not perceive upon the moon such a decided coincidence of 
appearances extending over any considerable portion of her surface ; 
but there are numerous limited areas where we behold mountainous 
ridges which partake strongly of the wrinkle character ; and in 
some cases it is difficult to decide whether the puckering agency or 
the exudative agency just discussed has produced the ridges. The 
district bordering upon Aristarchus and Herodotus, above referred 
to, is of this doubtful character ; and a similar district is that con- 
tiguous to Triesnecker (Plate XI.). There are, however, abundant 
examples of less prominent lines of elevation, which may, with 
more probability, be ascribed to a veritable wrinkling or puckering 
action ; they are found over nearly the whole lunar surface, some 
of them standing out in considerable relief, and some merely 
showing gentle lines of elevation, or giving the surface an 
undulating appearance. A close examination of our picture-map 
(Plate V.) will reveal very numerous examples, especially in the 
south-east (right-hand-upper) quadrant. Some of these lines of 
tumescence are so slightly prominent that we may suppose them 
to have been caused by the action indicated by Fig. 6 (p. 32), 
while others, from their greater boldness, appear to indicate a 
formative action analogous to that represented by Fig. 9 (p. 33). 



CHAPTER XI. 

CRACKS AND RADIATING STREAKS. 

We have hitherto confined our attention to those reactions of the 
moon's molten interior upon its exterior which have been accom- 
panied by considerable extrusions of sub-surface material in its 
molten or semi- solid condition. We now pass to the consideration 
of some phenomena resulting in part from that reaction and in part 
from other effects of cooling, which have been accompanied by 
comparatively little ejection or upflow of molten matter, and in 
some cases by none at all. Of such the most conspicuous examples 
are those bright streaks that are seen, under certain conditions of 
illumination, to radiate in various directions from single craters, 
and some of the individual radial branches of which extend from 
four to seven hundred miles in a great arc on the moon's surface. 

There are several prominent examples of these bright streak 
systems upon the visible hemisphere of the moon ; the focal craters 
of the most conspicuous are Tycho, Copernicus, Kepler, Aristarchus, 
Menelaus, and Proclus. Generally these focal craters have 
ramparts and interiors distinguished by the same peculiar bright or 
highly reflective material which shows itself with such remarkable 
brilliance, especially at full moon : under other conditions of illu- 
mination they are not so strikingly visible. At or nearly full moon 
the streaks are seen to traverse over plains, mountains, craters, and 
all asperities; holding their way totally disregardful of every 
object that happens to lay in their course. 

The most remarkable bright streak system is that diverging 



PLATE XVH 




' ^^)c J.'b-orytypc 



GLASS GLOBE 
CRACKED BY INTERNAL PRESSURE' 
ILLUSTRATING THE CAUSE OF THE BRIGHT STREAKS 
RADIATING FROM TYCHO. 



PLATE X!X. 



-i^ 




''VVbodburytype" 



FULL MOON 

EXHIBIT! r:G THE BRIGHT STREAKS 
RADIATING FROM TYCHO. 




^ 



CHAP. XI.] CRACKS AND RADIATING 8TREAK8. 151 

from the great crater Tycho. The streaks that can be easily 
individualized in this group number more than one hundred, while 
the courses of some of them may be traced through upwards of six 
hundred miles from their centre of divergence. Those around 
Copernicus, although less remarkable in regard to their extent than 
those diverging from Tycho, are nevertheless in many respects well 
deserving of careful examination : they are so numerous as utterly 
to defy attempts to count them, while their intricate reticulation 
renders any endeavour to delineate their arrangement equally 
hopeless. 

The fact that these bright streaks are invariably found diverg- 
ing from a crater, impressively indicates a close relationship or 
community of origin between the two phenomena : they are 
obviously the result of one and the same causative action. It is 
no less clear that the actuating cause or prime agency must have 
been very deep-seated and of enormous disruptive power to have 
operated over such vast areas as those through which many of the 
streaks extend. With a view to illustrate experimentally what we 
conceive to have been the nature of this actuating cause, we have 
taken a glass globe, and, having filled it with water and hermetically 
sealed it, have plunged it into a warm bath : the enclosed water, 
expanding at a greater rate than the glass, exerts a disruptive force 
on the interior surface of the latter, the consequence being that at 
the point of least resistance, the globe is rent by a vast number of 
cracks diverging in every direction from the focus of disruption. 
The result is such a strikingly similar counterpart of the diverging 
bright streak systems which we see proceeding from Tycho and the 
other lunar craters before referred to, that it is impossible to resist 
the conclusion that the disruptive action which originated them 
operated in the same manner as in the case of our experimental 
illustration ; the disruptive force in the case of the moon being that 
to which we have frequently referred as due to the expansion 
which precedes the solidification of molten substances of volcanic 
character. 



152 THE MOON, [chap. xi. 

On Plate XVITE. we present a photograph from one of many glass 
glohes which we have cracked in the manner described : a careful 
comparison between the arrangement of divergent cracks displayed 




FiQ. 42. 

ILLUSTRATIVE OF THE RADIATING CRACKS WHICH PRECEDE THE FORMATION OF THE 

BRIGHT STREAKS. 



by this photograph with the streaks seen spreading from Tycho 
upon the contiguous lunar photograph -will, we trust, justify us in 
what we have stated as to the similarity of the causes which have 
produced such identical results. 



CHAP. XI.] CRACKS AND RADIATING STREAKS. 153 

The accompanying figures will further illustrate our views upon 
the causative origin of the hright streaks. The primary action 
rent the solid crust of the moon and produced a system of radiating 




Fia. 43. 

ILLUSTRATIVE OP THE RADIATING BRIGHT STREAKS. 

fissures (Fig. 42) : these immediately afforded egress for the molten 
matter heneath to make its appearance on the surface simul- 
taneously along the entire course of every crack, and irrespective of 
all surface inequalities or irregularities whatever (Fig. 43). We 
conceive that the upflowing matter spread in hoth directions side- 



154 THE MOON. [chap. xi. 

ways, and in this manner produced streaks of very much greater 
width than the cracks or fissures up through which it made its way 
to the surface. 

In further elucidation of this part of our suhject we may refer to 
a familiar hut as we conceive cogent illustration of an analogous 
action in the behaviour of water beneath the ice of a frozen pond, 
which, on being fractured by some concentrated pressure, or by a 
blow, is well known to " star " into radiating or diverging cracks, 
up through which the water immediately issues, making its 
appearance on the surface of the ice simultaneously along the entire 
course of every crack, and on reaching the surface, spreading on 
both sides to a width much exceeding that of the crack itself. 

If this familiar illustration be duly considered, we doubt not it 
will be found to throw considerable light on the nature of those 
actions which have resulted in the bright streaks on the moon's 
surface. Some have attempted to explain the cause of these bright 
streaks by assigning them to streams of lava, issuing from the 
crater at the centre of their divergence and flowing over the surface, 
but we consider such an explanation totally untenable, as any idea 
of lava, be it ever so fluid at its first issue from its source, flowing 
in streams of nearly equal width, through courses several hundred 
miles long, up hills, over mountains, and across plains, appears to 
us beyond all rational probability. 

It may be objected to our explanation of the formation of these 
bright streaks, that so far as our means of observation avail us, we 
fail to detect any shadows from them or from such marginal edges 
as might be expected to result from a side-way spreading outflow of 
lava from the cracks which afforded it exit in the manner described. 
Were the edges of these streaks terminated by cliff-like or craggy 
margins of such height as 30 or 40 feet, we might just be able at 
low angles of illumination and under the most favourable circum- 
stances of vision, to detect some slight appearance of shadows ; but 
so far as we are aware, no such shadows have been observed. We 
are led to suppose that the impossibility of detecting them is due not 



PLATE XX. 






^i; 



43 



'¥' 













• ) 



0,:^v*>M;r^v. 



'.IJasmyth. 




' Woodburytype '.' 



Vv'ARGENTIN , 

JO 5 O 10 20 30 40 SO 60 70 SO 

lA 1 LES 

Scale • 



(l XJKIVEBSITY » 



CHAP. XI.] CRACKS AND RADIATING STREAKS. 155 

to their absence but to the height of the margins being so moderate 
as not to cast any cognizable shadow, inasmuch as an abrupt craggy 
margin of 10 or 15 feet high would, under even the most favour- 
able circumstances, fail to render such visible to us. Reference to 
our ideal section of one of these bright streaks (Fig. 45), will shew 
how thin their edges may be in relation to their spreading width. 

The absence of cognizable shadows from the bright streaks has 
led some observers to conclude that they have no elevation above 
the surface over which they traverse, and it has therefore been 
suggested that their existence is due to possible vapours which may 
have issued through the cracks, and condensed in some sublimated 
or pulverulent form along their courses, the condensed vapours in 
question forming a surface of high reflective properties. That 
metallic or mineral substances of some kinds do deposit on conden- 
sation very white powders, or sublimates, we are quite ready to 
admit, and such explanation of the high luminosity of the bright 
streaks, and of the craters situated at the foci or centres of their 
divergence is by no means improbable, so far as concerns their mere 
brightness. But as we invariably find a crater occupying the 
centre of divergence, and such craters are possessed of all the 
characteristic features and details which establish their true 
volcanic nature as the results of energetic extrusions of lava and 
scoria, we cannot resist the conclusion that the material of the 
crater, and that of the bright streaks diverging from it, are not 
only of a common origin, but are so far identical that the only 
difference in the structure of the one as compared with the other 
is due to the more copious egress of the extruded or erupted 
matter in the case of the crater, while the restricted outflow or 
ejection of the matter up through the cracks would cause its 
dispersion to be so comparatively gentle as to flood the sides of the 
cracks and spread in a thin sheet more or less sideways simul- 
taneously along their courses. There are indeed evidences in the 
wider of the bright streaks of their being the result of the outflow 
of lava through systems of cracks running parallel to each other, 



166 THE MOON. [chap. xi. 

the confluence of the lava issuing from which would naturally yield 
the appearance of one streak of great width. Some of those 
diverging from Tycho are of this class ; many other examples 
might be cited, among which we may name the wide streaks 
proceeding from the crater Menelaus and also those from Proclus. 
Some of these occupy widths upwards of 25 miles — amply sufficient 
to admit of many concurrent cracks with confluent lava outflows. 

We are disposed to consider as related to the forementioned 
radiating streaks, the numerous, we may say the multitudinous, 
long and narrow chasms that have been sometimes called " canals" 
or '* rills," but which are so obviously cracks or chasms, that it is 
desirable that this name should be applied to them rather than one 
which may mislead by implying an aqueous theory of formation. 
These cracks, singly and in groups, are found in great numbers in 
many parts of the moon's surface. As a few of the more con- 
spicuous examples which our plates exhibit we may refer to the 
remarkable group west of Triesnecker (Plate XI.), the principal 
members of which converge to or cross at a small crater, and thus 
point to a continuity of causation therewith analogous to the evident 
relation between the bright streaks and their focal craters. Less 
remarkable, but no less interesting, are those individual examples 
that appear in the region north of (below) the Apennines (Plate IX.), 
and some of which by their parallelism of direction with the 
mountain-chain appear to point to a causative relation also. There 
is one long specimen, and several shorter in the immediate 
neighbourhood of Mercator and Campanus (Plate XV.) ; and 
another curious system of them, presenting suggestive contortions, 
occurs in connection with the mountains Aristarchus and 
Herodotus (Plate XXI.). Others, again, appear to be identified 
with the radial excrescences about Copernicus (Plate VIII.). 
Capuanus, Agrippa, and Gassendi, among other craters, have more 
or less notable cracks in their vicinities. 

Some of these chasms are conspicuous enough to be seen with 
moderate telescopic means, and from this maximum degree of 



CHAP. XI.] CRACKS AND RADIATING STREAKS. 157 

visibility there are all grades downwards to those that require the 
highest optical powers and the best circumstances for their detection. 
The earlier selenographers detected but a few of them. Schroeter 
noted only 11 ; Lohrman recorded 75 more ; Beer and Maedler added 
55 to the list, while Schmidt of Athens raised the known number 
to 425, of which he has published a descriptive catalogue. We take 
it that this increase of successive discoveries has been due to the 
progressive perfection of telescopes, or, perhaps, to increased 
education, so to speak, of the eye, since Schmidt's telescope is a 
much smaller instrument than that used by Beer and Maedler, and 
is regarded by its owner as an inferior one for its size. We doubt 
not that there are hundreds more of these cracks which more 
perfect instruments and still sharper eyes will bring to knowledge 
in the future. 

While these chasms have all lengths from 150 miles (which is 
about the extent of those near Triesnecker) down to a few miles, they 
appear to have a less variable breadth, since we do not find many 
that at their maximum openings exceed two miles across ; about a 
mile or less is their usual width throughout the greater part of 
their length, and generally they taper off to invisibility at their 
extremities, where they do not encounter and terminate at a crater 
or other asperity, which is, however, sometimes the case. Of their 
depth we can form no precise estimate, though from the sharpness 
of their edges w^ may conclude that their sides approach perpen- 
dicularity, and, therefore, that their depth is very great ; we have 
elsewhere suggested ten miles as a possible profundity. In a few 
cases, and under very favourable circumstances, we have observed 
their generally black interiors to be interrupted here and there with 
bright spots suggestive of fragments from the sides of the cracks 
having fallen into the opening. 

In seeking an explanation of these cracks, two possible causes 
suggest themselves. One is the expansion of sub-surface matter, 
already suggested as explanatory of the bright streaks ; the other, 
a contraction of the crust by cooling. We doubt not that both 



158 



THE MOON. 



