UC-NRLF

35 S5fi

IN STARRY REALMS

IN STARRY REALMS

BY

SIR ROBP:RT s. BALL, D.SC., LL.D., F.R.S.

LOWNDEAN PROFESSOR OF ASTRONOMY AND GEOMETRY IN THE
UNIVERSITY OF CAMBRIDGE

AUTHOR OF "GREAT ASTRONOMERS," " IN THE HIGH HEAVENS," ETC.

WITH NUMEROUS ILLUSTRATIONS

CHEAP EDITION

LONDON: SIR ISAAC PITMAN & SONS, LTD
BATH AND NEW YORK

UNIFORM WITH THIS VOLUME

IN THE HIGH HEAVENS. By Sir
ROBERT BALL, D.Sc., LL.D., F.R.S.
With 40 illustrations.

GREAT ASTRONOMERS. By the

same author. With 66 illustrations.

BY LAND AND SKY. The Record of
a Balloonist. By the Rev. JOHN
M. BACON, M.A., F.R.A.S. With
four illustrations.

ASTRONOMY FOR EVERYBODY. By

Professor SIMON NEWCOMBE, LL.D.
With an Introduction by Sir ROBERT
BALL. Illustrated. A popular
exposition of the wonders of the
Heavens. In demy 8vo, cloth gilt,
gilt top, 3s. 6d. net.

PEEPS INTO NATURE'S WAYS.
Being chapters on insect, plant and
minute life. By JOHN J. WARD.
Illustrated from photographs and
photo-micrographs taken by the
author.

. -FIRST EDITION^ APRIL, 1892.
* KEP»IIWTED~ ]tfNE, 1892.

„ FEBRUARY, 1893.

„ FEBRUARY, 1898.

„ JUNE, 1904.

CHEAP EDITION, APRIL, 1906.
REVISED EDITION, OCT., iqo6.
REPRINTED. APRIL, 1907.
MAY 1908.
OCTOBER, 1909

PREFACE TO THE NEW EDITION.

LEAVING the general features of " In Starry Kealms "
unaltered, I have endeavoured to correct the present
edition and bring it, as far as possible, up to date.
This has been particularly necessary in the chapters on
"Showers of Shooting Stars" and on "Photographing
the Stars," but the latter subject would now need
volumes to itself. Probably the recent discoveries in
Radio-Activity will, at no distant date, cause some
modification to be required in our present views as to
the sustentation of the sun's heat. At present, however,
there is no adequate reason for making any alteration
of moment in the chapters here given on this subject.

ROBERT S. BALL.

CAMBRIDGE,

21st August, 1906.

335930

PREFACE TO THE ORIGINAL EDITION,

THE contents of this little volume have within the last
few years seen the light in various periodicals. Chapters
I — vn and xv — xvii appeared in Good Words; vm — X
in the Girl's Own Paper; xi, XIV, xvin, xix in various
newspapers ; XII, xm in A 1 ; xxi in Macmillan's Maga-
zine; xxn in the Contemporary Review; xxni in Long-
man's Magazine; while XX was a presidential address
delivered at the Midland Institute, Birmingham.

A few alterations have been found necessary in gather-
ing them into a consecutive whole, and each chapter has
been carefully revised.

With respect to the illustrations, several of which did
not accompany the papers in their original form, I am
indebted to the kindness of Mr. A. Cowper Eanyard, the
editor of Knowledge, for permission to reproduce the
photograph of the Moon, taken with the Great Eefractor
of the Lick Observatory (Frontispiece), Mr. Lassell's
picture of the Great Nebula in Orion, and drawings of
the Eev. F. Hewlett's photographs of sun-spots of
1882-3. To the Council of the Eoyal Society I owe
thanks for a similar favour in regard to the Krakatoa
diagrams.

The object of the book is to give the general reader
some sketches of specially interesting matters relating
to the different heavenly bodies. They may be regarded
as supplementary to a treatise on elementary astronomy
such as my little volume, " Starland."

I am indebted to my friends, Eev. Maxwell Close,
Dr. A. Eambaut, and Mr. L. Steele, for their aid in
revising the work.

EOBEET S. BALL.
OBSERVATORY, Co. DUBLIN.
April, 1892.

CONTENTS.

fRAPTKK F»O1

i. — THE HEAT OF THE SUN -., . 1

n. — EXPENDITURE AND SUPPLY . . . .15

in. — How THE HEAT is KEPT UP . . . .26

iv. — WHAT WE OWE TO THE SUN . . . .40

v. — THE CONSTANT FACE or THE MOON . ^. . 49
vi. — THE MOON'S HISTORY ..... 63

vii. — THE LUNAR WORLD ..... 77

vin. — A VISIT TO AN OBSERVATORY . . .97

ix. — AN EVENING WITH THE TELESCOPE , . .113
x. — NOTES ON NEBULA . . . . . .124

xi. — VENUS AND MERCURY . . 141

xn. — MARS AS A WORLD . , . . .149

xni. — THE GREATEST PLANET 180

xrv. — THE NAMES or THE PLANETS . . . ,195
xv. — A FALLING STAR 202

rill CONTENTS.

CHAPTEB PAUB

xvi.— FIRE-BALLS .... .216

xvii. — SHOWERS OF SHOOTING STARS . . . .228

xvin. — THE NUMBER OF THE STARS .... 248

xix. — THE EXTENT OF THE SIDEREAL HEAVENS . . 255

xx. — THE MOVEMENTS OF THE ST/J&S . . ,261

xxi. — PHOTOGRAPHING THE STARS . . .292

xxii. — AN ASTRONOMER'S THOUGHTS ABOUT KRAKATOA 319

xxin. — DARWINISM AND ITS RELATION TO OTHER

BRANCHES OF SCIENQB , 343

LIST OF ILLUSTRATIONS.

PHOTOGRAPH OF THE MOON . . . frontispiece
SUNSPOTS 10

THE SOLAR PROMINENCES 12

THE SOLAR CORONA . . . . . . .13

THE LUNAR Disc SHOWN AGAINST THE MAP OF EUROPE 51

LUNAR ORBIT COMPARED WITH CIRCUMFERENCE OF THE

SUN 52

VlEW FROM THE SURFACE OF THE MOON . . .81
LUNAR CRATERS COMPARED WITH THE AREA OF ENGLAND 89

THE MERIDIAN CIRCLE 107

THE SUN AND ITS ATTENDANT WORLDS . . .114

SATURN AND HIS RINGS 119

THE Q-REAT NEBULA IN ORION . . , * 120

THE GREAT SPIRAL NEBULA . . „ « .139
ORBITS OF THE EARTH AND MARS . .152

x LIST OF ILLUSTRATIONS.

fAni

VIEW OF MARS . . . . * . .155

WEIGHING THE PLANETS . . . . . .174

ORBITS OF JUPITER AND THE EARTH . . . .181

COMPARATIVE SIZES OF THE SUN AND THE PLANETS . 183

VIEWS OF JUPITER . . . . . . .185

KRAKATOA : ERUPTION OF, AUGUST 26-27 , , . 833

KRAKATOA : PROGRESS OF SKY PHENOMENA . . . 336

KRAKATOA : NORTHERN LIMIT OF SKY PHENOMENA . 339

CHAPTER L

THE HEAT OF THE SUN.

THERE is not in the whole range of modern science a more
fruitful subject for discussion than the sun and his con-
nection with the earth. In the first place it is the func-
tion of the sun's attracting power to constrain the earth
to follow that orbit in which she performs her annual
revolution. The same office is also filled by the sun in
the guidance of the several planets which, like our earth,
are controlled by the one great central power. There are
other functions in which the sun appears to be more
obviously our benefactor by contributing several of the
necessaries for our welfare. Chief among these is his
provision of the heat by which life is sustained. The
study of the benefits conferred on us by the sun in this
particular opens up most interesting questions. As the
great dispenser of light to the surrounding worlds, the
functions of the sun illustrate other branches of science.
Indications of the sun's interference in terrestrial affairs of
somewhat minor moment are not wanting. The swing of
the magnetic needle is connected in some occult manner
with sunbeams, and phenomena of this class seem destined,

B

2. IN STARRY REALMS,

at a time now perhaps not far distant, to become of great
scientific interest. I propose to devote this and the few
following chapters to the more important relations of the
sun to the earth. I shall strive to illustrate the different
functions which the sun has to perform, and I shall point
out how it is that he is enabled to send down his benefits
upon the dwellers on this earth with a profuseness that
shows no signs of exhaustion.

Let us begin by considering the extent to which we are
indebted to the sun for maintaining the earth in the same
orbit year after year. It would almost seem as if our
globe were always trying to escape from the thraldom of
the sun. If the sun were to withhold that attractive
power by which the earth is maintained in the course that
it at present follows, dire calamity must immediately result.
This globe of ours is hurrying along at a pace of eighteen
miles a second, and if the sun's attraction no longer re-
strained it the globe would not continue to revolve in a
circle, but would at once start off in a straight line on a
voyage through space. Every minute would take us more
than a thousand miles, and by the time a hundred days had
elapsed we should be twice as far from the sun as we are
at present. His light and his heat would be reduced to
one- fourth part of what we now enjoy. With every suc-
cessive minute the sun's influence would still further abate,
and it is almost needless to add that all known forms of
life would have to vanish from the globe. It is, therefore,
satisfactory to know that we possess sufficient grounds for
our belief that the sun's attraction will never decline from
what it is at this moment, and therefore that there is
no cause for apprehension that life may be chased from

THE HEA1 OF THE SUN. j

this globe by a dissolution of the bond of attraction
between the earth and sun.

There is, however, another aspect of the earth's relation
to the sun which requires such close attention that it must
necessarily form the chief subject with which we have to
deal. For the preservation of life on this earth it is not
only necessary that the distance of our globe from the sun
shall never greatly alter from what it is at present, but also
that the present amount of the radiation from the source
of heat shall continue without perceptible variation. These
considerations introduce questions which do not admit of
being summarily disposed of. They involve some problems
that are not at all simple. Indeed, it is only in modern
times that the subject has become properly understood.
We have here to consider not alone some matters of
primary significance as regards the continuance of life on
this earth, but also various scientific problems of the
highest interest and importance. I therefore propose to
discuss how it comes to pass that the sun's power to radiate
heat to the earth is maintained at a rate which appears to
be constant.

There seems to be sufficient reason for the belief that
the heat at present emitted from the sun is neither greater
nor less to any sensible extent than that which our lumi-
nary used to dispense ages ago. Where the vine and the
olive now grow, the vine and the olive were growing
twenty centuries back. We must not, however, place too
strong a reliance on what seems an obvious deduction from
such a fact. Darwin has taught us how by natural selec-
tion an organism can continue to preserve its adaptation
to the environment notwithstanding the gradual change

4 IN STARRY REALMS.

of the surrounding conditions. The facts, so far as they
are known to us, fail to show any grounds for supposing
that there have been any important changes in the climates
of the earth within historic times. We have geological
evidence far earlier than any historical testimony as to the
character of the climates which prevailed at various epochs
of remote antiquity. The records of the rocks demon-
strate unquestionably that our globe has passed through
many striking vicissitudes so far as heat and cold are con-
cerned. Those records prove that there have been periods
during which some of the fairest regions of this globe
were desolated by a frost so intense that they became
thickly cased with solid ice. There have also been periods
when conditions of an opposite character have prevailed.
The polar regions which now seem the perennial abode
of impenetrable ice once enjoyed a succession of long
and delightful summers, diversified by winters remarkable
alike for their brevity and their mildness. The Arctic
solitudes, now so dismal and so barren, then nourished
plants and animals that can only thrive under genial con-
ditions of climate.

No doubt the question as to the origin of these great
climatic changes, which have so frequently occurred in
the course of geological time, presents many difficulties.
Opinion is divided as to what the cause of these changes
may have been. I do not now enter into this subject,
because for our present purpose it suffices to note one
very important conclusion. Those who are competent to
pronounce on the question of the cause of the geological
variations of climate are in substantial accordance with
the view that several changes have not been due to any

THE HEAT OF THE SUN. 5

actual variations in the amount of heat radiated from the
sun. In other words, there is not the slightest reason for
the belief that the sun has been during geological times
either appreciably hotter or appreciably colder than at the
present moment.

One of the indications of the existence of sunbeams
throughout geological times is afforded by the eyes of
certain fossil animals. It may, for instance, be specially
noted that the great fish-like reptile, the Ichthyosaurus,
which lived during a remote period of this earth's history,
possessed an eye apparently unequalled in size and com-
plexity by the eye of any other animal living or extinct.

There is no reason for being surprised that the sun
should have attracted the earth by the force of gravi-
tation a million years ago with just the same intensity
as it does at present. The gravitation between two
masses does not necessarily involve anything in the
nature of expenditure. Two cannon balls, for instance,
placed at a certain distance apart attract each other with
a certain force, and that force remains the same after any
lapse of time, provided the masses of the cannon balls
and the distance between them continue unchanged. The
case is, however, totally different, if it be radiation of
heat and not attraction of gravitation that is the pheno-
menon in question. For example, let us regard one of
the cannon balls as a red-hot body dispensing heat around
so as to warm the objects in its vicinity. Here, evidently,
the question of time enters as an important element. The
hot body will, in general, be able to impart heat to a cold
one, so long as the one retains an excess of temperature
over the other. It is, however, in the nature of a heated

6 IN STARRY REALMS.

object to pour forth its heat by radiation, and ultimately
to cool down to the temperature of the bodies which sur-
round it. It thus appears that the dispensing of heat
from a body at a high temperature, being essentially of
the nature of expenditure, cannot be maintained without
the source being in some manner replenished. Even the
greatest body, if also the hottest body, only contains a
limited and perfectly definite quantity of heat. In the
process of cooling a heated object gradually pays out from
its stores of heat, until, at last, its temperature approxi-
mates to that of the surrounding objects. From that
time the radiation ceases to have appreciable amount, or>
to speak more accurately, the heat which the body loses is
restored by the heat which it receives by the radiation
of other objects. No matter how magnificent be the
dimensions of the radiating mass, or how exalted be its-
temperature, it cannot escape from the application of these
principles. It must ultimately cool down and cease to
operate as a source of heat to the other objects around.

It is unavoidable that the sun himself must run his course
in accordance with the doctrines we have been here con-
sidering. The great source of light and heat cannot escape
from the legitimate consequences of continuous expendi-
ture. Unless there is some process of restoration by
which the results of solar extravagance are neutralised,
impoverishment and ultimate exhaustion are inevitable.
If, therefore, the sun receives no supply of heat, or of
what is equivalent to heat, the time must ultimately arrive
when his stores of radiant energy shall have been all ex-
pended, and the great globe can then no longer remain
as a source of life and light to the worlds which circulate

THE HEAT OF THE SUN. f

around him. We know that the fire which gladdens our
hearth requires occasional renewal by fuel else it will go
out ; so too that glowing globe in the heavens which glad-
dens and beautifies the earth must also go out unless its
energies be recuperated from some source or other.

The amount of heat squandered by the sun is truly pro-
digious, The earth intercepts only an extremely small
proportion of the total radiation of sunbeams. It would be
easy to show that the sun distributes sufficient light and
heat to maintain two thousand million planets in the same
comfortable circumstances as those in which our earth is
placed. The greater part of that radiation is entirely
lost, or, at least, lost in so far as any of the planets are
concerned. Our fellow-worlds — Jupiter, Saturn, Yenus,
Mercury, and Mars — do, no doubt, intercept a little of the
heat that would otherwise depart altogether from our
system, but the total amount of radiant solar energy that
all the planets together can utilise is utterly inappreciable
when compared with that which streams away into space,
and has gone from us for ever. The quantity of heat
radiated from the sun is one of the most astounding facts
in nature. Let us consider with the help of a few illus-
trations the wealth of radiation which our great central
hearth pours forth. We shall make use of some of the
facts collected in Professor Young's well-known treatise
on the sun.

When an engineer is designing the furnaces to supply
a steam engine he has to arrange that the heating surfaces
of the boilers shall have an extent duly proportioned to
the work which the engine has to do. Each square foot
of boiler exposed to the flame is capable of generating so

8 IN STARRY REALMS.

much steam per hour, and may thus be regarded as the
equivalent of so much horse-power. Let us consider an
area of a single square foot on the sun's surface, and sup-
pose that all the heat which passes through it on its way
to outer space could be collected and applied to the gene-
ration of steam in a boiler. The generation of steam in
that boiler would be so copious that a mighty engine of ten
thousand horse-power might be maintained in continuous
action. Indeed a great Atlantic liner could be driven at
full speed at a heat expenditure not larger than that
which flows out through each square foot of the sun. It
would be easy to show that if the heat from an area on the
sun of an acre or two in extent could all be utilised by a
system of boilers, they would generate as much steam as
would suffice to sustain in full work every steam engine in
the world.

We may exhibit the quantity of heat radiated from the
sun in another way. Let us suppose that it is to be
entirely applied to the melting of ice, and that this ice
is disposed in a shell closely enveloping the whole sun.
Even if the ice had a thickness of 48J feet the radiation
would be sufficient to reduce it all to water in one minute.
Statements like this give some conception of the profuse
expenditure with which the sun pours forth its stores of
heat ; they also raise a desire to study the method by
which the exhaustion that would seem the obvious con-
sequence of such monstrous extravagance can be avoided.

In the first place it should be noticed that the enor-
mous size of the sun is a very important element in the
inquiry. A large body cools much more slowly than a
gmall one. The loss of heat by radiation takes place

THE HEAT OF THE SUN. 9

chiefly, if not wholly, from the surface of the heated
body, and the heat from those parts which are not on the
surface can only be dispersed by radiation after it has
travelled by conduction from the interior to the exterior.
Such at least would be the case if the body were a solid
one; if; however, it were either wholly or partly in a
liquid t>r in a gaseous condition, as the sun appears to be,
then the mode by which the heat would pass from the
interior to the exterior would be correspondingly modified.
There would, doubtless, be currents of convection in the
solar materials just as there are currents of convection
which distribute, throughout the bulk of the liquid, the
water that has been heated at the bottom of a kettle
placed on the fire. This does not contradict the statement
that we have made as to the necessity for the arrival of
the heat from the interior at the surface before it could be
dispersed by radiation. The mode of conveyance of the
heat must be different in a fluid from what it is in a solid,
but the general principle remains unaltered. The spots
on the sun are doubtless connected with the circulation of
the heat between the interior and the surface. The
character of these remarkable features on our great lumi-
nary is well shown by the reproductions of the Rev. F.
Hewlett's photographs of spots in 1882, 1883.

The extraordinary profusion with which heat is poured
forth from every square foot of the sun's surface may
perhaps be explained by the following illustration. Sup-
pose there are two concert-halls, built from like designs,
but with every dimension in one of the buildings double
that corresponding in the other. The area, for instance,
in one hall is twice as long and twice as wide as in the

10

IN STARRY REALMS.

November 19, 1882.

Area of spot about 3,462,750,000

square miles.

Fig. 1.— June 29, 1883.

THE HEAT OF THE SUN. \\

lesser hall. There will be twice as many rows of seats in
it, and each row will contain twice as many chairs.
Accordingly four times as many people can be accommo-
dated in the large hall as in the small one. The buildings
being after the same design the number of exit doors will
be of course the same in both halls. Each door of the large
hall will, in conformity with our supposition, be double as
wide and double as high as the corresponding door in the
small one. Let us now suppose that both these halls are
filled to their utmost capacity, and that in each of them
a panic breaks out among the audience from an alarm of
fire, or from some similar cause. Would the facilities of
escape be equal from the two buildings, and if not, which
of them would have the advantage ? Considering that
the two buildings have been erected from the same
designs, it might at first appear that the facilities for
a rapid emptying of the buildings should be equal in
both ; but this is not the case. No doubt the larger
building has double the width of door exit possessed by
the small one, but, on the other hand, four times as many
people have to push through these doors, and consequently
the crowding at the exits of the large room would be
double as great as at those of the small one. In a pre-
cisely similar way it would appear that if one of the
buildings had ten times the linear dimensions of the
other, it would have ten times as many rows, and each
row would have ten times as many seats, so that the whole
audience contained in the large hall would be a hundred-
fold that contained in the small one. The width of door
exit would, however, be only ten times as great, and
consequently the crushing and crowding and the diffi-

12 IN STARRY REALMS.

culty of exit would be ten times more perilous in the
larger building than it would be in the smaller one.

This illustration will show the contrast between the
escape of heat from a large body and the escape of heat
from a small one. For the purpose of our argument, the

Fig. 2.— The Solar Prominences.

sun's diameter may be represented as one hundred times
that of the earth. The surface of the sun will exceed
the surface of the earth in the proportion of 10,000 to 1,
and the volumes of the two bodies will be in the propor-
tion of 1,000,000 to 1. If the two bodies possessed
originally the same temperature, and were composed of
the same materials, the sun would possess a million times

THE HEAT OF THE SUN.

14 IN STARRY REALMS.

as much heat as the earth. If this heat is to be lost it
must be by passing out through the surfaces of the bodies.
The sun's surface is no doubt ten them and times that of
the earth ; but, on the other hand, there is a million times
as much heat to pass through the sun's surface as through
the earth's surface. Hence it follows that one hundred
times as much heat must emerge through every square
foot on the sun's surface as through every square foot on
the earth if the two bodies remain at equal temperatures.
The surface of the sun is in a state of the most tremen-
dous agitation. Look, for example, at Fig. 2, which
exhibits, on a small part of the sun's margin, some of the
glowing flames that leap from its surface. These out-
bursts are often tens of thousands of miles high, and they
indicate the fearful tempests with which the tiery globe is
convulsed. Such objects are known as the "prominences,"
but the surroundings of the great luminary include also
the corona. This is a faint pearly light extending to a
vast distance round the sun, and rendered visible when
the moon intrudes between the earth and the sun, so as to
form a total eclipse.

CHAPTER II.

EXPENDITURE AND SUPPLY.

NOTWITHSTANDING the sun's great mass and immense store
of heat, it cannot escape from the application of those
familiar laws of cooling that would be observed in a red-
hot poker when taken from the fire, or in a freshly- run
casting when drawn from its mould in the foundry. The
larger the body the longer is the time it will require
before it becomes cold. A knitting needle heated red-hot
and withdrawn from the fire will become cold in a few
minutes. An iron rail, after it has been drawn red-hot
through the rolling-mill, may remain hot for an hour.

Our first surmise might be that the luminary is
simply a vast white-hot body, glowing with incandescence
and parting with its heat in accordance with the ordinary
laws of cooling. On this supposition the fact that the
abatement in sun heat was not perceptible in historic
times might be accounted for in consequence of the
immense size of the body. Suppose, for instance, the sun
were merely a globe of white-hot iron pouring forth its
heat, could we then explain the maintenance of its radia-
tion for so many thousand years at the same rate as at
present ?

Ib IN STARR Y REALMS.

This is a question which cannot be decided by mere off-
hand reasoning ; we must submit it to the test of actual
calculation. As a foundation for this calculation, we
know the quantity of heat that would be contained in a
globe of white-hot iron of the same dimensions as the sun.
We also know the quantity of heat which represents the
sun's daily expenditure. It is therefore a simple matter
of arithmetic to find the number of days' supply which a
white-hot sun could contain. The result is not a little
startling ; it demonstrates that to supply the current
expenditure of solar radiation at its present rate the
temperature of the sun would have to decline through
some degrees every year if the constitution of the body
were what we have supposed. It seems that if the sun's
temperature were suffering an abatement at a rate any-
thing like so fast as this supposition would involve, then
the effects of cooling would be perceptible in a continuous
fall of the sun's efficiency as a radiator. It is therefore
obvious that our first notion, which suggests that the
sun is merely a white-hot globe cooling down, must be
abandoned.

Another conceivable origin of the sun's heat must also
be considered for a moment before receiving its dismissal
as utterly incapable of affording a solution of the problem.
As heat is generated by the consumption of fuel, it might
seem not unreasonable to suppose that there may be some
process analogous to combustion at present in progress in
the sun on a scale of sufficient magnitude. But there are
insuperable difficulties about such a view, which will be-
come apparent when we call to mind the actual nature
of combustion. The coal which is glowing so cheerfully

EXPENDITURE AND SUPPLY. 17

in the fireplace is the seat of a vehement chemical action.
The carbon which constitutes a great part of the coal is
uniting with the oxygen of the air, and as an incident
of the chemical union between these two substances
heat in large quantities is evolved. Hydrogen is
another important ingredient in the composition of
ordinary coal. When ignited hydrogen combines with
the oxygen which the air so plenteously supplies, the
union of the two gases produces the vapour of water, and
che process is accompanied by the liberation of large quan-
tities of heat. Thus the radiation of warmth from the
fireplace is the direct consequence of a chemical union
between different elements. It is, therefore, proper to
inquire whether the heat of the sun may not be sustained
by a somewhat similar action. Can it be that there is
some mighty combustion of fuel perennially in progress in
the sun, and that continuous radiation of heat is the con-
sequence ? This is a question which it is difficult to
answer in a simple manner. There can be no doubt that
chemical activities of the highest order must be in inces-
sant operation in the sun, but it seems impossible that any
appreciable proportion of the sun's radiation can be main-
tained by chemical action.

Indeed we can show conclusively that no attainable
amount of chemical action could be adequate for the main-
tenance of the sun's expenditure at the rate which has
been current for so many centuries. In the first place it
is necessary that the two elementary bodies should be
both present in sufficient abundance, and that their
combination should take place with such uniformity
that the daily radiation shall be preserved at a nearly

c

1 8 /AT STARR* REALMS.

constant rate. We are aware that ingenious machinery
is often applied for the purpose of stoking the fires under
steam boilers. It is claimed for these machines that they
administer the coal to the fire so regularly that the pro-
duction of steam is carried on with all desired uniformity.
If there were sufficient fuel in the sun for the mainte-
nance of its radiation, and if there were gas or other mate-
rial suitable for union with that fuel in sufficient quantity
to generate the necessary heat, it seems difficult to ima-
gine by what arrangements the combustion of the two
elements eager for union could be so controlled that the
supply of heat produced should remain practically con-
stant. How are we to suppose that the gas and the fuel
can be continuously brought together in the duly regulated
quantities ? These considerations alone suffice to render
it highly improbable that chemical union could afford an
effective explanation of sun heat.

There is a still more insuperable objection. Careful
experiments have taught us the precise quantity of heat
that can be extracted from a ton of coal when combined
with the requisite quantity of oxygen for combustion at
the best possible advantage. We are, therefore, able to
compute how much coal would have to be consumed every
day to generate enough heat for the maintenance of solar
radiation. We can show that the sun's daily expenditure
is so enormous that the supply of fuel that would be neces-
sary places this view of the origin of the sun's heat alto-
gether out of the question. Suppose that the whole sun
from its surface to its centre were a solid globe of coaL
Suppose that from some source or other a supply of oxygen
were available which was adequate for the combustion of

EXPENDITURE AND SUPPLY. 19

this globe. Suppose that enough of this coal were to be
burned every day to supply a quantity of heat equal to
that poured forth by the sun in the same time. It is then
a matter of simple calculation to find how long a sun so
constituted could last as a source of light and heat. It can
be easily shown that on this supposition and at the present
rate of expenditure all power of radiation would be totally
exhausted in about three thousand years. We have, how-
ever, the best reasons for knowing that the sun has already
dispensed his beams for a far longer period than that just
stated. Hence we learn that the supply of the sun's heat
cannot be due to the combustion of any substance with a
composition similar to coal.

It is, however, a reasonable question whether there
may not be some extraordinary, and to us unknown,
elements present in the sun which in the course of their
chemical union generate far more heat than those elements
with which we are acquainted, so that by the combustion
of these materials the solar energies might be recuperated.
From all we know of the composition of the celestial
bodies, it does not seem in the least degree probable that
any materials exist which possess the necessary qualities.
In fact, it is one of the triumphs of modern science to have
identified to a considerable extent the materials of which
the sun is composed with the familiar elements in our
earth. By the aid of spectrum analysis it has been dis-
covered that upwards of thirty of the elements on the
earth are present also in the sun. Among these we may
specially draw attention to iron, which appears to be one
of the most widely distributed of all the elementary bodies.
Some hundreds of lines in the solar spectrum have been

20 IN STARRY REALMS.

demonstrated to be connected with this element. I should,
however, say that it has by no means been proved that
there may not be some elements in the sun of a different
kind from those we find on the earth. There are indeed
many lines in the solar spectrum which have not up to the
present been identified with those arising from any terres-
trial element. There are, however, so many difficulties
attending the identification of lines under widely varying
Conditions that it would be rash to pronounce emphatically
as to whether all the ingredients in the sun may be in some
shape or other known to us on the earth. The constitu-
tion of the great luminary does not, however, give any
encouragement to the suggestion that the source of his
heat may be attributed to combustion in the ordinary
meaning of the word. Indeed, the difficulties attending
such a view seem so overwhelming that we may withdraw
this doctrine from any further consideration.

We have thus shown not only that the sun's heat would
be insufficiently explained by supposing the great globe to
be a cooling body, but also that the phenomena of combus-
tion offer no adequate suggestion towards the explanation
of the difficulty.

Our search for the source of sun heat must therefore be
conducted in some other direction, and, strange to say, we
shall find that source to be not in any immediately avail-
able heat, but rather in the transformation into heat of
something which was originally of a mechanical nature.

When a shooting star dashes into our atmosphere its
course is attended with an evolution of light and heat
owing to its friction through the air. We are thus able
to account for the enormous quantity of heat, or of what is

EXPENDITURE AND SUPPLY. 21

equivalent to heat, which exists in virtue of the rapid
motion of these little bodies. It is true that we only see
meteors at that supreme moment of their dissolution when
they dash into our atmosphere. It is, however, impossible
to doubt that, besides the meteoric streams with which we
are acquainted, there must be many shoals of meteors
which never collide with our earth. The other great
globes in our system must, like our globe, absorb multi-
tudes of meteors which they chance to encounter in their
roamings. The numbers of meteors gathered by a globe
will be doubtless greater the larger and more massive the
globe is, and this for a double reason. In the first place,
the dimensions of the atmospheric net which the globe
extends to entrap the meteors will, when other things are
equal, increase with the size of the globe. But there is also
another reason — the more massive the globe the more vehe-
ment will be its power of attraction, and the greater will be
the number of the meteors that are drawn into destruction
in its atmosphere. Of course this reasoning applies in a
special degree to the sun. We shall probably be correct
in the assertion that for every meteor that descends upon
this earth, thousands, if not millions of meteors will plunge
into the sun. As these objects pierce their way through
the sun's atmosphere light and heat will of course be
evolved. It has been conjectured that the friction of the
meteors which are incessantly rushing into the sun may
produce light and heat in sufficient quantity to aid in the
maintenance of the sun's ordinary expenditure by radia-
tion. It has been even supposed that the quantity of
energy thus generated may supply all that is wanted to
explain the extraordinary circumstance that from age to

22 IN STARRY REALMS.

age no visible decline has taken place in the intensity of
the solar radiation. Here again is a question which we
may submit to calculation. We have first of all to deter-
mine the heat which could be generated by a body of, let
us say, one pound in weight, falling into the sun after
having been attracted thither from a very great distance.
The result is not a little startling ; it shows us that such a
body in the course of its friction through the sun's atmo-
sphere might generate as much heat as could be produced
by the combustion of many times its own weight of coal
consumed under the most favourable conditions.

The stern rules of arithmetic enable us to pronounce
decisively on the issue as to whether the influx of meteors
can be entertained as an adequate source for the supply of
sun heat. We can estimate the quantity of heat which
would be contributed by a single meteor a pound in mass.
This determines the total mass of meteors that would
have to be entrapped in the sun's atmosphere every day
if the current expenditure of sun heat had to be defrayed
from this source alone. We are able to avoid expressing
the answer in millions of tons by the fortunate circum-
stance that the moon happens to present a unit of suitable
magnitude for the purpose.

Suppose that the moon were to be entirely crushed into
fragments, which were permitted to rain down upon the
sun after the manner of meteors, then it can be shown
that the total quantity of heat generated by this influx
would be sufficient to maintain the sun's expenditure at
the present rate for about a year. Stated in this way, we
are enabled to decide at once as to the plausibility of the
supposition that the meteoric source of supply is sufficient

EXPENDITURE AND SUPPLY. aj

for the sun's wants. If every year the sun devoured a
quantity of meteors whose collective mass would form a
globe equal to our moon, then the quantity of meteoric
matter roaming about our system must be enormously
greater than seems likely to be the case. Indeed, it can
be shown that if so mighty a mass of material is absorbed
into the sun in the course of twelve months, the dynamical
conditions of the solar system would be widely different
from what we find them. A quantity of matter so distri-
buted that the sun could daily abstract a supply sufficient
for its wants would undoubtedly affect the movements of
the other heavenly bodies. Were there such an abundance
of substance in the vicinity of the sun, the perturbations
of Mercury and of the other planets arising from this
cause would be assuredly appreciable. As we do not find
that the movements of the planets are so affected to the
necessary extent, it is incumbent on us to conclude that
the quantity of loose material in our system cannot be
nearly so great as this doctrine of sun heat would require.
We are hence forced to take a modified view of the capa-
city of meteors to supply the solar radiation. It may
possibly be true, nay, doubtless must be true, that the
meteors do contribute, to some small extent, to neutralize
the sun's perennial losses ; but it is wholly impossible that
they can contribute in any substantial degree to the solu-
tion of the problem as to how the loss of heat by the sun
does not become apparent.

Three suggested sources for the sun's heat have been
tested and found wanting. We have first considered it as
a mere glowing globe, gradually cooling and dispensing
its heat by radiation. This will not answer, for the sun

.14 IN STARRY REALMS.

would have become cold and dark ages ago if it had no
further heat than this supposition implies. We then
investigated whether there could be any sufficient supply
of something analogous to fire, so that the sun's heat
could be maintained by combustion. This, again, will
not stand the necessary test : it is wholly impossible that
material of the required character should be forthcoming
in sufficient abundance. Finally, we have examined the
notion that the sun's heat may be produced by the fric-
tion of meteors which dash into his atmosphere. This
source of supply may be a small rate-in-aid of the sun's
current expenditure, but cannot explain how the bulk of
that expenditure has to be provided for.

We have now to set forth the true explanation, which
has been universally adopted whenever the evidence in its
favour has been duly considered. The question is by no
means an easy one, but I shall strive to make it as clear
as circumstances will permit. A multitude of observations
point irresistibly to the conclusion that the sun is not a
solid body. In this respect we may contrast the sun with
the moon. Our satellite is eminently a solid as opposed
to a gaseous object ; every feature on the moon is perma-
nent and definitely marked, and can be observed from
year to year in the same place and with the same sur-
roundings. But there seems to be no permanent object
whatever visible on the sun. In fact, it would be a great
convenience to astronomers if there were some solid moun-
tain peak which would mark one definite locality on the
sun's globe. We are sadly in want of some such definite
feature to serve as an origin from which the longitude of
spots or other objects on the sun's globe could generally

EXPENDITURE AND SUPPLY. 25

be estimated. Nothing of the kind can, however, be dis-
cerned. We are obviously only looking at vast masses of
glowing cloud suspended in gaseous matter. The solar
vapours have but little more permanence than the unstable
clouds of our own atmosphere.

Our knowledge of the interior of the sun is necessarily
very imperfect, but there seems no reason to think that
the luminary is, even in any part, what may be regarded
as a solid body. There is, in fact, a strong presumption
in the opposite direction. To explain this it is necessary
to contrast certain characteristics of our earth with the
corresponding characteristics of the sun. Our globe, for
instance, is constituted from materials which are on the
whole so heavy that the earth weighs over five times as
much as would a globe of water of the same size ; whereas
the sun is hardly one and a half times as heavy as a globe
of water of equal volume. Thus we learn that the sun
is composed of materials which have about one-fourth of
the density of those of the earth. This points to the con-
clusion that the materials of the sun are much less closely
compressed, and that even the sun's interior can hardly
be regarded as consisting of solid substance.

When, therefore, we discuss the loss of heat from the
sun by radiation we ought not to compare it with that of
radiation from a solid globe ; we must rather take it to
resemble radiation from a gaseous globe. At first, per-
haps, the profound difference between the two cases may
escape attention. Indeed it is only in comparatively
recent times that the remarkable laws by which a globe
of gaseous matter parts with its heat have been thoroughly
explained.

CHAPTER ITI.

HOW THE HEAT IS KEPT UP.

IT will be necessary for me, at this part of my subject, to
recall to mind a few points in the doctrine of heat. In
ordinary language we occasionally employ the words heat
and temperature indifferently, but when we are considering
such a question as that now before us we must be more
particular about our words. The heat contained in a
body is one thing, and the temperature shown by that
body is a different thing ; in fact, it may well be that of
two bodies, the one with the higher temperature contains
less heat than that which shows the lower temperature.
You might take a pound of iron and a pound of water,
each at the same temperature, and then give them both
equal additions of heat. The iron will rise to a tempera-
ture far higher than the water. A foot-warmer filled with
hot water will give out far more heat than if it contained
merely the same weight of iron heated to the same tem-
perature. A pound of mercury and a pound of water at
the same temperature, as shown by a thermometer, would
contain very different quantities of heat, the difference being
very much in favour of the water. If equal weights of
mercury and water had very unequal temperatures, and

HOW THE HEAT IS KEPT UP. 27

you stirred the two together, the mixture would assume
nearly the temperature of the water, whatever that of the
mercury may have been. If the pound of water was cold
and the pound of mercury hot the mixture would be nearly
cold. If it were the other way, and the pound of water
was hot and the pound of mercury cold, the mixture would
be nearly as hot as the water.

These illustrations suffice to show that the quantity of
heat in a body and the temperature which that body
exhibits are not related in any very obvious manner.
Especially is this true in the case of gaseous bodies ;
indeed, the connection between the temperature of a gas
and the heat it contains presents sometimes the startling
anomaly that while the gas is losing its heat it may be,
nevertheless, gaining in temperature. Once we take into
view that the sun is, to a large extent at all events, com-
posed of gaseous material, the difficulties as to the supply
of the heat necessary for its daily radiation vanish.

The other suggested sources of sun heat which we have
already discussed had to be rejected, for the very simple
reason that they failed to explain why the sun did not
seem to get colder. Now we shall be able to account for
the fact — the strange fact, some may certainly think —
that the loss of heat does not necessarily involve a fall of
temperature. Indeed, it seems just as likely that if the
sun's temperature is changing at all at the present time it
may be rising instead of falling, though no doubt the
alteration must be extremely slow.

A fundamental doctrine in the theory of heat tells us
that when heat is imparted to a body it expands. There
are no doubt certain exceptions, but they need not here

*8 IN STARRY REALMS.

detain us. We have also the correlative truth that if a
body loses heat by radiation it follows as a necessary con-
sequence that it must be contracting in dimensions ; of
course it may usually be added that the body is also losing
temperature, but this is not invariably the case. After
heat has been lost by radiation without being compensated
by an accession of heat from some external source, the
total supply of heat, or of what is equivalent to heat, is of
course lessened. Owing, however, to the shrinking of
material which has taken place the quantity of heat which
remains in the body is distributed over a lessened volume.
It may therefore conceivably happen that though a body
is losing heat, and therefore contracting, yet that, as the
heat which remains is concentrated within a diminished
bulk, it may actually show a higher temperature than the
body had before the heat was lost.

The point I am here trying to explain may be illus-
trated as follows. Suppose that a man maintains a very
large establishment in town, with troops of servants and a
stable full of horses, a yacht, a place in the country, a
shooting-lodge in the Highlands, and a villa in the south
of Europe. He must have a very large income to main-
tain his position in this fashion, and the income is pro-
vided by a fine estate. Suppose that by some reverse of
fortune the man loses a part of his property, his income is
diminished in corresponding proportion, and he at once
begins to curtail his expenses. He sells his yacht, lets his
Highland shooting, throws up his villa, reduces his hun-
ters, dismisses half his servants, and curtails his establish-
ment in many other directions. He so lessens the area of
his expenses that he has brought them into conformity

HOW THE HEAT IS KEPT UP. 29

with his income. If, indeed, he be a wise man he will
take the opportunity of carrying on his retrenchments to
such a point that his expenses shall be even more conveni-
ently within his income than they were in the original
days of high living. With his moderate establishment,
and with an income which is abundant in comparison with
that establishment, the owner may feel a comfort and
enjoy a genuine prosperity which he never knew in those
bygone days that seemed more splendid. He may actu-
ally have more money in his pocket and be in a more
satisfactory financial condition.

In like manner when a great amount of sun heat has
been radiated into space and lost for ever, the sun accom-
modates itself to the altered circumstances by shrinking
inwards. Like the prudent man who has suffered a
reverse of fortune, the sun when it does shrink takes the
opportunity of so far reducing its bulk that, even though
it has less heat than it originally possessed, yet that heat
so shows itself on the reduced bulk that the orb of day
may be a more genial body than ever it was, notwith-
standing its apparent losses.

Matter in the solid state undergoes only a compara-
tively small diminution of its volume by its loss of heat.
For example, when a cannon-ball heated to redness cools
down through 100 degrees of temperature, the shrinking
of its volume amounts to but little more than a thousandth
part of the whole. No doubt in some small degree the
quantity of heat left in the cannon-ball does show to
slightly better advantage in the reduced bulk than it
would have done had the body remained at its original
dimensions. The gain, however, in temperature from this

30 IN STARRY REALMS.

source is not at all sufficient to counteract the direct loss
from radiation, and consequently the temperature falls and
the cooling of the body progresses.

But now let us suppose the case of the cooling of a
globe of gas. No doubt the condition supposed is one
that we can hardly reproduce in our laboratories, but we
can imagine a globe of gas in space not enclosed in any
envelope, but merely taking the shape that is given by
the mutual attraction of its particles. We can indeed
see bodies apparently of this description in our telescopes,
and we call them planetary nebulae. Imagine a globe of
this kind to be dispensing its heat by radiation. In con-
formity with the general law, the loss of heat must be
accompanied by contraction. It is in the nature of a gas
to contract for a given fall of heat, through a much greater
range than that through which a solid contracts. The
consequence is that though the globe has undoubtedly
parted with some heat by radiation, yet the diminished
bulk of the contracted gas displays to such advantage the
heat still remaining, that the temperature of the mass
actually rises instead of falls !

This seems a proposition so remarkable that one may well
hesitate before accepting it. There can, however, be no
doubt of its truth, and it leads to a very remarkable result.
Suppose a great volume of gas were permitted to part
with what heat it had by radiation. If the volume were
large enough, and if the gaseous bodies of which it was
composed were of a nature that would admit of their
condensation, then the following is the course which
events would take. I am first supposing that the tempera-
ture of the gas is but little above that of the surround-

HOW THE HEAT IS KEPT UP. 31

ing space. The loss of heat will therefore be at first a
very slow process, each loss involves a corresponding con-
traction of the volume, and as we have already pointed
out, this contraction must be attended with a rise of tem-
perature. As the temperature of the mass increases the
rate at which it parts with its heat will increase also ; it
follows that the contraction of the volume will proceed at
an accelerated pace, and that consequently the rising of
temperature will go on with increasing rapidity. We
thus find that though the temperature of the gas may
have been at first extremely low, yet, that as the loss of
heat proceeds the temperature may gradually ascend,
until at last it becomes sufficiently high to render the gas
visible by actual incandescence. As the process advances
still further the body may pass from a mere nebula into
a starlike object. With the increase of contraction the
pressure also increases, and materials which were origin-
ally gaseous will assume more and more a density resem-
bling that of solid bodies.

Here is indeed an astonishing result. We have found
that a nebula which may have been originally only a
single degree of temperature above the surrounding
medium has by the mere fact that it is losing heat grown
to the lustre of a star, and may have acquired the heat-
diffusing power of a sun. Of course, the capacity for
radiation depends on temperature rather than on amount
of heat.

The limit to this remarkable evolution has, however
to be stated. When the condensation has progressed
sufficiently for the body to have become transformed from
the gaseous state into a condition resembling that of a

ja IN STARRY REALMS.

liquid or a solid, the foundation of our argument slips
from us. It only applies to bodies so long as they remain
in the gaseous state, or, at all events, so long as they
conform to the laws which gaseous bodies follow, in so far
as the present matter is concerned. Once the condensa-
tion has brought the bodies originally gaseous into a solid
form, then the ordinary laws of cooling for a solid regulate
the further changes. Loss of heat will then entail a loss
of temperature, consequently the ultimate future of such a
body would be that after ages of perhaps nearly stationary
splendour the brilliance would begin to wane, the vivid
white would be succeeded by hues that indicated a lesser
temperature, and ultimately the globe would subside into
one of those dark masses with which we know that space
is largely tenanted.

This explanation will have prepared the way for the
reception of the true theory of the source of solar heat.
There can be no doubt that the sun is still largely, if
not wholly, of the gaseous or vaporous character. It
follows that the process of cooling is still guided by the
general principle, that though there may be loss of heat by
radiation, yet the temperature of the body, ever lessening
by contraction, may show no perceptible abatement. Thu&
we explain how the sun continues, from age to age, to
diffuse a warmth around which shows no evident symptoms
of decline. It is unquestionable that the sun must be
parting unceasingly with its capital of heat, but by the
operation of the laws we have set forth the inevitable loss
of heat need not at present involve the loss of tempera-
ture.

We can submit this doctrine to the same numerical

HOW THE HEAT IS KEPT UP. 33

test that we have applied to the other explanations which
have been offered of the source of sun heat. When we do
so the figures supply abundant confirmation of its truth.
Suppose that the sun were to pass by loss of heat and
consequent contraction into a globe which was less than
its present size by one ten-thousandth part of its diameter.
Such a change would, no doubt, be considerable if we
merely regarded its absolute dimension in miles. It
would amount to a shrinkage in the sun's diameter of
87 miles. But on so mighty a globe this alteration is
relatively insignificant ; indeed no measurements that
could be made at our observatories would be sufficiently
delicate to detect a change of this magnitude. Helmholtz
has, however, shown that if the sun were to undergo even
this small diminution of volume the quantity of heat that
would be thereby liberated for the purposes of radiation
would supply the sun's current rate of expenditure for
nearly two thousand years.

This point is so important, that for the sake of further
illustration, I shall present it from a somewhat different
point of view. Imagine two suns of similar materials and
of equal temperature, but so that the diameter of one was
a ten- thousandth part greater than that of the other,
which latter was as big as ours. Then, although these
two suns would be practically the same as far as sensible
warmth was concerned, and although the difference
between their dimensions would be too insignificant to be
appreciable to measurements, conducted from such a dis-
tance as that at which we are placed, yet, nevertheless,
the larger of these two suns would possess an excess of
heat, or its equivalent, over that contained in the smaller..

D

34 W STARRY REALMS.

sufficient to supply its entire radiation for two thousand
years.

If it be the case that the sun's temperature does remain
absolutely constant, then it would seem to follow that the
diminution of one ten- thousandth part of its diameter
takes place every two thousand years. We have no means
of knowing at present whether the actual contraction of
the sun takes place at this rate or at any other rate, either
somewhat higher or somewhat lower. If the contraction
goes on faster than we have stated, then the temperature
of the sun must be rising ; if slower, then the temperature
must be falling.

Here, then, we have a completely satisfactory explana-
tion as to how the sun is able to continue from age to age
its beneficent radiation. So long as the body is sufficiently
gaseous, so long will it obey the laws of cooling we have
indicated, and so long may the amount of its radiation be
undiminished. We have seen, however, that there must
be some limit to this process. As the contraction of
the sun's volume proceeds, the density of the body will
increase to such a degree that the luminary is less and
less to be regarded as a gaseous mass. At length the
time will come when the sun shall have parted with so
much heat that it passes to a large extent into the solid
condition. There can be no doubt that when this
state of things has arrived the heat-dispensing power of
the sun will be seriously impaired. The radiation from
a solid, as we have already had occasion to show, could
not be protracted with sensible uniformity from age to
age. We thus have no assurance of the ultimate perma-
nence *rf the sun as a source of heat to our system. Indeed

HOW THE HEAT IS KEPT UP. 35

we may rather anticipate that the character of the sun as
a dispenser of heat cannot be eternal. Doubtless the orb
of day contains a magnificent supply of either actual heat
or potential heat adequate for all the requirements of life
for a cycle of ages that must be reckoned by millions of
years. It is, however, impossible to overlook the fact that
this excessive 'expenditure must at length produce its
natural consequences, and that a bankruptcy of sunbeams
is the inevitable destiny of our system. I say inevitable
of course with this proviso, that the ordinary conditions
of nature as we now find them shall continue to exist.

Seeing that the sun must ultimately become a dark
globe, with no higher temperature than that of the sur-
rounding space, it is needless to add that life must ulti-
mately vanish from this earth. We, therefore, learn as a
simple consequence from the laws of science as we now
find them, that the duration of our earth as the abode of
organized beings has a term beyond which it cannot
extend.

It would be interesting to inquire whether some con-
firmation of these views may not be obtained from other
parts of space. We remember that cardinal doctrine of
astronomy which asserts that our sun is only a star ; and
as there are millions of stars, it is natural to compare the
present phase of our sun with the phases through which
other suns are passing. We may for the moment liken
the development of a sun to the growth of a tree, which,
after passing through every grade of youth, of maturity,
and of decrepitude, is finally overthrown to return to the
dustr from) whence it came. In a similar way our sun,
which is at present at the height of maturity, is destined

36 IN STARRY REALMS.

to pass into the decrepit state when its powers of radiat-
ing heat and light have become greatly impaired before it
passes finally into the death-like condition in which it will
have no longer any stores of light and heat to dispense.
When we look at a forest we see there are a multitude of
trees in every stage of development. There are some
which are still only young seedlings ; there are others
which are vigorous saplings ; there are a few splendid
trees of ample dimensions, and there are others whose
mighty trunks show symptoms of decay, while some have
already succumbed and are found prostrate and lifeless.
These several stages can be witnessed in a glance through
a forest glade. In a similar way when we look up at the
starry heavens we see a vast multitude of suns in every
phase of development ; there are some still in the entirely
gaseous state, they merely appear as stains of light on the
sky ; they are what we call the nebulae. There are others
again in which the gradual condensation of the gas has so
far advanced that a central and brighter part can be dis-
tinguished. In other nebulae this brighter part has
become starlike, and in yet others the transformation has
proceeded so far that the nebula seems merely a glow of
gaseous atmosphere around a brilliant star. In continuing
our search we find others in which the nebula has entirely
vanished, while the brilliant star which remains is a
striking illustration of the doctrine that the intense fer-
vour of sunlike bodies is due to their contraction from an
original gaseous form.

Thus we find the stars furnish us with many excellent
illustrations of epochs in the sun's history up to the
present time in which the sun has assumed the definite

HOW THE HEAT IS KEPT UP. 37

form of a star without being surrounded by a nebula to
any considerable extent. The aspect of the heavens will
also offer us many suggestions as to the future course of
the evolution through which the sun appears destined to
pass. There are many stars whose lustre seems declining
from what it must once have been. We are acquainted
with some stars which radiate quantities of light and
heat quite inconsiderable in proportion to their masses.
Such bodies as these have evidently advanced towards the
conditions of mere red-hot globes of solid matter, and
their further decline and ultimate extinction appear to be
impending. Unfortunately, however, our observations of
stars in stages still further advanced must necessarily be
of a very incomplete nature. Once a star has parted with
its lustre and become a mere mass of non-luminous
matter we are unable to see it. Of course I am speaking
here of stars properly so called and not of planets, which
are visible not from any light of their own, but in conse-
quence of the sun's light which falls upon them. The
stars are, however, usually speaking, millions of times as
far from us as are the planets. The beams of our sun are
utterly incapable of penetrating to the depths of space
with an intensity which would be sufficient to render dark
bodies visible. Our sun would, in fact, shed no more light
on a dark globe near the star Yega than Vega now sheds
on us. To see the dark stars is, therefore, impossible, but
we are not left entirely destitute of information in respect
to them. The dark stars, though invisible, are often very
massive ; they have the power of attracting objects in
their vicinity. It sometimes happens that the movements
of a bright star are influenced by the attraction of a dark

38 IN STARRY REALMS.

star which lies near it. When, therefore, we find a
difference between the place of a bright star, and the place
in which it ought to lie in conformity with calculation, we
can attribute the discrepancy to the attraction of some dark
star. We are thus able to learn something about these
dark stars, which, though never themselves seen, axefelt,
so to speak, by their action on stars that can be observed.
The more we meditate on this subject the more probable
does it become not only that dark extinguished suns
exist, but that they abound in immense numbers ; in fact
it might be fairly argued that these dead stars vastly
exceed in number the bright and living ones. Matter only
becomes visible to us across the abyss of stellar distance
during those episodes in its career, be they few or many, in
which it happens to be at a temperature of incandescence.
When the temperature has waned, so that the globe ceases
to glow, it may reasonably be held that the body has
"joined the majority."

I have been stating merely the results of calculations,
and I have made little or no attempt to indicate the pro-
cesses by which those calculations must be conducted. It
is therefore only right that I should refer the inquiring
student to some reliable work where he may be able to
find the principles on which such researches are made
fully set forth. The book I would suggest is Williamson
and Tarleton's " Dynamics," at the end of which will be
found an instructive chapter on this subject. I shall here
extract an interesting numerical fact, for the proof of
which reference to this book may be made. Let us sup-
pose that the materials of the sun were originally distri-
buted in a nebulous form, throughout the length, breadth,

HOW THE HEAT IS KEPT UP. 35.

and depth of space. If these materials were drawn close
together in virtue of their mutual attraction, a quantity
of heat would be developed in accordance with the laws
we have already explained. It is this quantity of heat
for which we desire to find an illustration. Imagine a
globe of water so vast that its mass shall be equal to that
of the sun. We can attach a perfectly definite idea to the
quantity of heat which would be required to raise this
globe of water from the temperature of ice to that of
boiling water. It can be shown that the quantity of heat
generated in the contraction of the nebula from dimen-
sions infinitely great down to the dimensions of a sun
would be not less than two hundred and seventy thousand
times as great as that which would be required to raise to
boiling heat an equal mass of water. The result is a
remarkable one. It clearly demonstrates that in the
transposition of the materials so large a quantity of heat
is liberated as to amount to several thousand times as
much as the sun now contains. Need we wonder that
this mechanical explanation of the source of the sun's
heat now finds almost universal acceptance ?

CHAPTER IV.

WHAT WE OWE TO THE SUN.

SOME of my readers may think that the subject on which
we have been engaged is rather abstruse, and they will
perhaps be willing to meditate a little on the simpler
matter as to what the sun actually does for us.

A few years ago it was my privilege to deliver at the
Royal Institution of Great Britain a Christmas course of
lectures addressed to an audience consisting mainly of
juveniles. In discoursing of the sun I endeavoured to
set before them the many indirect benefits which we de-
rive from his beams, and as an illustration taken from
domestic affairs I dwelt on the kindly providence with
which the sun ministers to our tea-table. I showed how
the heat of the fire came ultimately from sunbeams, how
the tea was wafted from China by the sun, how the water
for the kettle was brought into our house by solar rays,
how the flour was grown and ground by the same agency.
I even pointed out that it was the sun which gave white-
ness to the tablecloth and bright colours for the ladies'
dresses. I do not now refer further to these matters, for
they have been recently set forth in my little book called

WHAT WE OWE TO THE SUN. 41

" Star-Land," in which these lectures have been pub-
lished. There are, however, one or two points which
seemed more advanced than would have been suitable for
the readers of " Star-Land," and which I did not accord-
ingly there enter upon. These are of such importance
that I am glad to take this opportunity of alluding to
them.

It will be remembered that in Gulliver's renowned
Travels he visited the Grand Academy of Lagado. The first
professor into whose laboratory Gulliver was conducted
had been engaged for eight years upon " a project for
extracting sunbeams out of cucumbers, which were to be
put in phials, hermetically sealed, and let out to warm
the air in raw inclement summers. He did not doubt
that in eight years more he should be able to supply the
governor's gardens with sunshine at a reasonable rate."
This is not the only occasion on which Dean Swift has,
consciously or unconsciously, given us profound scientific
truth under a guise of absurdity. Within a page or two
of the passage we have quoted is the announcement of the
discovery xrf the two satellites of Mars by astronomers on
the flying island of Laputa. To this we shall return in a
later part of this book. The myth of the Lagado pro-
fessor about sunbeams and cucumbers is allied to a truth
still more remarkable.

It is perfectly certain that in the growth of the cu-
cumber plant the leaves do lay hold of the sunbeams,
extract the heat from them, and lay up that heat or its
equivalent in the fruit which is produced. It is equally
certain that the heat of the sunbeams could be extracted
again, could even be stored in phials hermetically sealed,

42 IN STARRY REALMS.

and be utilised when occasion might require. There is in
fact only one weak point in the scheme of the Lagado pro-
fessor : its economical aspect is unsatisfactory. The heat
of the sunbeams could only be recovered from cucumbers
at an utterly prohibitive cost. Nevertheless, the heat is
there, and in plants of other growths it becomes quite
possible not only to extract the latent sunbeams, but even
to do so with the highest profit and advantage. Sunbeams
may shine on a tree for a few decades, the leaves may each
summer garner the sunbeams which fall on them and
incorporate those sunbeams in the solid trunk of the tree.
In due time the tree is felled, logs of it are thrown on the
fire, and pleasant light and warmth radiate from the
hearth. It is not a mere flight of poetical fancy to regard
the light and heat from a fireplace as the sunbeams re-
transformed to the active state from a condition of inert-
ness. It is an undoubted scientific truth.

Let us ponder on the character of the process by which
the leaves of a plant contribute so largely to the building
up of its solid stem. Wood consists chiefly of the element
carbon in association with small quantities of mineral ingre-
dients. The growing tree draws its supplies of the neces-
sary material partly from the earth and partly from the
air. The earth provides, speaking generally, the mineral
jonstituents ; but carbon, which is the essential charac-
teristic of wood, and which is the principal source of the
heat- giving qualities of a log fire, is chiefly obtained from
the air.

The atmosphere contains a certain proportion of car-
bonic acid gas, which is composed of the element carbon
in combination with oxygen. To understand what takes

WHAT WE OWE TO THE SUN. 43

place by the action of the growing plant on the carbonic
acid of the air, we should note the circumstances under
which this gas is ordinarily produced in the combination
of oxygen and carbon. In the act of formation quantities
of heat are evolved ; in fact the production of heat in an
ordinary fireplace is the immediate consequence of the
combination between the carbon in the coal with the
oxygen of the air. The difference, then, between a quan-
tity of oxygen and carbon separately and the same quan-
tity of the two substances united in the form of carbonic
acid, simply is that the separate materials have the power
of producing not alone the carbonic acid, but of evolving
a quantity of heat during the process of combination. If
it be required to separate the carbon and the oxygen
already united in carbonic acid, then heat must be applied
during the process exactly in the same amount as would
be given out during the combination of the two elements.
The leaves of the tree have to extract carbon from the
carbonic acid in the atmosphere around them ; to do this
they require heat and light, and these they find supplied
by the beams from the sun. Each leaf of the plant is a
chemical laboratory in which sun heat is applied to the
splitting asunder of the atoms of carbon and oxygen held
so tightly in combination. It is the carbon that the leaves
want ; this material is transmitted to the growing trunk
in which the results of the operation are accumulated.
The oxygen not being required by the necessities of the
plant is returned to the atmosphere. It is well known
that the operation I have here indicated is a beneficent one,
not only to the plant which requires air for its growth,
but also to the animals, including man himself, to whom

44 IN STARRY REALMS.

respiration is a necessity. Animals absorb the oxygen
and give off carbonic acid, while plants remove this gas,
which is pernicious to animal life, abstract from it the
carbon they want, and restore the invigorating oxygen.

Thus the growing tree absorbs heat from the sunbeams,
and is enabled to acquire from the air that quantity ol
carbon which is essential for its increase. Afterwards,
when the tree is burned in the fire, the carbon returns
again to carbonic acid, and in doing so radiates forth again
the heat which had been absorbed in its manufacture.

This process becomes particularly interesting when
viewed in connection with our supply of fuel in the form
of coal. It is well known that coal is the remains of a
splendid vegetation, which from time to time has been
permitted to clothe large tracts on our globe. No one can
venture to express the thousands of years or the millions
of years that have elapsed since the last coal forests
nourished. Certain it is that the period was long ere man
came on this globe. The sun must have shone in those
ancient days as he does at present. His genial beams
were not, however, lost even so far as man was concerned.
The leaves on the great carboniferous forests captured
those beams, utilised them on a stupendous scale for the
production of carbon, and thus manufactured mighty seams
of coal. When we now put this coal on our fires, the heat
which it discharges is none other than that sun heat which
was laid by in the time of those primeval forests. The
light from our gas-burners is of course directly derived
from coal, but we may speak of it really as well as figura-
tively as artificial sunshine; indeed, we might even go
further, and show that almost every source of heat and

WHAT WE OWE TO THE SUN. 45

every source of light may ultimately be traced back to
sunbeams in one form or another.

Let us take as a splendid example the electric light,
which as ordinarily produced is ultimately due to sun-
beams. No doubt it is immediately derived from a pair of
carbon points, but the current necessary for the purpose is
obtained from a dynamo, which must be driven by some
source of power, usually a steam-engine. It follows that
the power of the steam-engine is the source of the electric
light ; but the steam-engine is driven by steam, and the
steam is made from water, and the water is heated by
coal : thus it is coal which actually generates the electric
light — the boiler, the water, the steam-engine, the dynamo,
the wires, and the carbon points are merely appliances by
which something which has been stored in the coal is
applied to the particular purpose of 'generating light.
The coal, as we have seen, has derived its potency from
the sunbeams which shone on the ancient forests at the
time of the formation of the coal measures. Our electric
lights to-night may be regarded as a resuscitation on a
feeble scale of sunbeams emitted from the orb of day
millions of years ago.

In such a case as that just considered, we seem to be
drawing on an accumulated hoard of sunbeams stored
away in the earth's interior ; but we can also utilise what
we may describe as the present radiation of the sun from
day to day for the production of artificial illumination.
For the prodigious energy of modern civilisation the Falls
of Niagara have to be rendered serviceable, even though
the process involves a lamentable sacrifice of their beauty.
The power of those Falls suitably applied to water-wheels

46 IN STARRY REALMS.

conld be made to turn dynamos to generate electricity
enough for the wants of a mighty people. It is easy to
show that even in such a case also sunbeams would be un-
questionably the effective agents. Niagara is supplied by
the rain which falls over a vast tract of country ; that rain
is brought thither by the action of warm sunbeams, which
beat down on the oceans, evaporate the water, and raise it
aloft ; then the winds waft the vapour over the globe until
it again descends in the form of rain. Thus we learn that
the power of Niagara, no less than the efficiency of the
coal-fields in the production of heat and light, is to be
attributed to the sun, the difference being that in the one
case we are utilising the sun's ancient beams, and in the
other the power is provided from the radiation of the sun
at the present time.

One more question may be briefly discussed. It will
be remembered that in what we have here said we are
merely discussing the natural consequences of those
laws of nature which are in operation around us. We
are tempted to push the inquiries back a stage earlier,
and examine how the materials of the sun came into the
nebulous state from which they have gradually become
transformed to the sun that we now have. It is quite
evident that the nebula cannot have existed from all time,
for were this so the sun must necessarily have long since
passed into that cold and inert condition which at present
is looked forward to in the dim future. It is, therefore,
interesting to inquire whether by the operation of natural
laws a vast glowing nebula could have been called into
being at some period not infinitely remote from the present
day. There is a possible, indeed a highly probable, ex-

WHAT WE OWE TO THE SUN. 47

planation of such an occurrence. Suppose that two bodies,
each dark and cold, were to strike together in space with
such velocities as are ordinarily possessed by celestial
bodies, then a vast generation of heat would be a conse-
quence of the collision. Each of the bodies possesses a
certain quantity of energy in virtue of its motion; but
an important law of mechanics tells us that, though the
motion may seem partly or wholly annihilated by the
collision, yet the energy is not lost; it must reappear
in one of the other garbs which energy can so rapidly
assume. In such a case as this it will immediately take
the form of heat, and the two bodies will be heated by
the blow. Other things being equal the quantity of
heat that will be produced depends upon the speed with
which the two colliding objects rush together. With
high speeds the heat that can be thus generated is much
greater than when the speeds are low. Thus, for in-
stance, if the speed be doubled the heat that would be
generated by the collision is increased fourfold ; or if
the speed of approach be increased tenfold, then the
consequent production of heat is made ten times ten, that
is, one hundredfold. Speaking generally, we might say
that the heat generated by the collision is proportional to
the square of the relative velocity with which the two
bodies fly together.

Considering the enormously high velocities with which
the heavenly bodies are animated, it is obvious that in
the possibility of collision we have a source of heat
adequate for the production of the most gigantic phe-
nomena. For example, let us take our own earth, which
has at present a speed of eighteen miles a second. Sup-

48 IN STARR Y REALMS.

pose that in the depths of space it should happen that
two globes, each as large as our earth and each having
a velocity of eighteen miles a second, rushed into direct
collision ; it can be shown that the energy which would
be transformed by the shock would be adequate, when
converted into heat, to raise the two bodies not merely
to incandescence but even to transmute them into a
gaseous nebula. Here then we have a possible explana-
tion as to how the nebula from which our sun has been
developed was originally formed. It may have been
produced, and most likely was produced, by the collision
between two other bodies. In the same way it may yet
conceivably happen that after the sun shall have grown
condensed and cold, and is journeying on with enormous
velocity through space, it may again come into collision
with some other body, and the transformation of the
energy from motion into heat may again generate another
nebula, and set in progress yet another evolution like that
through which we are at present passing.

CHAPTER V,

THE CONSTANT FACE OF THE MOON.

THE myriad bodies that adorn the skies are of varied
degrees of brilliancy and of the widest range in dimensions.
It seems a reasonable question to ask which of all the
celestial bodies is the smallest. I speak not now of
telescopic objects, I am only alluding to those which can
be seen by unaided vision. Many of the stars which lie
on the verge of visibility seem extremely minute, but then
we have the best reasons for believing that this apparent
smallness is merely a delusion. The stars, so far as they
are known to us, are found to be bodies far exceeding the
earth in size, so much so that many of them are comparable
with our sun itself. The planets are smaller than the
more important stars, notwithstanding the fact that the
apparent lustre of Venus or of Jupiter under favourable
circumstances exceeds that of any star. It is, however,
somewhat surprising to learn that, so far as is at present
known, the very smallest of all the orbs visible to the
unaided eye is our satellite, the moon. Seeing that the
illumination dispensed by the moon is greatly in excess of
that sent to us by all the other heavenly bodies put together,

5o IN STARRY REALMS.

the sun alone excepted, we may well pause before admit-
ting the truth of the somewhat startling fact that each
of these stars and planets, dim though they admittedly
are, must still be intrinsically larger and heavier than the
moon.

No doubt the telescope discloses to us multitudes of
celestial objects many of which do not possess one ten-
thousandth part of the bulk of the moon. But those
objects are all so faint that good instruments are required
to make them discernible. As to the real dimensions of
the moon T may as well mention, once for all, the few facts
that will be important for us. The diameter of our satellite
is rather more than one-fourth the diameter of the earth.
To express it a little more precisely, we may say that the
moon's diameter is two thousand one hundred and sixty
miles, that is to say about as far as from London to
the Caspian Sea, or from the Straits of Gibraltar to the
Crimea.

It is instructive to open a map of Europe and then draw
with a pair of compasses a circle representing the size of
the moon on the same scale as that of the map. This
circle will enable the dimensions of our satellite to be
properly appreciated. The British Islands are of compara-
tively insignificant extent if placed in the centre of the
figure.

In the figure on the opposite page we have indicated
the circle so as to include the larger part of Europe.
From the Ural Mountains the circumference passes north
into Sweden and Norway, then traversing the North Sea
it just touches the eastern parts of England. There it
crosses the Channel, and after passing through the centre

THE CONSTANT FACE OF THE MOON. 51

Fig. 4. — The Lunar Disc shown against the Map ol Europe.

of France enters the Mediterranean. Bisecting Sardinia
and grazing the south of Sicily the circle then keeps well
to the south of the Grecian Archipelago, shoots beneath
Asia Minor, and touches land again in Palestine. Here
it rises towards the north, including the Black Sea in its
ample interior, and just cutting off a corner of the
Caspian. Finally, the curve bends towards the Ural
Mountains, and completes its circuit of the Continent.
This little sketch will also show that when we look at the
full moon the objects there so conspicuous must be as
large, say, as Spain or the Black Sea.

The distance of the moon from the earth is, in round
numbers, two hundred and forty thousand miles; how-
ever, it varies somewhat, so that in extreme cases the
moon's distance may be twenty thousand miles more

52 IN STARRY REALMS.

or twenty thousand times less than its mean value. It
may be observed that a circle as large as the orbit of the
moon could be entirely included within the sun. This is
illustrated in Fig. 5. If we desire to find a convenient
standard of comparison between the distance of the moon

Fig. 5. — To compare the orbit of the moon with the size of the sun.

and the size of the earth, let us think of a cord wrapped
ten times round the earth at the equator, this would be
long enough to extend from the earth to the moon.

One of the earliest facts that any observer will notice,
is that the moon always turns the same face towards us.
The simplest observations suffice to give assurance on this
point. There are large and conspicuous marks on our

THE CONSTANT FACE OF THE MOON. 53

satellite which have been observed from all antiquity, and
whenever the surface of the moon can be seen, the same
marks are always discernible. I ought, however, to qualify
this remark by saying that the face of the moon is some-
times tilted a little, so that the objects found close to the
edge are not always unchanged. Speaking generally,
however, there is hardly a more striking fact in the celes-
tial movements than the constant face of our satellite. We
have little doubt that the moon, like the other heavenly
bodies, is globular in its form, but we have never been
actually able to see that the opposite side of the moon has
the hemispherical shape that we find possessed by the side
which is presented to us. As we have not the means of
holding up a looking-glass behind the moon there seems to
be no possibility of learning what is at the other side.

We may perhaps emphasize the remarkable attitude of
the moon towards the earth, by contrasting it with that of
some of the other celestial bodies. Consider, for instance,
the corresponding circumstances with regard to the sun.
It is true that our great luminary exhibits to us no per-
manent features like those which are engraved on the
moon's face, but the spots on the sun, transient though
they doubtless are, still endure long enough to testify con
clusively that the sun rotates on its axis. We learn in fact
from a vast accumulation of observations specially directed
to this one point that the period of the rotation of the
sun is certainly more than three weeks, and certainly less
than four. Hence that side of the great luminary which
is directed towards the earth must be constantly changing.
If we look on one hemisphere of the sun to-day, then a
fortnight ago it was the opposite hemisphere that was

54 IN STARRY REALMS.

turned towards us, and in about another fortnight we shall
have the original hemisphere restored. Each side of the
sun is thus equally accessible to our observations ; there is
no region withheld from our scrutiny as is the case in the
moon.

Similar language may be used with regard to the
attitude of the planets towards the earth. Mars is so much
farther from us than the moon that we cannot scrutinize
his ruddy globe so minutely as we are able to do that of
our own satellite ; but Mars turns round on his axis once
every twenty-four hours and a half, so that we are able to
form maps of his entire surface, and whatever incomplete-
ness those maps may possess arises from such causes as the
imperfection of our instruments or the distance of Mars ;
for we cannot assert that the movements of the planet are
such as to prevent our examination from being extended
over the whole of his globe. The giant orb of Jupiter
revolves with rapidity on its axis. Suppose an astronomer
looks at the great planet at eight o'clock in the evening,
observes the belts and other configurations of its surface,
and turns to other work until about one o'clock in the
morning ; if he now directs his telescope again to Jupiter
the panorama is entirely changed. The belts and marks
which he saw in the earlier observations have vanished, they
have in fact retired to the opposite side of the globe, while
all the objects that he sees in his second observation were
on the distant side of the globe at the time of the first
observation and were consequently invisible. Should his
ardour be equal to that of some famous astronomers, as,
for instance, to that of Sir William Herschel, whose
hours of observing were from dusk to dawn, our observer

THE CONSTANT FACE OF THE MOON. 55

might then, if the season were suitable, look at Jupiter
again at six o'clock ; in this case the hemisphere he last
saw would have vanished, while that presented to him at
eight o'clock on the previous evening would have been
restored. He would again find the same belts and marks
as those he saw on the first occasion, altered only by such
intrinsic changes as may have actually taken place on the
planet in the interval. These illustrations will suffice to
indicate how different are the circumstances of the moon's
motion from those that we find in the other celestial bodies.

If I were asked to offer a conjecture as to what the
other side of the moon may be like, I should not hesitate
to reply that in all probability it would present features of
a similar character to those on that side of our satellite
with which we are so familiar. The study of the sun and
planets shows that they are, generally speaking, symmet-
rical in form, and the circumstance that we happen to be
prevented from viewing one side of the moon does not
impair the analogy between it and the other orbs of heaven.
We should therefore expect to find on that distant side
of the moon indications of the same volcanic and other
phenomena that our telescopes show on that surface which
we actually see.

We have mentioned the case of other globes which
revolve on their axes, the revolution being conducted in
such a manner that the entire surface is more or less
known to us. The moon also revolves on its axis, the
peculiarity, however, consisting in the circumstance that
the time taken for the rotation about its axis is equal to
that of its revolution round the earth. An old paradox is
connected with this question, for as the moon does not

5o ZN STARRY REALMS.

change its aspect towards us, it has been sometimes
argued that it cannot be turning round. The difficulty,
such as it is, resolves itself more into a matter of words
than of actual fact. Let me try to explain it as follows.