[chap. XI. 



causes have been at work, one perhaps enhancing the other. Where, 
as in the cases we have pointed out, there are cracks which are so 
connected with craters as to imply relationship, we may conclude 
that an upheaving or expansive force in the sublunar molten 
matter has given rise to the cracks, and that the central craters 
have been formed simultaneously, by the release, with ejective 
violence, of the matter from its confining crust. The nature of the 
expansive force being assumed that of solidifying matter, the wide 




J.N. 

Fig. 44. 



extent of some chasms indicates a deep location of that force. And 
depth in this matter implies lateness (in the scale of selenological 
time) of operation, since the central portions of the globe would be 
the last to cool. Now, we have evidence of comparative lateness 
afforded by the fact that in many cases the cracks have passed 
through craters and other asperities which thus obviously existed 
before the cracking commenced ; and thus, so far, the hypothesis of 
the expansion-cracking is supported by absolute fact. 

It may be objected that such an upheaving force as we are 
invoking, being transitory, would allow the distended surface to 
collapse again when it ceased to operate, and so close the cracks or 
chasms it produced. But we consider it not improbable that in 
some cases, as a consequence of the expansion of sub- surface 



CHAP. XI.] 



CRACKS AND RADIATING STREAKS. 



159 



matter, an upflow thereof may have partially filled the crack, and by 
solidifying have held it open ; and it is rational to suppose that 
there have been various degrees of filling and even of overflow — that 
in some cases the rising matter has not nearly reached the edge of 
the crack, as in Fig. 44, while in others it has risen almost to the 




Fig. 45. 



surface, and in some instances has actually overrun it and produced 
some sort of elevation along the line of the crack, like that repre- 
sented sectionally in Fig. 45. It is probable that some of the 
slightly tumescent lines on the moon's surface have been thus 
produced. 

We have suggested shrinkage as a possible explanation of some 
cracks. It could hardly have been the direct cause of those com- 
pound ones which are distinguished by focal craters, though it may 
have been a co-operative cause, since the contracting tendency of 
any area of the crust, by so to speak weakening it, may have 



160 THE MOON. [chap. xi. 

virtually increased the strength of an upheaving force and thus 
have aided and localized its action. We see, however, no reason 
why the inevitable ultimate contraction which must have attended 
the cooling of the moon's crust, even when all internal reactions 
upon it had ceased, should not have created a class of cracks with- 
out accompanying craters, while it would doubtless have a tendency 
to increase the length and width of those already existing from any 
other cause. Some of the more minute clefts, which presumably 
exist in greater numbers than w^e yet know of, may doubtless be 
ascribed to this effect of cooling contraction. In this view we should 
have to regard such cracks as the latest of all lunar features. 
Whether the agency that produced them is still at work — whether 
the cracks are on the increase — is a question impossible of solution : 
for reasons to be presently adduced, we incline to believe that all 
cosmical heat passed from the moon, and therefore that it arrived at 
its present, and apparently final, condition ages upon ages ago. 

Besides the ridges spoken of on p. 157, and regarded as cracks 
up through which matter has been extruded, there are numerous 
ridges of greater or less extent, which we conceive are of the 
nature of wrinkles, and have been produced by tangential com- 
pression due to the collapse of the moon's crust upon the 
shrunken interior, as explained and illustrated in Chap. III. 
The distinguishing feature of the two classes of phenomena we 
consider to be the presence of a serrated summit in those of 
the extruded class, while those produced by '' wrinkling" action 
have their summits comparatively free from serration or marked 
irregularity. 



PLATE XXI. 










J.lSIasirLytlx 



' WoocLbiiry-cype" 



ARISTARCHUS & HERODOTUS 

70 £ 10 20 30 *0 SO 



Scale 



CHAPTER XII. 

COLOUR AND BRIGHTNESS OF LUNAR DETAILS : CHRONOLOGY OF 
FORMATIONS, AND FINALITY OF EXISTING FEATURES. 

Speaking generally, the details of the lunar surface seem to us 
to be devoid of colour. To the naked eye of ordinary sensitiveness 
the moon appears to possess a silvery whiteness : more critical 
judges of colour would describe it as presenting a yellowish tinge. 
Sir John Herschel, during his sojourn at the Cape of Good Hope, 
had frequent opportunities of comparing the moon's lustre with 
that of the weathered sandstone surface of Table Mountain, when 
the moon was setting behind it, and both were illuminated under 
the same direction of sunlight ; and he remarked that the moon 
was at such times " scarcely distinguishable from the rock in 
apparent contact with it." Although his observations had reference 
chiefly to brightness, it can hardly be doubted that similarity of 
colour is also implied ; for any diiference in the tint of the two 
objects would have precluded the use of the words " scarcely dis- 
tinguishable ; " a difference of colour interfering with a comparison 
of lustre in such an observation, though it must be remembered 
that he observed through a dense stratum of atmosphere. Viewed 
in the telescope, the same general yellowish-white colour prevails 
over all the moon, with a few exceptions offered by the so-called 
seas. The Mare Crisium, Mare Serenetatis, and Mare Humorum 
have somewhat of a greenish tint; the Palus Somnii and the 
circular area of Lichtenberg incline to ruddiness. These tints are, 
however, extremely faint, and it has been suggested by Arago that 



162 THE MOON. [chap. xii. 

they may be mere effects of contrast rather than actual colouration 
of the surface material. This, however, can hardly be the case, 
since all the " seas " are not alike affected; those that are slightly 
coloured are, as we have said, some green and some red, and con- 
trast could scarcely produce such variations. The supposition of 
vegetation covering these great flats and giving them a local colour 
is in our view still more untenable, in the face of the arguments 
that we shall presently adduce against the possibility of vegetable 
life existing upon the moon. 

^^It appears to us more rational to consider the tints due to actual 
colour of the material (presumably lava or some once fluid mineral 
substance) that has covered these areas ; and it may well be con- 
ceived that the variety of tint is due to different characters of 
material, or even various conditions of the same material coming 
from different depths below the lunar surface ; and we may 
reasonably suppose that the same variously-coloured substances 
occur in the rougher regions of the lunar surface, but that they 
exist there in patches too small to be recognized by us, or are 
** put out " by the brightness to which polyhedral reflexion gives 
rise. 

Seeing that volanic action has had so large a share in giving to 
the moon's surface its structural character, analogy of the most 
legitimate order justifies us in concluding not only that the 
materials of that surface are of kindred nature to those of the un- 
questionably volcanic portions of the earth, but also that the tints 
and colours that characterize terrestrial volcanic and Plutonian 
products have their counterparts on the moon. Those who have 
seen the interior and surroundings of a terrestrial volcano after a 
recent eruption, and before atmospheric agents have exercised their 
dimming influences, must have been struck with the colours of the 
erupted materials themselves and the varied brilliant tints conferred 
on these materials by the sublimated vapours of metals and mineral 
substances which have been deposited upon them. If, then, 
analogy is any guide in enabling us to infer the appearance of the 



CHAP. XII.] CHRONOLOGY OF FORMATIONS. 163 

invisible from that which we know to be of kindred nature and 
which we have seen, we may justly conclude that were the moon 
brought sufficiently near to us to exhibit the minute characteristics 
of its surface, we should behold the same bright and varied colours 
in and around its craters that we behold in and about those of the 
earth ; and in all probability the coloured materials of lunar 
volcanoes w^ould be more fresh and vivid than those of the earth 
by reason of the absence of those atmospheric elements which 
tend so rapidly to impair the brightness of coloured surfaces 
exposed to their influence. 

Situated as we are, however, as regards distance from the moon, 
we have no chance of perceiving these local colours in their smaller 
masses ; but it is by no means improbable, as we have suggested, 
that the faint tints exhibited by the great plains are due to broad 
expanses of coloured volcanic material. 

But if we fail to perceive diversity of colour upon the lunar 
surface, we are in a very different position in regard to diversity of 
brightness or variable light-reflective power of difi'erent districts 
and details. This will be tolerably obvious to those casual ob- 
servers who have remarked nothing more of the moon's physio- 
graphy than the resemblance to a somewhat lugubrious human 
countenance which the full moon exhibits, and which is due to the 
accidental disposition of certain large and small areas of surface 
material which have less of the light-reflecting property than 
other portions ; for since all parts seen by a terrestrial observer 
may be said to be equally shone upon by the sun, it is clear that 
apparently bright and shaded parts must be produced by differences 
in the nature of the surface as regards power of reflecting the light 
received. 

When we turn to the telescope and survey the full disc of the 
moon with even a very moderate amount of optical aid, the meagre 
impression as to variety of degree of brightness which the 
unassisted eye conveys is vastly extended and enhanced, for the 
surface is seen to be diversified by shades of brilliancy and dulness 

M 2 



164 THE MOON. [chap. xii. 

from almost glittering white to sombre grey : and this variety of 
shading is rendered much more striking by shielding the eye with 
a dusky glass from the excessive glare, which drowns the details in 
a flood of light. Under these circumstances the varieties of light 
and shade become almost bewildering, and defy the power of brush 
or pencil to reproduce them. 

We may, however, realize an imperfect idea of this characteristic 
of the lunar surface by reference to the self- drawn portrait of the 
full moon upon Plate IV. This is, in fact, a photograph taken 
from the full moon itself, and enlarged sufficiently to render 
conspicuous the spots and large and small regions that are 
strildngly bright in comparison with what may in this place be 
described as the " ground " of the disc. As an example of a wide 
and irregularly extensive district of highly reflective material, the 
region of which Tycho is the central object, is very remarkable. 
We may refer also to the bright ** splashes " of which Copernicus 
and Kepler are the centres. So brilliant are these spots that they 
can easily be detected by the unassisted eye about the time of full 
moon. Still brighter but less conspicuous by its size is the crater 
Aristarchus, which shines with specular brightness, and almost 
induces the belief that its interior is composed of some vitreous- 
surfaced matter : the highly-reflective nature of this object has 
often caused it to become conspicuous when in the dark hemisphere 
of the moon, unilluminated by the sun, and lighted only by the 
light reflected from the earth. At these times it appears so bright 
that it has been taken for a volcano in actual eruption, and no 
small amount of popular misconception at one time arose therefrom 
concerning the conditions of the moon as respects existing volcanic 
activity — a misconception that still clings to the mind of many. 

The parts of the surface distinguished by deficiency of reflecting 
power are conspicuous enough. We may cite, however, as an 
example of a detailed portion especially remarkable for its dingy 
aspect, the interior of the crater Plato, which is one of the darkest 
spots (the darkest well-defined one) upon the hemisphere of the 



OHAP. XII.] CHRONOLOGY OF FORMATIONS. 165 

moon visible to us. For facilitating reference to shades of 
luminosity, Schroeter and Lohrman assorted the variously reflective 
parts into 10 grades, commencing with the darkest. Grades 1 to 3 
comprised the various deep greys ; 4 and 5 the light greys ; 6 and 
7 white ; and 8 to 10 brilliant white. The spots Grimaldi and 
Eiccioli came under class 1 of this notation ; Plato between 1 and 
2. The " seas " generally ranged from 2 to 3 ; the brightest 
mountainous portions mostly between degrees 4 and 6 ; the crater 
walls and the bright streaks came between these and the bright 
peaks, which fell under the 9th grade. The maximum brightness, 
the 10th grade, is instanced only in the case of Aristarchus and a 
point in Werner, though Proclus nearly approaches it, as do many 
bright spots, chiefly the sites of minute craters, which make their 
appearance at the time of full moon. 

In photographic pictures produced by the moon of itself there is 
always an apparent exaggeration in the relation of light to dark 
portions of the disc. The dusky parts look, upon the photograph, 
much darker than to the eye directed to the moon itself, whether 
assisted or not by optical appliances. It may be that the real 
cause of this discrepancy is that the eye fails to discover the actual 
difference upon the moon itself, being insensible to the higher 
degrees of brightness or not estimating them at their proper 
brilliance with respect to parts less bright. On the other hand, it 
is probable that the enhanced contrast in the photograph is due to 
some peculiar condition of the darker surface matter afl'ecting its 
power of reflecting the actinic constituent of the rays that fall 
upon it. 

The study of the varying brightness or reflective power of 
different regions and spots of the lunar disc leads us to the con- 
sideration of the relative antiquity of the surface features ; for it is 
hardly possible to regard these variations attentively without being 
impressed with the conviction that they have relation to some 
chronological order of formation. We cannot, in the first place, 
resist the conviction that the brightest features were the latest 



166 THE MOON. [chap. xii. 

formed ; this strikes us as evident on pvimd facie grounds ; but it 
becomes more clearly so when we remark that the bright forma- 
tions, as a rule, overlie the duller features. The elevated parts of 
the crust are brighter than the ** seas " and other areas ; and it is 
pretty clear that the former are newer than the latter, upon which 
they appear to be super-imposed, or through which they seem to 
have extruded.* The vast dusky plains are in every instance more 
or less sprinkled with spots and minute craters, and these last 
were obviously formed after the area that contains them. One is 
almost disposed to place the order of formations in the order of 
relative brightness, and so consider the dingiest parts the oldest 
and the brightest spots and craters the newest features, though, in 
the absence of an atmosphere competent to impair the reflective 
power of the surface materials, we are unable to justify this 
classification by suggesting a cause for such a deterioration by 
time as the hypothesis pre-supposes. 

As we have entered upon the question of relative age of the 
lunar features, we may remark that there are evidences of various 
epochs of formation of particular classes of details, irrespective of 
their condition in respect of brightness, or, as we may say, fresh- 
ness of material. As a rule, the large craters are older than the 
small ones. This is proved by the fact that a large object of this 
class is never seen to interfere with or overlap a small one. Those 
of nearly equal size are, however, seen to overlap one another as 
though -several eruptions of equal intensity had occurred from the 
same source at different points. This is strikingly instanced in 
the group of craters situated in the position 35 — 141 on our map, 
the order of formation of each of which is clearly apparent. The 
region about Tycho offers an inexhaustible field for study of these 
phenomena of over-lapping or interpolating craters, and it will be 



* We meet a difficulty in reconciling this idea with the partial craters of which 
we have a conspicuous example in Fracastorius, No. 78, of our Map, which seem to 
be partially sunk below the contiguous surface. This looks as though the crater- 
rim belonged to an older epoch than the plain from which it rises. 