Suppose a horse is galloping round a circular course,
then a spectator at the centre of the course will have one
side of the horse turned towards him, while the other side
of the horse is invariably turned away. To this extent
therefore the movement of the horse round the course is
analogous to the movement of the moon around the earth.
When the horse has completed his circuit it will be found
that he has not only performed a revolution around the
course, but he has also rotated once in the same period.
It is obvious that the horse must at one moment or another
of his journey have faced every part of the horizon suc-
cessively. No matter how the course be placed there must
have been one point so situated that while passing it the
horse was going directly towards the north, while when he
has reached the opposite point of the circle he is galloping
towards the south. At one intermediate point he is for the
moment bound towards the east, but at the point diametri-
cally opposite his flight is towards the west. It is clearly
impossible that the horse can have faced north, south, east,
and west, without performing a movement of rotation.
Unless he rotated he would necessarily be always journey-
ing towards the same point of the compass. Suppose that,
at the moment when he was facing due north, his right
side being turned towards the spectator in the middle, we
bring him to the opposite side of the course, and again
place him facing due north. It is clear that the left side
of the animal will now be towards the spectator. In such

THE CONSTANT FACE OF THE MOON. 57

a case it is plain that the movement has not been at all
like that of the moon, the characteristic feature of which is
that the same face is always directed towards the earth.

It will not be a sufficient explanation to say that if the
moon rotated on an axis at all it must rotate in some
definite period, and if so, why should not that period be
identical with the revolution of the moon round the earth ?
Would it not be, it might be urged, as likely to be that period
as any other ? This will not satisfy the student of nature ;
the coincidence between these periods is so perfect that the
chances are many millions to one against its occurrence if
there were no physical reason for its existence. It is now
well known that there is such a reason, and as it affords an
instructive illustration of the mutual relations between the
earth and her satellite, I propose to enter into the matter
with some little detail.

I must ask you to think of a very ancient epoch ; how
ancient it is to be I cannot express by the use of ordinary
chronology ; it must have been hundreds of thousands of
years ago, probably many millions of years ago. At this
period, however, the moon was not like the object that we
know so well. Her surface now seems to be composed
of rocks or stone, or at all events of materials which are
hard and solid. But this was not always so. There are
abundant traces of the remains of past volcanoes on the
moon ; their craters are no longer active, but some of
them once had dimensions and energy transcending any of
the volcanoes on the earth that are now known to us. It
is thus certain that at some former time the moon must
have possessed stores of internal heat by which these
volcanic eruptions were produced. It therefore follows

58 IN STARRY REALMS.

that even if the moon be now entirely cold, it must once
have been heated. No one is likely to dispute this, but
yet the full consequences of such an admission are not
always readily perceived. For if the moon were once hot
it is plain that earlier still it must have been yet hotter,
and therefore the farther we look back into the depths of
time past, the hotter and hotter do we see that the moon
must have been. At last we discover an epoch, how
remote I do not pretend to say, when our neighbouring
globe, instead of being the quiescent object that we now
see must have contained many active volcanoes indicative
of an interior containing volumes of molten lava. Earlier
still we perceive the moon ever hotter and hotter, until
at last we seem to discern a time when it must have
consisted of a globe, partly, or even wholly, molten. This
seems an inevitable deduction from the fact that the moon
bears traces of having once possessed internal heat. Such
a view of the early condition of our satellite will explain
many of its characteristic features. At present, however
I am only concerned to account for the fact that the moon
now always turns the same face towards the earth.

Every one is familiar with the fact that the moon raises
tides on the earth ; these tides ebb and flow along our
coasts, and in virtue of them the satellite exercises a cer-
tain control on the movements of our globe. If the moon
had liquid oceans on its surface there cannot be a doubt
that the attraction of the earth would generate tides in the
oceans on the moon just as the attraction of the moon gene-
rates tides in the oceans of the earth. But there would be
a fundamental difference between the two cases ; the shores
of the lunar seas would be periodically inundated by tides

THE CONSTANT FACE OF THE MOON. 59

far vaster than any tides which the moon can create on the
earth. But it may be said that as the moon contains no
water it seems idle to talk of the tides that might have
been produced in oceans if they had existed. It is no
doubt true that the moon contains no visible liquid water
on its surface at the present time ; it is, however, by no
means certain that our satellite was always void of water ;
it is not at all impossible that spreading oceans may have
once occupied a large part of that surface now an arid
wilderness. The waters from those oceans have vanished,
but the basins they presumably filled are still left as
characteristic features on our satellite. For our present
argument, however, it is really not material that the moon
should ever have had oceans as we understand them. Have
we not seen that there was a time when the very mass of
the moon itself appears to have been largely, if not wholly,
liquefied ? The water at those remote periods must have
been suspended in the form of vapour around the more
solid parts of the glowing globe. But tides can be
manifested in other liquids besides that which forms our
seas. In fact if the basins of our great oceans were filled
with oil or with mercury, or even with molten iron instead
of water, the moon would still cause tides to ebb and flow,
no matter what the material might be, so long as it pos-
sessed to some extent the properties of a liquid. It need
not be a perfect liquid, for any material which is in some
degree viscous, like honey or treacle, would still respond
to tidal influence, though not, it may be well believed, with
the same alacrity and freedom of movement as would a
fluid of a more perfect character. In the molten moon
itself, throughout the very body of our satellite, the tidal

bo IN STARRY REALMS.

influence of the earth must have been experienced in these
primitive ages.

We can hardly describe the tides in such a case
as producing actual currents like those which our sailors
know so well; they must rather be regarded as throb-
bings or heavings throughout the mass of the moon
which would originate deformations of its constitution.
Whenever movements of this kind are produced friction
must inevitably result, and it is characteristic of friction
to act in such a way as tends to check the movement by
which it is caused. The effect of the tides in the wholly
or partially fluid moon would therefore incessantly tend
to adjust the moon's movement in such a manner that the
tides should not further disturb it. There would, no
doubt, be a high tide on two opposite meridians of the
moon and low tide in the intermediate regions, but those
conditions would be permanent. To take the simplest
supposition we may regard the high tide as present on the
central meridian on that side of the moon turned towards
the earth, and as that side remains constantly directed to-
wards the earth during the moon's monthly movements,
the high tide remains always at the same lunar locality
There is no ebb and flow, there is no distraction of the
material of the globe, which, having become once adjusted
to this condition, remains without future tidal change.

There cannot be a doubt that in ancient days when the
moon was sufficiently fluid, the action of the tides tended
without ceasing to the establishment of such an adjustment
between the rotation of the moon around its axis and the
revolution of the moon around the earth, that the two
should be brought to have equal periods. Friction would

THE CONSTANT FACE OF THE MOON. bi

incessantly operate until this adjustment had been effected,
and owing to the preponderating mass of the earth such
strenuous tides must have been evoked in the moon that
our satellite was brought under tidal control with compa-
rative facility. Hence it arose that in those early days
the habit of bending the same face incessantly towards
the earth around which it revolved was established on
our satellite.

Time passed on, the moon gradually dispensed its
excessive heat by radiation into space, and it gradually
became transformed from a molten globe to a globe with
a solid crust. It may be that the water was condensed
from vapour and then collected together into oceans on the
newly formed surface ; if so, these oceans would not have
any ebbing tide or flowing tide, for it would be constant
high tide at some places and constant low tide at others.
Such a state of things would at all events endure so long
as the adjustment of equality between the moon's rotation
and its revolution continued. In fact, should any depar-
ture from this adjustment have manifested itself corre-

J

spending tides would have begun to throb in the lunar
oceans, and their tendency wouid be to restore the adjust-
ment which was disturbed. This arrangement between
the two movements was necessarily stable when tidal con-
trol was always at hand to check any tendency to depart
from it.

It may be that the moon has now cooled so thoroughly
that not only is it hard and congealed on the exterior
as we see, but it seems highly probable that the heat
may have so entirely forsaken even the interior that
there is no longer any fluid in the globe of our satellite to

62 IN STARRY REALMS.

respond to tidal impulse. There is, therefore, in all
probability, no longer any actual tidal control. On the
other hand, however, there is nothing to disturb the
adjustment. It was, as we have seen, caused by the tides
which have done their work ; the consequences of that
work are still exhibited in the constant face of the
moon, which, now that it has been brought about, seems
likely to exist permanently aa a stable adaptation of
the movement.

CHAPTER VI.

THE MOON'S HISTORY.

THE doctrine which I shall here endeavour to set forth is
mainly due to the labours of Sir George H. Darwin of
Cambridge. I have mentioned how the tendency of the
tides on a tide-disturbed globe is to adjust the movements
of that globe in such a way that the tides shall no longer
ebb or flow, but that permanent high tide shall be estab-
lished in some places and permanent low tide in others.
If the rotation of the body be not fast enough the tide will
pull the body round in order to effect this object. If the
rotation of the body be too rapid then the influence of the
tide will tend to check the movement and bring down the
speed of rotation until the desired adjustment is obtained.
At present the earth is spinning too fast to permit the
high tides to remain at permanent localities, and con-
sequently tides are applied with the effect of checking the
rotation. The earth is, however, so vast, and the tides
generated by so small a body as the moon are relatively
so impotent, that their effects in reducing the speed of
the earth's rotation are insignificant. Nevertheless, small
though they are, they unquestionably exist, and there can-

64 IN STARRY REALMS.

not be a doubt that to some extent the earth is affected by
the unremitting action of the tides ; the consequence is
that the rapidity with which the earth rotates upon its
axis is gradually declining.

One result of this can be stated in a very simple
manner. The length of the day must be increasing
It is true that this gradual stretching of the dav
is very slow ; it is indeed quite inappreciable in so far
as our ordinary use of the day as a measure of time is
concerned. The alteration almost eludes any means of
measurement at our disposal. Even in a thousand years
the change is so small that the diminution in the length of
the day is only a fraction of a second. We can doubtless
afford to disregard so trifling a variation in our standard
of time so far as the period contemplated in mere human
affairs is concerned. In fact the change is absolutely devoid
of significance within such periods as are contemplated
since the erection of the Pyramids, or indeed since any other
human monument has been reared. We must not, how-
ever, conclude that the change in the length of the day has
no significance in earth history. Of late years it has be-
come known that this alteration in the day is connected
with an important chapter in the remote history of
this globe. In fact there is hardly a more interesting
doctrine in modern science than that which deals with this
subject. I have explained the phenomena somewhat fully
in my little volume entitled " Time and Tide," in which I
have endeavoured to sketch the remarkable evolution
through which the earth seems to have passed. I must,
\iowever, here give a few of the leading features in the
story, for it would be quite impossible to exclude it alto-

THE MOON'S &ISTOXY. 65

gether when the subject of our satellite is under discus-
sion.

The significance of the gradual elongation of the day by
the tides arises from the circumstance that the change
always takes place in one direction. In this form of effect
the tide differs from other more familiar astronomical
phenomena which sometimes advance in one direction and
then after the lapse of suitable periods return in the
opposite direction, and thus restore again the initial state
of things. But the alteration of the length of the day is
not of this character, it is not periodic, its motion is never
reversed, is never even arrested. Only one condition is
therefore necessary to enable it to obtain tremendous dimen
aions, and that is sufficient time in which it can operate.

There are many lines of reasoning which show the ex-
treme antiquity of our globe : the disclosures of geology are
specially instructive on this head. Think, for instance, of
that mighty reptile the Atlantosaurus, which once roamed
over the regions now known as Colorado. The bones of
this vast creature indicate an animal surpassing in propor-
tions the greatest elephant ever known. No one can count
the SBons of years that have elapsed since the Atlantosaurus,
whose bones are now to be seen in the museum at Yale
College, breathed its last. A still more striking concep-
tion of time than even the antiquity of this creature
affords is derived from the consideration that his mighty
form was itself the product of a long and immeasurable line
of ancestry, extending to a depth in the remote past far
beyond the limits of computation. I have mentioned this
illustration of the antiquity of the earth for the purpose of
showing the ample allowance of time that is available for

6fc [N STARRY REALMS.

tides to accomplish great work in earlier stages of our globe's
history.

As the evidence of the earth's crust proves that
our globe has lasted for incalculable ages, it becomes of
interest to think how far the gradual elongation of the day
may have attained significant proportions since very early
time. It may be that even in a thousand years the effect
of the tides is not sufficient to alter the length of the day
by so much as a single second. But the effect may be very
appreciable or even large in a million years, or ten million
vears. We have the best reasons for knowing that in
intervals of time comparable with those I have mentioned,
the change in the length of the day may have amounted
not merely to seconds or minutes, but even to hours.
Looking into the remote past, there was a time at which
this globe spun round in twenty-three hours instead of
twenty-four ; at a still earlier period the rate must have
been twenty hours, and the further we look back the more
and more rapidly does the earth appear to be spinning.
At last, as we strain our gaze to some epoch so excessively
remote that it must have been long anterior to those
changes which geology recognises, we see that our globe
was spinning round in a period of six hours or five hours,
or possibly even less. Here then is a lesson which the
tides have taught us : they liave shown that if the causes
at present in operation have subsisted without interruption
for a sufficiently long period in the past, the day must
have gradually grown to its present length from an initial
condition in which the earth seems to have spun round
about four times as quick!}7 as it does at preseut.

We should, however, receive a very inadequate im-

THE MOON'S HISTORY. 67

pression of what tides are able to accomplish if we
merely contemplated this change in the length of the day,
striking and significant though it doubtless is. The
student of natural philosophy is well aware that there is
no action without a corresponding reaction, and it is
instructive to examine in this case the form which the re-
action assumes. Our reasoning has been founded on the
supposition that it is the attraction of the moon on the
waters of our globe that gives rise to the tides. It is,
therefore, the influence of the moon which checks the
speed of the earth's rotation and adds to the length of the
day.

As the moon acts in this fashion on the earth, so, by
the general law that I have mentioned, the earth reacts
upon the moon. The form which this reaction assumes
expresses itself in a tendency to allow the moon gradually
to move farther and farther away from the earth than
the earth's attraction would permit if our globe were
a solid mass void of all liquid capable of being dis-
tracted by tides. It is, therefore, certain that the distance
of the moon, which is at present about two hundred and
forty thousand miles, must be gradually increasing ; but
we need not look for any appreciable change in the moon's
distance arising from this cause when only an interval of
a few centuries is considered. We need not expect to
measure the difference due to tides between the size of the
moon's orbit this month and the size of the orbit last
month. In fact, there are so many periodic causes of
change in the dimensions of the moon's orbit, that it
becomes impossible to detect the tidal influence even in the
course of centuries. Here, again, we have to remember

68 IN STARRY REALMS.

that in dealing with the history of our earth, as such
history is contemplated by the astronomer, we are to con-
sider not merely the thousands of years that include the
human period, not merely the millions of years that are
required by the necessities of geology, but also those un-
known periods anterior to geological phenomena to which
we have already referred.

In the course of such vast ages the reaction of the
earth on the moon's orbit has not only become per-
ceptible, it has become conspicuous ; it has not only
become conspicuous, but it has become the chief deter-
mining agent in making the moon's orbit as we find it
at the present day. WB have seen that as we look into
the past the length of the day seems ever shorter and
shorter ; and concurrent with this decline in the day is
the diminution in the moon's distance from the earth.
There was a tin^ w^hen the moon, instead of revolving at
a distance of two hundred and forty thousand miles, as it
does at present, revolved at a distance of only two hundred
thousand miles. As we think of epochs still earlier we
discern the moon ever closer and closer to the earth, until
at last, at that critical time in the history of the earth-
moon system, when the earth was quickly revolving in a
period of a few hours, our satellite seems to have been
quite close to the earth ; in fact, the two bodies were
almost in contact.

The study of the tides has therefore conducted us
to the knowledge of a remarkable configuration ex-
hibited in the primitive earth-moon system. The earth
was then spinning round rapidly in a day which was
only a few hours long, whilst close to the earth, or almost

THE MOON'S HISTORY. 69

in contact with it, the moon coursed around our globe, the
period of its revolution being shorter to such an extent
that the satellite completed its circuit in the same time as
the earth required for one turn round its axis. We cannot
indeed say that the earth and the moon in this wonderful
condition bore much resemblance to the earth and the
moon as we now know them. Even if the bodies had by
that time assumed the globular form, it seems certain that
they must have been composed of wholly or partially
molten material, very unlike the rigid globes that the
earth and moon now appear to be.

Up to this point our reasoning has been based on the
results of mathematical investigation. It is characteristic
of such investigations that they leave no loop-hole for
error when the conditions have been properly stated. We
are naturally anxious to push the research one step farther
and to learn how it was that the moon happened to
be found in such a remarkable situation in the immediate
vicinity of the earth. At this critical stage mathe-
matics seems to withhold its guidance, but at the same
time does not forbid an attempt to penetrate the mystery
of the moon's origin. The reasoning that has hitherto
guided our inquiries becomes no longer available, and any-
thing which we can add to the sketch already given of
earth-moon history must therefore be received on authority
possessing a different value from that which has charac-
terized the former stages of the inquiry. It is, how-
ever, almost impossible to resist the conjecture which
naturally arises when we endeavour to form a mental
picture of the condition of the earth and moon at this
most interesting epoch.

70 JN STARRY REALMS.

We must remember that the materials destined to
form the pair of allied planets did not then form
two solid bodies as they do at present ; they were both,
in all probability, incandescent masses glowing with
tervour, and soft, if not actually molten, or incohe-
rent, or even gaseous. These aggregations were close
together, and one of them was whirling around the other
in a period of a few hours, the duration of that period
being equal to the time in which the larger mass revolved
on its axis. In fact, the two objects, even though distinct,
seem to have revolved the one around the other as if they
had been bound together by rigid bonds. The rapid rota-
tion with which they were animated suggests a cause for
this state of things. It is well known that a fly-wheel,
when driven at an unduly high speeed, is liable to break
asunder in consequence of its rapid motion. If a grind-
stone be urged around with excessive velocity the force
tending to rend the stone into fragments may overcome
its cohesion, and it will fly into pieces, often projected with
such violence that fearful accidents have been the conse-
quence.

Viewing the earth as a rotating body, it must be sub-
ject to the law that there is a speed which cannot be ex-
ceeded with safety. With the present period of rotation
of once in every twenty-four hours the tendency to dis-
ruption is but small and consequently the earth retains its
integrity, though no doubt the protuberance at the equator
is the result of the accommodation of the shape of the globe
to the circumstances attending its revolution. But let us
suppose that the length of the day was greatly diminished,
or, what comes to the same *hing, that the speed with

THE MOON'S HISTORY. 71

which the earth rotates on its axis was greatly increased ;
it is then conceivable that the tension thus arising might
be too great for the coherence of the material to with-
stand. We believe that the earth could turn round with
double the speed that it has at present before this tension
approached the point at which disruption would ensue.
But supposing the day were to be so much shortened that
the period of rotation was only a very few hours instead
of twenty-four, there is then good reason to know that
the tension in the earth arising from this rapid rotation
would be so great that a rupture of the globe would be
imminent.

Provided with this conception, let us think of the initial
stage when the moon was quite close to the earth. Our
globe was then, as we know, spinning round so rapidly
that its materials were almost on the point of breaking
up in consequence of the strain produced by the rotation.
It is interesting to note that the tidal action of the sun
would also conduce to the rupture of our globe in the
critical circumstances we have supposed. It seems hardly
possible to doubt that such a separation of the glowing
mass did actually take place, a small fragment was dis-
carded, and gradually drew itself by the mutual attraction
of its particles into a globular form and thus became
the moon.

Such is the view of the origin of the moon which is
suggested by tidal evolution. Similar deductions from the
theory of the tides will enable us to offer a forecast of the
future career of the earth-moon system. Let us transfer
our attention from the remote past and endeavour to think
of the distant future. So long as the solar system shall

72 IN STARRY REALMS.

last without the intervention of other agents than those
known to us by the law of gravitation, it seems certain
that the progress of the earth-moon system must be in
substantial accordance with the principles here laid down.
We have seen that at the present moment the day is
becoming gradually longer and the moon is steadily
receding farther and farther from the earth. At present
these changes take place with extreme slowness, but in the
primitive periods of which we have already spoken, the
changes in the length of the day, and the changes in the
distance of the moon, proceeded at a rate far more rapid
than at present. As the moon has receded farther from
the earth its efficiency as a tide-producer has declined, and
consequently the rate at which the consequences of tidal
action have proceeded is continually lessening. It must
therefore be expected that the progress of tidal evolution
in the future will be ever getting slower and slower, so
that the periods of time required for the further develop-
ment of the phenomena far exceed those which have
elapsed in the course of the history already given. We
can, however, foreshadow what is to happen in the folldV-
ing manner. The length of the day will slowly increase ;
and we can indicate a state of things in the excessively
remote future towards which it may be said the system
is tending. The day will grow until it becomes not merely
twenty- five or twenty-six hours, but until it becomes
as long as two or throe of our present days. In fact, as
we stretch our imagination through ages so inconceivable
that I forbear to specify any figures which might charac-
terize them, we seem to discern that the length of the day
may go on ever getting longer and longer until at last a

THE MOON'S HISTORY. 73

stage is reached when the day is about fifty or sixty
times as long as our present day.

All this time, in accordance with the general law of action
and reaction, the moon must be gradually retreating ; the
orbit of the moon is destined to grow ever wider and wider ;
the distance of our satellite from the earth becoming ever
greater and greater until at last the period is reached to
which we have already referred, when the day is some
fourteen hundred hours long. As the orbit of the moon
is gradually enlarging, the time that the moon takes to
revolve around the earth must be continually on the
increase ; from the present month of twenty-seven days
the length of the month will gradually augment as the
ages roll by until at last when the moon has reached a
certain distance the period of its rotation will have
become double what it is at present, or indeed rather more
than double, and we shall have the day and the month
equal, each being about fourteen hundred hours long.
When this state of things is reached, the earth will always
turn the same face towards the moon, just as the moon
at present always turns the same face towards the earth.

We have already explained how the constant face of the
moon can be accounted for by the action of tides raised
in the moon by the attraction of the earth. Owing to the
small size of the moon the tides have already wrought
all that they were capable of doing, and have compelled
the moon to succumb to the conditions they imposed.
Owing to the great mass of the earth and the compara-
tively small mass of the moon the tides on the earth
raised by the moon have required a much longer period
wherein to accomplish their effects than was the case

74 IN STARRY REALMS.

when the earth raised tides on the moon. But small
though our satellite may be, yet the tides raised on the
earth have incessantly tended to wear down the speed of
our globe and reduce it to conformity with the law that
the two bodies shall bear the same face towards each
other. At present the earth turns round twenty- seven
times while the moon goes round once, so the tides have
still a gigantic task to accomplish. With unflagging
energy, however, they are incessantly engaged at the work,
and they are constantly tending to bring down the speed
of the earth ; constantly tending towards that ultimate
condition of things in which the earth and moon are des-
tined to revolve in a period of fourteen hundred hours as
if they were connected with invisible bonds.

If such a state of things as this were established then
it is plain that tides would no longer ebb and flow,
that is, at least, if we exclude from our consideration
the intervention of any other body. High tides must
prevail at some parts of the earth, and low tides at
other parts, but the position of these tides will remain
fixed. Where it is high tide it will always be high
tide; where it is low tide it will always be low tide.
When this state of things is reached, the moon will be
constantly visible in the same part of the sky from one
half of our globe, while the other half of our globe will
never be turned towards the moon. In fact, the
moon would always appear to us in a fixed position as
the earth would always appear to be if viewed by an
observer stationed on the moon. If there were any Luna-
rians whose residence was confined to the opposite side
of the moon, they could never see this earth at all, while

THE MOON'S HISTORY. 75

those who lived on this side of our satellite would always
be able to see the earth apparently fixed in the same part
of the sky. An observatory located at the middle of the
moon's disc, say near the crater Ptolemy, would always
have the earth in its zenith or very near thereto, while the
astronomer, let us say, in the Mare Crisium, would always
find the earth low down near his horizon.

In order to facilitate our reasoning I have assumed that
the moon is the only tide-producing agent ; this is, how-
ever, not the case. No doubt the ebb and the flow
around our coasts is generated mainly by the attraction
of the moon. It must not, however, be forgotten that a
portion of the tide is originated by the attraction of the
sun. These solar tides will still continue to ebb and flow
quite independently of the lunar tides, so that even if
the accommodation between the earth and the moon had
been completed some further tidal disturbance would not
be wanting. The effect of the solar tides will be to abate
still further the velocity with which the earth turns
round on its axis, and consequently a time must ultimately
arrive when the length of the day will be longer than
the time which the moon takes to revolve around our
earth.

It is here interesting to note that an adaptation of
a somewhat similar kind has already been detected in
another part of the solar system. Our neighbouring
planet Mars is attended in its revolution around the sun by
a pair of small satellites. The inner of these little moons
presents features unlike those to be seen in any other part
of our system at present, but resembling in a marked
degree the future which we may venture to prognosticate

76 IN STARRY REALMS.

for our earth- moon system when sufficient ages have
elapsed. Mars rotates on its axis in about half an hour
longer than our present day. The interior satellite to
which we have referred courses around the planet so
quickly that it completes a circuit in about seven hours
and a half, so that the Martial moon revolves three times
round its primary in the same time that Mars requires for
a single rotation. In the case of this planet and his
satellite the dimensions and the masses involved are much
smaller than are those in the case of the earth-moon sys-
tem. It seems, therefore, that we have in the adjustment
of Mars and his moon a sort of miniature representation
of the state of things to which the earth- moon system
tends in the excessively remote future.

CHAPTER VIL

THE LUNAR WORLD.

THE genuine man of science can never approach the study
of the moon without recalling that the orbit of our satellite
is a precinct specially associated with the name of Newton.
It was obvious to the clear vision of the great philosopher,
that some power resided in the earth by which the famous
apple was pulled down. The existence of this power was,
however, not Newton's discovery ; many a previous in-
vestigator, while pondering on the fact that bodies fall
downwards to the earth, perceived the obvious analogy to
the attraction of a piece of iron by a magnet. To demon-
strate the mere existence of that force which we call the
attraction of gravitation did not require the intellect of a
Newton. One of the discoveries which have given im-
mortality to the name of the great philosopher was con-
nected with the moon. He soared in thought far above
the apple-tree, and he asked himself whether the motion
of the moon may not be due to the same force as the fall of
the apple. When he had solved that problem, the scheme
of the universe lay open before him.

Let us consider this question of falling. Objects dropped

7« IN STARRY REALMS.

from a balloon which has ascended two or three miles into
the air will duly fall to the earth. A meteorite, which i&
often a lump of solid iron, may tumble down here from an
altitude of unknown miles. The tendency to fall to the
earth certainly exists at a height of ten miles, or fifty
miles, or hundreds ot thousands of miles. Why, then,,
does not the moon fall ? There is assuredly no material
structure to keep it up, no scaffolding to enable the moon
to resist the earth's attraction; why, then, does it not
tumble down ? We feel pretty confident that it is not
falling down, for the moon is not nearer to the earth now
than it was a twelvemonth ago ; for hundreds of years, or
thousands of years, our satellite has approached no nearer
to the earth. In fact, we have already seen that in so far
as there is any difference at all, the moon is getting
farther away from us instead of coining nearer. Nor can
we suppose that at the great distance of the moon, the
tendency to fall earthwards ceases to operate. No doubt
our satellite is a quarter of a million miles away, but
it is not that circumstance which preserves its position.
If a stone or a world or anything else were taken up a
quarter of a million of miles and simply let go, it would
unquestionably tumble down on the earth. In the fall of
a body from the height of the moon to the earth, it would
move very slowly at first ; so slowly indeed that the
movement performed in the first minute would not exceed
that fallen in fhe first second, where the body was let
drop near the earth. As the journey proceeded, the pace
would gradually mend, would gradually become rapid and
ever increasing ; the body would at last crash down on the
earth, after a journey lasting altogether about five days.

THE LUNAR WORLD. 79

But why does not the moon do this ? Or rather why has
not the moon done it ages ago P By what spell is that body
poised aloft in seeming defiance of the attraction by which
mother earth seeks to gather together all exterior objects
into her bosom ? We have read how the coffin of Mahomet
was poised without support in the mosque of the faithful,
from which all unbelievers were so rigidly excluded ; no
material support was necessary to sustain the remains of
the prophet, the body itself seemed ever on the point of
following the departed spirit to the realms of bliss. A
perennial miracle was indeed necessary to sustain the
revered sarcophagus in space. The infidel, no doubt, is
somewhat sceptical about this marvellous phenomenon, but
now, as ever, truth is stranger than fiction. Far over our
heads there is a vast globe larger and heavier than
millions of sarcophagi ; no material support is rendered to
that globe, yet there it is sustained from day to day, from
year to year, from century to century. What is it that
prevents the moon from falling ? That is the question
which now lies before us.

It is assuredly the case that the earth continually attracts
the moon. The effect of the attraction is not, however,
shown in actually drawing the moon closer to the earth, for
this, as we have seen, does not happen, but the attraction of
the earth keeps the moon from going farther away from
the earth than it would otherwise do. Suppose, for instance,
that the attraction of the earth were suspended, the moon
would no longer follow its orbit but would start off in a
straight line in continuation of the direction in which it
was moving at the moment when the earth's action was
intercepted. What Newton did was to show, from the

So /JV STARRY REALMS.

circumstances, the moon's distance and movement, that it
was attracted by the earth with a force of the same
description as that by which the same globe attracted the
apple ; the difference being that the intensity of the force
becomes weaker the greater the distance of the attracted
body from the earth. In fact, the attraction of the earth
on a ton of matter at the distance of the moon would be
withstood by an exertion not greater than that which
would suffice to sustain about three-quarters of a pound at
the surface of the earth. I do not, however, now enter
further into the subject, but in my little volume of lectures
entitled "Star Land," I have endeavoured to explain the
movement of the moon under the action of gravitation in
as simple a manner as possible.

The moon is entirely dependent upon the light from the
sun for her illumination. This is now well known, but
the ancients seem to have had the impression that the
moon must be self-luminous, at all events to a certain
extent. Nor can it be denied that a plausible reason for
such a supposition may be offered. The simplest con-
siderations suffice to show that the phases of the moon,
those interesting changes by which its light increases from
the faint crescent up to the half moon and then to the full
moon, are due to the aspects under which the sun-illu-
minated hemisphere is turned towards us. There is,
however, one condition in which the moon seems to present
a justification for the belief that it contains some intrinsic
light. In the early stages, while our satellite is still a
crescent, the larger part of its disc can be seen to glimmer
with a pale ashy light. This could not have come directly
from the sun, and hence it was supposed that it was

THE LUNAR WORLD.

Fi«r. 6. -View

the Surface of the Moon.
i.

Sz IN STARRY REALMS.

provided by the moon itself, and that therefore our satel-
lite could not be a wholly non-luminous object. But this
inference is not a correct one, and to prove this to be so, it
is only necessary to imagine yourself for a moment an
inhabitant of the moon at the time when this phenomenon
is witnessed. To the Lunarian the earth would then
present much the same aspect as the full moon does to us.
There would, however, be one important difference, for the
full earth would present to the moon a disc thirteen times
as large as the full moon exposes to us. The intrinsic
brilliancy of the two lighted surfaces being the same, it
therefore follows that the Lunarian illuminated by a
" full " earth would find the country around him thirteen
times as brilliant as this earth is at night, when the full
moon is above the horizon. Here then lies the explanation
of that phenomenon which is often spoken of as the old
moon in the new moon's arms. It is indeed produced by
sunlight, only that sunlight has not shone directly on
the moon, but has been reflected there from the earth.
We have, in fact, sufficient reasons for knowing that the
moon exhibits no light of her own, and would necessarily
be wholly invisible were it not that light is provided from
another source.

There is perhaps no more interesting question sug-
gested by the results of astronomical research than that as
to whether the other world0 around us are inhabited.
These worlds exist in teeming myriads, they are of all sizes,
and in every stage of development. Some of them are no
doubt smaller than our own, but many of them are far
greater and more splendid. As our globe is clothed with
verdure, and swarming with living creatures, it is surely of

THE LUNAR WORLD. 83

much interest to inquire whether some of these other globes
may not also be tenanted by organic life. To solve this
question we naturally turn first to the globe which is most
easily accessible to our instruments ; of course, this is the
moon, which is always at a distance of less than a hundredth
part of that of any other globe in the sky. The results of a
telescopic investigation, conducted from this point of view,
are, however, somewhat disappointing to those who would
expect to find inhabitants elsewhere. We cannot detect
on our satellite the slightest trace of organic life. Further
reflection will, however, show that little more could have
been expected. Even if the moon had contained living
objects resembling those on the earth, no telescope could
reveal them, for though it is no doubt true that the moon
is our closest neighbour in the celestial host, yet we must
recollect that it is still two hundred and forty thousand
miles away.

A moderate telescope will show the moon as if its
distance were only a tenth part of its actual amount. A
good telescope might reduce the apparent distance to about
one hundredth part, while when we employ one of the
greatest instruments to scrutinise our satellite we are able
to see it as if it were brought within a thousandth part of
that distance, by which it is actually separated from us.
An instrument capable of achieving so much lends greatly
augmented power to human vision. Each thousand miles
seems thus reduced to one mile, but as there are two
hundred and forty of these thousands in the distance of the
moon, the fact still remains that the utmost efforts of the
most potent lenses known to the astronomer can only
reduce the apparent distance of the moon to about two

84 fN STARRY REALMS.

hundred and forty miles. It follows that no object can be
visible on the moon even from our greatest observatories
unless it be large enough and distinct enough to be visible
with the unaided eye at that same distance of two hundred
and forty miles.

This at once shows that we need not expect to see
any objects on the moon, even with our best telescopes,
unless those objects have dimensions that must be veri-
tably colossal. An elephant could hardly be seen, even
under the most favourable circumstances, at a distance
of more than a couple of miles, so that the biggest ele-
phant would be utterly invisible on the moon, unless
our telescopes were a hundred times more powerful than
any instrument that has ever yet been constructed. In
fact, with our present appliances, no object would be
visible on our satellite unless it were as large as some great
building like a cathedral or town-hall. The loftiest trees
could not be discerned if they grew on the moon, though
it must be admitted that were there vast forests on our
satellite, which shed their leaves periodically, the varying
hues in the different seasons would doubtless be detected.
But phenomena of this kind have never been fully estab-
lished, and hence we are obliged to conclude that, so far
as direct telescopic observation goes, the evidence as to
the existence of organic life on our satellite must be
esteemed entirely negative.

There is, however, another way in which the question
can be studied, and which will suffice to show that, in all
probability, life of evt/ry type with which we are ac-
quainted must be almost certainly absent from the moon.
Every kind of life, whether animal or vegetable, requires

THE LUNAR WORLD. 85

both the presence of air and the presence of water ; we
do not of course say that in other parts of the universe
there may not be types of life for which neither air nor
water is essential ; nothing is, however, more clear than
the evidence which we are able to produce with reference
to the presence on, or absence from, the moon, of the sub-
stances we have named. First, with regard to water. I
have already had occasion incidentally to allude to this
subject; there are, no doubt, some reasons for thinking
that there may have been at one time water on the moon,
but it is now certain that there is no liquid on its surface,
nor indeed can I find much reason to believe that there is
even frozen water there, as has been sometimes supposed.
It is certainly a singular fact that two constituents which
are so abundant here should seem to be entirely wanting
in the moon, and it is an interesting speculation to con-
sider what has happened to the water on the moon if it
once existed there.

It is generally believed that as our satellite cooled down
the water penetrated into the interior, and was there
seized upon by the minerals which required water in
order that they might assume their appropriate crystalline
forms. The water on the moon has therefore, accord-
ing to this view, become transformed into a solid and
incorporated with the bodily texture of the globe. It
has even been surmised that a similar destiny awaits the
oceans on our own globe ; broad and deep though they
are, they yet may be inadequate to quench the thirst for
water possessed by so vast a mass of crystallizing mine-
rals as must exist in the interior of the globe. But
whether this be the explanation of the absence of liquid

86 IN STARRY REALMS.

water from the moon or not, the fact of that absence cannot
be questioned. The moon has been subjected to careful
scrutiny for centuries, yet no one has ever seen any
genuine ocean or sea, no one has ever seen any indication
of the present existence of water, and we are entitled to
assert that water, in a liquid form, is absent from the
surface of our satellite.

On the allied question as to the existence of air around
the moon something must now be said, and here let us
understand distinctly the problem which awaits solution.
I do not here enter on the vexed question as to the nature
of the boundary which separates the higher limits of our
atmosphere from the emptiness of space ; it is quite suffi-
cient for our present purpose to remark that the great
mass of the encompassing air lies within a few miles of
the earth's surface, though no doubt the more attenuated
portions extend with ever-declining density to a distance
above the surface which appears at present indeterminate.
It is, however, certainly known that an atmosphere is
found in the vicinity of many of the other globes in the
universe besides our earth, though there is the widest
difference both in its density and extent, as well as in its
material composition. The sun, for instance, is encom-
passed by a stupendous atmosphere comparable in depth
and density with his tremendous mass. Similarly the
other planets have gaseous envelopes. There is Mars,
the world that seems most like our own in other re-
spects, and it too is encompassed with an atmosphere of
some sort, though we have no reason to think it re-
sembles in any degree the atmosphere which is suitable
for our respiration. Nor are Yenus and Jupiter void of

THE LUNAR WORLD. 87

that gaseous vestment which seems appropriate to every
planet.

Here again our satellite the moon stands in striking
contrast with the other globes that are accessible to our
observation. If the moon ever possessed an atmosphere,
and this is a point on which we cannot feel any certainty,
it has at least now vanished. I should perhaps qualify
this statement with the remark that acute observers have
detected occasional indications of some gaseous material
on the moon in extremely limited quantity confined to
certain valleys or depressions. At first sight it may
perhaps seem difficult to imagine that the telescope could
be invoked to study the question as to whether there was
atmosphere on the moon ; for atmosphere seems so trans-
parent, and is indeed so invisible, that we might naturally
ask, how is it to be observed ? No doubt if there were
water on the moon, we might reasonably expect to see
clouds or vapours if air existed in which those vapours
could be suspended. But the air itself we could not ex-
pect to see ; how then can its absence be demonstrated by
the telescope? It may be true that we never could
observe material quite translucent, but what we might
expect to see are certain indications that would be per-
ceived if the atmosphere were present, and as we do not
find them we infer that the atmosphere does not exist.

The simplest method of demonstrating the absence of
atmosphere of sensible amount surrounding the moon is
by observing the phenomenon presented in what is called
the occultation of a star. As the moon wends its way
over the starry heavens it sometimes passes between the
earth and a star, and the phenomenon is often one of

38 IN STARRY REALMS.

considerable interest. With the aid of a good telescope
the moon is observed to approach close to the star, and
then to pass in front of it, whereupon instantly the star is
extinguished. The sudden character of this phenomenon
is that which generally strikes the observer, for he can,
in fact, observe with accuracy the second of time when
the extinction of the star takes place. This particular
class of observation can be made with such definiteness
and with such precision that it becomes of much value for
the determination of the position of the moon itself. If,
however, the moon were surrounded by an atmosphere, it is
quite clear that the phenomenon attending the occupation
of a star would be something wholly different from that
which we actually find it to be. In proof of this we need
only refer to the circumstances which can be observed
here when a star is in process of setting. As the star
gets lower and lower it gradually becomes fainter and
then passes to extinction. Indeed, though we talk of the
star " setting," no one has ever yet seen a star " set,"
the fact being that the star has become quite invisible long
before it apparently reaches the horizon. If there were
an atmosphere surrounding the moon at all comparable
with that surrounding our earth, the occulted star would
gradually decline in lustre and be extinguished long
before it reached the edge of the moon. Even with a
lesser degree of atmosphere the place of the star as well
as its appearance would be largely affected by refraction,
and this could not fail to be noticed by the discrepancies
it would produce in the position of the star.

From these various considerations it becomes certain
that there is no atmosphere surrounding the moon, which

THE LUNAR WORLD.

Cf fHCUSM MILCS.

Fig. 7. — Lunar Craters compared with area of England.

Ftolemaeus, diameter 115 miles; Alphonsus, diameter 83 miles; Arzachel,
diameter 65 miles.

90 IN STARRY REALMS.

is even a thousandth part so copious as the atmosphere
surrounding the earth. Seeing then that air and water
may, for all practical purposes, be said to be absent from
our satellite, it becomes at once plain that those forms in
which life is manifested here must be absent from our
neighbour. Strange indeed to us would seem the con-
ditions of a globe without air and without water. Let us
try for a moment to realise what we should find such a
a world to be like if we could procure the means of getting
there, and if we were able to dispense for once with such a
primal necessity as air to breathe. The surface of the
moon would appear to be an utter desert, at least in so far
as the absence of organic life is concerned ; we should see
around in every direction huge craters, the remains of
ancient volcanoes that are now never in eruption, but
which in the days of their activity sculptured the moon
into the form in which we now see it. Some of these vast
craters (Fig. 7) would be many miles in diameter, the
larger of them, in fact, upwards of a hundred miles across.
They would be generally surrounded by a range of lofty
cliffs, a mile or more in height, though on account of their
large size relatively to the comparatively small globe of
the moon, these bounding cliffs or " ramparts " as they
are generally called, would be often below the horizon
of the observer who was standing in the middle of the
crater. Then, too, we should occasionally see great ranges
of lofty mountains comparable with our Alps in altitude ;
they would, however, like all other lunar features,
possess a ruggedness and a sharpness transcending any-
thing to be found in the wildest regions on our globe.
All over this earth there are agents in operation

THE LUNAR WORLD. 91

tending to wear down and reduce the asperities on its
surface. Water in its varied forms is constantly acting
in this way ; the rains trickle down the slopes of the
mountains wearing down the materials and gradually
tending to smooth away irregularities. Frost is also a
potent disintegrator of rocks. It finds its way into their
crevices and with irresistible power the rocks are riven
asunder by the expansion which the water undergoes when
it passes into the form of ice ; thus great blocks of rock
are loosened from their sites, and the other agents effect
their complete disruption. The inveterate action of streams
and rivers is gradually transforming the appearance of
our globe, and in the course of ages a stream will cut a
deep valley through the hardest rocks. In fact, there is
scarcely a spot on our globe in which the features of the
landscape have not been largely affected by water and
weather in some of the numerous ways in which they
operate. Of course all such agents are absent from the
moon, and hence it is impossible for us to see on our*own
earth any tracts of country really resembling the lunar
surface.

It is true that we have volcanic districts and we have
rainless districts, but we should require a district not only
entirely volcanic, not only entirely rainless, but even devoid
of air itself, to reproduce the phenomena that we find in
the moon. There are no doubt some localities on our
globe which seem to suggest the mode in which some of
the characteristic lunar features may have arisen. We
have, for instance, in the Sandwich Islands the great
crater of Kilauea : at this wonderful spot the traveller
will see a large basin of molten lava surrounded by a range

92 IN STARRY REALMS.

of glowing cliffs ; if we could extinguish the internal fires,
by which at present this mighty cauldron is kept incande-
scent, the floor of lava would become congealed, and we
should then have a plain surrounded by a ring of cliffs,
and offering a resemblance to those objects which are seen
by hundreds on the moon. There are other localities in
the Sandwich Islands in which the fires have apparently
ceased, and where the craters now look as little liable to
eruption as are the extinct volcanoes in the moon. The
absence of air and water has had a distinctly preservative
effect so far as the features of lunar scenery are concerned.
As the volcanoes sculptured our satellite into form count-
less ages ago, so it has retained that form to the present
day.

If a building were once erected on the moon, it hardly
seems conceivable that it should ultimately fall into ruins,
if purely volcanic agents are wanting. Without atmo-
sphere and without water, what harm can time do to the
fabric ? In fact, with the exception of the expansion and
the contraction by the alternation of heat and cold, it does
not seem that any agent of destruction can exist. We find
it difficult to realise the circumstances of an airless globe.
Fires could not burn, for there is no air to support the
flame ; winds could not blow, for of course wind is only
the passage of air from one place to another ; there need
be no windows in the building, for there is no air to keep
out ; there need be no roof, in so far at least as a roof is
required as a protection against rain. Two out of the five
organs of sense with which we are endowed operate solely
by utilising certain properties which air possesses. Our
ears are exquisite contrivances bv which we are enabled to

THE LUNAR WORLD. 93

receive and to interpret the undulations which are trans-
mitted through the air. On an airless globe our ears
would be almost useless ; we could hear no sound trans-
mitted as those sounds are which usually reach us. I ought,
however, to add that certain audible waves admit of being
transmitted through other materials besides air, and con-
sequently in some exceedingly imperfect and indirect
method a sense analogous to that possessed by our ears
might be rendered available on an airless globe. The
other sense which would be useless to any inhabitants who
might dwell on our satellite or other airless globe would
be that of smell. Although much less is known about
this sense than about either the sense of vision or that
of hearing, yet it seems certain that our nasal organs
receive extremely minute particles, and when these come
in contact with our olfactory nerves the sensation appro-
priate to those nerves is occasioned. Whatever may
be the mechanism of this process, it can hardly be doubted
that air is the vehicle by which these particles, so minute
and so imponderable as to elude entirely all our other
senses, are wafted from their source. The sense of smell
would apparently be impossible on an airless globe like
the moon.

There is, however, another wholly different class of sen-
sations which would be experienced by a denizen of this
earth if he were translated to a small globe like the moon.
We must remember that in every fibre of our constitution
we have been specially adapted to the life on this
particular sphere. We have already seen how our senses
are adjusted in harmony with the particular atmosphere
with which the earth is surrounded : we have now to

94 IN STARRY REALMS.

notice another point, in which the texture of our bodies is
arranged to suit the material contents of this globe on
which we dwell.

It may seem strange to learn that the strength of
our bones and muscles has been adjusted not solely
with regard to the size of our bodies or the quantity of
matter they may contain, but with reference to the dimen-
sions and mass of the earth. Even though on another
globe there was an atmosphere exactly like our own
both in density and in composition, even though it
was supplied with water as ours is, even though it
provided us with abundance of suitable food and had a
climate agreeable to our constitution, yet it might be
wholly impossible for us to ezist there by reason of an
incompatibility between the strength of our frames and
the mass of the globe on which we stood. Thus, to take
the case of the moon, which only weighs about one-
eightieth part of the earth ; the gravitation with which
the moon would draw all bodies towards it would be much
less than the similar gravitation on the earth. The weights
of all objects would be reduced to about one-sixth part of
that which we find them to possess here. The buoyancy of our
bodies would be so great that athletic feats would be easy
on a body the size of the moon, which could never be
attempted on this globe by beings with muscles like ours.
If a man of twelve stone were to be transferred to the
moon, the weight of his body would be reduced to about
two stone. If his muscles and his frame remained the
same it would seem as if he would be able to jump over
a wail twelve feet high on the small globe without any
greater exertion than would be required to clear a wall

THE LUNAR WORLD. 95

two feet high on the earth. Looked at from every point
of view, it seems hardly possible that there can be any
life on the moon resembling the life that we know of on
the earth.

But though the want of atmosphere surrounding the
moon may deprive it of some features of interest, yet to
the user of a telescope on this earth the airless nature of
the moon is a distinct advantage. Suppose, for example,
that a Lunarian were to endeavour to study our earth with
the telescope, he would see little, comparatively speaking,
of the actual surface, for our atmosphere itself, so closely
enveloping the earth, would obstruct his vision, while the
clouds with which so large a part of our atmosphere is
often charged would form an impenetrable screen. It would
be only in a very imperfect fashion that the dweller on the
moon would be able to make out the features of the globe
to which the moon is a satellite. But the terrestrial
astronomer experiences no such difficulties. It is true that
our own atmosphere often interferes with us in a manner
with which every astronomer is only too painfully fami-
liar, but by suitable choice of opportunities these dis-
advantages may be largely obviated and beautiful pictures
of our satellite may be obtained. The want of atmosphere
on the moon enables us to see its features with exquisite
clearness and sharpness, and the shadows are cast with a
definition which we never find in terrestrial scenery.

There is another circumstance which makes the moon
an attractive and easy subject for the telescopic inquirer.
Owing to the constant face of the moon, he always finds the
craters in nearly the same position, and as our satellite
courses through her monthly vicissitudes the observer

96 IN STARRY REALMS.

learns to watch for the reappearance of the same object
m much the same position, just as in a garden we love to
watch for the blossoming of our old favourites as the spring
comes round. It is also a great advantage that the moon
is sufficiently near us to be well within the reach of tele-
scopes of moderate pretensions ; even a little instrument
that may be held in the hand will with the help of a map
of the moon afford much interest to the beginner in
seeking out the different craters and learning to identify
them by name*

CHAPTER VIII.

A VISIT TO AN OBSERVATORY.

IT is well known that in the United States there are many
more opportunities for educated women to gain useful
and remunerative employment than are found in Great
Britain. I was particularly struck with this when I saw
American ladies employed in doing work in the astro-
nomical observatories of their country, which on this side
would be almost exclusively performed by men. The
work they had to do was eminently suited to ladies ; it
required neatness and care, and a conscientious determi-
nation to perform it with unremitting accuracy and atten-
tion. How successful they have been is known to all
astronomers who have made themselves acquainted with
the great volume of excellent astronomical research that
flows from the American observatories.

I was much interested and entertained lately by reading
a paper written by one of the American astronomers to
whom I have referred. Two years ago a new astronomical
observatory was opened at Carleton College, Northfield,
Minnesota, and at the laying of the foundation-stone an
appropriate — a most appropriate — address was delivered

98 IN STARRY REALMS.

in celebration of the auspicious event. When I add that
this address was given by a lady it will, I think, show
that America, in respect to such performances, is much in
advance of the countries on this side of the Atlantic.

The address was published in the Sidereal Messenger,
and has been copied into the Observatory for December,
1886. It is signed Mary E. Byrd, and is well worth
reading. It admirably expresses the wide divergence that
there often is between the presumed nature of the work
that is performed in an observatory in the popular imagi-
nation, and the routine of somewhat prosaic detail that is
the actual fact.

I cannot resist quoting a passage from this address as
the best introduction to our present subject.

"It is commonly fancied that there is a great deal of
poetry and romance within the walls of an observatory.
All have read the ancient legend of Tycho Brahe, how he
went to the observatory in robes of state, as if the presence
of the stars was the presence of princes. And people
fancy that here at midnight, in starlit domes, you almost
hear the music of the spheres. They picture to them-
selves the observer seated at his telescope hour after hour
looking down, down, into deep lunar craters, feasting on
delicate nebulae and swift-flying comets, or revelling in
gorgeous star clusters. Here, they think, night after night
before his rapt vision there passes all the panorama of the
heavens multiplied and glorified a thousandfold by his
powerful lenses. I have sometimes wished that it were
so ; but it is work that goes on in an observatory, work as
stern as that of the factory."

The actual fact is, indeed, widely different from that

A VISIT TO AN OBSERVATORY. 99

supposed view of the duties of the astronomer which
are here so humorously portrayed. The popular notion
of the way in which astronomical discoveries are made is
often ver}' wide of the truth. It seems sometimes to be
thought that the astronomer in search of some crowning'
triumph sits gazing with ecstatic rapture through the
tube until suddenly some majestic, and hitherto utterly
unheard of, celestial body soars into his view, and he
immediately records an immortal discovery. The fact is
that when an astronomer goes into his observatory for
his night's work he finds it usually convenient to leave
all the ecstatic and most of the poetic portions of his
constitution outside. He arrays himself in costume, not
with a view to the sublimity of the universe, but to-
the effectual keeping out of the cold. It is not the
stupendous size of the celestial bodies that so often
appals him ; he is rather straining his attention to the
effort of hiding a tiny star behind the spider web of
his micrometer.

But though it may be generally true that the work of
the observatory is essentially a routine almost of a prosaia
character, yet the astronomer must indeed be of a callous
nature who does not feel the noble character of the occu-
pation to which his nights are devoted. I propose in these
pages to give a sketch of what the visitor to the obser-
vatory may reasonably expect to see, if his or her visit
be appropriately timed. It must be remembered that the
celestial objects are not always to be observed. It is no
doubt true that Saturn or the great nebula in Orion are
like the Matterhorn or the Falls of Niagara, always to be
seen if the visitor could only go to the right place. But

too IN STARRY REALMS.

very often no terrestrial observatory can be the right place
for seeing either of the objects named, and this for various
reasons. In the first place, it may be that the season of
the year is wrong. There is, for instance, no use in going
to look for the great nebula in Orion in summer, nor is
there any possibility of seeing Saturn when, as happens
every year, he is situated near the sun on the surface of
the heavens.

Then, too, it must be remembered that some objects
never rise at all in our latitudes. You need never expect
to see the Magellanic clouds by going to any observatory
in England. Nor can the astronomers in Australia ever
observe the companion to the pole star through the great
reflector at Melbourne. Even when the body you want to
see is " up/* it may be in a very unfavourable condition
for making observations. If it be the moon that you wish
to see, and even if it be up, and high up, it may be quite
unsuited for telescopic scrutiny from the simple fact that it
is " full " ; a condition in which the varieties of light and
shade which give to the moon-pictures their beauty and
their instructiveness are altogether wanting.

The planets, too, will often be accessible to the tele-
scope, but still be very unfavourably placed for disclosing
their real beauty as compared with the seasons when they
can be observed to advantage. The ring of Saturn may
be presented under an aspect in which it is too much
foreshortened, or the opposition of Mars may be one in
which his distance from the earth is too great to admit
of a truly effective telescopic picture being produced.
It will thus be obvious that to observe any of the celestial
bodies effectively not only must good instruments be

A VISIT TO AN OBSERVATORY. 101

available, but a careful forethought must be exercised
in choosing the appropriate times for each object. But
even when the best moment for observation has arrived,
and we are quite ready to secure the examination of the
celestial body under the most favourable auspices, there is
still the very important consideration of the weather. For
astronomical work a clear sky is indispensable. When we
find that a cloud or two can obscure from us the direct
image of the sun, we need hardly expect that stars or
planets or objects still fainter and more delicate will be
perceived through a dense curtain of watery vapour.
Fogs and mists, no less than more extensive clouds, must
be absent, and even nights that seem tolerably clear to the
ordinary spectator may, from atmospheric causes alone, be
very ill-adapted for any careful astronomical work.

From all these reasons it will be seen that the hours in
which really excellent work can be done in an observatory
are but few in comparison with the whole number of hours
in the year. When such hours do arrive astronomers
greatly prize them, and it may be readily believed that
during the time for which they have waited so long
they are often not too well pleased to be disturbed by
visitors who come on a star-gazing expedition.

Of all the impediments to astronomical work those pro-
duced by the atmosphere are the most vexatious, because
they do not admit of being predicted. The astronomer
may sometimes have even gone to the other side of the
earth to observe some rare phenomenon like a total eclipse
of the sun, or the still more occasional transit of Yenus
across the sun. The time when such an event will occur
admits of actual prediction ; all may be in readiness, when

<o* IN STARRY REALMS.

the incalculable clouds overcast the sky, and the whole
object of the enterprise is frustrated.

But even when not charged with clouds the atmosphere
is baneful to the practical astronomer, for air when clear-
est and purest still obstructs a great deal of light. It
makes the stars appear fainter than they would otherwise
be, while rays from very small stars are extinguished by
it, though those rays would have been quite sufficient to
render their source perceptible in our telescopes if we
<;ould observe without the intervention of the atmosphere.

An airless globe like the moon would, for merely tele-
scopic purposes, present the most favourable condition
conceivable, though how it would fare with the astro-
nomers involves questions of a different character. We
have not only the imperfect transparency of even the
purest skies to contend with, but there are other diffi-
culties. Even under the stillest and clearest sky it is
of the essence of the atmosphere to distort the places
of the celestial bodies. To see a star we have to
point the telescope, not at the real position of the star,
but in a somewhat different direction. Astronomers
«peak of this derangement as refraction ; they are obliged
•continually to bear it in mind, and their observations have
to be corrected for it so as to place the star in its true
instead of its apparent position. The amount of the dis-
placement of a celestial object depends, among other
things, upon the temperature of the air. If the tempera-
ture changes the amount of the displacement will change.
While, therefore, there is any fluctuation of temperature
the place of the star will appear to be continually dis-
turbed. The astronomer will say that "the stars are

A VISIT TO AN OBSERVATORY. 103

very unsteady to-night." As long as this lasts the value
of his observations is appreciably impaired ; sometimes
the unsteadiness is so great that he must desist from
working altogether.

To escape from the pernicious influences of the atmo-
sphere is at present the pressing need of practical
astronomy. In this respect of course there will be great
differences between one climate and another. But the
best method of evading the severe tax which the atmo-
sphere imposes on every astronomer's time and the injury
that it inflicts upon his measurements is to carry his
observatory to the top of a lofty mountain.

The ocean of air that lies over our heads is perhaps one
hundred miles or more in thickness. It is, however, in
the lowest mile or so that most of the mischief is done of
which the astronomer so sadly complains. A lofty mountain
peak which reared its head a mile above the earth's surface
would protrude through the most troublesome portion of the
atmosphere. An observatory perched on the summit of
this mountain will therefore be in a position to observe the
stars almost free from the atmospheric anxieties of the
astronomer at the bottom of that turbid atmospheric
ocean on which the mountain astronomer can now look
down.

At the present moment the attention of the astro-
nomical world is largely fixed on the bold experiment
which has been made in America to locate a telescope of
perhaps unsurpassed optical perfection on the top of a
mountain in one of the most exquisite climates on the
globe.

Mr. Lick, a Californian millionaire, committed to the

I04 IN STARRY REALMS.

care of the Lick Trust a great sum of money, and directed
them " to erect a powerful telescope, superior to and more
powerful than any telescope ever yet made, with all
the machinery appertaining thereto, and also a suitable
observatory,"

To carry out this trust a careful search was made to
obtain the most suitable locality, and an expedition was
sent to the top of Mount Hamilton, near San Jose, in
California. The observations showed that the air was
remarkably steady, and that there was a continued
succession of perfect nights. Then, too, it was found
that observations of objects very low down in the sky
were practicable to a far greater extent than in other
observatories in similar latitudes. Mr. Burnham, a dis-
tinguished American astronomer, who conducted these
preliminary tests of the capabilities of Mount Hamilton,
made many valuable discoveries of new double stars, many
of them being exquisitely beautiful and delicate objects,
during the six months that his stay lasted. These trials
having proved eminently satisfactory, Mount Hamilton was
decided upon as the seat of the Lick Observatory. Then
commenced the arduous task of constructing and equipping
a vast astronomical establishment on the summit of a
mountain four thousand feet in height, and twenty-six
miles distant from the nearest town. With true American
energy these difficulties have been severally vanquished.
The county of Santa Clara provided, and now maintains, a
magnificent road, which runs by a gentle slope the whole
way from San Jose to the top of Mount Hamilton. Then
the top of the mountain had to be cut off to make a level
platform large enough for the buildings. To do this no

A VISIT TO AN OBSERVATORY. 105

less than seventy thousand tons of material had to be re-
moved. When the question arose as to the erection of
the telescope, the first point to be settled was whether it
should be a reflecting telescope or be a refractor, — that
is, whether it should, like the great telescope of Lord
Rosse at Parsonstown in Ireland, consist of a large and
brilliant mirror at the end of a long tube, from which the
rays of light from the stars were to be reflected ; or
whether it should be founded on the more familiar prin-
ciple of refraction, in which powerful glass lenses are
made use of to concentrate the light and render faint
objects visible. There was much to be said on both sides,
but finally the trustees, after taking counsel with the
wisest astronomers all over the world, decided to erect a
great refractor.

The preparation of the object-glass was the next great
work to be accomplished. It was to consist of two pieces,
one of crown glass and the other of flint glass, and it
was to have a diameter of the unparalleled length of
thirty-six inches. The fabrication of the actual pieces of
glass on which the opticians could work was a matter of
the greatest difficulty, and sorely tried the patience of all
concerned ; to obtain the rough discs of glass alone no less
than six years were required, and they were only finally
adopted after twenty unsuccessful trials. At last, how-
ever, when Messrs. Alvan Clark, of Cambridgeport,
Massachusetts, did obtain suitable pieces of glass whereon
to employ their optical skill, another year of assiduous
work was necessary to give to the glasses the exact shapes
that would render the vision through them perfect. For
the execution of this great object-glass the Lick trustees

lot IN STARRY REALMS.

paid to Messrs. Alvan Clark a sum exceeding £10,000.
The length of the tube in which this pair of lenses
had to be mounted was adapted for a focus of fifty-six
feet two inches. On the 1st June, 1888, the celebrated
Lick Observatory was formally pronounced ready for
work, and handed over to the charge of the regents of the
University of California.

I take these particulars from the first volume of the
publications of the Lick Observatory. The charges of the
future publications will be generously provided by the
State of California, while the United States has pre-
sented the site. With consummate astronomical equip-
ment, with a staff" of practical astronomers that cannot
be surpassed in the world, we may confidently hope that
the Lick Observatory will fulfil the generous intention
of its founder ; indeed, we have already in some exqui-
site photographs and in other ways received earnest of
its success.

I have entered thus fully into the account of the Lick
Observatory, partly on account of its novelty and its
importance, but also because in one aspect of its work
it suitably illustrates the title at the head of this
chapter. Professor Campbell, the accomplished astro-
nomer who presides over the Lick Observatory, has
arranged that the resources of the great telescopes shall,
under suitable regulations, be available to those visitors
who may spend an evening at the Lick Observatory.

I propose to mention some of the objects which a visitor
to an astronomical observatory should specially wish to
see. He will first of all desire to learn by actual examina-
tion something of those marvellous instruments by which

A VISIT TO AN OBSERVATORY. 107

the astronomer has detected the movements and learned
the actual character of the heavenly bodies. He will see
the fundamental weapon with which inaccuracy is driven

Fig. 8. — The Meridian Circle.

from the field of astronomy. That weapon is the transit
instrument, or rather the more complete apparatus which
is found in all our modern observatories, and is known as
the meridian circle. This consists of a telescope of con-

io8 IN STARR Y REALMS.

siderable but not of the largest dimensions, mounted in a
particular way for a particular purpose. A view of the
meridian circle at Dunsink Observatory is given in the
adjoining sketch. The telescope is attached to an axis
about which it can revolve, just as a cannon can be
turned up or down in its bearings. The axis carries
two large circles. On these the utmost refinements
of mechanical skill must be exercised. The circles
in the instrument at Dunsink Observatory, the estab-
lishment over which I have had the honour to preside, are
about three feet in diameter. There is a rim of silver
let into the margin of the wheel ; this silver is graduated
by fine marks — marks indeed so fine that they require a
microscope for their detection. When the telescope is
pointed to a star, the position of the marks shows the
elevation of the instrument, and thus the height of the
star above the horiaon is determined. The telescope is
carefully adjusted so as to move in the plane of the
meridian. It can be pointed to the due north, then it
can be raised up to the point of the heavens vertically
overhead, then it can be turned downwards to the south.
But it is of the essence of the meridian circle that it
admits of no other movements ; it cannot be turned to the
east or to the west in the slightest degree.

A star or a planet, the sun or the moon, can there-
fore only be observed with the meridian circle at the
very instant of crossing the meridian. In fact, one of
the purposes of observing with the meridian circle is
to determine the time at which the celestial body is
on the meridian. To obtain the requisite accuracy, it
will not be sufficient merely to observe the star in

A VISIT TO AN OBSERVATORY. 109

the field of view of the telescope — we must have some
means of readily indicating the precise line down the
field which represents the meridian ; a thread or wire
has to be stretched across for this purpose. The magnify-
ing power of a large meridian circle is, however, so con-
siderable that any ordinary thread would look like a
coarse rope, and would be utterly unsuited for work
requiring nicety of observation. We must therefore
employ some fibre which shall be extremely fine, and
which shall yet be strong enough to admit of being
stretched into a perfect straight line, while a certain
degree of elasticity is also requisite in order to preserve
the straightness of the line with any permanence. These
various conditions are most completely complied with by
the beautiful thread of the spider. It is a somewhat
•delicate task to stretch one of these exquisite filaments
properly over the little circular framework at the eye-end
of the telescope, but when it has once been done the web
of the spider is sufficiently durable to fulfil many years of
service.

The method of observing the transit of a star will con-
sist of noting the time by the astronomical clock when it
passes behind the spider web. The due estimation of this
time taxes the skill of the practical astronomer; it de-
pends on the appreciation of the fractional part of a second
at which the transit takes place. Here, as in so many
other departments of science and of the arts, the aid of
electricity is invoked to give a subtleness to the work that
cannot otherwise be obtained. As the star passes behind
the spider web the astronomer taps a key which closes the
current, and impresses a mark on a revolving cylinder.

no IN STARRY REALMS.

Thus a record is obtained which makes the closest ap-
proach to absolute precision.

But for the purposes of the continuous observation of a
celestial body the meridian instrument is quite unfitted.
Et merely gives us a flying glimpse of the object as it
hurries through the field ; a glimpse sufficient for the
determination of the place of the object, but quite inade-
quate to enable us to examine the object with any close
attention. A totally different form of mounting for our
telescope is now required, which shall enable us to follow
the object persistently for hours together. Of course the
facility of movement in the telescope which this implies
can only be obtained by the sacrifice of some other
qualities. The equatorial — for so this form of mounting
is termed — is quite unsuited for the rigidly accurate pur-
poses of measurement, which it is the sole object of the
meridian circle to obtain. In the stand of the equatorial
clockwork is placed, by which the instrument, after it has
been once pointed to the star, is constrained to move so
that the object under examination shall continue steadily
in the field of the observer's view.

It is the equatorial that we are to visit when we come
to spend our evening at the observatory. We shall find
ourselves usually in a circular room covered by a dome.
This dome reposes on wheel work, so that it can be made
to revolve. In it is a shutter, which can be opened so as
to allow the telescope to be pointed to the sky. There is,
of course, no glass window in the opening ; even the most
transparent plate glass would impair the perfection of
the image. Nor will it be allowable to have the building
warmed by artificial heat. If the temperature inside

A VISIT TO AN OBSERVATORY. u»

were raised appreciably above that which is found out-
side, the heated air from the interior would pour out
through the opening, and the disturbance thence arising
would produce a mixture of air of different degrees oi
density, which would be quite sufficient to impair the
delicacy of the telescopic images. Wind, if unattended
with clouds, is, as a rule, not injurious to good seeing,
though the comfort of observatory work is greatly promoted
by a tranquil atmosphere.

And now as to the objects which the visitor to the obser-
vatory should specially ask to see. It must be borne in
mind that many of the most interesting objects to the
astronomer are almost completely devoid of effectiveness
from the merely spectacular point of view. A grand
nebula, for instance, will often have portions so faint, that
however we may find it necessary to represent them in
drawings, they are in the field of the telescope only to be
seen by most assiduous attention with an eye especially
trained for such work. The visitor will hardly feel con-
tented if he be desired to look at something which he
may only have a chance of seeing after he has steadily
gazed for several minutes at one place. There are, how-
ever, some splendid objects in the sky against which no
objections of the kind can be alleged, and it is to these
that we specially commend the attention of a visitor who
is anxious to see some of the wonders of the heavens.

The celestial objects which I have now specially in view
are three : they are the planet Saturn, the star cluster in
Perseus, and the great nebula in Orion. Any one who
has had the gratification of witnessing these objects
on a cloudless sky, and through a telescope of adequate

112 IN STARRY REALMS.

dimensions, will have obtained a fair notion of the glories
of the heavens. Nor is it often possible to compass all
these objects during a single visit to the observatory.
The moon, as if jealous of the other celestial bodies,
drenches the skies with such a flood of light that the
glories of the lesser bodies become well-nigh invisible. In
fact, just as the sun at midday prevents us from seeing
any of the stars, so the beams of the moon pale the smaller
stars to invisibility, and largely detract from the lustre of
the brighter ones. Some description of these objects will
occupy us in the following chapter.

CHAPTER IX.

AN EVENING WITH THE TELESCOPE.

THE opportunities for observing the planet Saturn are not
nearly so frequent as those in which the moon may be
advantageously observed. We must choose the time when
Saturn comes nearest to the earth. This will take place
when the earth lies nearly between Saturn and the sun.
Supposing that such an occurrence is now taking place,
the earth will in a short time have moved away from the
best position, and the distance from the earth to the
planet will be on the increase. When a year has elapsed
the earth will have returned to its original position, but
it will not then be exactly between Saturn and the sun,
for the great planet has moved. Like the earth, Saturn
also revolves round the sun, but the magnitude of his orbit
is much greater than that of the earth, and consequently
the time that Saturn requires to complete a single revo-
lution is about twenty-nine and a half of our years.
Hence it follows that Saturn will have moved in the
course of the year, so that the earth must pursue its
journey for another twelve or thirteen days before it will
again have resumed its position between the planet and
the sun.

H4 IN STARRY REALMS.

Saturn, like our earth and like the moon, is entirely
indebted to the sun for its supply of light. Bright as the
planet may seern, it has no intrinsic luminosity — all we
see is merely the reflected solar beams. Its globe is of
noble proportions. Were that globe divided into six
hundred equal parts, and were each of those parts rolled

Fig. 9. — The Sim and its Attendant Worlds.

into a globe, it would be a larger ball than this earth of
ours, eight thousand miles in diameter. A view of this
orbit of Saturn, as well as of some of the other planets, is
shown in the adjoining figure.

This mighty globe also revolves on its axis, but its day
is much shorter than ours, as each revolution of Saturn is
accomplished in ten hours and fourteen minutes. Owing
to its higher speed of rotation, the bulging out at the

AN EVENING WITH THE TELESCOPE. 115

equator of Saturn is much larger than in our earth ; in
fact the elliptical form of Saturn is so evident as to be
at once detected without any delicate telescopic measure-
ments. The surface of the globe presents but few
features of interest, and indeed there is little on it which
can be depicted in a drawing. It is of a nearly uniform
whitish yellow colour, occasionally, however, marked over
with faint bands. It is obvious that what we see is merely
the outside of a great casing of cloud, in which the entire
planet is shrouded. In fact it seems very doubtful
whether Saturn bears any resemblance to a solid body at
all. Our first impression might perhaps be that the
planet was a sort of rigid globe like our earth, covered by
a coating of cloud, much deeper and denser, and more
uniformly distributed than the somewhat intermittent
clouds of which we on the earth have so often to complain.
But it is not easy to see how far the interior of Saturn
can, with propriety, be likened to a dense globe like ours,
and for the following reason : — our earth is composed of
rocks and metals, and the entire weight of the globe is
above five times the weight that a globe of equal bulk of
water would have. It may seem a very difficult problem
to weigh the planet Saturn, and so to compare its mass
with that of an equal globe of water, but the task is not
beyond the resources of the practical astronomer. When-
ever a planet is provided with satellites, or little attendant
moons, it is possible to put the great globe into the weigh-
ing scales and to determine how heavy it is. I cannot
here delay to explain fully the details of the process ; suf-
fice it to say that it can be done with great accuracy,
and the result in the case of Saturn is truly astonishing.

U6 IN STARRY REALMS.

We learn that the globe has so little resemblance to our
earth that it is actually lighter than a vast globe of water
would be were its dimensions the same as those of Saturn
itself. Indeed, if we could imagine a model of the planet
just as large and just as heavy as the planet actually is,
and if that globe were cast into a great ocean of water of
suitable dimensions, the mighty planet would float buoy-
antly on the surface. It seems hard to reconcile these
facts with the belief that there can be much solid matter
in the interior of the planet.

It is impossible in any picture to represent the exquisite
delicacy of Saturn as revealed in a great telescope, but
to make our description plain we have to present a
sketch which will at least give a general notion of the
ring system, and may perhaps stimulate the reader to
secure some opportunity for observing these objects in a
telescope.

We must notice that the rings are not fastened to the
globe. There is a tendency for them to fall down on the
globe of the planet, but that tendency is neutralised by
their rapid rotation around the globe. The rings are
shown slightly turned towards us in the picture. Were
we able to look squarely at them they would be circles ;
when we view them edgewise they are found to be so
extremely thin that they elude our vision almost entirely,
except in powerful instruments. We speak of them as
rings in the plural, because it will be seen that they are
threefold. The two outer are separated by a broad line
of demarcation which can be traced the entire way round.
These two outer rings are apparently of the same general
nature, and there are some other dark lines or divisions

AN EVENING WITH THE TELESCOPE. 117

of a somewhat fainter description, of which one at least,
in the outer ring, is a permanent feature of the system.

To see the most delicate part of this beautiful structure
requires a telescope of considerable power ; it has only been
discovered in comparatively recent years, and is the third
ring of the system. It extends from the inner margin
of the second ring, half-way in towards the globe of
the planet. The name assigned to this mysterious but
lovely object is the crape ring, and the appropriateness
of the designation will be apparent when the hue of
the object as well as its curious semi-transparency is
noticed.

The structure of the rings of Saturn suggests problems
that have exercised the profoundest mathematicians. It
was speedily seen that the rings could not be thin plates
of solid material. No doubt a superficial glance at their
appearance will suggest that such is their nature. How-
ever, it can be demonstrated by mechanical principles that
the very existence of the rings would be impossible if
they were what this notion implies. In this instance,
as in many others, the pen of the mathematician has
proved a more potent instrument than the telescope.
What the astronomer could hardly expect to find out in
his observatory has been demonstrated by actual calcula-
tion made by pen and paper. It has been proved that
this wondrous set of rings consists of a multitude of small
objects, each pursuing its own voyage round the planet
like a little moon. These bodies are so numerous and so
close together that the most powerful telescope can hardly
be said to recognise their individual existence, though
occasionally no doubt the rings do seem subdivided in a

US IN STARRY REALMS.

way which renders to the true view of their nature some
degree of telescopic confirmation.

It is only in this way that we can offer any reasonable
account of the semi-transparency of the crape ring. The
little moonlets (if I may for the moment coin a word)
which go to make up the rings are, in the outer of the
structures, so close together that they reflect what is
generally seen as a continuous sheet of light ; but in the
crape ring the moonlets are either not so numerous or not
so large as in the outer rings : the consequence is they do
not appear as a continuous sheet of light ; we are able in
some degree to see between them, and this is how we explain
the semi-transparency of the crape ring.