OVERLAPPING CRATERS 



so 10 20 



50 60 70 



CHAP. XII.] CHRONOLOGY OF FORMATIONS. 167 

found, with very few exceptions, that the smaller crater is the 
impinging or parasitical one, and must therefore have been formed 
after the larger, upon which it intrudes or impinges. There are 
frequent cases in which a large crater has had its rampart inter- 
rupted by a lesser one, and this again has been broken into by one 
still smaller ; and instances may be found where a fourth crater 
smaller than all has intruded itself upon the previous intruder. 
The general tendency of these examples is to show that the craters 
diminished in size as the moon's volcanic energy subsided : that 
the largest were produced in the throes of its early violence, and 
that the smallest are the results of expiring efforts possibly 
impeded through the deep-seatedness of the ejective source. 

Another general fact of this chronological order is that the 
mountain chains are never seen to intrude upon formations of the 
crater order. We do not anywhere find that a mountain chain 
runs absolutely into or through a crater ; but, on the other hand, 
we do find that craters have formed on mountain chains. This 
leads unmistakably to the inference that the craters were not 
formed before their allied mountain chains ; and we might assume 
therefore that the mountains generally are the older formations, 
but that there is nothing to prove that the two classes of features, 
where they intermingle, as in the Apennines and Caucasus, were 
not erupted cotemporaneously. 

Upon the assumption that the latest ejected or extruded matter 
is that which is brightest, we should place the bright streaks 
among the more recent features. Be this as it may, it is tolerably 
certain that the cracks, whose apparently close relation to the 
radiating streaks we have endeavoured to point out, are relatively 
of a very late formative period. We are indeed disposed to 
consider them as the most recent features of all ; the evidence in 
support of this consideration being the fact that they are sometimes 
found intersecting small craters that, from the way in which they 
are cut through by the cracks, must have been in situ before the 
cracking agency came into operation. It is in accordance with our 



168 THE MOON. [chap. xii. 

hypothesis of the moon's transition from a fluid to a solid body to 
consider that a cracking of the surface would he the latest of all 
the phenomena produced by contraction in final cooling. 

The foregoing remarks naturally lead us to the question whether 
changes are still going on upon the surface of our satellite : whether 
there is still left in it a spark of its volcanic activity, or whether 
that activity has become totally extinct. We shall consider this 
question from the observational and theoretical point of view. 
First as regards observations. This much may be affirmed indis- 
putably — that no object or detail visible to the earliest seleno- 
graphers (whose period may be dated 200 years back) has altered 
from the date of their maps to the present. When we pass from 
the bolder features to the more minute details we find ourselves at 
a loss for materials for forming an inference ; the only map pre- 
tending to accuracy even of the larger among small objects being 
that of Beer and Maedler, which, truly admirable as it is, is not 
very safely to be relied upon for settling any question of alleged 
change, on account of the conventional system adopted for exhibit- 
ing the forms of objects, every object being mapped rather than 
drawn, and shown as it never is or can be presented to view on the 
moon itself. This difficulty would present itself if a question of 
change were ever raised upon the evidence of Beer and Maedler's 
map : it may indeed have prevented such a question being raised,, 
for certainly no one has hitherto been bold enough to assert that 
any portion or detail of the map fails to represent the actual state 
of the moon at the present time. 

In default of published maps, we are thrown for evidence on this 
question upon observations and recollections of individual observers 
whose familiarity with the lunar details extends over lengthy periods. 
Speaking for ourselves, and upon the strength of close scrutinies 
continued with assiduity through the past thirty years, we may say 
that we have never had the suspicion suggested to our eye of any 
actual change whatever having taken place in any feature or minute 
detail of the lunar surface; and our scrutinies have throughout 



CHAP. XII.] CHRONOLOGY OF FORMATIONS. 169 

been made with ample optical means, mostly with a 20-inch 
reflector. This experience has made us not unnaturally in some 
slight decree sceptical concerning the changes alleged to have been 
detected by others. Those asserted by Schroeter and Gruithuisen 
were long ago rejected by Beer and Maedler, who explained them, 
where the accuracy of the observer was not questioned, by varia- 
tions of illumination, a cause of illusory change which is not 
always sufficiently taken into account. A notable instance of this 
deception occurred a few years ago in the case of the minute bright 
crater Linnet which was for a considerable period declared, upon 
the strength of observations of very promiscuous character, to be 
varying in form and dimensions almost daily, but the alleged 
constant changes of which have since been tacitly regarded as due 
to varying circumstances of illumination induced by combinations 
of libratory effects with the ordinary changes depending upon the 
direction of the sun's rays as due to the age of the moon. This 
explanation does not, however, dispose of the question whether the 
crater under notice suffered any actual change before the hue and 
cry was raised concerning it. Attention was first directed to it by 
Schmidt, of Athens, whose powers of observation are known to be 
remarkable, and whose labours upon the moon are of such extent 
and minuteness as to claim for his assertions the most respectful 
consideration.* He affirmed in 1866 that the crater at that date 
presented an appearance decidedly different from that which it had 
had since 1841 : that whereas it had been from the earlier epoch 
always easily seen as a very deep crater, in October, 1866, and 
thenceforward it presented only a white spot, with at most but a 
very shallow aperture, very difficult to be detected. Schmidt is one 
of the very few observers whose long familiarity with the moon 

* We are informed by a friend, who has lately visited Athens, that Schmidt's 
detail drawings of the Moon, comprising the work of forty years, form a small 
library in themselves. The map embodying them is so large (6 ft. 6 in. in diameter) 
and so full of detail that there is small hope of its complete publication, unless 
there should be such a wide extension of interest in the minute study of our satellite 
as to justify the cost of reproducing it. 



170 



THE MOON, 



[chap. XII. 



entitles him to speak with confidence upon such a question as that 
before us upon the sole strength of his own experience ; and this 
case is but an isolated one, at least it is the only one he has 
brought forward. He is, however, still firmly convinced that it is 
an instance of actual change, and not an illusion resulting from 




Fig. 46. 



some peculiar condition of illumination of the object. It should be 
added also on this side of the discussion than an English observer, 
the Kev. T. W. Webb, while apparently indisposed to concede the 
supposition of any notable changes in the lunar features, has yet 
found from his own observations that, after all due allowance for 
differences of light and shade upon objects at different times, there 
is still a "residuum of minute variations not thus disposed of" 
which seem to indicate that eruptive action in the moon has not 
yet entirely died out, though its manifestation at present is very 
limited in extent. It appears to us that, if evidence of continuing 
volcanic action is to be sought on the moon, the place to look for it 



CHAP. XII.} CHRONOLOGY OF FORMATIONS, 171 

is around the circumference of the disc, where eruptions from any 
marginal orifice would manifest itself in the form of a protruding 
haziness, somewhat as illustrated to an exaggerated extent in the 
annexed cut (Fig. 46). 

The theoretical view of the question, which we have now to 
consider, has led us, however, to the strong belief that no vestige 
of its former volcanic activity lingers in the moon — that it assumed 
its final condition an inconceivable number of ages ago, and that 
the high interest which would attach to the close scrutiny of our 
satellite if it ivere still the theatre of volcanic reaction cannot be 
hoped for. If it be just and allowable to assume that the earth and 
the moon were condensed into planetary form at nearly the same 
epoch (and the only rational scheme of cosmogony justifies the 
assumption) then we may institute a comparison between the con- 
dition of the two bodies as respects their volcanic age, using the 
one as a basis for inference concerning the state of the other. We 
have reason to believe that the earth's crust has nearly assumed its 
final state so far as volcanic reactions of its interior upon its 
exterior are concerned : we may affirm that within the historical 
period no igneous convulsions of any considerable magnitude have 
occurred ; and we may consider that the volcanoes now active over 
the surface of the globe represent the last expiring efforts of its 
eruptive force. Now in the earth we perceive several conditions 
wherefrom we may infer that it parted with its cosmical heat (and 
therefore with its prime source of volcanic agency) at a rate which 
will appear relatively very slow when we come to compare the like 
conditions in the moon. We may, we think, take for granted that 
the surface of a planetary body generally determines its heat dis- 
persing power, while its volume determines its heat retaining 
power. Given two spherical bodies of similar material but of 
unequal magnitude and originally possessing the same degree of 
heat, the smaller body will cool more rapidly than the larger, by 
reason of the greater proportion which the surface of the smaller 
sphere bears to its volume than that of the larger sphere to its 



172 THE MOON. [chap. xii. 

volume — this proportion depending upon the geometrical ratio 
which the surfaces of spheres hear to their volumes, the contents of 
spheres heing as the cubes and the surfaces as the squares oi their 
diameters. The volume of the earth is 49 times as great as that 
of the moon, hut its surface is only 13 times as great; there is 
consequently in the earth a power of retaining its cosmical heat 
nearly four times as great as in the case of the moon ; in other 
words, the moon and earth heing supposed at one time to have had 
an equally high temperature, the moon would cool down to a given 
low temperature in ahout one -fourth the time that the earth would 
require to cool to the same temperature. But the earth's cosmical 
heat has without doubt been considerably conserved by its vaporous 
atmosphere, and still more by the ocean in its antecedent vaporous 
form. Yet notwithstanding all this, the earth's surface has nearly 
assumed its final condition so far as volcanic agencies are con- 
cerned : it has so far cooled as to be subject to no considerable dis- 
tortions or disruptions of its surface. What then must be the 
state of the moon, which, from its small volume and large propor- 
tionate area, parted with its heat at the above comparatively rapid 
rate ? The matter of the moon is, too, less dense than the 
earth, and from this cause doubtless disposed to more rapid 
cooling ; and it has no atmosphere or vaporous envelope to retard 
its radiating heat. We are driven thus to the conclusion that the 
moon's loss of cosmical heat must have been so rapid as to have 
allowed its surface to assume its final conformation ages on ages 
ago, and hence that it is unreasonable and hopeless to look for 
evidence of change of any volcanic character still going on. 

We conceive it possible, however, that minute changes of a non- 
volcanic character may be proceeding in the moon, arising from the 
violent alternations of temperature to which the surface is exposed 
during a lunar day and night. The sun, as we know, pours down 
its heat unintermittingly for a period of fully 300 hours upon the 
lunar surface, and the experimental investigations of Lord Kosse, 
essentially confirmed by those of the French observer, Marie Davy, 



CHAP. XII.] CHRONOLOGY OF FORMATIONS. 173 

show that under this powerful insolation the surface becomes heated 
to a degree which is estimated at about 500° of Fahrenheit's scale, 
the fusing point of tin or bismuth. This heat, however, is entirely 
radiated away during the equally long lunar night, and, as Sir John 
Herschel surmised, the surface probably cools down again to a 
temperature as low as that of interstellar space : this has been 
assumed as representing the absolute zero of temperature which has 
been calculated from experiments to be 250° below the zero of 
Fahrenheit's scale. Now such a severe range of heat and cold can 
hardly be without effect upon some of the component materials of 
the lunar surface.* If there be any such materials as the vitreous 
lavas that are found about our volcanoes, such as obsidian for in- 
stance, they are doubtless cracked and shivered by these extreme 
transitions of temperature ; and this comparatively rapid succession 
of changes continued through long ages would, we may suppose, 
result in a disintegration of some parts of the surface and at length 
somewhat modify the selenographic contour. It is, however, 
possible that the surface matter is mainly composed of more 
crystalline and porous lavas, and these might withstand the fierce 
extremes like the " fire-brick " of mundane manufacture, to which 
in molecular structure they may be considered comparable. Lavas 
as a rule are (upon the earth) of this unvitreous nature, and if they 
are of like constitution on the moon, there will be little reason to 
suspect changes from the cause we are considering. Where, how- 
ever, the material, whatever its nature, is piled in more or less 
detached masses, there will doubtless be a grating and fracturing 
at the points of contact of one mass with another, produced by 
alternate expansions and contractions of the entire masses, which in 
the long run of ages must bring about dislocations or dislodgments 
of matter that might considerably affect the surface features from 
a close point of view, but which can hardly be of sufficient magni- 



* It is conceivable that the alleged changes in the crater Linne may have 
been caused by a filling of the crater by some such crumbling action as we are 
here contemplating. 



174 THE MOON. [chap. xii. 

tude to be detected by a terrestrial observer whose best aids to 
vision give him no perception of minute configurations. And it 
must always be borne in mind that changes can only be proved by 
reference to previous observations and delineations of unquestion- 
able accuracy. 

Speaking by our own lights, from our own experience and 
reasoning, we are disposed to conclude that in all visible aspects 
the lunar surface is unchangeable, that in fact it arrived at its 
terminal condition ceons of ages ago, and that in the survey of its 
wonderful features, even in the smallest details, we are presented 
with the sight of objects of such transcendent antiquity as to render 
the oldest geological features of the earth modern by comparison. 



CHAPTER XII L 

THE MOOK AS A WORLD: DAY AND NIGHT UPON ITS SURFACE. 

A WIDE interest, if not a deep one, attaches to the general ques- 
tion as to the existence of living beings, or at least the possibility 
of organic existence, on planetary bodies other than our own. The 
question has been examined in all ages, by the lights of the science 
peculiar to each. With every important accession to our astrono- 
mical knowledge it has been re-raised : every considerable discovery 
has given rise to some new step or phase in the discussion, and in 
this way there has grown up a somewhat extensive literature ex- 
clusively relating to mundane plurality. It will readily be under- 
stood that the moon, from its proximity to the earth, has from the 
first received a large, perhaps the largest, share of attention from 
wanderers in this field of speculation : and we might add greatly to 
the bulk of this volume by merely reviewing some of the more 
curious and, in their way, instructive conjectures specially relating 
to the moon as a world — to imaginary journeys towards her, and to 
the beings conjectured to dwell upon and within her. This, how- 
ever, we feel there is no occasion to do, for it is our purpose merely 
to point out the two or three almost conclusive arguments against 
the possibility of any life, animal or vegetable, having existence on 
our satellite. 

We well know what are the requisite conditions of life on the 
earth ; and we can go no further for grounds of inference ; for if w^e 
were to start by assuming forms of life capable of existence under 
conditions widely and essentially different from those pertaining to 



1^6 THE MOON. [chap. XIII. 

our planet, there would be no need for discussing our subject 
further : we could revel in conjectures, without a thought as to 
their extravagance. The only legitimate phase of the question we 
can entertain is this : — can there be on the moon any kind 
of living things analogous to any kind of living things upon 
the earth? And this question, we think, admits only of a 
negative answer. The lowest forms of vitality cannot exist 
without air, moisture, and a moderate range of temperature. It 
may be true, as recent experiments seem to show, that organic 
germs will retain their vitality without either of the first, and with 
exposure to intense cold and to a considerable degree of heat ; and 
it is conceivable that the mere germs of life may be present on the 
moon.* But this is not the case with living organisms themselves. 
We have, in Chapter Y., specially devoted to the subject, cited the 
evidence from which we know that there can be at the most, no 
more air on the moon than is left in the receiver of an air-pump 
after the ordinary process of exhaustion. And with regard to 
moisture, it could not exist in any but the vaporous state, and we 
knew that no appreciable amount of vapour can be discovered by 
any observation (and some of them are crucial enough) that we are 
capable of making. We may suppose it just within the verge of 
possibility that some low forms of vegetation might exist upon the 
moon with a paucity of air and moisture such as would be beyond 
even our most severe powers of detection : but granting even this, 
we are met by the temperature diifficulty ; for it is inconceivable 
that any plant-life could survive exposure first to a degree of cold 
vastly surpassing that of our arctic regions, and then in a short time 

* Is it not conceivable that the protogerms of life pervade the whole universe, 
and have been located upon every planetary body therein ? Sir William Thomson's 
suggestion that life came to the earth upon a seed-bearing meteor was weak, in so 
far that it shifted the locus of life-generation from one planetary body to another. 
Is it not more philosophical, more consistent with our conception of Creative omni- 
potence and impartiality, to suppose that the protogerms of life have been sown 
broadcast over all space, and that they have fallen here upon a planet under con- 
ditions favourable to their development, and have sprung into vitality when the 
fit circumstances have arrived, and there upon a planet that is, and that may be for 
ever, unfitted for their vivification. 




< 
or 
o 

cc: 

13 



O 

22: 



CHAP. XIII.] THE MOON AS A WORLD. Ill 

(14 days) to a degree of heat capable of melting the more fusible 
metals — the total range being equal, as we have elsewhere shown, 
to perhaps 600 or 700 degrees of our thermometric scale. 

The higher forms of vegetation could not reasonably be expected 
to exist under conditions which the lower forms could not survive. 
And as regards the possibility of the existence of animal life in any 
form or condition on the lunar surface, the reasons we have adduced 
in reference to the non-existence of vegetable life bear still more 
strongly against the possibility of the existence of the former. We 
know of no animal that could live in what may be considered a 
vacuum and under such thermal conditions as we have indicated. 

As to man, aeronautic experiences teaches us that human life is 
endangered when the atmosphere is still sufficiently dense to support 
12 inches of mercury in the barometer tube ; what then would be 
his condition in a medium only sufficiently dense to sustain one- 
tenth of an inch of the barometric column? We have evidence 
from the most delicate tests that no atmosphere or vapour 
approaching even this degree of attenuation exists around the 
moon's surface. 

Taking all these adverse conditions into consideration, we are in 
every respect justified in concluding that there is no possibility of 
animal or vegetable life existing on the moon, and that our satellite 
must therefore be regarded as a barren world. 

******* 

After this disquisition upon lunar uninhabitability it may appear 
somewhat inconsistent for us to attempt a description of the scenery 
of the moon and some other effects that would be visible to a spec- 
tator, and of which he would be otherwise sensible, during a day 
and a night upon her surface. But we can offer the sufficient 
apology that an imaginary sojourn of one complete lunar day and 
night upon the moon affords an opportunity of marshalling before 
our readers some phenomena that are proper to be noticed in a 
work of this character, and that have necessarily been passed over 
in the series of chapters on consecutive and special points that have 



178 THE MOON. [chap. xiii. 

gone before. It may be urged that, in depicting tbe moon from 
such a standpoint as that now to be taken, we are describing scenes 
that never have been such in the literal sense of the word, since no 
eye has ever beheld them. Still we have this justification — that we 
are invoking the conception of things that actually exist ; and that 
we are not, like some imaginary voyagers to the moon, indulging 
in mere flights of fancy. Although it is impossible for a habitant 
of this earth fully to realize existence upon the moon, it is yet 
possible, indeed almost inevitable, for a thoughtful telescopist — 
watching the moon night after night, observing the sun rise upon a 
lunar scene, and noting the course of eiBfects that follow till it sets 
— it is almost inevitable, we say, for such an observer to identify 
himself so far with the object of his scrutiny, as sometimes to be- 
come in thought a lunar being. Seated in silence and in solitude 
at a powerful telescope, abstracted from terrestrial influences, and 
gazing upon the revealed details of some strikingly characteristic 
region of the moon, it requires but a small effort of the imagination 
to suppose one's self actually upon the lunar globe, viewing some 
distant landscape thereupon ; and under these circumstances there 
is an irresistible tendency in the mind to pass beyond the actually 
visible, and to fill in with what it knows must exist those accessory 
features and phenomena that are only hidden from us by distance 
and by our peculiar point of view. Where the material eye is 
baffled, the clairvoyance of reason and analogy comes to its aid. 

Let us then endeavour to realize the strange consequences which 
the position and conditiqns of the moon produce upon the aspect 
of a lunar landscape in the course of a lunar day and night. 

The moon's day is a long one. From the time that the sun rises 
upon a scene* till it sets, a period of 304 hours elapses, and of 
course double this interval passes between one sunrise and the 
next. The consequences of this slow march of the sun begin to 

* Our remarks have general reference to a region of the moon near her equator j 
near the poles some of the conditions we shall describe would be somewhat 
modified. 



CHAP. XIII.] THE MOON AB A WORLD. 179 

show themselves from the instant that he rises above the lunar 
horizon. Dawn, as we have it on earth, can have no counterpart 
upon the moon. No atmosphere is there to reflect the solar beams 
while the luminary is yet out of actual sight, and only the glimmer 
of the zodiacal light heralds the approach of day. From the black 
horizon the sun suddenly darts his bright untempered beams upon 
the mountain tops, crowning them with dazzling brilliance Vv^hile 
their flanks and valleys are yet in utter darkness. There is no 
blending of the night into day. And yet there is a growth of 
illumination that in its early stages may be called a twilight, and 
which is caused by the slow rise of the sun. Upon the earth, in 
central latitudes, the average time occupied by the sun in rising, 
from the first glint of his upper edge till the whole disc is in sight, 
is but two minutes and a quarter. Upon the moon, however, this 
time is extended to a few minutes short of an hour, and therefore, 
during the first few minutes a dim light will be shed by the small 
visible chord of the solar disc, and this will give a proportionately 
modified degree of illumination upon the prominent portion of the 
landscape, and impart to it something of the weird aspect which so 
strikes an observer of a total solar eclipse on earth when the scene 
is lit by the thin crescent of the re -appearing sun. This impaired 
illumination constitutes the only dawn that a lunar spectator could 
behold. And it must be of short duration ; for when, in the course 
of half an hour, the solar disc has risen half into view the lighting 
would no doubt appear nearly as bright to the eye as when the 
entire disc of the sun is above the horizon. In this lunar sunrise, 
however, there is none of that gilding and glowing which makes the 
phenomenon on earth so gorgeous. Those crimson sky-tints with 
which we are familiar are due to the absorption of certain of the 
polychromous rays of light by our atmosphere. The blue and 
violet components of the solar beams are intercepted by our enve- 
lope of vapour, and only the red portions are free to pass ; while on 
the moon, as there is no atmosphere, this selective absorption does 
not occur. If it did, an observer gazing from the earth upon the 

N 2 



180 THE MOON. [chap. xiii. 

regions of the moon upon which the sun is just rising would see 
the surface tinted with rosy light. This, however, is not the case : 
the faintest lunar features just catching the sun are seen simply 
under white light diluted to a low degree of brightness. Only upon 
rare occasions is the lunar scenery suffused with coloured illumina- 
tion, and these are when, as we shall presently have to describe, 
the solar rays reach the moon after traversing the earth's atmos- 
phere during an eclipse of the sun. 

This atmosphere of ours is the most influential element in 
beautifying our terrestrial scenery, and the absence of such an 
appendage from the moon is the great modifying cause that affects 
lunar scenery as compared with that of the earth. We are 
accustomed to the sun with its dazzling brightness — overpowering 
though it be— subdued and softened by our vaporous screen. Upon 
the moon there is no such modification. The sun's intrinsic 
brilliancy is undiminished, its apparent distance is shortened, and 
it gleams out in fierce splendour only to be realized, and then 
imperfectly, by the conception of a gigantic electric light a few feet 
from the eye. And the brightness is rendered the more striking 
by the blackness of the surrounding sky. Since there is no atmo- 
sphere there can be no sky-light, for there is nothing above the 
lunar world to diffuse the solar beams ; not a trace of that moisture 
which even in our tropical skies scatters some of the sun's light 
and gives a certain degree of opacity or blueness, deep though it be, 
to the heavens by day. Upon the moon, with no light-difi'using 
vapour, the sky must be as dark or even darker than that with 
which we are familiar upon the finest of moonless nights. And 
this blackness prevails in the full blaze of the lunar noon-day sun. 
If the eye (upon the moon) could bear to gaze upon the solar orb 
(which would be less possible than upon earth) or could it be 
screened from the direct beams, as doubtless it could by intervening 
objects, it would perceive the nebulous and other appendages which 
we know as the corona, the zodiacal light, and the red solar pro- 
tuberances : or if these appendages could not be viewed with the 



CHAP. XIII.] THE MOON AS A WORLD 181 

sun above the horizon they would certainly be seen in glorious per- 
fection when the luminary was about to rise or immediately after 
it had set. 

And, notwithstanding the sun's presence, the planets and stars 
would be seen to shine more brilliantly than we see them on the 
clearest of nights ; the constellations would have the same configu- 
rations, though they would be differently situated with respect to 
the celestial pole about which they would appear to turn, for the 
axis of rotation of the moon is directed towards a point in the con- 
stellation Draco. The stars would never twinkle or change colour 
as they appear to us to do, for scintillation or twinkling is a 
phenomenon of atmospheric origin, and they would retain their full 
brightness, down even to the horizon, since there would be no haze 
to diminish their light. The planets, and the brighter stars at 
least, would be seen even when they were situated very near to the 
sun. The planet Mercury, so seldom detected by terrestrial gazers, 
would be almost constantly in view during the lunar day, manifest- 
ing his close attendance on the central luminary by making only 
short excursions of about two (lunar) days' length, first on one side 
and then on the other. Venus would be nearly as continuously 
visible, though her wanderings would be more extensive on either 
side. The zodiacal light also, which in our English latitude and 
climate is but rarely seen and in more favourable climes appears 
only when the sun itself is hidden beneath the horizon, would upon 
the moon be seen as a constant accompaniment to the luminary 
throughout his daily course across the lunar sky. The other 
planets would appear generally as they do to us on earth, but, never 
being lost in daylight, their courses among the stars could be traced 
with scarcely any interruption. 

One planet, however, that adorns the sky of the lunar hemisphere 
which is turned towards us deserves special mention from the con- 
spicuous and highly interesting appearance it must present. We 
allude to the earth. To nearly one-half of the moon (that which 
we never see) this imposing object can never be visible ; but to the 



182 THE MOON. [chap. xiii. 

half that faces us the terrestrial planet must appear almost fixed 
in the sky. A lunar spectator in (what is to us) the centre of the 
disc, or ahout the region north of the lunar mountains Ptolemy and 
Hipparchus, would have the earth in his zenith. From regions 
upon the moon a little out of what is to us the centre, a spectator 
would see the earth a little declining from the zenith, and this 
declination would increase as the regions corresponding to the (to 
us) apparent edge of the moon were approached, till at the actual 
edge it w^ould be seen only upon the horizon. From the phenomena 
of libration (explained in Chap. VI.) the earth would appear from 
nearly all parts of the lunar hemisphere to which it is visible at all 
to describe a small circle in the sky. To an observer, however, 
upon the (to us) marginal regions of the lunar globe, it would 
appear only during a portion of the lunar day — ^being visible in 
fact only in that part of its small circular path which happened to 
lie above the observer's horizon : in some regions only a portion of 
the terrestrial disc would make its brief appearance. From the 
lunar hemisphere beyond this marginal line the earth can never be 
seen at all. 