Nor is the Saturnian system wanting in other attractions
which would render it of great interest, even were the
supreme feature of the ring system absent. The planet is
attended by no fewer than eight moons, some of which are
easily visible in the most modest telescope, while others
demand the employment of exceptionally powerful instru-
ments.

I hope that every one of my readers will obtain some
opportunity of observing Saturn at a suitable time and
with a telescope of sufficient power ; assuredly they will
be delighted and fascinated with the spectacle. But I
)ught also to add a word of warning. I have before now
met with people who were woefully disappointed with the
planet, even when the circumstances were most favourable.

I remember many years ago taking a bright little boy
of six to the Dublin Zoological Gardens. It was his first
visit, and all the way he prattled to me delightfully cf
what he expected to see ; the principal object of his desires

AN EVENING WITH THE TELESCOPE.

119

being an eagle. Perhaps he had been duly instructed in
the semi. fabulous stories of children being borne away by
eagles ; probably he was also acquainted with the adven-
tures of Sinbad the Sailor and the Roc ; at all events,
we made our way to where there was a large cage con-

Fig. 10.— Saturn and his Rings.

taining several fine sea eagles. I awaited his rapturous
delight, but in vain. I saw the poor little fellow assume
a crestfallen aspect. "They are so small," was all he
said, but I could easily divine that he had expected to
see something about twenty feet high, and that one of
his pretty childish idols had been smashed to pieces.

I JO IN STARRY REALMS.

On a more recent occasion I visited the Falls of Niagara.
When we reached the station in the middle of the night,
one of my fellow-passengers, who was not by any means a
child, remonstrated with the conductor, who came round to
tell us that we were at Niagara Falls. " That cannot be,"
said the individual in question, " for I know two things —
first, that the station is not two miles from the falls ; and
secondly, that the noise of the falls can be easily heard
twenty miles away, and as I hear nothing I know that
this cannot be the station." But it was nevertheless.
The conception of the thunders of Niagara in the mind of
the tourist bore no more relation to the actual fact than
did the Roc which the boy expected to the actual eagle
which was all I could show him. Nor did I fail to observe
the utterly disgusted and disappointed demeanour of my
unhappy fellow-traveller the next day, when he perceived
the contrast between the wretched trickle of the real Nia-
gara and the splendour of that ideal cataract which all this
journey had been taken in the hope of seeing. Had the
Atlantic Ocean been seen pouring down from the moon it
would not have done more than realise the expectations
of volume and altitude which so ruthlessly collapsed
before the fact.

It is obvious that properly to appreciate natural scenery
persons must either be naturally gifted with intuitive
taste, or else they must wait until experience has taught
them what to see, and what it is reasonable to expect to
see. Let not any one, therefore, be disappointed if their first
glimpse of Saturn falls far short of what they anticipated,
the beauties of it are not so glaring that they can be dis-
cerned without nice and careful observation. Remember

AN EVENING WITH THE TELESCOPE. 121

that the crape ring is so subtle an object that multitudes
of astronomers gazed at Saturn for ages and never saw it
at all. Even the immortal William Herschel, with his ex-
cellent instruments and with his indefatigable perseverance,
never noticed the crape ring. Let the casual visitor bear
this in mind, and not expect to see Saturn exhibited as a
vast panorama which he that runs can read ; let him rather
expect to find an exquisitely wrought miniature, which
will demand the closest attention, but which will reveal
ever new beauties to those who know how to woo the real
loveliness of nature in the only way in which it can be
won.

Widely different, indeed, are the attractions of the next
object we have to mention from those of which we have
just been speaking. Saturn is a planet lighted by a sun,
while the globular cluster of stars in the constellation of
Perseus is itself a group of suns.

Girdling the entire heaven is that beautiful but some-
what irregular band of light, the Milky Way. It is com-
posed of myriads of stars, too small and too faint, by
reason of their vast distance, to be severally visible ; but
their different rays unite to give us the beautiful " Via
Lactea." This is a star cluster on the grandest scale, but
the several components are too much scattered to furnish
us with a brilliant or effective telescopic picture.

In the sword handle of Perseus there are two densely-
packed groups of stars, which form the two celebrated
clusters. They are visible to the unaided eye as faintly
luminous spots, but in a grand telescope they unfold into
spectacles of the most gorgeous sublimity. Each of the
two clusters — and they lie close together — is sufficient to

122 IN STARRY REALMS.

crowd the field of view with a multitude of brilliant stars,
in which the curious eye will find charming configura-
tions ; in one of these clusters notably a horseshoe, in the
other a beautiful system of triangles. Many of the stars
are of a ruddy hue, and as contrasted with other clusters
the smaller and inconspicuous stars are comparatively
absent. "We can, as it were, see right through the
entire group at every part to the space beyond. Such
a collection of gems, and so exquisitely set, will extort
admiration from everybody ; I do not remember to have
seen any one disappointed at this spectacle. But it requires
some previous acquaintance with a few facts in astronomy
to fully realise all that the picture represents. Unhappily
we are not able to supply the most important piece of
information which will naturally be asked : we are not
able to give the actual dimensions of this system. The
case is in this respect very different from that of the moon
or of Saturn. There the arts of the astronomer have been
successful in the attempt to measure and even to weigh ;
but in the stellar regions proper our knowledge of the
weights and the distance is always scanty, and not infre-
quently entirely wanting. This is so in the case of the
cluster in Perseus. We do not know the distance by
which it is separated from us. The methods which astro-
nomers are in the habit of using in such investigations
have, I believe, never been yet applied to the cluster in
Perseus. The belief no doubt exists that the only methods
available would be inapplicable to such an object.

In seeking the distance of a fixed star, the ordinary
method is to select some other star in the neighbourhood
apparently, but which is really very much farther behind,

AN EVENING WITH THE TELESCOPE. 123

though in the same line of sight. By a careful series of
measures made in the course of an entire year, there is an
apparent displacement of one of these stars relatively to
the other, caused by the fact that the earth has been ever
changing its position in the course of its annual revolution
around the sun. The method fails if the star's distance be
beyond a certain limit, as it actually is with the great
majority of the bodies to which the method has been ap-
plied. The difficulty of applying this process to the
cluster in Perseus is, that we have no clue to guide us in
the choice of the pair of stars which would be suitable ;
one of them must lie in the cluster, the other must be far
behind it. If we had any means of identifying a pair of
stars which were certainly so situated, the inquiry would
be certainly undertaken ; but it would not improbably
happen, if a pair of stars were chosen at random, that they
would be both in the cluster, and thus the attempt would
be abortive. It is doubtless this feeling which has pre-
sented astronomers from devoting their attention to an
irduous and protracted series of observations, of which
the result would not improbably be futile.

CHAPTER X.

NOTES ON NEBULJE.

I 4M attempting in these pages to give a conception of
the -varied nature of the objects which the skies offer to
our contemplation. In the moon we have a body of the
most solid description, evidently cold and hard. We
then saw that Saturn was entirely destitute of those rigid
features which gave the moon its beauty. The charms of
Saturn lie in quite another direction. Then we passed
from these sun-illuminated bodies to a group of suns
themselves in the glorious star cluster. Now we look
to an entirely different class of objects.

The telescope, ever an ally in the study of the heavens,
is in this part of the science absolutely indispensable. In
other branches of astronomy we can learn something with-
out its aid. Indeed, many great astronomical discoveries
were made long before the telescope was invented. But
ere this memorable event in the history of science it was
impossible for us to know anything of the existence of the
nebulae. It is indeed true that there is one of these ob-
jects which can be just detected by the naked eye. It

NOTES ON NEBULAE. 125

lies in the constellation of Andromeda, where, on a clear
and dark night, a faint spot of light can just be discerned
by a good eye. But a mere glimpse gives us really no
adequate notion of the true character of the object. It
might only, so far as the naked eye discloses its nature, be
a cluster of stars like that we have already discerned in
Perseus, or like the similar group that, under the name of
the Beehive, is comparatively familiar in the constellation
of Cancer. With the single exception of the nebula in
Andromeda, all the objects so called are entirely telescopic,
yet how important a constituent the nebulse form in the
contents of the heavens will be shown by a look at some of
the lists of these objects. There are now several thousands
of nebulae known, and their positions in the sky, as
well as the details of their appearances, are set forth in
the catalogues.

It will therefore be proper that during our evenings at
the observatory more opportunity should be taken to
examine these mysterious nebulae. An exceptionally fine
night should be chosen for this purpose. The sky should
be clear and bright, and the moon should be absent. In-
deed, when the moon is present the light it scatters over
the sky is sufficient entirely to extinguish the faint nebulae,
and greatly to impair the lustre and the beauty of the
brighter ones. A good test of the suitability of a night
for such purposes is found in the visibility of the Milky
Way. If it be seen clearly spanning the sky, then the
night will usually be favourable for such observations.
From the same considerations we may infer that it will
not do to choose nights in the middle of summer. Then
the twilight glow over the sky in our northern latitudes

126

IN STARRY REALMS.

Fig. 11.— The Great Nebula in Orion.

has a similar effect to the moonbeams, in so far as it
greatly detracts from the telescopic effectiveness of the
nebulaB. In fact, these objects are most of them so faint
that the caution I have already given as to the liability to
disappointment must be especially remembered. Many
of them can only be made out by careful watching, and
for the fainter ones an eye specially trained to such work is
required. It is for nebulae that the leviathan reflectors have
been chiefly constructed. The great mirrors are specially

NOTES ON NEBULA. 127

adapted to grasp all the feeble rays of light that these ob-
jects diffuse, and concentrate them so as to produce an
image bright enough to admit of being observed.

The most glorious constellation of stars in the firmament
is undoubtedly that of Orion. This splendid group is seen
in the south during the winter months, and towards the
close of January it is situated in a very convenient posi-
tion for observing early in the evening. The group is
specially characterized by the number of unusually bright
stars which it includes, and the three stars in the centre,
forming the so-called Belt of Orion, is as well known a
celestial figure as the sky contains. Directly under the
belt are three much smaller stars nearly in a line, which
points straight upwards to the middle star of the belt.
Those three lower stars are usually known as the sword
handle of Orion, this being the position which they occu-
pied in the fanciful old sketches of the constellation. The
three stars of the sword handle of Orion are plunged in
the Great Nebula. This object cannot be seen by the un-
assisted eye, though doubtless around the central star a little
haziness is perceptible, and even the slightest telescopic
aid will suffice to indicate that the central star of the
sword handle is attended by a surrounding glow of light,
which renders it quite unlike other stars. This can indeed
be sufficiently shown with an ordinary opera-glass, one
glance through which will awaken in the beholder a keen
desire to study the object under more favourable condi-
tions. But to do justice to the object, telescopes of large
power are desirable.

To realise fully the magnificence of the Great Nebula,
the observer who is being introduced to the object for the

:28 IN STARRY REALMS.

first time should not, strange to say, direct the telescope
at the nebula ; the instrument should rather be pointed at
the heavens, just a little to the west of the nebula. The
clock driving the equatorial should not be started, and the
observer should take his seat and look through the eye-
piece before the nebula has entered the field. He will
see, no doubt, a few stars on the black background, which
gradually pass in procession across his field of view.
This is merely the ordinary diurnal journey of the heavens,
by which all the objects move slowly from east to west ; I
ought rather to say appear to move, for of course the
motion on the heavens is only apparent, the fact being
that it is the earth which is turning around.

After the observer's eye for a minute or so has become
familiarised with the dark aspect of the heavens under
ordinary circumstances, he will begin to perceive on the
eastern side (it will appear in the telescope no doubt as on
the western side) a faint dawn of light. Gradually there
will steal across his field of view a sort of ghostlike lumi-
nosity that is in marked contrast to the darkness in the
rest of the field ; as the seconds move on, this object will
disclose itself until the full splendour of the Great Nebula
comes into view ; then the entire field will be filled with
the light, and then it will gradually advance and gradu-
ally pass away again to emphasize the contrast between
the brilliance of the nebula and the darkness of the sky.
Unless this method is adopted, the full interest of a tele-
scopic yiew of the Great Nebula is not attained, for when
the entire field is full of the glow the beginner will hardly
recognise the nebula. He will be apt to think that the
fainter part of the field he sees is the ordinary ground-

NOTES ON NEBULAE. 129

work of the sky, and this illusion can only be dispelled by
enabling him to witness the actual contrast in the way I
have described. The central portions of the nebula are,
however, so brilliant and so wonderfully marked with
interesting detail, that even a small instrument will suffice
to reveal much of its beauties.

In the centre of the nebula is the star known to astro-
nomers as Theta Orionis, the most prominent star of
the sword handle. To the eye this looks like an ordinary
star, but the telescope speedily dispels that notion. Theta
Orionis is found to consist of four, or rather six, stars
all so close together that the unaided eye fails to
distinguish them separately. A structure so complex
gives to this star quite a special, indeed a unique,
interest, wholly apart from the marvellous nebula of
which it is the focus. We must dwell a little on the
peculiarities of this star. We are familiar with stars
which are called double ; there are indeed some ten thou-
sand objects so designated known to astronomers and
duly registered in catalogues. Some of these are no
doubt only casual doubles. It happens that two stars lie
nearly in the same line of vision : they are thus found to
be very close together in the heavens. Such objects are
merely said to be optically double, and so far as their
physical nature is concerned they are of comparatively
little interest, though their practical utility in facilitating
the discovery of the distance of the nearer of the pair
ought not to be overlooked. It will, however, often hap-
pen that two stars are not merely apparently near each
other on the sky, but are actually quite close together, in
comparison, that is, to the immense distance at which they

K

130 IN STARRY REALMS.

are both separated from our system. This we know
because we often see the stars actually revolving one
around the other, not, it is true, with any very rapid
motion, in the ordinary acceptation of the word. One of
the stars may require many years to complete a revolution
around the other, but the fact that such revolution has
been noticed is sufficient in the cases where it occurs to
prove the connection existing between these stars. We
must also, of course, remember that these stars are suns of
stupendous magnitude, and from a view of our own system,
in which the great planets take many years to complete
one of their mighty journeys around the central orb, it is
not in the least to be wondered at that the periods of
revolution of these double stars should demand no less
intervals of time.

Many of these double stars are objects of extreme tele-
scopic beauty ; sometimes they offer to our admiration a
delightful contrast of colours ; perhaps one will be topaz
colour and the other bluish, or on rare occasions a pair of
emerald gems will be seen with an invisible band of mutual
connection. Sometimes triple stars are found, in which
three stars are obviously in alliance; but multiple stars
of greater complexity are comparatively rare ; and so
marvellous a spectacle as Theta Orionis, in which no
fewer than six stars are obviously an allied group, is
almost unique. It is not a little remarkable that we
find the most exquisite multiple star which the sky
can show, beautifully framed or set in the centre of
the grandest of the nebulae. Of course it might conceiv-
ably happen that the apparent concourse of these objects
was fortuitous. The actual phenomenon could be accounted

NOTES ON NEBULAE. 131

for by the belief that the Great Nebula was either very
much nearer or very much farther than the multiple star,
and that they chanced to lie in the same line of sight,
and had no other connection. But to me it appears that
this view is quite at variance with every reasonable pro-
bability; that the most wondrous multiple star should
have happened to lie in line with the very centre of the
most wondrous nebula, would have been a coincidence,
against the occurrence of which the probabilities were
almost infinite. There can scarcely be any doubt that the
multiple star and the Great Nebula are part of the same
system, and that the star is, in truth, placed in the middle
of the nebula, as it actually appears to be.

And now as to the composition of this mysterious object.
Here, indeed, the terrestrial analogies seem to render us
but little assistance. While we were discoursing about
the moon, we could appeal to the volcanoes, both active
and extinct, on the globe, as offering some clues to the
nature of the lunar craters. So also when we were
speaking of Saturn we were able to derive some assistance
in our attempt to understand the appearance of its globe
from the analogy of terrestrial clouds. No similar re-
source is open when we study the nebulae; we look in
vain for natural phenomena on this globe which shall
render the needful clue.

The word nebula means, of course, a little cloud, but
the expression is apt to be a misleading one. In a sense
no doubt they are little, inasmuch as the patch of the sky
which a nebula covers would be small compared with one
of our ordinary clouds. Indeed, a nebula which covered
as large an apparent part of the sky as the size of the

f32 IN STARRY REALMS.

moon would be ranked as a large object of its class, while
even the greatest of them is perhaps not more than ten or
twelve times as great. Nor is the word cloud, as applied
to nebula, an appropriate one. What we mean by a
cloud is only a vast mass of watery vapour raised by
the sun from the sea, and poised aloft until such time as
it shall be again dispersed into invisible water, or until it
shall descend to the earth as rain. Such clouds are of
course within the limits of our atmosphere, and are rarely
more than a few miles above the earth's surface. The
light which renders clouds visible only comes from
reflected sunbeams, and consequently at night clouds be-
come invisible, though the astronomer is often only too
unpleasantly made acquainted with their presence by the
opacity with which they shut out the stars from his
view.

Utterly different in all respects are the nebulae. They
are not masses of watery vapour. It may no doubt pos-
sibly be that water in some form is there, but it is not
water which we see. We are looking at some gaseous
material of a bluish hue. The light with which it glows
is no reflected sunlight. The nebula is indeed indebted
to no foreign source for that weird — I had almost said
ghost-like — radiance which it gives forth. The light
comes from the nebula itself. But how, it may well be
asked, should a purely gaseous substance be able to radiate
forth light ? It is easy for us to comprehend how stars
or suns or comparatively solid bodies can, in virtue of their
tremendous temperature, glow with heat like red-hot or
white-hot iron. It is true that flame is gas in an incan-
descent state, but in flame a vehement chemical union of

NOTES ON NEBULAE. !33

oxygen with some other substance is in progress, and this
is the source of the heat and the light that flame gives
forth. We cannot regard the Great Nebula in Orion as
originating in anything resembling flame.

We can, however, in our physical laboratories arrange
an experiment which seems to throw some light on the
composition of the nebula. Into a glass tube a small
quantity of hydrogen gas is admitted, the air having been
previously extracted. Then, by means of two wires, one
at each end of the tube, an electric current is transmitted
through the gas. Here there is no combustion ; the gas
is merely the vehicle by which the electricity flows from
one pole to the other. In doing so the gas instantly begins
to glow with an intense bluish light, and a very beautiful
effect is produced, which can be renewed or terminated at
will by simply making or breaking the electric current.
It would seem as if the gas we see in the nebula were in a
condition somewhat analogous to the gas in the tube. I
do not mean that the passage of electricity through the
nebula is the source of its luminosity. There is, indeed,
no ground for such a supposition. It is the property of
electricity when passing through a conductor to warm
that conductor ; thus we know that if a powerful current
be transmitted through a wire of the most infusible of all
metals, platinum, the wire will not only get warm, but it
may become red hot, white hot, and even melt under the
influence of the heat which is generated. In those beau-
tiful incandescent electric lamps which are now happily
coming into such extensive use a current of electricity
flows through a filament of carbon, and kindles that exqui-
site incandescence which is maintained while the current

•34 IN STARR Y REALMS.

flows. It would appear that so long as the electricity is
flowing through the glass tube its action on the gas is to
impart a very high temperature. It is in consequence of
this temperature that the gas glows. Now we can offer a
reasonable account of the luminosity of the Great Nebula
in Orion. The particles of gaseous or vaporous material
of which it is formed are of an extremely high tempera-
ture, sufficient to enable them to glow with the brilliancy
which renders them visible.

It is now almost twenty years since a marvellous acces-
sion to our knowledge of such objects as the Great Nebula
in Orion was made by Sir William Huggins. I have used
hydrogen as an illustration in describing the character of
the nebula, but I have now to add that the presence of
hydrogen is no mere fiction but a substantial verity. Truly
we here open up one of the most marvellous chapters
which science has to disclose. The chemist can analyse
the different substances on the earth with his test tubes,
and he can tell the elements of which they are composed.
But in this old-fashioned chemistry it was at least reason-
able for the chemist to demand a portion of the substance
he was expected to analyse. Unless he were provided
with a sample, how could it be possible for him to grind
it up or submit it to the various operations of his labora-
tory P In these modern days the chemist can perform
operations of which his predecessors never even dreamed.
No doubt the old method is still used — nay, is indeed at this
moment cultivated with greater skill and means than in
any previous age — but side by side with the old method,
and as an invaluable supplement thereto, the new method
of chemical research, called spectrum analysis, has been

NOTES ON NEBULA. 135

created, and has already conducted to many profoundly
interesting discoveries in the most varied branches of
science.

In the application of the spectroscopic method it is not
indispensably necessary that we actually have a fragment
of the substance ; all we require is a beam of light which
that substance can be made to yield when heated to a suf-
ficiently high temperature. No doubt this statement
should receive some more precise qualifications, but for
our present purpose it will indicate the nature of spectrum
analysis with sufficient accuracy.

To begin with a simple case, the colour of a light will
often afford an indication of its character. Thus the red
light seen in displays of fireworks is due to the presence
of the element strontium. The ghastly yellow hue pro-
duced by burning common salt with spirits of wine is
equally characteristic of sodium. Rarely, however, in
nature is a simple unmixed light presented to us, as it is
no doubt in the two cases I have mentioned. Suppose
that a number of distinct hues are blended into a single
beam, we could hardly expect to recognise the combina-
tion they produce. We must have some method for disen-
tangling the several ingredients so that they can be tested
separately.

The spectroscope gives the means of effecting the required
decomposition. A beam of light is passed through a
wedge-shaped piece of glass called a prism, or more fre-
quently through a whole series of prisms. If the light
under examination be a sunbeam, then the prism unfolds
a beautiful series of hues : the red, orange, yellow, green,
blue, indigo, and violet forming all the colours of the rain-

136 IN STARRY REALMS.

bow. Thus we demonstrate the highly composite charac-
ter of a sunbeam ; but the light from the nebula in Orion,
with which we are at present concerned, is of a much
more simple character.

When a beam of the nebular light is transmitted
through the prisms, it declares at once that the object
from which that light has come is totally different from a
star like the sun. Instead of the beautifully coloured
band, decked in all the glowing hues of the rainbow,
the nebular beam is seen to be composed simply of six
or seven widely separated strips. It is important to test
the character of the light in these strips. Fortunately
this can be done in a way that is completely satis-
factory. We can produce artificial lights from known
sources, and observe them through the spectroscope simul-
taneously with the light of the nebula.

There are in the composition of this globe some sixty
or seventy different elementary substances, and under
suitable conditions each one of these substances can afford a
perfectly characteristic spectrum. Thus the way of making
the comparison with the nebula is to try the different ele-
ments one after another, until one can be discovered which
pours forth a light that behaves under the prism aa
does the light from the nebula. Pursuing this inquiry,
Sir W. Huggins found that when hydrogen gas was ignited
to incandescence by the passage of electricity, it emitted
light which, after passage through the prisms, came to-
coincidence with one of the lines in the spectrum of
nebula ; and the hydrogen character of two of the other
lines has been since demonstrated. It was thus established
that hydrogen is one of the constituents of the Great

NOTES ON NEBULA. 137

Nebula in Orion. Further confirmation of this important
discovery was forthcoming when the photographs of the
spectrum of the Great Nebula were subsequently obtained.
On those photographs lines were present which are con-
stituted by light of such a nature as to be wholly invisible
to the eye, though perceptible on the photographic plate.
It is of the greatest interest to discover that these invi-
sible rays from the nebula are also indicative of the pre-
sence of hydrogen. Thus we obtain a beautiful confirmation
of the fact that the nebula is partly composed of glowing
hydrogen.

There are, however, some remaining lines, the character
of which has not yet been ascertained.

It would be a little premature to assert that there must
be some substance in the Great Nebula not at present
known to us on the earth. This would be, no doubt, one
interpretation of the facts. We must, however, admit the
possibility of another explanation. It is frequently found
that the lines yielded by an incandescent material vary to
some extent when the physical conditions of temperature
and of pressure are modified. It is, therefore, not impos-
sible that the unknown lines in the spectrum of the Great
Nebula may be due to some element known to us, but
which has not yet been tested under the conditions which
would make it yield the particular rays we are speaking of.

The composition of a nebula as disclosed to us by these
researches is very instructive. Here we are looking at
an object which seems to lie at the very limits of the
visible universe — an object so remote that our attempts to
fathom its distance are quite unsuccessful; yet in this
inconceivably distant part of our system we find at least

138 IN STARRY REALMS.

one ingredient which we know well on the eurth. Pre-
vious to actual trial no one would have expected, I think,
to find the Great Nebula largely constituted from such a
familiar element as hydrogen. This gas enters into the
composition of water, and is thus an element of extreme
abundance on the earth. That an element so common with
us here should also be abundant in these awfully distant
regions of the universe is one of the most astonishing
facts that modern science has revealed.

As the eye follows these ramifications of the Great
Nebula, ever fading away in brightness until it dissolves
in the blackness of the sky ; as we look at the multitudes
of bright stars which sparkle out from the depths of the
great glowing gas ; as we ponder on the marvellous out-
lines of a portion of the nebula, we are tempted to
ask what the true magnitude of this object must really be.
Here, again, we have to confess that science is unable to
satisfy this very legitimate curiosity. The only means
of learning the true length and breadth of a celestial
object depends upon our first having discovered the dis-
tance from us at which the object is situated. Unhappily
we are, as I have said, entirely ignorant of what this
distance may be in the case of the Great Nebula in Orion.
Our ordinary methods of conducting such an inquiry are
hardly applicable to such an object, and its position so
near the equator introduces fresh difficulties into the
problem. We shall, however, certainly not err on the side
of exaggeration if we assert that the Great Nebula must
be many millions of times larger than that group of bodies
which we call the solar system.

There are many other nebulae which would be well

NOTES ON NEBUL&.

I4o IN STARRY REALMS.

worthy of attentive examination. Here, for instance, in
Fig. 12 we represent one of the most famous known as
" the Great Spiral," but we must not linger any longer
over these objects. A treatise would be necessary to
convey what is known about them.

Our evening at the observatory has now been spent :
we have looked at the most remarkable objects which the
universe contains, and we have, let us hope, acquired a
fresh appreciation of the sublimity of that scheme of crea-
tion in which our earth plays a small though dignified
part

CHAPTER XL

VENUS AND MERCURY.

AMONG the planets of our system Venus is one which has
always been most tantalizing to astronomers. Notwith-
standing the fact that when this beautiful globe is seen at
its best it is far brighter than any star or any other planet^
yet as a telescopic object Venus is often disappointing. It
shows, no doubt, the beautiful crescent which charmed
Galileo, when he first directed the newly invented tube
towards it, and the crescent of the evening star is still one
of the most pleasing telescopic spectacles, which speci-
ally delights the beginner who for the first time finds
himself in the possession of a telescope. But it is the
very brilliancy of Venus which often leads to disappoint-
ment. We see a radiant object, a beautiful gem of light,
but the brightness tends to prevent us from seeing the
actual features on the planet. In this respect we can
contrast Venus with another globe, namely, the planet
Mars. This body is our next neighbour on the outside,
just as Venus is the next neighbour on the inside. It is,
therefore, of particular interest for us to learn all we can
about the two bodies which are similarly situated witK
regard to the benefits which the sun dispenses so liberally

142 IN STARRY REALMS.

It is well known that we are far better acquainted with
the aspect of Mars, in so far as the topographical features
of its surface are concerned, than we are with Venus, not-
withstanding that Mars is a smaller globe than Venus, and,
generally speaking, much farther away. We can even
study the surface of Jupiter, with considerable detail^
though under every combination of circumstances this
great planet is never less than twice as far from the earth as
is Venus. It is, however, only proper to add that it is the
vast bulk of Jupiter and the correspondingly great size of
his features which render him so discernible in spite of his
vast distance. We are able to speak of the belts of Jupiter,
to talk of the oceans and continents on Mars, and even
the " canals " on the latter globe are duly set down on
our maps. Many of the features on these two globes can
be represented with perfect confidence, and have conveyed
to us much valuable information with regard to our neigh-
bours in the solar system.

The case is very different with regard to Venus. The
reflected sunbeams which radiate so charmingly from
this planet bear to us but little information with re-
gard to the actual details of that globe from which
they come. Few astronomers have that confidence in
the perfection of their instruments and the accuracy of
their eyesight which will enable them to make out recog-
nisable features in the Evening Star. It is true that
certain of the older astronomers did indicate the exist-
ence of certain discernible marks on the planet. It was
sometime thought that darker patches could be ob-
served which were permanent features of the globe. It
was also thought that when Venus appeared as a narrow

VENUS AND MERCURY. 143

crescent of light, the irregularities near the horns of the
crescent could be attributed to mountains on the surface ;
while some astronomers had the hardihood to attempt to
calculate the elevation of the mountains which would give
rise to the features that were noticed. It can hardly be
said at any time that these observations and inferences com-
manded any very large degree of credence among those
competent to judge. From the facts which have recently
come to light, and which form the subject of this sketch, it
seems certain that even the small amount of belief which
was accorded to these researches must now be almost
entirely withheld.

There are many reasons why it should be of special
interest to learn all that we can of the topography of this
planet, which lies so near us. It so happens that Venus
has a bulk which is very nearly the same as that of the
earth. It seems also to be quite certain that Venus is
clothed with an atmosphere, for it would be otherwise im-
possible to explain the girdle of light which surrounds
the planet when she is in front of the sun's disc. The
measures of heat and light from the great luminary which
are received by Venus are, no doubt, in excess of those
which fall to the earth's share ; but still it seems that Venus
is a world so like our own in many respects that every
element of intellectual curiosity excites us to learn all
we can about another world to which we are linked by so
many affinities. There seems no prospect that we shall
ever perceive the features of the planet as fully and aa
minutely as we shall learn those of Mars. However, one
step has now been taken, and the results are pregnant with
interest.

I44 IN STARRY REALMS.

Viewed as a world, there is no more important point
for consideration than the length of the day as it would
appear to an inhabitant of Venus. This has long been a
matter of much uncertainty. The only means we have of
determining the period of rotation of a distant globe on
its axis is by watching some mark or object on the globe,
which we can recognise with certainty, and then observ-
ing the time which it requires to go round to the oppo-
site side, and back to where it was originally situated.

On Mars this process presents but little difficulty. There
are many features on that globe which possess the desired
attributes of definiteiiess and sharpness ; and accordingly
the period of rotation of Mars on its axis, or the day of
that planet, is known with all desirable accuracy. It is
known even to the fraction of a second, and it happens by
a curious coincidence not to be very different from the
length of our own day. A similar method can be applied
to some of the other celestial bodies, though there are
individual peculiarities which often give rise to difficulties.
For instance, we know that the sun turns on its axis in
about twenty-five of our days. This is learned by the
observations of those spots by which the surface is fre-
quently marked. As, however, the sun is not a solid
body, and as the spots are merely apertures through a
covering of luminous clouds, they do not possess either
the defmiteness or the permanence which would enable us
to rely on them for any very minute accuracy. Each spot
is also too transient to enable any large number of conse-
cutive rotations to be watched by its aid ; and unless a
large number of rotations can be included in the observa-
tions as in the case of Mars, any very great precision in

VENUS AND MERCURY. I4J

the determination of the period of rotation is impossible.
Indeed, it may be remarked that the sun does not rotate
in the same manner as a solid globe would necessarily
have to do.

The difficulty about finding the length of the day of
Venus simply arose from the fact that the marks on the
planet seemed too vague and faint to be relied upon for
the purpose. It is true that certain observations of this
kind had been made in former years, and it had been con-
jectured that the length of the day on Venus was about
equal to that of the earth. The question would be, of
course, settled if there were some great mountains stand-
ing out from Venus which every one could see through
their telescopes ; or if there were any black spot or other
unmistakable mark, it would answer the purpose. Unfor-
tunately, there are no such features discernible — none, at
least, that are perceptible to ordinary eyes furnished with
ordinary telescopes and used in those latitudes on the
earth where astronomical observations are most abundant.
Much of what we have said with regard to Venus may be
applied to the planet Mercury, which lies still nearer to
the sun. The period of rotation of both these planeta
has been until lately a matter of much uncertainty.

We are, however, recently indebted to the keen eyes of
Professor Schiaparelli, of Milan, for a minute scrutiny
of Mercury, on which he has discerned features suffi-
ciently recognisable to enable him to solve the great
problem of the length of the day on our neighbouring
world. He has shown that the earlier notions are appa-
rently erroneous. It would have been something to have
learned even thus much ; but the positive results at which

L

146 M STARRY REALMS.

Schiaparelli has arrived are so striking and full of inte-
rest that they constitute one of the most important addi-
tions to our knowledge of the solar system that has been
brought to light for many a long day. It ought, however,
to be added that some competent observers are unable to
convince themselves of the accuracy of Schiaparelli' s in-
vestigation.

Now, however, Schiaparelli has shown that the constant
face which the moon turns to the earth seems to .have a
parallel in the revolution of Mercury around the sun. It
is not that Mercury always directs the same face to the
earth. That would be utterly inconceivable on any
rational hypothesis, and would not be at all analogous
to the behaviour of the moon. It is to be remembered
that Mercury revolves around the sun. This planet, there-
fore, stands in the same relation to the sun that the moon
stands to the earth. The singular fact to which I desire
to call attention is that Schiaparelli's researches show
that Mercury constantly keeps the same face directed to-
wards the sun. The period of rotation of Mercury on its
axis is, accordingly, of the same length as the period of its
revolution around the sun. We never see the other side of
the moon, because the moon always moves so as to keep the
same face towards us. In a similar manner, only one
side of Mercury is ever visible from the sun. The other
side is sedulously averted from that luminary.

The side of Mercury which is turned away from the sun
is, therefore, eternally in dark night, while the favoured
side is perennially suffused with the glory of an uninter-
mittent day, much brighter and hotter than any day we
know, inasmuch as Mercury is a planet so much nearer te

VENUS AND MERCURY. 147

the sun than we are. Schiaparelli's attention has also been
given to Yenus. He has certainly shown that the period
of rotation of this planet is very much longer than has
been hitherto supposed. However, the difficulties of the
observations are so great that the results are not so defi-
nite as in the case of Mercury. It seems, however, hard
to resist the conclusion that Yenus, like Mercury, revolves
so as always to show the same face to the sun.

But to my mind the significance of Schiaparelli's
investigation does not lie so much in the mere perception
of certain features in the planetary movements. It is the
interpretation of these phenomena which offers the chief
points of interest. Indeed, there has never been any
time at which these discoveries would have been more
acceptable in the scientific world than at the present
moment. They furnish us with a vivid and unexpected
illustration of the doctrine of tidal evolution, to which we
have elsewhere (p. 60) given attention.

We revert to the case of the moon for a suggestion as
to the cause of the remarkable peculiarities shown in the
movement of Mercury, and presumably true in the case
of Yenus. We have explained that the constant face of
the moon is a monument of tidal action. It would be
an infinitely improbable coincidence if there were no
physical cause to account for it, and the tides afford a
perfectly satisfactory explanation.

In a precisely similar way we can account for the fact
that Mercury always bends the same face to the sun.
The matter is fortunately not a little simplified by the
fact that Mercury is certainly devoid of any consider-
able satellite, and very likely has no satellite what-

148 IN STARRY REALMS.

ever. The sun is the only tide- raising globe that can
affect Mercury. We, of course, also experience sun-
raised tides on this earth, but to us they seem of com-
paratively little importance, because they are much less
than the tides raised by the moon. There is, however,
a double reason for regarding the sun Braised tides on
Mercury as of special importance to that globe. In the
first place, as there are no perceptible satellite- raised tides,
there the solar tides are the determining agents in tidal
phenomena, and, secondly, as Mercury is closer to the sun
than we are, the sun -raised tides will be much larger and
stronger than those which we experience. It seems, there-
fore, certain that the constant face which Mercury directs
towards the sun is a consequence of tidal action. In fact,
it might almost have been anticipated from tidal pheno-
mena alone that Mercury would have been found to move
in this manner. The solar tidal control on Mercury must
be much more vigorous than that on Venus, but any
attempt on either planet to escape from the thraldom that
requires it to keep the same face constantly to the sun
would be checked with exemplary energy.

It will thus be seen that the results of the delicate
observations of an Italian astronomer seem to illustrate in
a striking manner one of the most interesting of modern
astronomical doctrine*.

CHAPTER

MARS AS A WORLD.

£T is rather curious that the planets should so closely
simulate the guise of the fixed stars. Saturn and Mars,
two of the most celebrated planets, have often a superficial
resemblance to Aldebaran or Betelgeuse. Yet it is diffi-
cult to emphasize sufficiently how wide is the actual
difference between them. The planets are not self-lumi-
nous bodies like the stars ; they are merely orbs revolving
around the sun, and indebted, like our earth, to the sun
for the benefit of his light and heat. These planets are
bodies which must be generally, if not always, smaller
than the fixed stars ; and their apparent brilliancy, not-
withstanding their small size and the fact that they
exhibit only reflected light, is simply a consequence of
their comparative nearness.