The lunar spectator whose situation enabled him to view the 
earth would see it as a moon ; and a glorious moon indeed it must 
be. Its diameter would be four times as great as that of the moon 
itself as seen by us, and the area of its full disc 13 times as great. 
It would be seen to pass through its phases, just as does our 
satellite, once in a lunar day or a terrestrial month, and during 
that cycle of phases, since 29 of our days would be occupied by it, 
the axial rotation would bring all the features of its surface 
configuration into view so many times in succession. But the 
greatest beauty of this noble moon would be seen during the lunar 
night, in considering which we shall again allude to it ; for when it 
is full-moon to the earth it is new-earth to the moon. At lunar 
midnight this globe of ours is fully illuminated ; as morning 
nears, the earth-moon wanes, its disc slowly passing through the 
gibbous phases until at sunrise it would be just half-illuminated. 



CHAP. XIII.] THE MOON AS A WORLD. 183 

During the long forenoon it assumes a crescent which narrows and 
narrows till at midday the sun is in line with the earth and the 
latter is invisible, save perhaps by a thin line of light marking its 
upper or lower edge, accordingly as the sun is apparently above or 
below it. In the lunar afternoon an illuminated crescent appears 
upon the opposite side of the terrestrial globe, and this widens and 
widens till it becomes a half disc by lunar sunset and a full disc by 
lunar midnight. 

The sun in his daily course passes at various distances, some- 
times above and sometimes below, the nearly stationary earth. 
Obviously it will at times pass actually behind it, and then the 
lunar spectator would behold the sublime spectacle of a total solar 
eclipse, and that under circumstances which render the phenomenon 
far more imposing than its counterpart can appear from the earth ; 
for whereas, when we see the moon eclipse the sun, the nearly 
similar (apparent) diameters of the two bodies render the duration 
of totality extremely short — at most 7 minutes — a lunar spectator, 
the earth appearing to him four times the diameter of the sun, and 
he and the earth being relatively stationary, would enjoy a view of 
the totality extending over several hours. During the passage of 
the solar disc behind that of the earth, a beautiful succession of 
luminous phenomena would be observed to follow from the refrac- 
tions and dispersions which the sunbeams would suffer in passing 
tangentially through those parts of our atmospheric envelope 
which lie in their course ; those, for instance, on the margin of the 
earth, as seen from the moon. As the sun passed behind the 
earth, the latter would be encircled upon the in-gomg side wdth a 
beautiful line of golden light, deepening in places to glowing 
crimson, due to the absorption, already spoken of, of all but the 
red and orange rays of the sun's light by the vapours of our 
atmosphere. As the eclipse proceeded and totality came on, this 
ruddy glow would extend itself nearly, if not all, around the black 
earth, and so bright would it be, that the whole lunar landscape 
covered by the earth's shadow would be illuminated with faint 



184 THE MOON. [chap. xiii. 

crimson light,* save, perhaps, in some parts of the far distance, 
upon which the earth had not yet cast its shadow, or off which the 
shadow had passed. Although the crimson light would prepon- 
derate, it would not appear bright and red alike all around the 
earth's periphery. The circle of light would be, in fact, the ring 
of twilight round our globe, and it would only appear red in those 
places where the atmosphere chanced to be in that condition favour- 
able for producing what on earth we know as red sunset and sunrise. 
We know that the sun, even in clear sky, does not always set and 
rise with the beautiful red glow, which may be determined by 
merely local causes, and will therefore vary in different parts of the 
earth. Now a lunar spectator watching the sun eclipsed by the 
earth, would see, during totality and at a coup d'oeil, every point 
around our world upon which the sun is setting on one side and 
rising upon the other. To every part of the earth around what is 
then the margin, as seen from the moon, the sun is upon the 
horizon, shining through a great thickness of atmosphere, reddening 
it, and being reddened by it wherever the vaporous conditions 
conduce to that colouration. And at all parts where these con- 
ditions obtain, the lunar eclipse-observer would see the ring of light 
around the black earth -globe brilliantly crimsoned ; at other parts 
it would have other shades of red and yellow, and the whole effect 
would be to make the grand earth -ball, hanging in the lunar sky, 
like a dark sphere in a circle of glittering gold and rubies. 

During the early stages of the eclipse, this chaplet of brilliant- 
coloured lights would be brightest upon the side of the clisap' 
pearing sun ; at the time of central eclipse the radiance (supposing 
the sun to pass centrally behind the earth) would be equally 
distributed, and during the later stages it would preponderate upon 



* We see this reddening during an eclipse of the moon (when the event we are 
describing — an eclipse of the sun visible from the moon — really takes place). The 
blood-red colour has often struck observei's very forcibly, and it has indeed been 
suggested that the appearance may be the innocent and oft-repeated fulfilment of 
the prophetic allusion to the moon being " turned into blood." 




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ll; 



CHAP. XIII.] THE MOON A8 A WORLD, 185 

the side of the reappearing sun. We have endeavoured to give a 
pictorial realization of this phenomenon and of the effect of the 
eclipse upon the lunar landscape, hut such a picture cannot hut 
fall very, very far short of the reality. (See Plate XXIV.) 

And now for a time let us turn attention from the lunar sky to 
the scenery of the lunar landscape. Let us, in imagination, take 
our stand high upon the eastern side of the rampart of one of the 
great craters. Height, it must he remarked, is more essential on 
the moon to command extent of view than upon the earth, for on 
account of the comparative smallness of the lunar sphere the dip of 
the horizon is very rapid. Such height, however, would he attained 
without great exercise of muscular power, since equal amounts of 
climbing energy would, from the smallness of lunar gravity, take a 
man six times as high on the moon as on the earth. Let us choose, 
for instance, the hill-side of Copernicus. The day begins by a 
sudden transition. The faint looming of objects under the united 
illumination of the half-full earth, and the zodiacal light is the 
lunar precursor of daybreak. Suddenly the highest mountain peaks 
receive the direct rays of a portion of the sun's disc as it emerges 
from below the horizon. The brilliant lighting of these summits 
serves but to increase, by contrast, the prevailing darkness, for they 
seem to float like islands of light in a sea of gloom. At a rate of 
motion twenty-eight times slower than we are accustomed to, the 
light tardily creeps down the mountain-sides, and in the course of 
about twelve hours the whole of the circular rampart of the great 
crater below us, and towards the east, shines out in brilliant light, 
unsoftened by a trace of mountain- mist. But on the opposite side, 
looking into the crater, nothing but blackness is to be seen. As 
hour succeeds hour, the sunbeams reach peak after peak of the 
circular rampart in slow succession, till at length the circle is com- 
plete and the vast crater-rim, 50 miles in diameter, glistens like a 
silver-margined abyss of darkness. By-and-by, in the centre, 
appears a group of bright peaks or bosses. These are the now 



186 THE MOON. [chap. xiii. 

illuminated summits of the central cones, and the development of 
the great mountain cluster they form henceforth becomes an 
imposing feature of the scene. From our high standpoint, and 
looking backwards to the sunny side of our cosmorama, we glance 
over a vast region of the wildest volcanic desolation. Craters from 
five miles diameter downwards crowd together in countless numbers, 
so that the surface, as far as the eye can reach, looks veritably 
frothed over with them. Nearer the base of the rampart on which 
we stand, extensive mountain chains run to north and to south, 
casting long shadows towards us; and away to southward run 
several gi'eat chasms a mile wide and of appalling blackness and 
depth. Nearer still, almost beneath us, crag rises on crag and 
precipice upon precipice, mingled with craters and yawning pits, 
towering pinnacles of rock and piles of scoriae and volcanic debris. 
But we behold no sign of existing or vestige of past organic life. 
No heaths or mosses soften the sharp edges and hard surfaces : no 
tints of cryptogamous or lichenous vegetation give a complexion of 
life to the hard fire-worn countenance of the scene. The whole 
landscape, as far as the eye can reach, is a realization of a fearful 
dream of desolation and lifelessness — not a dream of death, for 
that implies evidence of pre-existing life, but a vision of a world 
upon which the light of life has never dawned. 

Looking again, after some hours' interval, into the great crateral 
amphitheatre, we see that the rays of the morning sun have crept 
down the distant side of the rampart, opposite to that on which we 
stand, and lighted up its vast landslipped terraces into a series of 
seeming hill-circles with all the rude and rugged features of a 
terrestrial mountain view, and none of the beauties save those of 
desolate grandeur. The plateau of the crater is half in shadow 
10,000 feet below, with its grand group of cones, now fully in 
sight, rising from its centre. Although these last are twenty 
miles away and the base of the opposite rampart fully double that 
distance, we have no means of judging their remoteness, for in the 
absence of an atmosphere there can be no aerial perspective, and 



CHAP. XIII.] THE MOON AS A WORLD. 187 

distant objects appear as brilliant and distinct as those which are 
close to the observer. Not the brightness only, but the various 
colours also of the distant objects are preserved in their full 
intensity ; for colour we may fairly assume there must be. 
Mineral chlorates and sublimates will give vivid tints to certain 
parts of the landscape surface, and there must be all the more 
sombre colours which are common to mineral matters that have 
been subjected to fiery influence. All these tints will shine and 
glow with their greater or less intrinsic lustres, since they have not 
been deteriorated by atmospheric agencies, and far and near they 
will appear clear alike, since there is no aerial medium to veil them 
or tarnish their pristine brightness. 

Cin the lunar landscape, in the line of sight, there are no means 
of estimating distances ; only from an eminence, where the inter- 
vening ground can be seen, is it possible to realize magnitude in a 
lunar cosmorama and comprehend the dimensions of the objects it 
includes. 

And with no air there can be no diffusion of light. As a con- 
sequence, no illumination reaches those parts of the scene which 
do not receive the direct solar rays, save the feeble amount reflected 
from contiguous illuminated objects, and a small quantity shed by 
the crescent earth. The shadows have an awful blackness. As we 
stand upon our chosen point of observation, we see on the lighted 
side of the rampart almost dazzling brightness, while beneath us, 
on the side away from the sun, there is a region many miles in 
area impenetrable to the sight, for there is no object within it 
receiving sufficient light to render it discernible ; and all around 
us, far and near, there is the violent contrast between intense 
brightness of insulated parts and deep gloom of those in equally 
intense shadow. The black though starlit sky helps the violence 
of this contrast, for the bright mountains in the distance around 
us stand forth upon a background formed by the darkness of inter- 
planetary space. The visible effects of these conditions must be in 
every sense unearthly and truly terrible. ) The hard, harsh, glowing 



188 THE MOON. [chap. xiii. 

light and pitchy shadows ; the absence of all the conditions that 
give tenderness to an earthly landscape ; the black noonday sky, 
with the glaring sun ghastly in its brightness ; the entire absence 
of vestiges of any life save that of the long since expired volcanoes 
— all these conspire to make up a scene of dreary, desolate 
grandeur that is scarcely conceivable by an earthly habitant, and 
that the description we have attempted but insufficiently pourtrays. 
A legitimate extension of the imagination leads us to impressions 
of lunar conditions upon other senses than that of sight, to which 
we have hitherto confined our fancy. We are met at the outset 
with a difficulty in this extension ; for it is impossible to conceive 
the sensations which the absence of an atmosphere would produce 
upon the most important of our bodily functions. If we would 
attempt the task we must conjure up feelings of suffocation, of 
which the thoughts are, however, too horrible to be dwelt upon ; 
we must therefore maintain the delusion that we can exist without 
air, and attempt to realize some of the less discomforting effects of 
the absence of this medium. Most notable among these are the 
untempered heat of the direct solar rays, and the influence thereof 
upon the surface material upon which we suppose ourselves to 
stand. During a period of over three hundred hours the sun pours 
down his beams with unmitigated ferocity upon a soil never 
sheltered by a cloud or cooled by a shower, till that soil is heated, 
as we have shown, to a temperature equal nearly to that of melting 
lead ; and this scorching influence is felt by everything upon which 
the sun shines on the lunar globe.y But while regions directly 
isolated are thus heated, those parts turned from the sun w^ould 
remain intensely cold, and that scorching in sunshine and freezing 
in shade with which mountaineers on the earth are familiar would 
be experienced in a terribly exaggerated degree. Among the 
consequences, already alluded to, of the alternations of temperature 
to which the moon's crust is thus exposed, are doubtless more or 
less considerable expansions and contractions of the surface 
material, and we may conceive that a cracking and crumbling of 



CHAP. XIII.] THE MOON A8 A WORLD. 189 

the more brittle constituents would ensue, together with a grating 
of contiguous but disconnected masses, and an occasional dislocation 
of them. We refer again to these phenomena to remark that if an 
atmospheric medium existed they would be attended with noisy 
manifestations. fThere are abundant causes for grating and 
crackling sounds, and such are the only sources of noise upon the 
moon, where there is no life to raise a hum, no wind to murmur, 
no ocean to boom and foam, and no brook to plash. Yet even 
these crust- cracking commotions, though they might be felt by the 
vibrations of the ground, would not manifest themselves audibly, 
for without air there can be no communication between the grating 
or cracking body and the nerves of hearing. Dead silence reigns 
on the moon : a thousand cannons might be fired and a thousand 
drums beaten upon that airless world, but no sound could come 
from them : lips might quiver and tongues essay to speak, but no 
action of theirs could break the utter silence of the lunar scene. 

At a rate twenty-eight times slower than upon earth, the shadows 
shorten till the sun attains his meridian height, and then, from the 
tropical region upon which we have in imagination stood, nothing 
is to be seen on any side, save towards the black sky, but dazzling 
light. The relief of afternoon shadow comes but tardily, and the 
darkness drags its slow length along the valleys and creeps 
sluggishly up the mountain-sides till, in a hundred hours or more, 
the time of sunset approaches. This phenomenon is but daybreak 
reversed, and is unaccompanied by any of the gorgeous sky tints 
that make the kindred event so enrapturing on earth. The sun 
declines towards the dark horizon without losing one jot of its 
brilliancy, and darts the full intensity of its heat upon all it shines 
on to the last. Its disc touches the horizon, and in half an hour 
dips half-way beneath it, its intrinsic brightness and colour 
remaining unchanged. * The brief interval of twilight occurs, as in 
the morning, when only a small chord of the disc is visible, and 
the long shadows now sharpen as the area of light that casts them 
decreases. For a while the zodiacal light vies with the earth-moon 



190 THE MOON. [chap. xiii. 

high in the heavens in illuminating the scene ; but in a few hours 
this solar appendage passes out of view, and our world becomes the 
queen of the lunar night. 