The apparent resemblance between planets and stars is
quickly dispelled when a telescope is directed towards
them. The star, as we have stated, shows merely a
bright point of light. It is far otherwise when we look
at the planets. Each of them exhibits in any instrument,
even of moderate power, a characteristic appearance by
which the planet is at once distinguished from the stars,

ISO IN STARRY REALMS.

while the several planets can each be discriminated from
the others.

There is, however, a crucial test which will detect every
object of a planetary nature. The very word planet means
a wanderer ; and the term is appropriately employed to
designate the wandering stars which move about over the
surface of the heavens. These bodies pass from one con-
stellation to another in the course of their circuits around
the sky, and they are thus widely distinct from the so-
called fixed stars by which the constellations are them-
selves formed. In the course of a few weeks, or even less,,
it will be easy to observe, even without telescopic aid, the
actual changes in the positions of the planets ; while, with
the assistance of a good equatorial telescope, a single hour
is usually sufficient to disclose enough movement in a
planet to show that the object is something quite different
from a star.

It is my object in this chapter to convey a sketch of
what is known, or can be reasonably conjectured, with
regard to Mars as a world. This globe is of particular
interest to us ; for it is natural to feel curious with regard
to the neighbouring globe, which is in many respects
placed in much the same conditions as is our earth. It
would seem that our globe occupies an intermediate posi-
tion, so to speak, in the general system of the planets.
I do not now refer only to the fact that there are some
planets which are nearer the sun than we are, and that
there are others more distant. This is undoubtedly true ;
but there are other circumstances of a still more signi-
ficant character. This world is a good deal larger than
some of the planets, while it is a good deal smaller than

MARS AS A WORLD. 151

others. We are attended by a single moon ; and if in
this respect we are disposed to envy the superior endow-
ments of a planet which has two moons, like Mars, four
moons, like Jujriter, or eight, like Saturn, we should be
consoled by the reflection that though Venus is a globe as
large as we are, yet she has no attendant moons at all,
and that Mercury is in a similar solitary condition.
In physical constitution, also, it will shortly appear how
our earth is more solid than this planet or less solid than
that ; how it has more air and clouds than some of these
bodies, and less air and clouds than others ; how the earth,
viewed as a habitable world, seems to be in the meridian
of life, while there are other globes still in the phase of
early youth, or in a more advanced old age. All these
various circumstances give to the study of the worlds
which form the sun's family a quite exceptional interest
to us. We may expect to learn from this study some
facts which will throw light on that question so pregnant
with interest, as to the possible habitability of the other
globes in space.

Of all the planets the one that comes into the most
favourable position for telescopic scrutiny from the earth
is unquestionably that globe known to the ancients by the
name of Mars.

Mars revolves around the sun, and accomplishes its
journey in a period of 687 days. It moves in a path
which we shall not greatly misrepresent if we describe it as
a circle with a radius of about one hundred and fifty-one
millions of miles. The true shape of the path is, however,
oval or elliptical, so that the planet is sometimes about
one-tenth of the distance just stated nearer to the sun, and

'5*

IN STARRY REALMS.

as often the same distance farther from the sun. The
orbit of this planet is thus farther out from the sun than
is that of our earth ; and in the annexed figure (Fig. 13)
we have drawn the relative sizes of the two orbits, both
represented in their actual form. It will thus be seen
how similarly the two bodies are circumstanced, in so far
as their relations with the sun are concerned. It is, how-
ever, obvious that the earth occupies a more favoured

Fig. 13.— Orbits of Earth and Mara.

position for the enjoyment of the light and heat dispensed
from the common source.

In another respect also the resemblance between this
planet and the earth is very notable. Our globe rotates
on its axis, and accomplishes each complete rotation in
one sidereal day ; that is, in 23 hrs. 56 min. 4 sec. of
solar time. In like manner does Mars revolve on it?
axis. By assiduous watching of the varied points on its
surface, which are seen gradually to turn round, to become

MARS AS A WORLD. 153

invisible, and then to reappear, it has been possible to
discover the period of his rotation with much accuracy.
This period is singularly close to that of our earth ; there
is not three-quarters of an hour's difference, the duration
of the rotation of Mars being the greater, or stated exactly,
24 hrs. 37 min. 23 sec.

It is, however, the telescopic aspect of the planet itself
which is especially interesting. With the single excep-
tion of the moon, there is no other body in the universe
that our telescopes can investigate so closely as they can
the planet Mars. It is now more than two hundred and
fifty years since it began to receive that assiduous atten-
tion from astronomers which has been continued to the
present hour. Among the earliest observations we have
are those of Fontana, at Naples. His feeble telescope was
sufficient to show that on the ruddy face of the planet were
certain more or less definite markings.

The nature of these marks on the globe has been now
so carefully studied that there can be no doubt that the
chief of them indicate permanent features on the planet.
In fact, astronomers have prepared several charts or actual
maps of the continents and oceans, or rather of the dark
regions which are presumably oceanic, and of the more
ruddy or often orange -coloured portions which seem to be
land.

The entire surface of the planet is in some degree known
to us ; in this respect there is a notable difference between
Mars and the moon. Of the moon we are only acquainted
with that face of it which is turned towards the earth,
and the other side is eternally secluded. But as Mars
rotates, we see now one side of it and now another, and

154 IN STARRY REALMS.

the various features of it are disclosed under every variety
of aspect and foreshortening as they advance from the
margin, and then cross its surface to recede again from
view by the progress of its perpetual rotation. We have
not, however, to rely solely on the rotation of the planet
to present us with a view of its different aspects. It
fortunately happens that sometimes the north pole
of the planet and sometimes the south pole is tilted to-
wards us.

To understand the different circumstances under which
the planet is thus presented, it will be necessary to enter
a little into the nature of its movements as represented in
Fig. 13. This drawing shows two orbits which, at a
superficial glance, appear to be circles, but which, on a
more careful examination, are found to be ellipses. The
inner of the two represents the path of the earth, the
outer shows the orbit of Mars. At the centre the sun is
placed. The earth completes its motion round the sun in
365 days, while Mars requires 687 days for its journey.
It will therefore happen, sometimes, that the earth, as it
were, overtakes Mars, and comes between the planet and
the sun. This phenomenon is called the opposition of
Mars ; and it is on the occasion of an opposition of the
planet that the greatest facilities for making a telescopic
scrutiny of its surface are enjoyed. This will be obvious
from a glance at the figure, which shows that at the
moment of opposition the earth must be closer to Mars
than the bodies can generally be at other times. Remem-
bering also that our point of view is on the earth, which,
in such a case, lies between Mars and the sun, it follows
that at the best hours of observation, in the middle of the

MARS AS A WORLD. 155

night, the planet must be on the meridian, and at its
highest in the sky.

The oppositions of Mars succeed each other at intervals
of 780 days. It therefore follows that about every two
years and two months the planet occupies a favourable
position for being observed. It must, however, be noticed
that all oppositions are not equally advantageous. Owing

Fig. 14.— View of Mars, Sept. 10th, llh. 20m., 1887.

to the high eccentricity in the shape of the orbit of
Mars it lies in one region of its path much closer to
the earth's circuit than it is when elsewhere. To ex-
press the results with numerical accuracy, we may take
the average distance from the earth to the sun to be
about ninety-three millions of miles. Then the distance
between Mars and the earth at opposition may some-
times be as much as sixty-three millions of miles, and

t5<> IN STARRY REALMS.

sometimes may be as low as thirty-four millions of
miles. It will thus appear that the best oppositions
will exhibit the planet at a distance which is but little
more than half that of the unfavourable oppositions. The
most convenient method for expressing any particular
point of the earth's orbit is by the day at which the earth
is annually due there. On August 26th the earth is in
that part of its path which lies nearest to the orbit of
Mars. If, therefore, an opposition of the planet should
occur on or near to that date, the earth and tho planet
will then be separated by the least distance possible.
On the other hand, when the earth is passing through
that part of its path which it traverses every February
22nd, the conditions are reversed. Should an opposition of
the planet then occur, the distance between the two bodies
will attain a greater value than is possible when the
opposition occurs at any other date in the course of the
year. It therefore follows that the nearer the oppositions
are to August 26th, the better they are, and the nearer
they are to February 22nd, the worse they are.

It is a noteworthy fact that the relations between the
periodic times of Mars and the earth are such that seven-
teen revolutions of Mars are accomplished in nearly thirty-
two years. There is a still more approximate relation
expressed in the fact that twenty-five revolutions of the
planet are almost exactly completed in forty-seven revolu-
tions of the earth. Hence it follows that the relative
positions which Mars and the earth occupy to-day they
will again regain in forty-seven years. Thus we sec.
that any favourable opposition of Mars will be succeeded
at intervals of thirty-two years and forty-seven years by

MARS AS A WORLD. 157

oppositions which are also favourable. As an illustration
of the use that may be made of these numbers, we shall
predict the occurrence of an approaching favourable oppo-
sition. There was one in 1877, there must, therefore,
have been a good opposition thirtj'-two years previously,
that is, in 1845. It must similarly be followed in
forty-seven years by another good opposition, which
brings us to 1892. We may therefore anticipate in the
autumn of 1892 another good view of our neighbouring
planet.

We have already spoken of the rotation of Mars, and it
follows that this planet has an axis about which that rota-
tion is performed ; the planet being in this respect, as in so
many others, a body analogous to our earth. The plane of
the orbit of Mars is inclined at a small angle to that of the
earth. This angle is under two degrees. It varies slightly,
and is at present decreasing with extreme slowness. The
value for the year 1890 is 1° 51' 1", and the decline in its
value is at the rate of two or three seconds per century.
So far as the mere aspect of Mars from our earth is con-
cerned, this small angle of inclination possesses an inap-
preciable influence. The case would be very different,
however, from the point of view of an inhabitant of Mars.
If there were absolutely no inclination between the two
orbits, then, whenever an opposition of Mars took place,
it would happen that the earth would, of course, be seen
actually projected on the disc of the sun when viewed
from Mars. As, however, there is some inclination, it
will usually happen that the Martial inhabitant will not
see the earth in front of the sun during opposition, for
the same reason that we do not see Venus on the sun**

rsS IN STARRY REALMS.

face every time that she is in conjunction. She is usually
either over the sun or under the sun, and only on rare
occasions have we the pleasing spectacle of a transit of
Venus. So rare, indeed are the transits of Venus, that
though the present generation has witnessed two, namely
in 1874 and 1882, there can be no recurrence of the
phenomenon until the years 2004 and 2012 have come
round. The Martial inhabitant will also find that a
transit of the earth in front of the sun is an occasional
and noteworthy spectacle. It has been pointed out by
Mr. Marth that such an occurrence might have been
witnessed from Mars on the 12th of November, 1879.

We may, however, for the present purpose ignore this
slight inclination of Mars' orbit to our own. But we
have now to consider the very different matter of the incli-
nation of the axis about which Mars' rotation is performed
to that of its actual orbit. We may here again advan-
tageously refer to the parallel case of our own globe.
The earth, in its annual progress round the sun, turns
now one pole and now another towards the sun, thus
producing the vicissitudes of the seasons. In a similar
manner the equator of Mars is inclined to the plane of its
orbit, and thus we sometimes are enabled to see one pole
of the planet and sometimes another. It happens that
when the planet is in opposition in our autumn, the
southern hemisphere is turned towards the earth. The
planet is then seen under the most favourable conditions,
in so far as its apparent dimensions are concerned ; for, as
we have already pointed out, it is under such circum-
stances at its least distance from the earth. It thus follows
that the most complete series of telescopic pictures of the

MARS AS A WORLD.

'59

planet which we possess are those in which the south pole
is exhibited.

Many astronomers have given to us carefully finished
sketches of the appearance of the planet on such occasions.
Among these I now specially invite attention to the
remarkable series of drawings obtained in the opposition
in September, 1877, by Mr. N. E. Green, who enjoyed the
privilege of observing Mars under singularly favourable
conditions in the beautiful skies of Madeira. The tele-
scope he employed was a reflector of the Newtonian form,
with a mirror thirteen inches in diameter, figured by Mr.
With, of Hereford. The magnifying power he most
usually employed was one of about 250 diameters. In
Mr. Green's memoirs he gives us representations of twelve
of his drawings selected from a much larger number. The
pictures have been chosen to show the planet at intervals
of two hours. This arrangement secures that the plane
shall be displayed under every different aspect, for after
twelve views at intervals of two hours have been depicted,
the thirteenth would merely be a repetition of the first, as
the rotation of the planet on its axis is so nearly twenty-
four hours. Mr. Green also suggests the caution with
which all drawings of planets, and, indeed, all drawings
of celestial objects generally, should be received. He
truly remarks that " they are necessarily exaggerations.
When the eye has been taxed to the utmost to observe
the form of some delicate marking, any shade which the
hand could apply to a drawing, and which could be
trusted to remain as a memento of the observation, would
be far beyond the strength of the reality. But the value
of the representation need not thereby be unduly depre-

£6o IN STARRY REALMS.

oiated, for if a proportionate amount of force be given to
all the details, it is still useful as expressing the general
character of the object, and valuable as exhibiting a fair
relation between its several parts."

As soon as the prominent features on the planet came
to be recognised, it was found necessary to devise means
by which their positions on the globe could be conve-
niently indicated. The analogy of our own earth showed
that the proper method of accomplishing this was to
adapt to Mars a system of latitudes and longitudes. As
to the former, there was but little difficulty. Once the
rotation of the planet was sufficiently observed, its axis
became known, and also its equator. The latitude of an
object on Mars is, of course, its angular distance from the
equator. But the longitude presents a difficulty as to the
zero from which it should be measured. This difficulty
is not exclusively found on Mars. We have it on our own
globe. Englishmen naturally measure all longitudes from
the meridian of Greenwich ; but national susceptibilities
arise, which prevent us from actually asserting that all ter-
restrial longitudes are measured in the same manner that
we use.

On Mars the difficulty is of a somewhat different nature.
The first essential for the zero of longitude is that it shall
be at all events a mark so clearly denned that there shall
be no ambiguity. This is the point about which there
has been some difference of opinion among astronomers.
But I think there can be little doubt that the proposition
made by Mr. Knobel, in 1884, is the correct one. He
points out that there is on the face of the planet one spot
so very clearly defined and so very easily observed with

MARS AS A WORLD. it>i

moderate telescopes, that it should obviously be adopted
as the zero from which all longitudes should be measured.
The nomenclature of the different objects is a little con-
fusing. The zero oi the longitudes was called the Oculus
by Madler, the Terby Sea by Green, and the Lacus Solis
by Schiaparelli. The zero now mentioned is not that
which Mr. Green had employed. Professor Schiaparelli,
in his charts of the appearance of Mars in 1881-82, places
the meridian of 90° through what he has called the Lacus
Solis, and which, following Mr. Knobel, is the zero of
longitudes. The Oculus or the Lacus Solis, or the Terby
Sea, whichever name be adopted, is to be regarded as the
Greenwich on the planet Mars. In my " Atlas of Astro-
nomy " I have given a map of Mars, in which the nomen-
clature has been determined from a careful comparison
of the different authorities.

Mr. Green has provided us with an excellent reason for
referring to his picture of Mars on September 10th. He
tells us that it " was without comparison the grandest
obtained at Madeira. This drawing also, of all the series,
most effectually recalls the impression made at the tele-
set ^ ; this sight of Mars was felt at the time to be a rich
reward for eyery effort, and will remain, while memory
lasts, a constant delight and satisfaction."

The interpretation of the various marks which pictures
of the planet exhibit involves questions that must still
be regarded as of a somewhat speculative character. It is
so natural for us to conjecture that the objects on the
planet may be of identical nature with the objects on the
earth, that we are accustomed to employ language on the
subject that is apt to express more than the extent of our

M

i6» IN STARRY REALMS.

information will actually warrant. In the first place, the
most striking feature presented in Mr. Green's drawings,
as well as in every similar series of pictures, is the remark-
able white region near the pole of the planet. In the case be-
fore us it is the southern pole that is exposed to view. It
does not, however, appear that the white region is situated
quite centrally with reference to the axis. Mr. Green gives
good reasons for believing that the true south of the end
axis of rotation of Mars lies just within the eastern edge
of the white cap, as it appeared in September, 1877. The
phenomena of our own poles at once suggest an explana-
tion of the presence of these white regions on Mars. Can
it be that this planet, which resembles our earth in so
many other respects, resembles it also in the possession
of polar regions crowned with solid masses of peren-
nial snow and ice? Apart from the argument founded
on analogy, the strongest evidence that can be adduced in
support of the ice explanation of the white region arises
from the undoubted variability of size of the region over
which the white fields extend, corresponding with the
Martial season.

The existence of an atmosphere surrounding the planet
has also been asserted, perhaps I might say demonstrated.
It is, however, certainly very much less ample than the
atmosphere enveloping this earth in its density and
extent. The most reliable indications of air around Mars
are afforded by the indistinctness of the planetary features
as they approach the margin by the gradual rotation of
the planet. In fact, Mr. Green speaks of the atmospheric
ring of bluish white surrounding the interior edge of the
disc. The atmosphere also sometimes contains masses of

MARS AS A WORLD. 163

cloud, though here, again, the thickest clouds ever seen
on Mars fall very far short of the dense cloud masses with
which our globe is so frequently shrouded. There can
be no doubt that portions of the permanent features on
the planet are temporarily obscured by the intervention
of clouds. Perhaps it would be more correct to regard
these objects in the atmosphere of Mars as rather in the
nature of mists or fogs than of clouds in the ordinary
sense of the word.

Still greater caution must be used in the attempt to
interpret the varied hues of the different permanent
features of the face of the planet. The general appear-
ance of the surface of the disc, as represented in Mr.
Green's drawings, is of " a warm yellow, heightening into
orange " in some of the regions. W^ith these are strongly
contrasted the dark portions which Mr. Green well de-
scribes as being of " a greenish grey, partly due to con-
trast with the orange."

It is customary to speak of these latter portions as lakes,
or seas, or oceans, according to their size and surround-
ings ; while the yellow and warmer portions generally are
spoken of as islands or as continents. There can be no
inconvenience in using this language so long as we clearly
remember that the distinction between land and water on
the planet is purely of a conjectural character.

Those oppositions of Mars in which the northern hemi-
sphere of the planet is turned towards the earth, are, as
already explained, of a disadvantageous character for a
minute scrutiny of the planet's surface. It is then at an
exceptionally great distance from the earth. Mr. Knobel
has well observed that, as the planet is then near itr

ib4 IN STARRY REALMS.

perihelion and receiving a larger supply of sun heat, the
meteorological condition of its surface may be very dif-
ferent from what is found when the opposition occurs at
a different part of the planet's orbit. Still, it is so im-
portant to study the aspect of Mars when the north pole
is tilted into view, that, notwithstanding the small appa-
rent size of the planet when near its aphelion, such oppo-
sitions merit careful attention.

A careful study of Mars was made during the opposition
of the planet, in 1884, by Mr. E. B. Knobel, and his
drawings are given in Vol. xlviii. of the Memoirs of
the Royal Astronomical Society. Although I do not
reproduce Mr. KnobePs careful examination of many
isolated features on Mars, I give here an abstract of
the more general remarks on the planet to which his
researches have conducted him. It is impossible to look
with care at any of the good drawings of the planet and
not be struck with the fact that there is a wide discrepancy
between the appearances of the northern and of the
southern hemispheres. The various dark markings, the
so-called seas, which are so conspicuous and so well
marked in the southern hemisphere, become replaced in
the northern hemisphere by objects of a much more ambi-
guous character. Indeed, the only object in the northern
hemisphere which can be regarded as thoroughly marked
and identified is that portion of the so-called "Kaiser Sea'*
which extends north of the equator. This remarkable
Martial feature is by far the most conspicuous and well-
defined marking that its globe presents. Even in quite
small telescopes the Kaiser Sea is well seen when hardly
any other object of the planet admits of being recognised.

MARS AS A WORLD. 16$

It is shown in Fig. 14 as the dark pointed region extend-
ing towards the north pole.

Around the north pole, as around the south, is also to
be found the accumulation of white material which so
irresistibly reminds us of snow. I have often been struck,
when observing Mars with the 12-inch refractor of the
Dunsink Observatory, with the extremely sharp boundary
line by which this white region is marked off from the
body of the planet.

I must now discuss another discovery, the result of the
acute observations of Schiaparelli, the director of the
Milan Observatory, to whose labours much of our know-
ledge of the topography of the planet is due.

In September, 1877, when Mars occupied that particular
situation relatively to the earth in which the distance
between the two globes was almost at its lowest point,
and when, consequently, the apparent magnitude of the
planet attained its highest value, Professor Schiaparelli,
availing himself of the clear atmosphere in which his
observatory was situated, made a remarkable observation
on the planet. He showed that the regions heretofore
spoken of as continents appeared to be intersected by dark
streaks, which must have been about sixty miles wide,
and which often attained a length of thousands of miles.
During the opposition of 1881-1882, Schiaparelli again
recognised the presence of the canals, but on this occasion
it would seem that a very extraordinary phenomenon was
perceived. Several of the canals were observed to be
double, that is to say, there were a pair of canals seen
separated by an interval of two hundred miles or more,
and running parallel to each other throughout their whole

166 IN STARRY REALMS.

course. It is, however, a doubtful point as to the actual
nature of these objects. Many assiduous astronomers,
who have enjoyed the aid of excellent telescopes, have
failed to see them. It would, at all events, seem that
there are no permanent markings on the planet which
appear like the twin canals. If we admit their existence
at all — and this is a point on which some astronomers
entertain considerable doubts — they might be regarded
rather in the light of certain periodic phenomena con-
nected with the changes of the seasons in the planet than
as permanent configurations on the globe.

I am, however, fortunately able to discuss here some
observations made by very capable observers during the
opposition of 1888. An excellent account of the results
which have been thus obtained is found in the Observatory
for September, 1888, by Mr. E. W. Maunder, of the
Roj^al Observatory, Greenwich. It must first be remarked
that the opposition was not a favourable one, in so far as
the apparent size of the planet was concerned, but still
there were many very interesting observations recorded,
and some very beautiful sketches of the planet were
obtained. The existence of the canals has been asserted
by Dr. Terby and M. Perrotin, and the latter astronomer,
provided with a superb telescope at Nice, has also detected
the remarkable phenomenon of the double canals. Prof.
Schiaparelli himself has again confirmed their existence,
and, in the enthusiasm of a brilliant telescopic view, he
declares that the Martial canals had all the distinctness
of an engraving on steel, with the magical beauty of a
coloured painting.

Several theories have been put forward to account for

MARS AS A WORLD. 167

these extraordinary " canals." We naturally try to obtain
from terrestrial phenomena some clue to their explanation.
The late Mr. Proctor, to whom we are indebted for the
first successful attempt to construct a useful map of Mars,
regarded the canals as rivers ; but, as Mr. Maunder has
so well pointed out, the objections to this view are insu-
perable. In the first place, they often intersect each
other, a phenomenon which is almost an impossibility for
two rivers. The curious straightness of the canals is any-
thing but suggestive of the sinuosities of a river. We
also frequently find the canals running sheer across the
continents, connecting one sea with another. It is need-
less to say that the rivers on our globe do not display any
phenomena of this description. For the present, all we
can say is, that the " canals " present problems of a very
mysterious nature, which have not yet been solved. We
must anxiously await further really favourable oppositions
of the planet, in the hope that the quickened attention to
the subject, and the admirable optical powers now at the
disposal of astronomers, will permit of some satisfactory
results being arrived at.

If we have reluctantly been compelled to leave the con-
figuration of Mars' surface in a somewhat ambiguous and
unsettled condition, it is satisfactory to turn to the very
clear and well-defined discovery of the satellites of the
planet. In this we have the pleasure of recording the
most interesting of pure telescopic discoveries that have
been made during this century. As our earth is attended
by one moon, as Jupiter is attended by four, Saturn by
eight, while the outer planets — Uranus and Neptune — are
also dignified by the companionship of satellites, it seems

I6« IN STARRY REALMS.

rather strange that the remaining planets — Mercury,
Venus, and Mars — should be entirely unprovided with
whatever benefits the possession of attendant moons is
competent to bestow. So far as Mercury and Yenus are
concerned, it may, indeed, be contended that the pre-
sumption that they ought to have moons had but little
substantial basis. It was obvious that the outer planets
had, on the whole, more satellites than the inner planets,
and as the earth had only one moon, while Yenus and
Mercury both revolved in orbits inside that of the earth, it
was obvious that there was but slender ground for expect-
ing that Mercury or Yenus should be found provided with
attendant orbs. But the case of Mars was different. As
Mars revolved in a path between that of the earth and
Jupiter, and as the earth had one moon and Jupiter had
four, it could be plausibly contended that Mars ought to
have at least a couple. But before the year 1877 this
conjecture was entirely devoid of any telescopic corrobora-
tion, and the " moonless Mars " was the phrase in which
one of the five great planets of antiquity was somewhat
contemptuously spoken of.

While the British Association was holding its annual
session at Plymouth, in 1877, a telegram arrived from
the United States, stating that Professor Asaph Hall had
achieved the superb feat of discovering two satellites
to Mars, and had shown that these satellites were
objects of the utmost interest. I well remember the
enthusiasm which this announcement produced among
the philosophers then assembled at Plymouth. Of course
we were all impatient to learn the details of the process
of search, and the particulars of the observations. These

MARS AS A WORLD. ttxj

were not long in forthcoming, and Professor Hall himself
described his great work in a complete memoir published
at Washington in the following year. It is to this paper
that we turn for full information on the subject.

It has been often said that the astronomer's best assis-
tant is his wife, but perhaps few of them have the same
confession to make as Professor Asaph Hall does when he
tells his readers that at first he thought the prospect of
discovering new satellites to Mars was very discouraging.
Other astronomers, and skilful astronomers, too, had tried
to find the satellites, and they had not been successful.
Why, then, should he hope to succeed in an enterprise
which had been already pronounced desperate ? He tells
us that he felt this so keenly that he might have aban-
doned the search had it not been for the encouragement
of his wife.

In 1877 the planet approached exceptionally close to the
earth, and this it was which indicated to Professor Hall
that the right time for making the search had arrived. In
this respect he, of course, only enjoyed the same facility
which every other astronomer possessed of choosing the
most favourable moment for making the attempt.

There were, however, two circumstances which were
greatly in favour of the success of the search which Pro-
fessor Hall was enabled to make. In the first place, the
United States Naval Observatory is situated in the pure
skies of Washington, and the more northward latitude
of the observatory gives it also a distinct advantage over
the similar observatories in Europe. Another factor in
Professor Hall's success was the use which he enjoyed of
a superb object glass, two feet in diameter. This magni-

1 70 IN STARRY REALMS.

ficent instrument is one of the very finest in the world j
perhaps, indeed, its optical perfection is at this moment
hardly surpassed anywhere. Like the great Lick tele-
scope already referred to, it is a monument of the con-
summate optical skill of the Messrs. Alvan Clark, of"
Cambridge Port, in Massachusetts.

But though these were necessary conditions in Pro-
fessor Asaph Hall's work, yet it would be absurd to
attribute his success either to the locality of his workshop
or to the perfection of his tools. A man's genius is best
shown by the right use which he makes of his oppor-
tunities. Many faculties are required to make a skilful
observer, and those faculties require careful training and
long experience before they can be trusted to do valuable
and reliable astronomical work. The practical astronomer
is, indeed, an artist of a special kind. He must have
cultivated delicacy and quickness of perception, he must
have a nice manual dexterity, he must conduct his work
with much forethought and tact. It must be all care-
fully organized and planned. He must have anticipated
his difficulties, and decided how they shall be overcome
or evaded. Unless he possess these qualities, and many
more, he will not be a discoverer of the satellites of Mars,
no matter how superb the telescope he is provided with,
or how glorious the climate in which he is situated.

It was in the middle of August, 1877, when the planet
was blazing in the glories of opposition, that Professor
Hall discovered the satellites. Tiny objects they indeed
were. Their feeble light was so faint that, to render
them visible at all, the fiery planet itself had to be moved
out of the field of view, or otherwise screened off. At

MARS AS A WORLD. 17*

first Professor Hall was greatly puzzled by the extra-
ordinary behaviour of one of these little moons. It first
appeared at one side of the planet, and then at the other,
in a manner so incomprehensible that Professor Hall
began to think there must be two or three bodies instead
of a single one.

Nor was such a supposition unnatural. It was, indeed,
the only alternative to an explanation which at first
seemed wildly improbable. This latter supposition, which
ultimately turned out to be the correct one, showed that
the inner of the two moons accomplished a feat entirely
without parallel in the solar system. It actually com-
pleted three revolutions around Mars in less time than
Mars required for a single rotation on his own axis. To
appreciate the significance of this statement, let us refer
to the condition of our own earth-moon system. The
moon requires about twenty-seven days to complete its
circuit of our earth, so that the revolution of the moon is
twenty-seven times as long as the rotation of the earth.
In a similar way, though not always to the same extent,
the periods of revolution of all the satellites that had been
previously discovered were each much larger than the
period of rotation of their primaries.

Contrary, however, to the invariable precedent exhibited
in the case of every other satellite of our system, the
inner satellite of Mars exhibits, to our astonishment, the
spectacle of a little moon galloping round the planet
three times for every single rotation that the planet itself
accomplishes. The outer of the two Martial satellites
follows the analogy of the other similar bodies in our
system. Its period of revolution is, however, only about

172 JN STARRY REALMS.

a day and a quarter, which is not much longer than the
period of rotation of the planet.

The names of the two satellites were suggested by a
passage in Homer, where Deimos and Phobos were sum-
moned, as the attendants of Mars, to yoke his steeds,
while he put on his armour. Deimos is the outer and
more conspicuous of the two, Phobos is the inner and
more interesting.

Immediately after Professor Hall's great discovery, the
powerful telescopes all over the world were directed to
Mars, and many observers were fortunate enough to
obtain measurements of the position of the satellites, and
thus to contribute to our knowledge of their movements.

One of the functions of the astronomer is to measure
and to weigh the masses of the celestial bodies. Now,
difficult as it may seem actually to weigh a mighty planet,
yet in many cases the task is a comparatively simple one.
It is true we do not seek to evaluate the actual mass of
the planet in tons. We are at present only striving after
a relative knowledge — a knowledge of the mass of the
planet as compared with the mass of the earth or the sun.
Our task is an easy one when we have to deal with planets
attended by one or more satellites. It becomes a very
difficult one when the planet has no satellite that can be
observed.

Let me endeavour to give some explanation of the
method we adopt when we seek to weigh a great planet —
such, for example, as our earth against the sun. We
shall suppose that the earth revolves around the sun in a
nearly circular orbit, and that the moon revolves around
the earth in an orbit nearly circular also. We may

MARS AS A WORLD. 173

further suppose, with sufficient accuracy, that the dis-
tance between the earth and the sun is 400 times the
distance between the earth and the moon. The celebrated
law of Kepler tells us that if two satellites be revolving
around the same primary, the squares of the periodic
times will be proportional to the cubes of the mean dis-
tances. I must ask you, then, in order to conduct the
calculation, to assume the existence of a fictitious satellite
revolving around our earth at a distance which is 400
times as great as that of the moon at present. This would
move much more slowly than the moon, and it would
require a great many months indeed to complete one revo-
lution. How many months are required can be discovered
by Kepler's law just stated. As the moon goes round in
one month, the fictitious moon would require a number of
months, which is determined by first multiplying 400 by
itself twice, that is by obtaining the number 400 X 400
X 400, and then taking the square root of it. We for-
tunately find that for this particular problem the actual
nature of the work is materially facilitated. Each of the
several factors, 400, is itself found by multiplying 20 by
itself, or as we would say the square root of 400 is 20,
hence the square root of the cube of 400 is found by
merely multiplying 20 by itself twice over, and that gives
us the answer 8,000. Hence we learn that if there were
a fictitious moon revolving at a distance 400 times that
of our present moon, its periodic time would be 8,000 of
our present months.

We have now placed the problem in the following
aspect. Our earth revolves around the sun in a year, or,
for our present purpose, we may, with quite sufficient

• 74

STARRY REALMS.

0V
*/

o/

01

cy\

s

if

\

MARS AS A WORLD. 175

accuracy, speak of the period as thirteen lunar months.
We have, therefore, two systems to compare. In one of
these the period of revolution is 8,000 months. In the
other, the period of revolution is thirteen months. In
each case the distance from the central body to the
revolving body is the same. The first is, of course, the case
of the earth and the fictitious moon, the second is the case
of the sun and the earth. The advantage of this method
of treatment is that we have by an artifice brought the
two distances into equality, so that they need not be
further considered.

Look now at the two figures exhibited on the previous
page. They represent equal circles. In each case there
is a great attracting body in the centre, while the revolv-
ing body describes the circumference of the circle. The
circumstances, it will be seen, are quite similar, except
that there is a marked difference between the periods of
the two revolutions. To what can this difference be due P
As the distances are the same, the discrepancy in the
periodic times can be only attributed to the difference in
masses between the central bodies. We can show that the
mass or size of the revolving body has no appreciable influ-
ence in the matter. At a specified distance from its
primary, a satellite will revolve in substantially the same
time, whether its mass be only an ounce or a million of tons.
Start any object around our earth with the right velocity
and at the right distance, and it will perform the circuit
in 8,000 months. We have a quite analogous pheno-
menon on our earth. A heavy body and a light body will
fall to the ground in the same time. Take a bullet in one
hand and a cork in the other, drop them at the same

«7& IN STARRY REALMS.

instant, and you will find that they reach the ground at
the same moment. It is true that with very light bodies
there is some difference, but this is caused by the resist-
ance of the air. There is no resistance to the motion of
satellites in the way we are now considering. It is there-
fore plain that the masses of the satellites can in no
degree account for the huge disparity between the periods
of their revolutions. Our search has now become greatly
narrowed, for the only remaining point of difference be-
tween the two systems is found in the masses of the
primaries. To this, therefore, we must look for an
originating cause of the differences of the periods of revo-
lution of the two satellites. If the two central masses
had been equal, there can be no doubt that the two
periodic times would also have been the same. The value
of this reasoning consists in the deduction which it enables
us to make. We know the periodic times, and from that
knowledge we are enabled to calculate the masses of the
two central bodies, by which the satellites that were re-
volving in those periodic times have been controlled.

This point is of such fundamental importance in astro-
nomy, that I feel I must pursue it a little further, so as to
enable us actually to solve the majestic problem of weigh-
ing our earth against the sun.

In the first place, it can be shown that the greater the
mass of the sun, the greater must be the rapidity with
which the planet revolves, in order to sustain its position
in a circular orbit at a specified distance. The planet
would, if unacted upon by any force, pursue a rectilinear
path and move on continuously with an unaltered velocity.
The presence of a central force constantly compels the

JtfARS AS A WORLD. irj

planot to swerve from a rectilinear path, and thus con-
strains it to move in a circular orbit. The more rapid the
motion, the more intense must be the force which is con-
tinually drawing the planet aside from the rectilinear
path, and consequently, the greater must be the mass of
the central sun by which the attraction is produced. The
proportion is, however, not a very simple one. If the mass
of the sun be doubled, the speed of the planet will not be in-
creased in the same proportion. The true law is that the
mass of the central attracting body will be proportional to
the square of the velocity, or inversely proportional to the
square of the periodic time. Hence it follows that if the
mass of the central sun be increased fourfold, the speed of
the satellite will be doubled, or what comes to the same
thing, the periodic time of the satellite will be reduced to
one-half.

Returning now to our figure. We have seen that the
two bodies revolve in periods of 8,000 months and thir-
teen months respectively ; that their distances from the
primaries are equal ; and from these data it is required to
conclude the relative masses of these primaries. One of
these numbers is about 615 times the other. Hence we
have to determine the relative masses of two central
attracting bodies, such that the relative periodic times of
their planets at equal distances shall be 615. We have
shown that the masses of the central bodies must be
inversely proportional to the squares of the periodic times ;
hence we learn that the more rapidly moving of these two
bodies must be controlled by a central force which is
615 X 615 times greater than that necessary for the
slower motion. We have used approximate numbers in

N

«;8 IN STARRY REALMS.

making this computation, so that the result thus obtained
is rather in excess of what would have been the result if
we had retained several small elements in the calculation
which we have thought it convenient to discard in such an
elementary sketch. The final result, as obtained by the
most trustworthy methods, is that it would take 326,800
•earths, all rolled into one, to form a mass as ponderous
as the sun.

It will thus be seen from this illustration, that the
observation of satellites in attendance upon a planet
enables us to weigh that planet. Hence we see that the
discovery of the satellites of Mars and the observation of
their periods, conducted us to an accurate knowledge of
the mass of the ruddy planet himself. I say an accurate
knowledge, for it must not be presumed that we did not
know something of the weight of this planet before. The
different departments of astronomy are so closely inter-
woven, that the influence of each planet is often felt under
circumstances that would hardly be credited by one who
had not the means of following out the complete details of
the subject. Our earth, in pursuing its annual progress
round the sun, is, of course, mainly guided by the all-
compelling and powerful attraction of the great central
luminary. But every planet also draws the earth in some
degree. The capacity of the planet to make the earth
swerve to the side of its path is dependent on the mass of
the planet. If the planet be great, its influence will be
correspondingly large. By incessant observations of the
sun, astronomers have learned, with the utmost minute-
ness of detail, the nature of the path which the earth
follows. They are actually able to disentangle the mar-

MARS AS A WORLD. 179

rellous complexities of its motion, and to assign to each
of the causes of disturbance its due share of responsibility.

It thus comes to pass that we learn how much Mars
affects the path of the earth, and hence we are able to dis-
cover the mass of the planet itself. Is not this a mar-
vellously recondite inquiry ? Yet it shows the perfection
to which astronomy, both theoretical and practical, has in
fact advanced, when I have to add, in conclusion, that the
mass of the planet Mars, discovered by this extremely
delicate and elaborate process, was found to be practically
coincident with the accurate value speedily and surely
proved when once the satellites had become known.

A curious literary point was soon remarked in connec-
tion with the satellites of Mars. We should not naturally
turn to " Gulliver's Travels " as an authentic manual of
astronomy, yet, strange to say, in that remarkable work
the astronomers, in the flying Island of Laputa are
found to have already anticipated Professor Hall. These
people, we are assured, " are very bad reasoners, and
vehemently given to opposition, unless when they happen
to be of the right opinion, which is seldom the case."
Nevertheless, we learn that they have " discovered two
lesser stars or satellites, which revolve about Mai's,
whereof the innermost is distant from the centre of the
primary planet exactly three of his diameters, and the
outermost five. The former revolves in the space of ten
hours, and the latter in twenty-one and a half." This
was certainly a marvellous anticipation of the actual facts
of the case. The astonishing feature is that the periodic
time attributed by Swift to the interior planet, should
have been so near correct.

CHAPTER XIIL

THE GREATEST PLANET.

MARS is, as we have seen, much smaller than the earth.
We now turn to a planet which is very much greater —
which is, in fact, the greatest of all planets — the majestie
globe of Jupiter. This superb world far exceeds in bulk
any other body in our system, the sun alone excepted.
Nay, if all the other planets could be welded into one,
their united mass would fall short both in bulk and in
weight of the stupendous mass of Jupiter. Our earth has
a diameter not one-tenth part that of Jupiter, and the
diameter of Jupiter is more than twenty times as great as
the diameter of Mars. The path in which the giant planet
pursues his stately course is shown in Figure 16, while the
inner of the two circles is the orbit of the earth, both
drawn to scale. The actual diameter of Jupiter's orbit is
about five times that of the earth. It should, however, be
noticed that though these orbits are nearly circular, and
though I often find it convenient to speak of them as
actual circles, yet that when very careful measures are
made, they are found to be ellipses instead of circlea
Thus, if we take the earth's distance from the sun as 100,
the distance of Jupiter from the same centre would be

THE GREATEST PLANET. 181

/

M JUPITER
/

SUN \ 1

\ ^ /

/

Fig. 16.— Orbits of Jupiter and the Earth.

sometimes as much as 545 and sometimes as low as 495.
Under the most favourable combination of circumstances
we are still left at an inconveniently great distance from
Jupiter when we wish to study his surface. He will still
be about four times as far from us as we are from the sun.
Hence it follows that in our telescopic scrutiny of the
great planet it is only objects of colossal proportions on
his globe that can be at all visible. In this respect we
are placed much more disadvantageously with regard to
Jupiter than with regard to Mars, though, on the other
hand, the much greater size of Jupiter more than com-

182 IN STARRY REALMS.

pensates for his distance in so far as the size and general
effectiveness of his telescopic picture is concerned.

A view showing the relative dimensions of the chief
planets in our system compared with the sun is given in
Fig. 17.

Jupiter is obviously flattened at the poles and pro-
tuberant at the equator. Our earth is also shaped in
a similar fashion, but the extent of the protuberance on
the earth is much less than that on Jupiter. Nor is
it difficult to explain the reason of this peculiarity of
shape, and of the excessive degree in which Jupiter is
affected by it. We have already seen that Mars revolves
on its axis, as does our earth, and we might probably
assert that rotation of this kind was a universal attribute
of celestial bodies — at all events, Jupiter possesses it in a
very marked degree. Though our little globe — for little
it must be regarded beside Jupiter — takes 24 hours to go
round, yet Jupiter accomplishes his complete spin in less
than half as much — in fact, in 9 hrs. 55£ mins.

Owing to the rapidity of Jupiter's rotation and to the
immense size of the planet, the actual speed of the surface
of the body must be enormously great. A point on the
equator of Jupiter moves about twenty- seven times as fast
as a point on our equator. The protuberance at the
equator of the great planet is to be ascribed to this fact.
As the body is revolving so rapidly, the yielding materials
of which it is made accommodate themselves to the shape
which nature demands, and the characteristic aspect of
the planet is the result.

We have seen that Mars is a body, in some respects, at
all events, resembling or analogous to our earth. In fact,

THE GREATEST PLANET.

as Herschel wrote years ago, the resemblance between the
earth and Mars appeared to be the most striking analogy
in the whole solar system. But a very brief examination
of Jupiter shows that we have here a body which, while
still a planet, is quite unlike our earth, and still more

Fig. 17.— Comparative Sizes of the Sun and the Planets

unlike Mars. It becomes, therefore, of special interest to
study Jupiter, as it affords an instructive chapter in
planetary history.

The general aspect of the great planet is fortunately
within the compass of moderate telescopic power. It has
been scrutinized with unintermitting zeal since first

184 IN STARRY REALMS.

instruments have been directed to the sky. Of recent
years the powers of great instruments have been devoted
to the study of Jupiter, and consequently we have at
our disposal an ample body of facts and an extensive col-
lection of drawings and measurements.

The appearance of the planet in a telescope of any reason-
able pretensions generally shows several markings on his
surface (see Fig. 18). Especially noticeable under ordinary
circumstances are a pair of belts parallel to the planet's
equator, one to the north and the other to the south. These
features are well represented in the cut, which delineates
a series of telescopic appearances of the planet. As we
watch the globe from hour to hour, various changes in
its appearance force themselves on our notice. No doubt
these are partly to be accounted for by the rotation of the
planet on its axis. Certain features gradually pass away
into invisibility on one side, while new features come in
on the other. Even the actual appearance of them at the
centre of the disc is more or less affected by the varying
degrees of foreshortening under which they are exposed.
But after all allowance has been made for such mere
apparent changes, it is speedily seen by the attentive
observer that the actual marks themselves are not perma-
nent, that they are waxing and waning, now entirely
disappearing, while ever and anon fresh features break
forth. Even the two belts themselves are not constant ;
the uniformity of the belts is often interrupted, while
their margins undergo considerable changes. Sometimes,
indeed, the belts are not to be discerned at all, and some-
times, instead of merely a pair of belts, the globe reveals
several. The longer the surface of Jupiter becomes

THE GREATEST PLANET.

185

studied, the more confident will the observer become that
he is not looking at any solid objects — he is looking at a
vast mass of clouds by which the entire globe of Jupiter
is surrounded. He will see that these clouds are so thick
and so dense that even his greatest telescope is never able

Fig. 18. — Views of Jupiter.

to pierce down through them and obtain a glimpse of the
solid interior of the globe. Indeed, it seems very doubt-
ful whether the words " solid interior " can be rightly
used with regard to this planet.

Although we do not seem to see anything which can be
regarded as solid on Jupiter, yet we can hardly draw the

1 86 IN STARRY REALMS.

inference that it possesses nothing of a permanent cha-
racter on its surface. It is true that we search in vain on
the great planet for anything so definite as the Kaiser
Sea, or one or two other objects that might be mentioned
on Mars. But there is at least one mark on Jupiter which
has been continuously observed for several years, and is
known as the great red spot on Jupiter. In 1879 it had
attained remarkable distinctness and such permanence
that, though it has undergone no infrequent changes in
brightness and appearance, it is a feature on the planet at
the present time. A remarkable phenomenon in connec-
tion with the proof of the absence of anything like rigidity
on the surface of Jupiter is strikingly brought out by the
behaviour of the red spot and of a white spot in its
vicinity. The rotation of the red spot, as computed in
the Olsercatory for 1886, was accomplished in 9 hrs.
55 mins. 39 sees., while that of the white spot was 9 hrs.
50 mins. 10 sees. Thus it follows that the red spot took
5J minutes longer to complete its circuit of the planet
than was required for the white one. If the globe of
Jupiter were really rigid, and if the spots were permanent
objects on that globe, then, of course, the period of rota-
tion concluded from either spot must be the same as that
concluded from the other, and both must be equal to that
of the rotation of the planet on its axis. In fact, to
suppose that there could be any difference would be
similar to saying that the day which is twenty-four hours
long in London was five minutes less than twenty-four
hours in Paris. It is obvious, therefore, when we find
the objects on Jupiter differing by five minutes in the
periods they give for the rotation, that one or both of

THE GREATEST PLANET. 187

these objects must have some proper motion on its own
account, and can in no case be structurally a part of the
rigid globe of the planet. In the "Companion to the
Observatory " for 1886 tables will be found which show
the times at which both the red spot and the white spot
were to be expected during that year on the central meri-
dian of Jupiter. Considering that these objects are of
such an unstable material, we can hardly be surprised at
the editor's complaint that the motion of the white spot
has appeared of late to be irregular.

It has sometimes been thought that the great red spot
may be a protuberance from the partially solid interior of
the planet, which has been forced up through the over-
lying stratum of clouds, and thus brought into visibility.
So long as it lasts it will provide us with a tolerably
definite point on Jupiter from which longitudes can be
measured, and thus take the place which Dawes' forked
bay provides for us on Mars.

There is another instructive line of reasoning by which
we can demonstrate that the Jupiter which we see is
certainly not to be regarded as a solid or rigid body. Its
bulk is so enormous that it is fully 1,200 times as big as our
earth. So much for the measurement of the giant planet ;
now as to its weight. We have already explained in con-
nection with the satellites of Mars how it is feasible, by
observations of the satellites, to discover the mass of the
planet to which those satellites are appended. In the appli-
cation of this method to the determination of the mass of
Jupiter we have a choice of no fewer than four fine satel-
lites from which to make the observations. The mass oi
Jupiter can also be obtained by other methods of inquiry,

i88 IN STARRY REALMS.

so that it can be regarded as one of the most accurately
known elements of our solar system. To express the
result in the form best adapted for our present line of
reasoning we may say that the mass of Jupiter is 310
times as great as the mass of the earth.

Truly Jupiter is a noble globe. If we imagine a stu-
pendous pair of weighing scales constructed, and that
Jupiter was placed in one pan, we should find that it
would require 310 globes, each as ponderous as our earth,
to be placed in the other pan before the beam began to
turn. Yet while we are impressed with the gigantic
mass of Jupiter, we recall the fact that the planet is 1,200
times as large, and then we are driven to inquire why
it is that though Jupiter is 1,200 times as big as our
earth it is only 310 times as heavy.

It is quite obvious that Jupiter must be a world of a
kind very different indeed from that on which we stand.
Our world is made up of rocks and minerals, largely also
of iron, and all these materials together produce a globe
of very high density, between five and six times the
weight of a globe of water of equal dimensions. But the
case with respect to Jupiter is widely different. If we
take the earth as a standard of comparison, we see that as
Jupiter exceeds us in bulk 1,200-fold, he should also be
about 1,200 times as heavy, if the materials of his con-
stitution were similar to and in the same state as those on
the earth. But he only weighs about a quarter as much
as this hypothesis would require. It therefore follows
that Jupiter cannot be organized on anything like the
same plan as our globe. His average density is only one-
fourth as much as ours. Jupiter only weighs about once

THE GREATEST PLANET. 189

and a quarter as much as a globe of water the same size.
This is quite incompatible with the supposition that his
bulk is largely made up of materials at all resembling in
density the solid substances on the earth. The true
explanation must no doubt be that the mighty planet is,
on his exterior at all events, largely composed of great
oceans of clouds and vapours of density and thickness ;
that these vast clouds weigh but little in comparison with
their bulk ; and that thus the apparent size of Jupiter is
swollen to dimensions greatly exceeding those to which he
would retire if the materials now in vapour were con-
densed into more liquid or solid forms.

When we ponder on the inflated size of Jupiter, and
when we apply to his globe the laws of heat, of gases, and
of vapours, which we have learned from experiments, we
are conducted to a very important discovery. There is
only one method of accounting for this stupendous mass
of impenetrable clouds which so persistently envelop the
planet. Clouds cannot exist except there be heat to
produce the requisite vapours. If the heat be absent, the
clouds will resume the liquid state. We are therefore
forced to the conclusion that the vast mass of clouds
enveloping the great planet can only be sustained by the
supposition that there must be some great source of heat
available on Jupiter. It is natural to inquire whence this
heat can be derived. Let us first compare or contrast the
condition of Jupiter with that of our own earth.

We have no doubt a liberal supply of clouds supported
in our atmosphere. But our clouds are neither so copious
nor so continuously impenetrable as those on Jupiter. An
astronomer on the latter planet, who gazed at us through

190

IN STARRY REALMS.

a telescope of sufficient power, would no doubt generally
find a large part of our globe obscured by suspended
masses of vapour. He would also certainly see some
traces of those belts of cloud, parallel to the equator,
which mark the trade wind zones. And he would be
struck with the instability and fickle character of the
cloudy screen which in the course of a few hours would
reveal or obscure whole kingdoms or oceans. The obser-
vations of the clouds alone would make the telescopic
scrutiny of our world from some distant world a singu-
larly interesting and instructive research. There are
perhaps some regions on the globe which would be almost
continuously quite free from cloud, and these the Jovian
astronomer could draw without any special choice of
opportunities. But to trace the majority of the outlines
of the continents and the oceans would be an arduous,
if even a possible, task ; at the least it would require the
patient astronomer to wait until openings in the clouds
enabled the outlines of the coasts to be clearly appre-
hended. But on Jupiter the clouds are so massive, that
no amount of patience ever enables us to see through
them ; the chance is never given to us. We are there-
fore compelled to suppose that the heat by which the
vapours to form clouds are manufactured, must be much
more abundant and intense than it is on our globe. The
heat that provides moisture for our atmosphere is obtained
from the sun. The warm rays beating down on our vast
oceans create great volumes of water vapour, which ascend
thousands of feet, and are then borne by winds to other
less genial climes, where chills reduce the transparent
water vapour to that opaque material of visible steam or

THE GREATEST PLANET. 191

cloud. For an explanation of all terrestrial clouds, we
require no other heat than that which the sun dispenses
so liberally and unceasingly.

In endeavouring to understand the clouds of Jupiter,
the first attempt which we naturally make is to explain
their origin in a manner similar to that of the clouds on
the earth. Jupiter revolves around the sun as this world
does, and like this world Jupiter is also illuminated by
the sun and warmed by him. Can it not therefore be
that the sunbeams on Jupiter generate the vast cloudy
covering just as they do here ?

This is a point on which a simple calculation will throw
much light. The average distance of the great planet
from the sun is 5£ times as great as the distance of the
earth. It can easily be shown that the intensity of the
heat received from any source decreases as the distance of
the source increases. The rate of decrease is indeed a
rapid one. If the distance be doubled, the supply of heat
is not alone halved, it is reduced to one-fourth of its
original intensity. In fact, we may express the law
generally in that form in which nature loves to work,
that the intensity of the heat received from a source
varies as the inverse square of the distance. It therefore
follows that to compare the sun heat received on Jupiter
with the sun heat received on the earth, we must square
the number 5£ ; that is, multiply it by itself. The result
is very close to 27, and hence we learn the significant fact
that the intensity of sun heat on Jupiter is only one
twenty-seventh part of that which is received on the earth.

We cannot believe that the sun's rays, enfeebled to a
twenty-seventh part of the intensity in v*hich we know

192 IN STARRY REALMS.

them, can possibly be adequate to evaporate enough fluid
on Jupiter to give rise to that vast mass of clouds which
surrounds him. More clouds require for their explana-
tion a larger supply of heat than that on the earth. Yet
the sun only conveys to the great planet less than four
per cent, of the heat on the same area that he sends to us.
There is only one interpretation to be put on these facts.
We must admit that the sun's heat is utterly inadequate
to the accomplishment of the work in progress on Jupiter,
and hence we must look to the presence of some other
source of heat.

Whence can Jupiter derive heat if not from the sun ?
Assuredly there is no other body in the universe which
can radiate even the millionth part of what he gets from
the sun. The stars are no doubt hot enough, but they
are so enormously distant that the influence of their heat
is evanescent, and need not for a moment be considered.
The other planets and their satellites can of course render
no assistance. Like Jupiter himself, they are recipients
of heat from the sun in varying degrees, according to
their distance. The only heat they can dispense will be
of the very feeblest, and be quite imperceptible. It i&
certainly true that Jupiter cannot receive heat from any
external body to any appreciable extent, except from the
sun, and yet the utmost that the sun can do is quite inade-
quate to account for the warmth of the great planet.

At first it might be thought that we had arrived at a
paradox, but this is not really so. There is a most
rational explanation of the phenomenon entirely consi^
tent with all the facts.

There are many reasons for knowing that the diffeient

THE GREATEST PLANET. 193

bodies of our solar system were originally much hotter
than they are at present; they have all been steadily
cooling, and the process has been going on for uncounted
ages. The small bodies have cooled more rapidly than the
large ones. Thus the moon, which is one of the smallest
of the bodies in our system, appears to have parted with
almost all its heat. The sun, at the other extreme, on
account of his gigantic dimensions, amounting as they do
to more than a thousand times the size of Jupiter, which
is itself larger than all the rest of our system together, is
large enough to have retained so much heat that he still
glows with excessive fervour.

Between the sun and the moon as the extreme limits
of size, may be ranked the other bodies of the system.
We find that many of the different bodies show more heat
or less heat, according to their size. Thus, our earth
being so much larger than the moon, and so much smaller
than the sun, might be expected to have still retained
some of its heat, though the glowing stage in the pro-
gress of cooling has long since been passed ; and this is what
we do actually find. Our globe shows many indications
of a high interior temperature. We have the well-known
phenomena of volcanoes, and of hot springs, to show us
that there are stores of heat beneath our feet, and our
belief in internal heat is confirmed by the gradual increase
of temperature that is found in every mine, the deeper it
penetrates into the earth.

Jupiter, being 1,200 times as big as the earth, has not
yet cooled to the same extent. It still retains vast stores
of internal heat, which are sufficient to preserve in the
vaporous form the oceans that shall one day roll upon its

194 IN STARRY REALMS.

surface. No longer need the dense clouds of the giant
planet be a mystery ; they are merely an indication of a
certain stage in the process of cooling, through which
Jupiter, like every other globe in our system, has to pass.
They are of interest and instruction to us. In the present
appearance of Jupiter, we may see what our earth was
once in those early days ere it had cooled to such an
extent that the oceans passed from the vaporous to the
fluid form in which we now find them.

Such studies convey to our minds an imposing idea of
the magnificence of the universe. We have only been speak-
ing of a few of the bodies belonging to our own system,
and such systems are spread in countless myriads through
space. They raise in us a majestic conception of the
extent and splendour of that universe we see, which must
itself be only an inconceivably small fraction of the entire
extent of the universe, of which the greater part lies for
ever beyond our view.

CHAPTER XTV

THE NAMES OF THE PLANETS

I PROPOSE in this chapter to discuss some points which
seem specially interesting with respect to the names of
the celestial bodies. The study of the heavens in bygone
days possessed an importance of a wholly different kind
from that which we now attribute to it. It was not
then supposed that among the stars there could be worlds
comparable in importance with our globe. In the belief
of the ancients our earth was the central body of the
universe. The sun, moon, and stars were merely regarded
as objects placed in the heavens for the purpose of minis-
tering to human wants. The relations of the sun and
moon to the inhabitants of the earth were, of course,
obvious, but the other bodies also had their terrestrial
influences which were to be discovered by occult science.
Accordingly a system of astrology was created for the
purpose of interpreting the movement of the stars.

It is, perhaps, hardly necessary to say that we now only
regard these notions of the old astrologers as curiosities.
We remember that our earth occupies but an insignificant

iqb IN STARRY REALMS.

portion of space, and that the stars and planets are gene-
rally globes far greater than that on which we dwell. The
movements of the heavenly bodies are no longer believed
to contain indications of human affairs. We do not cast
the horoscope, nor do we now think that the career of a
man is decided by the configuration of the planets at the
moment he happened to be born. It is, however, inter-
esting to note the different ways in which traces of the
old astrological beliefs still survive among us. In former
days, when any great undertaking was in contemplation,
the stars were consulted to know if the auspices were
favourable. Though we do not at the present day think
this necessary, yet we do at least admit that we ought
carefully to consider the prospects of the enterprise.
Astrology has provided us with the word, for consider is
derived from the Latin word sidus, a star, and signifies
literally that we consult the stars. Should the under-
taking not turn out fortunately, we often describe it as
ill-starred; and here we recognise that a good star has not
favoured our efforts, but that they have been under the
malign influence of an evil one. When a serious mis-
fortune occurs we may sometimes speak of it as a disaster.
This is a word derived from the Greek, and signifies that
our star has been unfavourable. Need I add that it is the
same Greek root that gives us the first half of the word
astronomy ?

Remembering the importance of the stars to the ancient
astrologers, it is not unnatural that they watched them
with the closest attention. They observed that the SUB
and the moon changed their places on the heavens, so that
these bodies were fitly described as " wanderers." It seem*

THE NAMES OF THE PLANETS. 197

to have been known from the remotest antiquity that there
were also some starlike objects which moved about. Of
these the earliest to be observed was, doubtless, the planet
Venus. Its brilliancy, as the evening star or the morn-
ing star, seems to have attracted attention from all intel-
ligent nations of which we have any record. As Venus
continually changed its place, and so far resembled the
sun and the moon, the astrologers credited this planet
with a significant intervention in terrestrial matters. But
there were other planets also to be discerned by those who
carefully watched the sky. Three bright objects, Jupiter,
Saturn, and Mars, showed by their movements that they
were bodies wholly different from ordinary stars. The
list of planets known to the ancients was completed by
the discovery of Mercury. It is impossible to reflect on
this achievement without admiring the acuteness of obser-
vation which disclosed the nature of this rarely seen
object, and identified its successive appearances. Mercury
seems to have been discovered independently by two or
three nations at dates antecedent to those of exact history.
It will thus be seen that there were in all five planets,
namely, Jupiter, Saturn, Venus, Mars, and Mercury. If
to these we add the sun and the moon we have the total
seven "wanderers" with which the astrologers used to
conduct their mystic operations.

There is no doubt that the ancient significance of tne
number seven is attributable to the fact that there were
believed to be neither more nor less than seven of these
bodies. The most striking illustration that we can give
of the survival of astrological notions is found in the
connection between the names of the seven days of the

198 W STARRY REALMS.

week and the names of the seven wanderers. In fact, it
appears that the use of a week of seven days has been
confined to those races who had recognised these seven
planets. The Incas of Peru seem to have detected no
other planet than Yenus, and their week had nine days,
to accord, apparently, with one-third of the duration of
the moon's revolution around the earth. The Aztecs of
Mexico, who were also in ignorance of the planets, Yenus
alone excepted, employed a week of thirteen days.

Among the ancient nations of the old world, not only
the number of the days in the week, but the names of
those days, were associated with the names of the seven
wanderers to which they respectively corresponded. The
ancient astronomers, though ignorant of the true rela-
tions of the celestial bodies, had yet been enabled to
arrange the wanderers in their true order of distance from
the earth, at least in so far as it is possible to place bodies
m such an order when their distances are incessantly
changing. The dimness of Saturn and the slowness of
his movements justified them in regarding him as the
most remote of the seven. Next came Jupiter, and then
came Mars. It is to be remembered that, according to
the ancients, the sun was classified as a wanderer with the
bodies we now speak of as planets. His distance was less
than that of Mars and greater than that of Yenus. Mer-
cury was nearer to us than Yenus, while the moon was the
nearest body of all to the earth. "We thus write the seven
wanderers, in order of their distances, as follows — Saturn,
Jupiter, Mars, Sun, Yenus, Mercury, Moon. This is not,
however, the order in which these planets stand when
they represent the seven days of the week. The succes-

THE NAMES OF THE PLANETS 199

gion of the week days is arrived at in the way we shall
now mention.

In ancient times the day and night together were
divided, as at present, into twenty-four hours. Each
one of these hours was consecrated to one of the planets.
The order in which the hours were appropriated to the
different wanderers was simply the order of their dis-
tances, and each day bore the name of the wanderer to
which its first hour was devoted. As a beginning the first
hour is consecrated to the most distant object, Saturn, and
accordingly the corresponding day is Saturn-day, or
Saturday ; the second hour belongs to Jupiter, the third
to Mars, the fourth to the Sun, the fifth to Yenus, the
sixth to Mercury, and the seventh to the Moon. Then
the eighth hour begins with Saturn again, the ninth with
Jupiter, and so on until after twenty-one hours the list
of the wanderers has been repeated three times over. The
twenty-second hour begins, of course, with Saturn again,
the twenty-third belongs to Jupiter, the twenty-fourth to
Mars, and thus the whole twenty-four hours of Saturday
are complete. The twenty-fifth hour, which is the first
hour of the next day, falls to the wanderer next to Mars,
that is to the Sun. The first hour of the day next after
Saturday is accordingly consecrated to the Sun, and as the
name of each day is derived from that of the planet pre-
siding over its first hour it follows that the name of the
day following Saturday is to be the Day of the Sun, that
is, of course, Sunday. You may follow the same calcula-
tion throughout.

The following is an equivalent but shorter process.
Write around a circle the names of the seven wanderer*

200 IN STARRY REALMS.

in the order of their distances, and then read them by
beginning with Saturn, and skipping two each time. They
will come into the following order : — Saturn, Sun, Moon,
Mars, Mercury, Jupiter, Yenus, and then Saturn again.
Now we have reached the precise order of the days in the
week. The meaning of Saturday and Sunday we have
already explained, Monday is Moonday, and the French
Lundi means also the Day of the Moon. Tuesday is con-
nected with Mars; in fact, the French word Mardi,
meaning Tuesday, is obviously Mars' Day. Wednesday,
or Mercredi in French, is Mercury Day. Thursday is
derived from Thor, a deity analogous to Jupiter. Fri-
day is shewn to be the day of Venus. Thus not only
the seven days of the week, but the very names of the
days themselves are directly related to the names of the
seven planets.

It is a question of much interest as to why the
names of the heathen deities should have been as-
signed to the planets. How comes it, for instance, that
the God Jupiter should have given a name to the great
planet ? The question is not without some difficulty, but
the origin of it appears to be as follows. There are at
the present day — and doubtless ever .have been — races
who worshipped the sun as a deity. Considering that the
sun was only one of the seven wanderers, it was not
unnatural that devotional homage should also have been
rendered to the other similar bodies. This was all the
more natural because the movements of these bodies were
believed to be intimately associated with human affairs.
That globe which we now style the planet Jupiter was
itself regarded as a deity, and as such was worshipped. A

THE NAMES OF THE PLANETS. 201

more refined creed afterwards separated the intangible
deity from the actual globe, which thus caine to be re-
garded merely as a symbol. The name, however, by which
the planet was designated was borne both by the material
body and by the deity of which that body was an emblem.
In modern days the naming of planets is quite a familiar
operation. Every now and then a new planet is dis-
covered, and it is the privilege of the discoverer to assign
to the new object a name which other astronomers shall
recognise. At lease this is generally the case, but not
invariably ; for when William Herschel immortalized
himself by discovering the planet now called Uranus, he
proposed to name it the Georgium Sidus in honour of
King George the Third. As this was the first planet ever
found in addition to the five bodies of this kind which
had been known from all antiquity, a wonderful interest
was aroused among astronomers all over the world ; but
the old planets had borne the old classical names, and it
was thought, especially by Continental astronomers, that
it would be a little incongruous to bring the name of
-George the Third into the same category as the heathen
divinities. Accordingly they set aside the wishes of the
discoverer, and established the precedent which has since
been generally followed, of choosing names for the newly
discovered planets from classical mythology. Though the
number of the planets is now more than 300, the re-
sources of ancient literature seem not yet to give signs
of exhaustion. Let Hebe and Hecuba, Diana and Sappho,
which I select at random from the list of minor planet^
serve as modern examples of how the heavenly bodies are
named

CHAPTER XV.

A FALLING STAR.

EVERY one who has occasionally taken a nocturnal walk
in the open country will probably have seen what is called
" a shooting star." Perhaps I might rather say that
unless the observer be very inattentive he will have
noticed dozens, or scores, or hundreds of these objects,
either bright or faint, with long streaks or with short.

For the due exhibition of a shooting star, that part of
the sky where it is displayed should, of course, be free
from cloud, and the silvery streak will seem all the more
vivid if the moon be absent. No telescope is needed.
This is, indeed, tne one branch of astronomical observation
in which the unaided eye can advantageously dispense
with optical assistance.

Our present knowledge as to the natural history of the
shooting stars has been mainly acquired during the last
hundred years. The first important step in the compre-
hension of these bodies was to recognise that the brilliant
flash of light was caused by some object which came from
without and plunged into our air. This was known at

A PALLING STAR. 203

the end of the last century, largely by the labours of the
philosopher Chladni in 1794. But even his sagacity did
not prevent him from making some serious mistakes about
the nature of shooting stars. It has been reserved for the
present generation to organize a multitude of facts into a
connected whole, and thus contribute a very interesting
chapter to modern astronomy.

Could an ordinary shooting star tell us its actual
history, the narrative would run somewhat as follows : —

" I was a small bit of material, chiefly, if not entirely,
composed of substances which are formed from the same
chemical elements as those you find on the earth. Not
improbably I may have had some iron in my constitution,
and also sodium and carbon, to mention only a few of the
most familiar elements. I only weighed an ounce or two,
perhaps more, perhaps less — but you could probably have
held me in your closed hand, or put me into your waistcoat
pocket. You would have described me as a sort of small
stone, yet I think you would have added that I was very
unlike the ordinary stones with which you were familiar.
I have led a life of the most extraordinary activity ; I
have never known what it was to stay still ; I have been
ever on the move. Through the solitudes of space I have
dashed along with a speed which you can hardly conceive.
Compare my ordinary motion with your most rapid rail-
way trains, place me in London beside the Scotch express
to race to Edinburgh ; my journey will be done ere the
best locomotive ever built could have drawn the train out
of the station. Pit me against your rifle bullets, against
the shots from your one hundred-ton guns ; before the
missile from the mightiest piece of ordnance ever fired

204 M STARRY REALMS.

shall have gone ten yards I have gone 1,000 yards. I do
not assert that my speed has been invariable — sometimes
it has been faster, sometimes it has been slower ; but I
have generally done my million miles a day at the very
least. Such has been my career, not for hours or days,
but for years and for centuries, probably for untold ages.
And the grand catastrophe in which I vanished has been
befitting to a life of such transcendent excitement and
activity ; I have perished instantly, and in a streak of
splendour. In the course of my immemorial wanderings I
have occasionally passed near some of the great bodies in
the heavens ; I have also not improbably in former years
hurried by that globe on which you live. On those occa-
sions you never saw me, you never could have seen me,
not even if you had used the mightiest telescope that
has ever been directed to the heavens. But too close an
approach to your globe was at last the occasion of my
fall. You must remember that you live on the earth
buried beneath a great ocean of air. Viewed from out-
side space your earth is seen to be a great ball, every-
where swathed with this thick coating of air. Be-
yond the appreciable limits of the air stretches the open
space, and there it is that my prodigious journeys
have been performed. Out there we have a freedom to
move of which you who live in a dense atmosphere have
no conception. Whenever you attempt to produce rapid
motion on the earth, the resistance of your air largely
detracts from the velocity that would be otherwise attain-
able. Your quick trains are impeded by air, your artillery
ranges are shortened by it. Movements like mine would
be impossible in air like yours.

A FALLING STAR. 205

" And this air it is which has ultimately compassed my
destruction. So long as I merely passed near your earth,
but kept clear of that deadly net which you have spread,
in the shape of your atmosphere, to entrap the shooting
stars, all went well with me. I felt the ponderous mass
of the earth, and I swerved a little in compliance with
its attraction ; but my supreme velocity preserved me,
and I hurried past unscathed. I had many narrow
escapes from capture during the lapse of those countless
ages in which I have been wandering through space.
But at last I approached once too often to the earth. On
this fatal occasion my course led me to graze your globe
so closely that I could not get by without traversing the
higher parts of the atmosphere. Accordingly, a frightful
catastrophe immediately occurred. Not to you ; it did
you no harm ; indeed, quite the contrary. My dissolution
gave you a pleasing and instructive exhibition. It was
then, for the first time, that you were permitted to see
me, and you called me a shooting star or a meteor.

" You are quite familiar with the disasters associated
with the word collision. Some of the most awful acci-
dents you have ever heard of arose from the collision of
two railway trains on land or of two ships in the ocean.
You are thus able to realise the frightful consequences of
a collision between two heavy bodies. But in the collision
which annihilated me I did not impinge against any other
heavy body. I only struck the upper and extremely rare
layers of your atmosphere. I was, however, moving with
a speed so terrific that the impulse to which I was ex-
posed when I passed from empty space even into thin air
was sufficient for my total disruption.

2ot> IN STARRY REALMS.

" Had the speed with which I entered your atmosphere
been more moderate — had it been, for instance, not greater
than that, of a rifle bullet, or even only four or five times
as fast, this plunge would not have been fatal to me. I
could have pierced through with comparative safety, and
then have tumbled down in my original form on the
ground. Indeed, on rare occasions something of this
kind does actually happen. Perhaps it is fortunate for
you dwellers on the earth that we shooting stars do gene-
rally become dissipated in the upper air. Were it not so,
the many thousands of us which would be daily pelting
down on your earth would introduce a new source of
anxiety into your lives. Fortunately for you, we dart in
at a speed of some twenty miles or more a second. Un-
fortunately for us, we learn that it is the ' pace which
kills/

" When from the freedom of open space I darted into
the atmosphere, I rubbed past every particle of air which
I touched in my impetuous flight, and in doing so I ex-
perienced the usual consequence of friction — I was warmed
by the operation. If you rub a button on a board it will
become warm. If you rub two pieces of wood together
you can warm them, and you could even produce fire if
you possessed the cunning skill of some people whom you
are accustomed to speak of as savages. Nor need you be
surprised to find that I was warmed by merely rubbing
against air. If you visit a rifle range and pick up a
fragment of a bullet which has just struck the target you
will find it warm ; you will even find it so hot that you
will generally drop it. Now whence came this heat ?
The bullet was certainly cold ere the trigger was pulled,

A FALLING STAR. *o't

No doubt there is some heat developed by the combustion
of the gunpowder, but the bullet cannot be much warmed
thereby ; it is, indeed, protected from the immediate effect
of the heat of the powder by the wad. The bullet is
partly warmed by the friction of rubbing against the
barrel of the rifle, but doubtless it also receives some heat
by the friction of the air and some from the consequence
of its percussion against the target. You need not, then,
wonder how it is that when I am checked by your atmo-
sphere I, too, am heated.

" Remember that I move a hundred times as swiftly as
your rifle bullet, and that the heat developed in the check-
ing of the motion of a body increases enormously when
the velocity of the body increases. Your mathematicians
can calculate how much. They tell you that the amount
of heat potentially contained in a moving body varies
as the square of the velocity. To give an illustration
of what this means, suppose that two rifles were fired at a
target, and that the sizes of the bullets and the ranges
were the same, but that the charge in one of the rifles was
such that its bullet had twice the initial velocity of the
other. Then the mathematician will say that the heat
developed during the flight of the rapid bullet might be
not alone twice but even four times as great as that de-
veloped in the slower bullet. If we could fire two bullets
one of which had three times the speed of the other, then,
under similar circumstances, the heat generated ere the
two bullets were brought to rest would be nine times
greater for the more rapidly flying bullet than for the
other one. Now you can readily comprehend the im-
mense quantity of heat that will have been produced ere

208 IN STARRY REALMS.

friction could deprive me of a speed of twenty miles a
second. That heat not merely warmed me, but I rapidly
became red-hot, white-hot, then I melted, even though
composed of materials of a most refractory kind. Still
friction had much more to do, and it actually drove me
oft into vapour, and I vanished. You, standing on your
earth many miles below, never saw me — never could
have seen me — until this supreme moment, when, glow-
ing with an instantaneous fervour, I for a brief second
became visible. You shouted, ' Oh ! there is a shooting
star.'

"Nature knows no annihilation, and though I had
been driven off into vapour and the trial by fire had
scattered and dispersed me, yet in the lofty heights of
the atmosphere those vapours cooled and condensed.
They did not, they never could again reunite and repro-
duce my pristine structure. Here and there in wide
diffusion I repassed from the vaporous to the solid form,
and in this state I wore the appearance of a streak of
minute granules distributed all along the highway I had
followed. These granules gradually subsided through
the air to the earth. On Alpine snows, far removed from
the haunts of men and from the contamination of chim-
neys, minute particles have been gathered, many of which
have unquestionably been derived from the scattered
remains of shooting stars. Into the sea similar particles
are for ever falling, and they have been subsequently
dredged up from profound depths, having subsided
through an ocean of water after sinking through an ocean
of air.