At this sunset time the earth, nearly in the zenith of us, will be 
at its half-illuminated phase, and even then it will shed more light 
than we receive upon the brightest of moonlight nights. As the 
night proceeds, the earth-phase will increase through the gibbous 
stages until at midnight it will be " full," and our orb will be seen 
in its entire beauty. It will perform at least one of its twenty-four- 
hourly rotations during the time that it appears quite full, and the 
whole of its surface features will in that time pass before the lunar 
spectator's eye. At times the northern pole will be turned towards 
our view, at times the southern ; and its polar ice-caps will appear 
as bright white spots, marking its axis of rotation. If our lunar 
sojourn were prolonged we should observe the northern ice-caps 
creep downwards to lower latitudes (during our winter) and retreat 
again (during our summer) ; and this variation would be perceptible 
in a less degree at the southern pole, on account of the watery area 
surrounding it. The seas would appear (so far as can be inferred) 
of pale blue-green tint ; the continents parti- coloured : and the 
tinted spots would vary with the changing terrestrial seasons, as 
these are indicated by the positions and magnitudes of the polar 
ice-caps. The permanent markings would be ever undergoing 
apparent modification by the variations of the white cloud- belts 
that encircle the terrestrial sphere. Of the nature of these 
variations meteorological science is not as yet in a position to 
speak : it would indeed be vastly to the benefit of that science if 
a view of the distribution of clouds and vapours over the earth's 
surface, as comprehensive as that we are imagining, could really be 
obtained. 

It might happen at " full-earth," that a black spot with a fainter 
penumbral fringe would appear on one side of the illuminated disc 
and pass somewhat rapidly across it. This would occur when the 
moon passed exactly between the sun and the earth, and the 



CHAP. XIII.] THE MOON AS A WORLD. 191 

shadow of the moon was cast upon the terrestrial disc. We need 
hardly say that these shadow-transits would occur upon those 
astronomically important occasions when an eclipse of the sun is 
beheld from the earth. 

The other features of the sky during the long lunar night would 
not differ greatly from those to which we alluded in speaking of its 
day aspects. The stars would be the more brightly visible, from 
the greater power of the eye-pupil to open in the absence of the 
glaring sun, and on this account the milky-way would be very 
conspicuous and the brighter nebulse would come into view. The 
constellations would mark the night by their positions, or the hours 
might be told off (in periods of twenty-four each) by the successive 
reappearances of surface features on certain parts of the terrestrial 
disc. The planets in opposition to the sun would now be seen, 
and a comet might appear to vary the monotony of the long lunar 
night. But a meteor would never flash across the sky, though 
dark meteoric particles and masses would continually bombard the 
lunar surface, sometimes singly, sometimes in showers. And these 
would fall with a compound force due to their initial velocity added 
to that of the moon's attraction. As there is no atmosphere to 
consume the meteors by frictional heat or break by its resistance 
the velocity of their descent, they must strike the moon with a force 
to which that of a cannon-ball striking a target is feeble indeed. 
A position on the moon would be an unenviable stand-point from 
this cause alone. 

The lunar landscape by night needs little description : it would 
be lit by the earth-moon sufiiciently to allow salient features, even 
at a distance, to be easily made out, for its moon {i.e. the earth) 
has thirteen times the light-reflecting area that ours has. But the 
night illumination will change in intensity, since the earth-moon 
varies from half-full to full, and again to half-full, between sunset 
and the next sunrise. The direction of the light, and hence the 
positions of the shadows, will scarcely alter on account of the 
apparent fixity of the earth in the lunar sky. A slight degree of 



192 THE MOON. [chap. xiii. 

warmth might possibly be felt with the reflected earth-light ; but 
it would be insufficient to mollify the intensity of the prevailing 
cold. The heat accumulated by the ground during the three 
hundred hours' sunshine radiates rapidly into space, there being 
no atmospheric coat to retain it, and a cooling process ensues that 
goes on till, all warmth having rapidly departed, the previously 
parched soil assumes a temperature approaching that of celestial 
space itself, and which has been, as we have stated, estimated at 
between 200° and 250° below the Fahrenheit zero. If moisture 
existed upon the moon, its night-side would be bound in a grip of 
frost to which our Arctic regions would be comparatively tropical. 
But since there is no water, the aspect of the lunar scenery 
remains unmodified by effects of changing temperature. 

Such, then, are the most prominent effects that would manifest 
themselves to the visual and other senses of a being transported 
to the moon. The picture is not on the whole a pleasant one, 
but it is instructive ; and our rendering of it, imperfect though it 
be, may serve to suggest other inferences that cannot but add to 
the interest which always attaches to the contemplation of natural 
scenes and phenomena from points of view different from those 
which we ordinarily occupy. 



CHAPTER XIV. 

THE MOON AS A SATELLITE: ITS RELATION TO THE EARTH 

AND MAN. 

Apart from the recondite functions of the moon considered as 
one of the interdependent members of the solar family, into which 
it would be beyond our purpose to inquire, there are certain means 
by which it subserves human interests and ministers to the wants 
of civilized man to which we deem it desirable to call attention, 
especially as some of them are not so self-apparent as to have 
attracted popular attention. 

The most generally appreciated because the most evident of the 
uses of the moon is that of a luminary. Popular regard for it is 
usually confined to its service in that character, and in that 
character poets and painters have never tired in their efforts to 
glorify it. And obviously this service as a *' lesser light " is 
sufficiently prominent to excite our warmest admiration. But 
moonlight is, from the very conditions of its production, of such a 
changeable and fugitive nature, and it affords after all so partial 
and imperfect an alleviation of night's darkness, that we are fain 
to regard the light-giving office of the moon as one of secondary 
importance. Far more valuable to mankind in general, so estim- 
able as to lead us to place it foremost in our category of lunar 
offices, is the duty which the moon performs in the character of a 
sanitary agent. We can conceive no direful consequences that 
would follow from a withdrawal of the moon's mere light ; but it 
is easy to imagine what highly dangerous results would ensue if 



194 THE MOON. [chap. xiv. 

the moon ceased to produce the tides of the ocean. Motion and 
activity in the elements of the terraqueous globe appear to be 
among the prime conditions in creation. Rest and stagnation are 
fraught with mischief. While the sun keeps the atmosphere in 
constant and healthy circulation through the agency of the winds, 
the moon performs an analogous service to the waters of the sea 
and the rivers that flow into them. It is as the chief producer of 
the tides — for we must not forget that the sun exercises its tidal 
influences, though in much lesser degree — that we ought to place 
the highest value on the services of the moon : but for its aid as 
a mighty scavenger, our shores, where rivers terminate, would 
become stagnant deltas of fatal corruption. Twice (to speak 
generally) a day, however, the organic matter which rivers deposit 
in a decomposing state at their embouchures is swept away by the 
tidal wave ; and thus, thanks to the moon, a source of direful 
pestilence is prevented from arising. Rivers themselves are pro- 
videntially cleansed by the same means, where they are polluted 
by bordering towns and cities which, from the nature of things, 
are sure to arise on river banks ; and it seems to be also in the 
nature of things that the river traversing a city must become its 
main sewer. The foul additions may be carried do\\Ti by the 
stream in its natural course towards the ocean, but where the 
river is large there will be a decrease in velocity of the current 
near the mouth or where it joins the sea, thus causing partial 
stagnation and consequent deposition of the' deleterient matters. 
All this, however, is removed, and its inconceivable evils are 
averted by our mighty and ever active " sanitary commissioner," 
the moon. We can scarcely doubt that a healthy influence of less 
obvious degree is exerted in the wide ocean itself ; but, considering 
merely human interests, we cannot suppress the conviction that 
man is more widely and immediately benefited by this purifying 
office of the moon than by any other. 

But the sanitary service is not the only one that the moon 
performs through the agency of the tides. There is the work of 



CHAP. XIV.] THE MOON AS A SATELLITE. 195 

tidal transport to be considered. Upon tidal rivers and on certain 
coasts, notwithstanding wind and the use of steam, a very large 
proportion of the heavy merchandize is transported by that slow 
but powerful " tug " the flood-tide ; and a similar service, for 
which, however, the moon is not to be entirely credited, is done by 
the down-flow of the ebb-tide. Large ships and heavily-laden rafts 
and barges are quietly taken in tow by this unobtrusive prime 
mover, and moved from the river's mouth to the far -up city, and 
from wharf to wharf along its banks; and a vast amount of 
mechanical work is thus gratuitously performed which, if it had to 
be provided by artificial means, would represent an amount of 
money value which for such a city as London would have to be 
counted by thousands, possibly millions, of pounds yearly. For 
this service we owe the moon the gratitude that we ought to feel 
for a direct pecuniary benefactor. 

In the existing state of civilization and prosperity, we do not, 
however, utilize the power of the tides nearly to the extent of their 
capabilities. Our coal mines, rich with the ** light of other days" 
— for coal was long ago declared by Stevenson to be "bottled sun- 
shine " — at present furnish us with so abundant a supply of power- 
generating material that in our eagerness to use it upon all possible 
occasions we are losing sight, or putting out of mind, many other 
valuable prime movers, and amongst them that of the rise and fall 
of the waters, which can be immediately converted into any form of 
mechanical power by the aid of tide-mills. Such mills may be found 
in existence here and there, but for the present they are generally 
outrivalled by the steam engine with all its conveniences and adapta- 
bilities ; and hence they have not shared the benefits of that in- 
ventive ingenuity which has achieved such wonders of mechanical 
appliance while steam has been in the ascendant. But it must be 
remembered that in our extravagant use of coal we are drawing from 
a bank into which nothing is being paid. We are consuming an 
exhaustible store, and the time must come when it will be needful 
to look around in quest of ** powers that may be." Then an im- 

o 2 



196 THE MOON. [chap. xiv. 

petus may be given to the application of the tides to mechanical 
purposes as a prime mover.* For the people of the British Islands 
the problem would have an especial importance, viewing the extent 
of our seaboard and the number of our tidal rivers. The source of 
motion that offers itself is of almost incalculable extent. There is 
not merely the onward flowing motion of streams to be utilized, but 
also the lift of water, which, if small in extent, is stupendous in 
amount ; and within certain limits it matters little to the mecha- 
nician whether the ** foot-pounds" of work placed at his disposal 
are in the form of a great mass lifted to a small height or a small 
mass lifted to a great height. There is no reason either why the 
utilization of the tides should be confined to rivers. The sea-side 
might well become the circle of manufacturing industry, and the 
millions of tons of water lifted several feet twice daily on our shores 
might be converted, even by schemes already proposed, to furnish 
the prime movement of thousands of factories. And we must not 
forget how completely modern science has demonstrated the inter- 
convertibility of all kinds of force, and thus opened the way for the 
introduction of systems of transporting power that, in such a state 
of things as we are for the moment considering, might be of im- 
mense benefit. Gravity, for instance, can be converted into elec- 
tricity; and electricity gives us that wonderful power of trans- 
mitting force without transmitting (or even moving) matter, which 
power we use in the telegraph, where we generate a force at one end, 
of a wire and use it to ring bells or deflect needles at the other end, 
which may be thousands of miles away. What we do with the 
slight amount of force needful for telegraphy is capable of being 
done with any greater amount. A tide-mill might convert its 
mechanical energy by an electro-magnetic engine, and in the form 
of electricity its force could be conveyed inland by proper wires and 
there reconverted back to mechanical or moving power. True, 
there would be a considerable loss of power, but that power would 

♦ About 100 years ago London was supplied with water chiefly by pumps worked 
by tidal mills at London Bridge. 



CHAP. XIV.] THE MOON AS A SATELLITE. 197 

cost nothing for its first production. Another means ready to hand 
for transporting power is by compressed air, which has already done 
good service ; another is the system so admirably worked out by 
Sir W. Armstrong, of transmitting water-power through the agency 
of an " accumulator," now so generally used at our Docks and else- 
where for working cranes and such other uses. And as the whole 
duty of the engineer is to convert the forces of nature, there is a 
rich field open for his invention, and upon which he may one day 
have to enter, in adapting the pulling force of the moon to his 
fellow man's mechanical wants through the intermediation of the 
tides. 

Another of the high functions of the moon is that by which she 
subserves the wants of the navigator, and enables him to track his 
course over the pathless ocean. Of the two co-ordinates. Latitude 
and Longitude, that are needful to determine the position of a ship 
at sea (or of any standpoint upon the earth's surface) the first is 
easily found, inasmuch as it is always equal to the altitude of the 
celestial pole at the place of observation. But the determination of 
the longitude has always been a difficult problem, and one upon 
which a vast amount of ingenuity has been expended. When it was 
first attacked it was soon discovered that the moon was the object 
of all others by which it could be most accurately and, all things 
considered, most readily determined. We must premise that the 
longitude of one place from another is in eff'ect the difference 
between the local times at the two places, so that when we say that 
a place or a ship is, for instance, seven hours, twenty-four minutes, 
ten seconds, west of Greenwich, we mean that the time-o'-day at 
the place or ship is seven hours twenty-four minutes ten seconds 
earlier than that at Greenwich. Hence, finding the longitude at sea 
or at any place and moment means finding what time it is at Green- 
wich at that moment. Of course this could be most easily done if 
we could set a timekeeper at Greenwich and rely upon its keeping 
time during a long sea voyage ; and this plan appeared so feasible 
that our Government long ago offered a prize of ^620, 000 for a time- 



198 THE MOON. [chap. xiv. 

keeper which would perform to a stated degree of accuracy after a 
certain sea voyage. One John Harrison did make such a timekeeper, 
that actually satisfied the conditions, and obtained the prize : and 
chronometers are now largely used for longitude, their construction 
having been brought to great perfection, especially in England, 
owing to a continuance (in a less liberal degree, however) of 
Government inducement. But chronometers are not entirely to be 
relied on, even where several are carried, which in other than 
Government ships is rarely the case : recourse must be had to the 
heavenly bodies for check upon the timekeeper. And the moon is, 
as we have said, the body that best serves the requirements of the 
problem. 

The lunar method for longitude amounts practically to this. The 
stars are fixed ; the sun, moon, and planets move amongst them ; 
the sun and planets with very slow rates of apparent motion, the 
moon with a very rapid one. If, then, it be predicted that at a 
certain instant of Greenwich time the moon will be a certain dis- 
tance from a fixed star, and if the mariner at sea observes ivhen the 
moon has that exact distance, he will know the Greenwich time at 
the instant of his observation.* The moon thus becomes to him as 
the hand of a timepiece, whereof the stars are the hour and minute 
marks, the whole being, as it were, set to Greenwich time. The 
requisite predictions of the distance (as seen from the earth's 
centre) of the moon from convenient fixed stars, or from the sun, or 
any of the principal planets — whose calculated places are so 
accurate that they may for this purpose be used as fixed stars— are 
given to the utmost exactness in the navigators' vade mecum, the 
" Nautical Almanac," for every third hour, day and night, of 
Greenwich time (except for a few days near new-moon, when the 
moon cannot be seen) ; and from these given distances the navigator 



* The sun and planets are comparatively useless for this object, because of their 
slow movement among the stars ; the change of their positions from hour to hour 
is so small as to render uncertain the Greenwich times deducible therefrom. 
Their use would be comparable to taking the time from the hour-hand of a clock. 



CHAP. XIV.] THE MOON AS A SATELLITE. 199 

can, by a simple process of differencing, obtain the Greenwich time 
corresponding to the distance which he may have observed.* Then 
knowing, as he does by other observations easily obtained, the local 
or ship's time of his observation, he takes the difference between 
this and the corresponding Greenwich time, and this difference is 
his longitude from Greenwich. Of course the whole value of this 
method depends upon the exactitude of the predicted distances 
corresponding to the given Greenwich times. These distances are 
obtained by tables of the moon's motions, which must be found 
from observations. The motions in question are of an intricacy 
almost past comprehension, on account of the disturbing forces to 
which the moon is subjected by the sun and planets. The powers 
of the profoundest mathematicians, from Newton downwards, have 
been severely exercised in efforts to group them into a theory, and 
represent them by tables capable of furnishing the requisite exact 
predictions of lunar positions for nautical purposes. Accurate ob- 
servations of the moon's place night after night have, from the 
dawn of this lunar method for longitude, been in urgent request by 
mathematicians for the purposes specified, and it was solely to pro- 
cure these observations that the Observatory at Greenwich was 
established, and mainly for their continued prosecution (and for the 
stellar observations necessary for their utilization) that it is sus- 
tained. For two centuries the moon has been unremittingly 
observed at Greenwich, and the tables at present used for making 
the ** Nautical Almanac" (those formed by Prof. Hansen) depend 
upon the observations there obtained. The work still goes on, for 
even now the degree of exactitude is not what is desired, and 
astronomers are looking forward with some interest to new lunar 
tables which were left complete by the late M. Delaunay, formerly 
the head of astronomy in France, based upon a theory which he 
evolved. This use of the moon is the grandest of all in respect of 
the results to which it has led. 

* Certain corrections are necessary to clear his observed distance of the effects pf 
parallax and refraction ; upon these, however, we cannot enter here, 



200 THE MOON. [chap. xiv. 

Then, too, regarding the moon as a timekeeper, we must not 
forget the service that it renders in furnishing a division of time 
intermediate between the day — ^which is measured by the earth's 
rotation — and the year, which is defined by the earth's orbital 
revolution. Notwithstanding the survival of lunar reckoning in 
our religious services, we, in our time and country, scarcely need 
a moon to mark our months ; but we must not forget that with 
many ancient people the moon was, and with some is still, the 
chief timekeeper, the calendars of such people being lunar ones, 
and all their events being reckoned and dated by " moons." To 
us, however, the moon is of great service in this department by 
enabling us to fix dates to many historical events, the times of 
occurrence of which are uncertain, by reason of defective records or 
by dependence upon such uncertain data as *' lives of emperors," 
years of this or that king's reign, or generations of one or another 
family. The moon now and then clears up a mystery, or decides a 
disputed point in chronology, by furnishing the accurate date of an 
ancient eclipse, which was a phenomenon that always inspired awe 
and secured for itself careful record. The chronologer is continually 
applying to the astronomer for the date and place of visibility of 
some total eclipse, of which he has found an imperfect record, 
veritable as to the fact, but dated only by reference to some year 
of a so-and-so's reign, or by some battle or other historical 
occurrence. The eclipses that occurred near the time are then 
examined, and when one is found that tallies with recorded condi- 
tions in other respects (such as the time of day and the place of 
observation), its indisputable date becomes a starting-point from 
which the chronologer works backwards and forwards in safety. 
There is one famous eclipse — that predicted by Thales six centuries 
before Christ, which put an end to the battle between the Medes 
and Lydians by the terror its darkness created in both armies — 
which is most intimately associated with ancient chronology, and 
has been used to rectify a proximate date (the first year of Cyrus 
of Babylon) which forms the foundation of all Scripture chronology. 



CHAP. XIV.] THE MOON AS A SATELLITE. 201 

Sacred and profane history alike are continually receiving assist- 
ance from the accurate dates which the moon, by having caused 
eclipses of the sun, enables the astronomer to fix beyond cavil or 
doubt. 

The . mention of eclipses reminds us, too, of the use which the 
moon has been in increasing, through them, our knowledge of the 
physical condition of the sun. If the moon had never intervened 
to cut off the blinding glare of the solar disc, we should have been 
to this day left to assume that the sun is all-contained by the 
dazzling globe that we ordinarily see. But, thanks to the moon's 
intervention, we now know that the sun is by no means the mere 
naked sphere we should have suspected. Eclipses have taught us 
that it is surrounded by an envelope of glowing gases, and that it 
has a vast vaporous surrounding, beyond its glowing atmosphere, 
which appears to be composed of matter streaming away from the 
sun into surrounding space. With these discoveries still in their 
infancy, it is impossible to foresee the knowledge to which they 
will eventually lead, but they can hardly be barren of fruit, and 
whatever they ultimately teach will be so much insight gained into 
the sublimest problem that human science has before it — the deter- 
mination of the source and maintaining power of the light and heat 
and vivifying agency of the sun. In according our thankful reflec- 
tions to the moon for these revelations, we must not forget that, 
should there be inhabitants upon our neighbouring worlds. 
Mercury, Venus, and Mars, which have no satellites, they, the 
supposed inhabitants, can gain no such knowledge upon the 
surroundings of the ruler of the solar system. On the other 
hand, any rational being who may be supposed to dwell upon 
Saturn or Jupiter, would, through the intervention of their 
numerous moons, have, in the latter case especially, far more 
abundant opportunities of acquiring the knowledge in question 
than we have. 

Finally, there is a use of the moon which touches us, author and 
reader, very closely. It has taught us of a world in a condition 



202 THE MOON. [chap. xiv. 

totally different from our own ; of a planet without water, without 
air, without the essentials to life development, hut rather with the 
conditions for life destruction ; a planet left hy the Creator — for 
wise purposes that we cannot fully know — as it were hut half-formed, 
with all the igneous foundations fresh from the cosmical fire, and 
with its rough-cast surface in its original state, its fire and mould- 
marks exposed to our view. From these we have essayed to resolve 
some of the processes of formation, and thus to learn something of 
the cosmical agencies that are called forth in the purely igneous 
era of a planet's history. We trust that we, on our part, have 
shown that the study of the moon may he a benefit not merely to 
the astronomer, but to the geologist ; for we behold in it a mighty 
" medal of creation " doubtless formed of the same material and 
struck with the same die that moulded our earth ; but while the 
dust of countless ages and the action of powerful disintegrating and 
denuding elements have eroded and obliterated the earthly impres- 
sion, the superscriptions on the lunar surface have remained with 
their pristine clearness unsullied, every vestige sharp and bright as 
when it left the Almighty Maker's hands. The moon serves no 
second-rate or insignificant service when it teaches us of the variety 
of creative design in the worlds of our system, and exalts our esti- 
mation of this peopled globe of ours by showing us that all the 
planetary worlds have not been deemed worthy to become the 
habitations of intelligent beings. 



Keflections upon the uses of the moon not unnaturally lead our 
thoughts to some matters that may be regarded as abuses. These 
mainly take the form of superstitions, erroneous beliefs in the 
moon's influence over terrestrial conditions, and occasionally of 
erroneous ideas upon the moon's functions as a luminary. The 
first-mentioned are almost beneath notice, for they include such 
mythical suspicions as that the moon influences human sanity and 
other affections of mind and body; that the moon's rays have ft 



CHAP. XIV.] THE MOON AS A SATELLITE. 203 

decomposing effect upon organic matter ; that they produce blind- 
ness by shining upon a sleeper's eyes ; that the moon determines 
the hours of human death, which is supposed to occur with the 
change of the tide, etc. All such, having no foundation on fact, 
are put beyond our consideration. The third matter we have men- 
tioned may also be dismissed in a very few words. The erroneous 
ideas upon the moon's functions as a luminary, to which we allude, 
are those which are manifested by poets and painters, and even 
historians, who do not hesitate to bring the moon upon a scene in 
any form and at any time they please without reference to actual 
lunar circumstances. It is no uncommon thing to see, in a picture 
representing an evening scene, a moon introduced which can only 
be seen in the morning — a waning moon instead of a waxing one ; 
and astronomical critics have, indeed, caught artists so far tripping 
as to put a moon in a picture representing some event that occurred 
upon a date when the moon was new, and therefore invisible. 
Writers take the same liberties very frequently. A newspaper 
correspondent, during the Franco-Prussian war, described the full 
moon as shining upon a scene of desolation on a particular night, 
when really there was no moon to be seen. One of the most flagrant 
cases of this kind, however, occurs in Wolfe's ballad on " The death 
of Sir John Moore," where it is written that the hero was buried 
*' By the struggling moonbeam's misty light." But the interment 
actually took place at a time when the moon was out of sight. We 
mention these abuses of the moon in the hope of promoting a better 
observance of the moon's luminary office. They who wish to bring 
the moon upon a scene, not knowing ipso facto that it was there, 
should first take the advice of Nick Bottom in the " Midsummer 
Night's Dream," and make sure of their object by consulting an 
almanac. 

The second of the specified abuses to which the moon is subject 
refers to its supposed influence on the weather ; and in the extent 
to which it goes this is one of the most deeply rooted of popular 
errors. That there is an infinitesimal influence exerted by the 



204 THE MOON. [chap. xiv. 

moon on our atmosphere will be seen from the evidence we have 
to offer, but it is of a character and extent vastly different from 
what is commonly believed. The popular error is shown in its 
most absurd form when the mere aspect of the moon, the mere 
transition from one phase of illumination to another, is asserted 
to be productive of a change of weather ; as if the gradual passage 
from first quarter to second quarter, or from that to third, could of 
itself upset an existing condition of the atmosphere ; or as if the 
conjunction of the moon with the sun could invert the order of the 
winds, generate clouds, and pour down rains. A moment's reason- 
ing ought to show that the supposed cause and the observed effect 
have no necessary connection. In our climate the weather may be 
said to change at least every three days, and the moon changes — 
to retain the popular term — every seven days ; so that the proba- 
bility of a coincidence of these changes is very great indeed : when 
it occurs, the moon is sure to be credited with causing it. But a 
theory of this kind is of no use unless it can be shown to apply in 
every case ; and, moreover, the change must always be in the same 
direction ; to suppose that the moon can turn a fine day to a wet 
one, and a wet day to a fine morrow indiscriminately, is to make 
our satellite blow hot and cold with the same mouth, and so to 
reduce the supposition to an absurdity. If any marked connection 
existed between the state of the air and the aspect of the moon, it 
must inevitably have forced itself unsought upon the attention of 
meteorologists. In the weekly return of Births, Deaths, and 
Marriages, issued by the Registrar -General, a table is given, show- 
ing all the meteorological elements at Greenwich for every day of 
the year, and a column is set apart for noting the changes and 
positions of the moon. These reports extend backwards nearly a 
quarter of a century. Here, then, is a repertory of data that ought 
to reveal at a glance any such connection, and would certainly have 
done so had it existed. But no constant relation between the moon 
columns and those containing the instrument readings has ever 
been traced. Our meteorological observatories furnish continuous 



CHAP. XIV.] THE MOON AS A SATELLITE. 205 

and unbroken records of atmospheric variations, extending over 
long series of years : these afford still more abundant means for 
testing the validity of the lunar hypothesis. The collation has 
frequently been made for special points in the inquiry, and certainly 
some connection has been found to obtain between certain positions 
of the moon in her orbit and certain instrumental averages ; but so 
small are the effects traceable to lunar influence, that they are 
almost inappreciable among the grosser irregularities that arise 
from other and as yet unexplained causes. 

^ The lunar influences upon our atmosphere most likely to be 
detected are those of a tidal character, and those due to the radiation 
of the heat which the moon receives from the sun. The first would 
be shown by the barometer, which may be called an " atmospheric 
tide gauge." Some years ago Colonel Sir Edward Sabine instituted 
a series of observations at St. Helena, to determine the variations 
of barometric indications from hour to hour of the lunar day. 
The greatest differences were found to occur between the times 
when the moon was on the meridian, and when it was six hours 
away from the meridian ; in other words, between atmospheric 
high tide and low tide. But the average of these differences 
amounted only to the four-hundredth part of an inch on the instru- 
ment's scale; a quantity that no weather observer would heed, 
that none but the best barometers would show, and that can have 
no perceptible effect on weather changes. The distance of the 
moon from the earth varies, as is well known, in consequence of 
the elliptical form of her orbit : this variation ought also to 
produce an effect upon the instrument's indications ; but Colonel 
Sabine's analysis showed that it was next to insensible ; the mean 
reading at apogee differing from that at perigee by only the two- 
thousandth part of an inch. Schubler, a German meteorologist, 
had arrived at similarly negative results some years previously. 
Hence it appears that the great index of the weather is not 
sensibly affected by the state of the moon ; the conclusion to be 
drawn with regard to the weather itself is obvious enough. As 



206 THE MOON. [chap. xiv. 

regards the lieat received from the moon, we know, from the recent 
experiments of Lord Rosse in England, and Marie Davy in France, 
elsewhere alluded to, that a degree of warmth appreciable to the 
highly sensitive thermopile is exerted by the moon upon the earth 
near to the time of full moon, when the sun's rays have been 
pouring their unmitigated heat upon the lunar surface continuously 
for fourteen days. And as it is improbable that the whole of the 
heat sent earthwards from the moon reaches the earth's surface, 
we must infer that a considerable amount is absorbed in the higher 
atmosphere, and does work in evaporating the lighter clouds and 
thinning the denser ones. The effect of this upon the earth is to 
facilitate the radiation of its heat into space, and so to cool the 
lower atmospheric strata. And this effect has been shown to be a 
veritable one by an exhaustive tabulation of temperature records 
from various observatories, which was undertaken by Mr. Park 
Harrison. The general conclusion from these was, that the 
temperature at the earth's surface is lower by about 2J degrees at 
moon's last quarter than at first quarter ; the paradoxical result 
being what would naturally follow from the foregoing consideration. 
The tendency of the full moon to clear the sky has been remarked 
by several distinguished authorities, to wit, Sir John Herschel, 
Humboldt, and Arago ; and in general the clearing may be accepted 
as a meteorological fact, though in one case of close examination it 
has been negatived. It cannot be doubted that a full moon some- 
times shows a night to be clear that would in the absence of the 
moon be called cloudy. 

When close comparisons are made between the moon's positions 
and records of rain-fall and wind-direction, dim indications of 
relation exhibit themselves, which may be the feeble consequences 
of the change of temperature just spoken of; but in every case 
where an effect has been traced it has been of the most insignifi- 
cant kind, and no apparent connexion has been recognized between 
one effect and another. Certainly there is nothing that can support 
the extensive popular belief in lunar influence on weather, and 



CHAP. XIV.] THE MOON AS A SATELLITE. 207 

nothing that can modify the conviction that this belief as at 
present maintained is an absurd delusion. Yet its acceptance is so 
general, and runs through such varied grades of society, that we 
have felt it our duty to dwell upon it to the extent that we have 
done. 



CHAPTER XV. 

CONCLUDING SUMMARY. 

Having arrived at the conclusion of our subject, it appears to us 
desirable that we should recall to the reader, by a rapid review, 
its salient features. 

Our main object being to attempt what we conceive to be a 
rational explanation of the surface details of the moon which 
should be in accordance with the generally received theory of 
planetary formation, and with the peculiar physical conditions of 
the lunar globe — the opening of our work was a summary of the 
nebular hypothesis as it was started by the first Herschel and 
systemised by Laplace. Following these philosophers we en- 
deavoured to show how a chaotic mass of primordial matter 
existing in space would, under the action of gravitation, become 
transformed into a system of planetary bodies circulating about a 
common centre of gravity ; and further, how, in some cases, the 
circulating planetary masses would themselves become sub-centres 
of satellitic systems; our earth being one of these sub-centres 
with only one satellitic attendant — to wit, the moon, the subject of 
our study. 

The moon being thus considered as evolved from the parent 
nebulous mass, and existing as an isolated and compact body, we 
had next to consider what was the effect of the continued action of 
the gravitating force. By the light of the beautiful ** mechanical 
theory of heat " we argued that this force, not being destructible, 
but being convertible, was turned into heat ; and that whatever 



CHAP. XV.] CONCLUDING SUM3IABY. 209 

may have been the original condition of the parent nebulous mass, 
as regards temperature, its planetary offspring became elevated to 
an intense degree of heat as they assumed the form of spheres 
under the influence of gravitation. 

The incandescent sphere having attained its maximum degree of 
heat by the total conversion thereinto of the gravitating force it 
embodied, we explained how there must have ensued a dispersion 
of that heat, by radiation into surrounding space, resulting in the 
cooling and consequent solidification of the outermost stratum of 
the lunar sphere, and subsequently in the continuation of the cool- 
ing process downwards or inwards to the centre. And here we 
essayed to prove that in this second stage of the cooling process, 
when the crust was solid and the subjacent portion of the molten 
sphere was about to solidify, there would come into operation a 
principle which appears to govern the behaviour of certain fusible 
substances, and which may be concisely termed the principle of 
pre-solidifying expansion. We adduced several examples of the 
manifestation of this principle, soliciting for it the careful con- 
sideration of physicists and geologists, and looking to it as furnish- 
ing the key to the mystery of volcanic action upon the moon, since, 
without needing recourse to aqueous or gaseous sources of eruptive 
power, it afforded a rationale of the ejection of the fluid and semi- 
fluid matter of the moon through the soHdified crust thereof, and 
also of the dislocations of that crust, unattended by actual 
ejection of subsurface matter, of which our satellite presents a 
variety of examples, and which the earth also appears to have 
experienced at some period of its formative history. 

Arrived at this stage of our subject we thought it needful to 
introduce some pages of data and descriptive detail. Accordingly 
in one chapter we discussed the form, magnitude, weight, and 
density of the moon, and the force of gravity at its surface : and 
the more soundly to fix these data in the mind, we devoted a few 
lines to explanation of the methods whereby each has been ascer- 
tained. We. then examined the question (so important to our sub- 



210 THE MOON. [chap. xv. 

ject) of the existence or non-existence of a lunar atmosphere, giving 
the evidence, which may he regarded as conclusive, in proof of the 
absence of both air and water from the moon, and, therefore, refut- 
ing the claim of these elements to be considered as sources or 
influencers of the moon's volcanic manifestations. A general coup 
d'oeil of the lunar hemisphere facing the earth next engaged our 
attention, and we considered the aspect of the disc as it is viewed 
by the naked eye and with telescopes of various powers. From this 
general survey we passed to the topography of the moon, tracing 
briefly the admirable labours of those who have advanced this sub- 
ject, and, by aid of picture and skeleton maps, placing it within 
the reader's power to become more than sufficiently acquainted for 
the purposes of this work with the names and positions of detailed 
objects and features of interest. Special descriptions of interesting 
and typical spots and regions were given in some few cases where 
such appeared to be called for. 

These descriptive matters disposed of, we proceeded to discuss 
the various classes of surface features with a view to explaining the 
precise actions which appear to us to have led to their formation. 
Naturally the craters first demanded our attention. We pointed 
out the reasons for regarding the great majority of the circular 
formations of the moon as craters, as truly volcanic as those of 
which we have examples, modified by obvious causes, upon the 
earth ; and, tracing the causative phenomena of terrestial volcanoes, 
we showed how the explanations which have been offered to account 
for them scarcely apply to those of the moon : and thus, driven to 
other hypotheses, we endeavoured to demonstrate the probability of 
the lunar craters having been produced by eruptive force, generated 
by that pre-solidifying expansion of successive portions of the 
moon's molten interior, which we enunciated in our third chapter. 
The precise cause of phenomena which resulted in the production 
of a crater of the normal lunar type, with or without the significant 
central cone, were then illustrated by a series of step-by-step 
diagrams with accompanying descriptive paragraphs. And after 



CHAP. XV.] CONCLUDING SUMMARY. 211 

treating of craters of the normal type we pointed out and explained 
some variations thereupon that are here and there to be met with, 
and likewise those curious complications of arrangement which 
exhibit craters superimposed one upon another and intermingled 
in strange confusion. 

From craters manifestly volcanic we passed to the consideration 
of those circular formations which, from their vastness of size, 
scarcely admit of satisfactory explanation by a volcanic hypothesis. 
We summarized several proffered theories of their origin, and 
pointed out what we considered might be a possible key to the 
solution of the selenological enigma which they constitute, without 
however, expressing ourselves entirely satisfied with the validity of 
our suggestion. The less mysterious features presented by peaks 
and mountain ranges were then discussed to the extent that we con- 
sidered requisite, viewing their comparatively simple character and 
the secondary position they occupy in point of numerical import- 
ance upon the moon. At greater length we dealt with the cracks 
and chasms and the allied phenomena of radiating streaks, pointing 
out with regard to these latter the strikingly beautiful correspond- 
ence in effect (and therefore presumably in cause) between them 
and crack- systems of a glass globe *' starred " by an expanding 
internal medium. 

The more notable objects and features of the lunar surface being 
disposed of, we had next to say a few words upon some residual 
phenomena, chiefly upon the colour of lunar surface details, and 
upon their various degrees of brightness or reflective power. And, 
inasmuch as varying brightness seemed to us to be related to 
varying antiquity, we were thence led to the question of the 
chronology of selenological formations, and to the disputation upon 
the continuance of volcanic action upon the moon in recent years. 
We regarded this question from the observational and the infer- 
ential points of view, and were led to the conclusion that the 
moon's surface arrived at its terminal condition ages ago, and that 
it is next to hopeless to look for evidence of existing change. 



212 THE MOON. [chap. xv. 

Thus far our work dealt with the moon as a planetary body 
merely. It occurred to us, however, that we might add to the 
interest attaching to our satellite were we to regard it for a time as 
a world, and consider its conditions as respects fitness for habita- 
tion by beings like ourselves. The arguments against the possi- 
bility of the moon being thus fitted for human creatures, or, 
indeed, for any high organism, were decisive enough to require 
little enforcing. It appeared to us, nevertheless, that much might 
be learnt by imagining one's self located upon the moon during a 
period embracing one lunar day (a month of our reckoning), with 
power to comprehend the peculiar circumstances and conditions of 
such a situation. We therefore attempted a description of an 
imaginary sojourn upon the moon, and pointed out some of the 
more striking aspects and phenomena which we know by legitimate 
inference would be there manifested. We trust, that while our 
modest efforts in the chapter referring to this branch of our subject 
may prove in some degree entertaining, they may be in a greater 
degree instructive, inasmuch as certain facts are brought into 
prominence which would not unnaturally be overlooked in contem- 
plating the moon from the earth, the only real stand-point that is 
available to us. 

In our final chapter we considered the moon as a satellite, and 
sought to enhance popular regard for it on account of certain high 
functions which it performs for man's benefit on this earth ; but 
which are in great risk of being overlooked. We showed that, not- 
withstanding the moon's occasionally useful service as a nocturnal 
luminary, it fills a far higher office as a sanitary agent by cleansing 
the shores of our seas and rivers through the agency of the tides. 
We pointed out the vast amount of absolutely mechanical work and 
commercial labour which the same tidal agency executes in trans- 
porting merchandize up and down our rivers — an amount that, to 
take the port of London alone, represents a money value pei' annum 
that may be reckoned in millions sterling, seeing that if our river 
was tideless all transport would have to be done by manual or 



CHAP. XV.] CONCLUDING SUMMARY. 213 

steam power. We then hinted at the stupendous reservoir of 
power that the tidal waters constitute, a form of power which has 
not as yet been sufficiently called into operation, but which may be 
invoked by-and-by, when we have begun to feel more acutely the 
consequences of our present prodigal use of the fuel that was 
stored up for us by bountiful nature ages upon ages ago. The 
moon's services to the navigator, in affording him a ready means of 
finding his longitude at sea ; to the chronologist and historian, as 
a timekeeper, counting periods too vast for accurate reckoning by 
other means ; to the astronomer and student of nature, in revealing 
certain wonderful surroundings of the solar globe, which, but for 
the phenomena of eclipses caused by the moon's interposition, 
would never have been suspected to exist — these were other 
functions that we dwelt upon, all too briefly for their deserts ; and, 
lastly, we spoke of the moon as a medal of creation fraught with 
instructive suggestions, which it has been our endeavour to bring 
to notice in the course of this work. And from uses we passed to 
abuses, directing attention to a few popular errors and wide- spread 
illusions relating to lunar influence upon, and in connection with 
things terrestrial. This part of our work might have been con- 
siderably expanded, for, in truth, the moon has been a misunder- 
stood and misjudged body. Some justice we trust we have done to 
her : we have brought her face to the fireside ; we have analysed 
her features, and told of virtues that few of her admiring beholders 
conceived her to possess. We have traced out her history, fraught 
with wonderful interest, and doubtless typical of the history of 
other spheres that in countless numbers pervade the universe : and 
now, having done our best to make all these points familiar, we 
commend the moon to still further study and still more intimate 
acquaintance, confident that she will repay all attentions, be they 
addressed to her as 

A PLANET, A WORLD, OE A SATELLITE. 



BRADBURV, AUNEW, <!c OO., I'KINTKRS, WHITKFRIARS. 




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