" The motes by which a sunbeam through a chink in a

A FALLING STAR. 2O<,

closed shutter is rendered visible, are no doubt mainly of
•organic origin, but they must also frequently comprise
the meteoric granules. These motes gradually subside
upon the tops of your bookcases or into other congenial
retreats to form that dust of which good housekeepers
have such a horror. It is certain that the great majority
of the particles of which ordinary dust is constituted have
purely terrestrial sources which it would be impossible to
«ndow with any romantic interest. It is equally certain
that in a loathed dust-heap are many atoms which, con-
sidering their celestial origin and their transcendent
voyages, would have merited a more honoured resting-
place."

The discovery of the height at which the streaks of
shooting stars are produced is an important element in
obtaining any precise knowledge of these bodies. To de-
termine it requires the joint action of at least two observers
situated at stations widely distant. It will thus be seen
that the occasions on whioh such observations can be
obtained are mainly fortuitous. We cannot foretell the
occurrence of conspicuous shooting stars so as to enable
the two observers to arrange a combined system of obser-
vations. We can no doubt in certain cases predict the
advent of a shower of shooting stars, but then the very
profusion in which the meteors appear tends to baffle any
preconcerted agreement, because it is so difficult to insure
that identical streaks shall be observed at the two stations.
It does, however, sometimes happen that a shooting star
observed in one place can by a time comparison be proved
to be identical with a shooting star observed in another
place. If the two observers have each possessed the skill

210 IN STARRY REALMS.

and taken the pains to note accurately the spot of the
heavens where the object appeared, or rather the point
either of the beginning or of the termination of its
track, then the discovery of its distance is rendered
practicable.

It is an interesting application of some few propositions
in Euclid to determine the meteor's height from these
observations. We shall, however, here be content with
describing two special cases, in the first of which the
general problem has been somewhat simplified. I shall
suppose that an observer at London sees a shooting star,
and notices that the luminous streak commences at the
point of the heavens exactly over his head. It also hap-
pens that an observer who resides at Bristol sees a shoot-
ing star, and notices that it lies to the east, and that the
point where the streak commences is at an elevation of
forty- five degrees, that is half-way from the horizon to
the zenith. On subsequent comparison these observers
find that their observations were made at the same
moment ; and as neither of them saw any other shooting
star about the same time it is obvious that they must have
both observed the same object. We have now to consider
a triangle of which the three corners are respectively
London, Bristol, and the shooting star (or rather the
point at the commencement of its track) . It is plain that
this triangle has a right angle and equal sides, and that
consequently the distance from London to the shooting
star must be the same as from London to Bristol. Thus
the height of a shooting star has been found. Of course
the observer at London will not usually be so fortunate as
to see the object directly over his head, and the observer

A FALLING STAR. 21 r

at Bristol will not often find the angle of elevation to be
half a right angle. The calculation therefore will usually
be not quite so simple as in the case we have supposed,
but it presents no real difficulty.

In addition to this somewhat imaginary illustration 1
shall mention an actual instance, and I naturally take
a recent meteor that suits my purpose. It is one which
has been fully described by the well-known astronomer,
Mr. W. F. Denning, who was himself one of the obser-
vers, and who afterwards calculated the position of the
meteor's track. From his account I gather the following
particulars.

A careful observer of these bodies, who resides at Leeds,
Mr. D. Booth, was keeping watch on the evening of
January 2nd, 1888, when at lOh. 58m. P.M. a splendid
meteor, which appeared to him to be as bright as Jupiter,
travelled across the heavens, through the constellation
Aries, towards the south. On the same evening Mr. Den-
ning, who was on the look out at Bristol, saw the same
bright meteor in the northern part of the sky, in the con-
stellation Draco. That the object seen at Leeds and the
object seen at Bristol were the same is obvious from the
fact that the times noted by the two observers were iden-
tical. Nor was there any other bright meteor recorded
at either place which would admit of the possibility of
confusion. No doubt the parts of the sky in which each
observer located the meteor were utterly different. From
Leeds it appeared high up in the south-western quarter of
the heavens, from Bristol it was seen in the north, almost
under the Pole. But the different situations do not imply
that there were two distinct objects; they are merely

212 IN STARRY REALMS.

aspects of the same object obtained from widely different
points of view. It is, in fact, the very difference between
these positions which renders it possible to discover the
true situation of the luminous streak.

The point at which the meteor appeared to vanish from
Bristol was at an elevation of nearly 30°, a little to the
west of north. Lay, therefore, a map of England on the
table, fasten with a pin a piece of thread at Bristol, and
then stretch the thread up towards the north at an inclina-
tion of one- third of a right angle above the surface of the
map. The observation at Bristol then assures us that the
vanishing point of the meteor was somewhere along that
line. The observation at Leeds tells us that the vanishing
point of the meteor was towards the south- west. A little
study of the map shows us that the line of sight from
Bristol to the meteor passes nearly over Chester, and that
Chester is south-west from Leeds. Hence we see that the
vanishing point must have been directly over Chester,
and the slope of the incline from Bristol indicates that the
height of the meteor must there have been about sixty
miles. If any inhabitant of Chester were fortunate enough
to have noticed that bright meteor he would apparently
have seen it terminate at a point exactly over his head.

The terminal point of the meteor is usually much better
observed than the initial point. Nor is this a matter for
surprise. The attention of an observer is often directed
towards the object by a bright light, while he may be
looking in quite another direction. He turns round and,
of course, follows the glorious object to its close at a point
the situation of which he can note accurately. It seems
in this case that the meteor was observed at a much earlier

A FALLING STAR. 213

part of its flight at Bristol than at Leeds. We may, how-
ever, conclude, from a study of both observations, that
the body commenced its brilliant career at a point ninety-
eight miles high above a spot to the west of Appleby.

We have thus the means of determining the actual path
which the meteor has traversed, and I would suggest to
the reader that he should construct, for his own informa-
tion, a little model, which will give him a clear picture of
the career of this curious visitor. Put a map of England
on a board and stick through it and into the board two
straight wires (knitting needles will do admirably), one
near Appleby and the other at Chester. A string is then
to be stretched from one of these needles to the other, but
the height at which its two ends are to be fastened
to the needles will of course depend upon the scale of the
map. The little model that is now before me is made
with the map from Bradshaw's Railway Guide. On the
scale of 'that map ninety-eight miles will be nearly 4|-
inches. Accordingly I have fastened one end of the
string to the knitting needle through Appleby at the
height of 4f inches. Similarly sixty miles correspond
on this map to about 3 inches, and therefore the other
extremity of the string is to be secured 3 inches over
Chester, and an instructive model is complete. We can
at once learn from it the actual length of the meteor's
path. The distance between the two extremities of the
string is a little over 5 inches, and consequently we see
from the scale of the map that the leDgth of flight is over
100 miles. Mr. Denning, from his accurate methods of
calculation, states it at 109 miles.

A glance at the model will also explain what might

214 IN STARRY REALMS.

otherwise appear to be paradoxical, and that is the asser-
tion by Mr. Booth at Leeds that the meteor was rather
swift, while Mr. Denning from Bristol assures us that the
motion was very slow. The apparent discrepancy vanishes
when we see how the course of the meteor actually lies.
The observer from Leeds sees the celestial rocket moving
squarely across his line of sight from right to left. He
could hardly have observed it under more favourable cir-
cumstances so far as the direction of the motion is con*
cerned. But to the observer at Bristol the aspect of the
meteor's path was wholly different. The object was mov-
ing towards him. In fact, if we continue the line of flight
sufficiently far, we find it sloping downwards to the Bristol
Channel, and finally touching earth in Devonshire. From
Bristol therefore the track was extremely foreshortened ;
and consequently during its whole flight the meteor ap-
peared from Bristol to traverse but a comparatively small
part of the heavens : to an observer there this motion
.would seem very slow when compared with such a view
of the object as was presented from Leeds. It will also
be noted that at the centre of its journey the meteor must
have been about 200 miles distant from Bristol, and the
greatness ot this distance is another reason why it should
appear to move slowly.

The spectacle was witnessed by Mr. Backhouse at
Sunderland; he describes the meteor to have been as bright
as the star Sirius, and it appeared near the constellation
of Orion. It would lead me too far to pursue this matter,
so I shall dismiss it with the remark that these facts can
be shown to tally with the path ascertained by the
observations at Leeds and at Bristol.

A FALLING STAR. 215

The track of this meteor may be taken as fairly repre-
sentative of the course pursued by those more splendid
shooting stars which are often called fire-balls. They
move, however, in every direction. They come from the
east, and from the west, from the north, and from the
south. There is no hour of the night at which they have
not occasionally been seen. Even in daylight it has hap-
pened not once or twice, but on several occasions, that a
brilliant meteor has forced itself upon our astonished
notice. They generally first make their appearance at a
height which is between 50 and 100 miles above the
ground. They hurry down their inclined path, but gene-
rally become extinguished while still at least 20 miles
aloft. In their more ambitious flights meteors have
been known to span a kingdom. Nor are even greater
strides unrecorded. The length of a continent may be
Compared with the track of that terrific meteor of 5th
September, 1868, which broke into visibility at a great
height above the Black Sea, and had not expended its
stupendous energy until it passed over the smiling vine-
yards of France.

CHAPTER XVI

FIRE-BALLS.

GREAT fire-balls are much more numerous than any one
would suppose who had not paid attention to the subject.
Nor need this be a matter for surprise if it be remembered
that when a fire-ball does arrive it is only by a favourable
combination of circumstances that any particular indivi-
dual is privileged to witness the exhibition. Let us
examine the conditions that are necessary. In the first
place, the observer who desires to look out for meteors
will naturally choose some station which commands the
most uninterrupted view of the sky on all sides. The
deck of a ship on the open sea, or, better still, the summit
of a lofty mountain, will represent ideal sites for the
purpose. But only a very small fraction of the entire
atmosphere of our globe is within the sphere of observa-
tion from one locality. If a fire-ball plunges into the air
it may perhaps be seen if in any part of its track it is
above the horizon. With sufficient precision we may
assert that the proportion of the total atmosphere which
is above the horizon at any one place is -j-J-^th of the whole,
and that anything which may occur in the remaining 499

FIRE-BALLS. 21"

parts is of course occluded from view. Even were the sky
so perfectly clear as to permit an UL intermitting watch
being maintained from one year's end to the other, we
could not expect from any single station to see more than
a mere fraction of the total number of fire-balls that had
actually descended. It must also be remembered that the
fire-balls which do travel above the horizon will have
many chances of escaping notice, for an observer cannot
be always on the look out. He will be indeed a diligent
astronomer and situated in a favoured clime who should
spend a thousand hours in the year on the watch for shoot-
ing stars. Even this will, however, only amount to one
hour out of every eight. We may reasonably suppose that
meteors and fire-balls will be as likely to arrive in any
one of the seven hours during which an observer is not on
the watch as in the single hour during which he is present.

This will explain why it is that though fire-balls are
really so very abundant, yet that the opportunities enjoyed
by any individual for seeing them are comparatively in-
frequent. As a random example of the yearly crop of
fire-balls, I take from the middle of 1877 to the middle of
1878. A list of the fire-balls noticed during this period
will be found in that storehouse of valuable information,
the Reports of the British Association. In the year
referred to I see that eighty-six great fire-balls have been
recorded. They have appeared in various localities, both
in the old hemisphere and in the new. The most arduous
observer may think himself fortunate if he had even seen
one of them.

As to the brilliant light from some of these great fire-
balls, there are numerous statements. We are not infre-

2i8 IN STARRY REALMS.

quently told that even the beams of the full moon are
ineffectual in comparison with the blaze of the meteor ;
and we find a high authority asserting that one of these
bodies displayed a flash as " blinding as the sun." But
our knowledge of the actual illuminating power of meteors
is almost unavoidably of rather an inaccurate description.
We could measure the light, no doubt, with all practical
precision if we knew when and where to expect it, for
then we could bring a suitable photometric apparatus to
bear at the critical moment. In the absence of any pre-
cise information, we are forced to make the most of such
occasional comparisons as may be available. On the 29th
July, 1878, a fire-ball was seen which created so splendid
an illumination that " the smallest objects were visible at
Manchester." An eye-witness states that when at its best
the fire-ball had the lustre of a powerful electric light seen
from a distance of thirty or forty yards, though at the
moment the fire-ball must certainly have been forty-five
miles away. We can make a comparative estimate of the
intrinsic intensity of a light which, when received from a
distance of forty-five miles, has a brilliancy equal to that
of a known source thirty-five yards away. It is thus
shown that the fire-ball must have emitted more light
than five million electric lamps.

Fortunate indeed would the astronomer have been who,
guided by some miraculous prescience, had gone to the
ancient city of York on the evening of the 23rd of Feb-
ruary, 1879, and on the tower of the glorious minster
spent the night in observation of the heavens. It would
have been his privilege to witness a majestic meteor under
circumstances of almost unique magnificence.

FIRE-BALLS. 219

Unhappily, there was no expectant astronomer at once
ready to observe and competent to record the great event
on the morning of the 24th. The time of its occurrence
was also the most unpropitious, so far as the attention of
casual observers was concerned. Did a nocturnal fire-ball
desire that its splendours should be witnessed by as few
persons as possible, it could choose no hour so favourable
as about three o'clock in the morning. Those who sit up
latest have at last got to sleep ; those who rise the earliest
have not yet awakened. It was at seven minutes befora
three that such few stragglers as the streets of York still
contained saw a pear-shaped ball of fire travelling across
the sky. It drenched the ancient city with a flood of
light. The superb front of the minster never before
glowed with a more romantic illumination. The unwonted
brilliancy streamed through every aperture in every win-
dow in the city ; every wakeful eye was instantly on the
alert ; every light sleeper started up suddenly to know
\vhat was the matter. Even those whom the blaze of
midnight light had failed to awaken were only per-
mitted to protract their slumbers for another minute and
a half — only until an awful crash, like a mighty peal of
thunder, burst over the town, shaking the doors, the win-
dows, and even the houses themselves. The whole city
was thus alarmed. Every one started at the noise. But
that noise was not a clap of thunder. Nor was it pro-
duced by an earthquake. It was merely the explosion of
the fire-ball which flung itself against the atmosphere after
its immeasurable voyage through space.

Let us imagine a wayfarer in the streets of Newcastle
on the same morning. He is struggling on his way in

220 IN STARRY REALMS.

dense darkness, and through a raging storm of snow.
An instantaneous transformation scene takes place. Sud-
denly there is light above and around which renders the
white mantle of the city as bright as a summer day.
The observer at once sees that this illumination is not
lightning. A flash of lightning lasts for an incon-
ceivably small fraction of a second. But while a man
could count fifty the town glows with this strange illu-
mination. And strange it is, for the source of the light
is not visible. As the snowstorm would have hidden the
sun itself at mid- day, so in the dead of night it hides the
great meteor. An exquisite phase of the phenomenon
was presented by the changes in the hue of the light,
which passed from a brilliant white to a beautiful blue ere
it. disappeared.

Perhaps the most remarkable instance of the explosion
of a meteor is recorded in the case of the great fire-ball so
widely observed in America on the 21st December, 1876.
The movements of this superb object have been carefully
studied by Professor H. A. Newton and Professor D
Kirkwood. For the prodigious span of a thousand miles
this meteor tore over the American continent with a speed
of some ten or fifteen miles a second. It first appeared
over Kansas at a height of seventy-five miles. Thence it
glided over the Mississippi, over the Missouri ; it passed
to the south of Lake Michigan ; it made a short voyage
over Lake Erie, and it cannot have been very far from
the Falls of Niagara, when by becoming invisible all
further traces of its movements were lost. While passing
a point midway between Chicago and St. Louis a violent
explosion shivered the meteor into a cluster of brilliant

FIRE-BALLS. 221

balls of fire, which seemed to chase each other across the
sky. This cluster must have been about forty miles long
and five miles wide. The detonation by which the explo-
sion was accompanied was a specially notable incident of
this meteor. It was not only heard with terrific intensity
in the neighbourhood, but the volume of sound was borne
to great distances. Bloomington, in Indiana, is one hun-
dred and eighty-five miles from the actual point in the
sky where the meteor was rent in pieces, yet about the
neighbourhood of Bloomington not only was the sound of
a frightful explosion heard, but the shock of the concus-
sion was actually felt to such a degree that some of those
who experienced it thought an earthquake must have
happened.

The tremendous volume of sound that must have been
emitted is forcibly presented to us when we consider the
interval of time that elapsed between the moment when
the inhabitants of Bloomington saw the gorgeous proces-
sion of fire-balls streaming over the heavens, and the
moment when the appalling crash burst on their ears.
Every one is familiar with the fact that the flash of a
gun is seen ere the report is heard, and the greater the
distance of the gun the longer is the interval by which
the light and the noise are separated. Sound takes five
seconds to travel a mile ; light travels so quickly that the
time necessary to traverse a mile, or a hundred miles, or a
thousand miles, is utterly inappreciable by ordinary
measurement.

The celestial spectacle had excited the utmost astonish-
ment ; doubtless it had been discussed and notes had been
compared by those whose good fortune had permitted them

»aa IN STARRY REALMS.

to see it. But the immediate excitement was over, friends
had parted for the night ; some of them had entered their
houses ; others had renewed their walk homewards, and
had travelled nearly a mile on their journey ; vehicles
had driven a couple of miles ; trains had run half-a-
dozen miles ; columns of newspapers had been read. Many
who had seen the meteor had already forgotten it, when
their ears were deafened by the arrival of the awful crash.
The waves of sound had to travel a distance as great as
from London to Liverpool, and even at the rate of a mile
every five seconds this cannot be done in less than a
quarter of an hour. Probably many of those who both
heard the noise and saw the light found it hard to believe
in their connection. We are indebted to the care of one
observer at Bloomington, who by looking at his clock
when the fire-balls were seen, and again when the explo-
sion was heard, has added an important particular to our
knowledge of this great meteor.

Nor need we feel much astonishment at the stupendous
phenomena, both of light and of sound, which accompany
Vhe advent of a splendid and detonating fire-ball. I must
here give a few figures which will show how great is the
store of energy possessed by a meteor in virtue of the
amazing velocity with which it is animated. I shall sup-
pose that the material element of the meteor consists of a
small mass of stone or of some more or less metallic
material which weighs one pound. We shall examine
the circumstances under which we must endeavour by
mechanical agents to project this small body with the
meteoric speed of, let us say, twenty -five miles a second.
Our utmost attempt? with a cannon could hardly produce

FIRE-BALLS. 223

a speed of one-hundredth part of the required amount,
and even for this a charge of gunpowder, certainly not
less than a quarter of a pound, would be consumed. If a
cannon of amazing strength and of ideal efficiency could
be conceived, in which all the energy of the exploding
powder should be usefully concentrated upon communicat-
ing velocity to the projectile, we should find that to double
the speed the charge must be quadrupled. To treble the
speed we should have to increase the charge ninefold. If
the speed were to be increased ten times, we must put one
hundred times as much powder into the cannon. Finally,
if we could procure a piece of ordnance strong enough and
gunpowder rapid enough to impart meteoric velocity to the
missile, a charge would be necessary which is ten thousand
times as great as the quarter of a pound that is sufficient
under the conditions of ordinary artillery. This argument
shows 'us that the energy of an entire ton weight of gun-
powder would be required to impart to a stone one pound
in weight the velocity of an ordinary fire-ball.

Whatever may have been the original source whence
the meteor acquired its energy (and this is a point on
which I do not at present make any remark), that energy
will be retained so long as the meteor retains its velocity.
Even if the meteor moved through empty space for a
million years, it would still be able to restore, if its motion
were arrested, the precise quantity of energy which it had
originally received. Hence, when the supreme moment
has arrived, the meteor expends in the throes of its disso-
lution all the energy it contains. Everything that the
energy liberated by the explosion of a ton of gunpowder
can dr that meteor weighing only a pound is competent

2*4 W STARRY REALMS.

to accomplish. If the weight of the missile were greater
than one pound, the velocity remaining the same, the
available energy would be increased in the same propor-
tion. For example, if the meteor weighed ten pounds,
one hundred pounds, or one thousand pounds, it would
discharge as much energy as could be awakened by the
detonation of ten tons, of one hundred tons, or of one
thousand tons of gunpowder. Need we wonder, then,
that gorgeous lights and that majestic thunders have
occasion ally accompanied the annihilation of large
meteors ?

There is another instructive method for obtaining a due
estimate of the potency of a meteor to produce tremendous
effects notwithstanding its comparatively small size. In
our present illustration we shall employ that great source
of energy which is familiar in the steam-engine. Let us
think of a powerful steamship, the engines of which have,
let us say, 8,200 horse-power. Let us conceive such
appropriate mechanism as would permit, the Titanic power
of these engines to be concentrated on the single duty of
imparting velocity to a small piece of matter one pound in
weight. It can then be shown that the piece of matter
would, after the lapse of sixty seconds, have acquired a
meteoric speed. So long as the meteor retained that
speed it would retain the energy the engine had given to
it, nor could the meteor surrender its rapid motion except
the energy were transformed into some other form. li
then the meteor lost its speed by piercing its way for a
minute through our atmosphere it would reproduce all the
energy that 8,200 horse-power can exert in the same time.

Suppose that in the elevated regions of our atmosphere

FIRE BALLS. 225

in the dead of night all the vigour of a set of engines,
whose collective strength was equal to that of forty thou-
sand horses, was concentrated on the production of an
electric light ; suppose, further, that the dazzling glare
thus created was accompanied by the music from an
orchestra of fog-horns blown by another forty thousand
horses, surely a scare would be produced which the most
pretentious fire-ball might be content to emulate. Yet
the figures we have already given will prove that a meteor
Dnly ten pounds in weight, which a child could carry,
bears in the mere swiftness of its flight a capacity adequate
to such a display.

The glory of a meteor is often so evanescent that we
just get a glimpse and it is gone. The sky resumes its
ordinary aspect ; the familiar stars are there, and even the
very situation of the brilliant streak has become unrecog-
nisable. But this is not always so ; it sometimes happens
that the brief career of the meteor leaves a notable trace
behind it, so that for seconds and for minutes the sky is
diversified by an unwonted spectacle. The path of the
meteor leaves a stain of pearly light on the sky to mark
the highway pursued by our celestial visitor.

In its fearful career the meteor is often rent to frag-
ments, reduced to dust, dissolved into vapour. The glow-
ing atoms of the wreck lie strewn along the path, just
as the ghastly remnants of Napoleon's mighty army
limned out the awful retreat from Moscow.

A pencil-shaped cloud of meteoric debris, perhaps eighty
or a hundred miles in length, and four or five miles in
diameter, thus hangs po;sed in air. It is at night. The
«un has sunk so far below the horizon that there is no

226 IN STARRY REALMS.

trace of the feeblest twilight glow. An ordinary cloud
would, of course, be invisible except as concealing the
stars ; no beams of light fall upon it ; there is nothing to
render it luminous. So, too, the meteoric streak will
often pass instantly into invisibility, but, as I have said,
this is not always the case. There is a well-authenti-
cated instance in which the trail of a superb meteor
remained visible for nearly an hour. I have en-
deavoured up to the present to explain the various
phenomena presented to us in the fall of a meteor, but
here, for the first time, we have to note a circumstance
for which it is not easy to account. We can explain why
it is that the long meteoric cloud should be there, but we
cannot so easily explain why we should be able to see it.
Whence comes this beautiful pearly luminosity ? Ii
seems that the meteoric dust must glow with some in-
trinsic luminosity. I have only heard one attempt to
offer a rational explanation of the true character of this
interesting phenomenon. It is as follows.

There are certain substances which are called phos-
phorescent, because they possess the power of actually
absorbing and retaining the light to which they have
been exposed, and then gradually dispensing it after-
wards. Phosphorescent materials have some useful appli-
cations as ingredients in the so-called " luminous paint.'*
A gate covered with this preparation will absorb the sun-
light during the day, and during the night will radiate
forth its store in a feeble glow. Clock faces have been
similarly illuminated, and match-boxes are made so that
the hand shall be guided to them in the dark by a radiance
more ghostlike than beautiful. The persistent streak of

FIRE-BALLS. ^^^

a meteor can be explained by the supposition that some
particles of these phosphorescent substances have been
present in the meteoric mass. These particles have been
ignited to brilliance during the progress of the meteor,
and they still continue to glow until their store of
luminous energy shall have become exhausted.

The meteoric streak is subjected to the same influences
as those which affect an aqueous cloud in the lofty regions
of the atmosphere. It must be dissipated by air currents,
and accordingly we observe that the trail, which was at
first so nearly straight, becomes bent and curved, some-
times even serpentine, ere its outlines have gradually
softened out and all visible traces of the meteor have
vanished.

CHAPTER XYIL

SHOWERS OF SHOOTING STARS.

I HAVE thought it convenient to illustrate the leading
phenomena of meteors by reference to some of the
more imposing of these bodies, but it must not be sup-
posed that the smaller meteors, and even the tiniest of
shooting stars, are unworthy of our close attention. In-
deed, the smaller shooting stars, by their greater fre-
quency, have taught us much more about meteors than
we could have ever learned from the great fire-balls.
Even regarded from the merely spectacular point of view,
the splendours of the latter are sometimes eclipsed by
the gorgeous showers of comparatively small meteors, to
which we shall presently have to refer.

We have spoken of dazzling fire-balls which generate
for a brief moment a light which eye-witnesses, with
possibly a pardonable exaggeration, have ventured to
compare with the beams of the sun himself. Other
meteors are described as being as bright as the full moon.
Descending still lower in the scale of splendour, we read
of fire-balls as bright as Venus or Jupiter, as bright as
Sirius, or as a star of the first magnitude. With each stej>

SHOWERS OF SHOOTING STARS. a»9

downwards in brilliancy we find the meteors to increase
in numerical abundance. Shooting stars as bright as the
stars of the second or third magnitude are comparatively
frequent ; they are still more numerous of the fourth and
fifth magnitudes. Every night brings its tale of shooting
stars whose brightness is just sufficient to impress the
unaided eye. Nor do the shooting stars which even
the most attentive eye can detect represent a fraction
of their entire number. As there are telescopic stars
which the unaided eye cannot see, so it might fairly be
conjectured that, as we can trace meteors of successive
stages of brightness down to the limit of unaided eye
visibility, so there may be meteors still and still smaller
which would be detected could we only direct a telescope
towards them. If it be impossible to turn a telescope
with sufficient dexterity to scan a visible shooting star,
how, it may well be asked, can we use the telescope to
discover shooting stars which the unaided eye cannot
see?

We must here depend entirely, or almost entirely,
on the chapter of accidents. The observer who really
sought to discover telescopic shooting stars would gene-
rally find that many hours of watching were rewarded
with but very meagre results. No doubt if, at properly
appointed times, he directed his telescope to certain
particular constellations, he would have more prospect of
success than if he merely pointed his telescope at random.
But though the experience of actually looking out for
telescopic shooting stars has not led to much, yet every
astronomer who is in the habit of making nightly obser-
vations with a good telescope, in almost any branch of

230 IN STARRY REALMS.

astronomical work, will frequently find a bright streak
of light flash across his field. This is a meteor, and a
comparison with any stars which may happen to be in the
field of view will probably show him that the object was
far too small to have been seen with the naked eye. We
must remember that the field of view of a large telescope
is but an extremely small fraction of the entire extent of
the heavens. It would be easy to show, by the doctrine
of chances, that if a telescopic shooting star were to dive
to extinction into the air, the chance against its being
seen by any particular telescope at that moment directed
to the sky would be at least fifty thousand to one ; but
every astronomer knows that the perception of a tele-
scopic shooting star is a common incident in the obser-
vatory. If, therefore, we reflect that for every one that
is seen there must be thousands which dart in unseen, we
obtain an imposing idea of the myriads of shooting stars
that daily rain in upon our globe.

The world is thus pelted on all sides day and night,
year after year, century after century, by troops and
battalions of shooting stars of every size, from objects not
much larger than grains of sand up to mighty masses
which can only be expressed in tons. In the lapse of
ages our globe must thus be gradually growing by the
everlasting deposit of meteoric debris. Looking back
through the vistas of time past;- it becomes impossible to
estimate how much of the solid earth may not owe its
origin to this celestial source. ^ J

It will greatly promote our comprehension of these
bodies if we group them into classes so as to discover
some of the laws by which their movements are guided.

SHOWERS OF SHOOTING STARS. 231

The first and most important truth with regard to the
recurrence of the meteors is their occasional appearance
in what are known as "meteoric showers." During such
displays it sometimes happens that shooting stars in
shoals break forth simultaneously, so as to produce a
spectacle which we now regard as of the utmost beauty
and interest, but which in earlier times has often been
the source of the direst terror and dismay.

Let me, for the sake of illustration, give some account
of one of these great showers of shooting stars. It
occurred on the 13th November, 1866. Doubtless many
of those who read these lines will remember that event.
It dwells in my memory along with one or two other
superb astronomical spectacles that it has been my privi-
lege to witness. I have never seen a total eclipse of the
sun under favourable circumstances. This is, I apprehend,
about the most sublime of all the occasional phenomena
which the heavens present to us. I have, however, seen
the great comet of 1858, the shower of shooting stars in
1866, and the transit of Venus in 1882. The last was of
interest rather from its rarity and its delicacy than
from its actual appearance regarded as a spectacle. Of
the phenomena in the heavens which I have seen, I must
give the first place to the wonderful shower of shooting
stars in November, 1866. Although I have previously
had occasion to describe my experience as an eye-witness
of this event, yet I thin1* it will bear telling again.

In the year 1866 I occupied the position of astronomer
to the late Earl of Rosse, who is specially known to fame
as having been the builder of the greatest telescope the
w<;yld has ever seen. This grand instrument at the time

232 IN STARRY REALMS.

I was in charge of it was devoted to the observation of
the nebulae, a branch of astronomical work for which the
vast size of the great reflector made it eminently suited.
In clear weather it was my duty, and I may truly add,
my delight, to scan the heavens during the long winter
nights, for the purpose of sketching and of measuring those
dim, faint nebulae which seem to lie on the confines of the
visible universe. The giant telescope is in the open air ;
it swings between two walls of castellated masonry, and
by ladders and galleries of ingenious construction the
observer is enabled to reach the mouth of the telescope in
all its positions. I say mouth of the telescope, for a
reflector of this description is not employed like the ordi-
nary telescope, through which we look. In the reflector
we must get access to the top and gaze down on the
reflections of the stars in the great mirror below. As the
telescope is sixty feet long it thus follows that the ob-
servers are sometimes sixty feet high in the air, and as
the telescope is placed in a very open situation in Lord
Rosse's beautiful demesne at Parsonstown, the position for
observing the shooting star shower was an exceptionally
favourable one. Beside the observer an attendant stands,
whose duty it is to move the gallery backwards and for-
wards, so as to keep the observer conveniently placed near
the eye-piece of the telescope.

The memorable night between November 13th and
14th, 1866, was a very fine one ; the moon was absent, a
very important consideration in regard to the effective-
ness of the display. The stars shone out clearly, and
I was diligently examining some faint nebulae in the
eye-piece of the great telescope when a sudden exclama-

SHOWERS OF SHOOTING STARS. 233

tion from the attendant caused me to look up from
the eye-piece just in time to catch a glimpse of a fine
shooting star, which, like a great sky-rocket, but with-
out its accompanying noise, shot across the sky over
our heads. About this time I was joined at the telescope
by Lord Oxmantown, as he then was, but who is now the
present distinguished Earl of Kosse, and we resumed our
observations of the nebulae, but a grander spectacle soon
diverted our attention from these faint objects. The great
shooting star which had already appeared was merely the
herald announcing the advent of a mighty host. At first
the meteors came singly, and then, as the hours wore on,
they arrived in twos and in threes, in dozens, in scores,
in hundreds. Our work at the telescope was forsaken ;
we went to the top of the castellated walls of the great
telescope and abandoned ourselves to the enjoyment of the
gorgeous spectacle.

To number the meteors baffled all our arithmetic ;
while we strove to count on the one side many of them
hurried by on the other. The vivid brilliance of the
meteors was sharply contrasted with the silence of
their flight. We heard on that marvellous night no
sounds save those .with which we were familiar. The
flights of the celestial rockets were attended with no
noises that we could hear. The meteors were no doubt
somewhat various as to size, but the characteristic feature
of this shower, as contrasted with another great shower I
have also seen, was the remarkable brilliance of the shoot-
ing stars. It was their exceptional splendour even more
than their innumerable profusion that gave to the shower
its peculiarity. As to the actual hrilliancv of the meteors

234 W STARRY REALMS.

I am enabled to give the accurate estimate made by Mr.
Baxendell at Manchester, where the shower was well seen.
Out of every hundred of these meteors ten were brighter
than a first magnitude star, and two or three of them were
brighter than Sirius. Fifteen out of each hundred were
between the first and second magnitudes, and twenty-five
were between the second and third magnitudes, while the
remainder were smaller. These results may be placed
in a somewhat more simple aspect in a different way.
Think of the brightness of the seven stars in that most
familiar of all the constellations, the Great Bear ; about
half of the meteors noticed during the continuance of
the shower were as bright as, or brighter than, the stars
of the Great Bear. The remaining half of the visible
meteors must be compared with stars of a fainter de-
scription.

I have described how that great November shower of
stars began, but I have not asserted that the display came
upon us entirely by surprise. I certainly was surprised
at its magnificence, but we confidently anticipated that a
shooting star shower of some notable kind would occur on
that very night. How, it may well be asked, could we
know that such a spectacle might be expected ? The story
is a wonderful romance in modern science.

We expect that the sun will rise to-morrow morning.
Now why do we so expect it ? Without entering into any
profound disquisition on the subject, we may say that the
practical grounds of this expectation depend upon the fact
that we have always found that the sun does rise, and
that as this operation has continued with unfailing regu-
larity for untold ages, we have reason to anticipate a

SHOWERS OF SHOOTING STARS. 2^

repetition of the phenomenon to-morrow as well as on the
following mornings. It was on similar grounds that we
were able to predict the occurrence of that great No-
vember shower.

Just thirty-three years previously, in the year 1833, a
splendid shower of shooting stars had been witnessed in
the «ame month of November and on the same day of the
month. That two great showers should both have occurred,
at the same epoch of the year, after an interval of thirty-
three years, was in itself a circumstance not a little
remarkable. But this might have been regarded as merely
a coincidence, if we had only been acquainted with these
two showers, and if we did not know their true relations.
Researches into history have, however, brought to light
the interesting fact that these two showers were not mere
isolated events, but that they were only the two latest
members of a long and connected series of great November
showers of meteors. As we look back through the records
of the past we find occasional mention of what can only
have been great displays of shooting stars. Within the
last century or two such gorgeous phenomena were wit-
nessed in an age when scientific knowledge enabled the
spectacle to be in some degree appreciated, but as we peer
back still earlier and earlier we find the records of these
great events assume a different complexion. Many cen-
turies ago the advent of a great shooting- star shower
was viewed with terror by a superstitious and ignorant
people. Such reords of the event as have been preserved
are tinged with such credulity, and so devoid of accurate
description, that all we can elicit are the facts that on
the dates specified certain celestial phenomena were wit-

«3<> IN STARRY REALMS.

nessed which we now know to have been showers of shoot-
ing stars.

The earliest of the records is nearly a thousand years
old, and from that period down to the present the earth
has probably been the scene of about thirty or more
superb showers belonging to the system we are now con-
sidering. Whether all these displays were actually wit-
nessed we do not know. Thick and cloudy weather would
be sufficient to have obscured even the most splendid of
these showers. Bright moonlight would have greatly
impaired the effectiveness of others. Even those which
were seen may not have been all recorded. Even all that
have been recorded may not yet have disclosed themselves
to the diligent search of Professor H. Newton and the
other astronomers who have laboured at this interesting
subject. The records which have been found are not
always easy to interpret. In the days when astrologers
taught, and when the people believed, that the configura-
tions of the stars were designed to shadow forth the vicis-
situdes of human affairs, it was not likely that any very
lucid interpretation would be given to such an event as a
shower of shooting stars. Such phenomena have been
regarded as miraculous, and they were often thought to
be portents conveying and threatening divine wrath.
They were occasionally interpreted as gracious manifesta-
tions of divine approval.

Some important facts with regard to ancient shooting-
star showers have, however, survived the thousand and
one casualties to which historical records are exposed. A
careful discussion of those which are sufficiently accurate
to be intelligible discloses to us the remarkable law

SHOWERS OF SHOOTING STARS. 237

of a thirty-three year period in the occasional recurrence
of grand shoo ting -star showers. The chief qualifications
of this statement would be twofold. In the first place,
the interval has been occasionally thirty-four years. In
the second place, it sometimes happens that at the return
of the thirty-three year period the passage of the meteors
makes no visible display, two consecutive years are
rendered memorable by great showers. At present this
particular shower occurs about the 15th November ; but
in earlier ages we find the date to shift slowly towards
the commencement of the year. Thus the display which
took place in A.D. 1698 was on the 9th of November ; while,
looking back still farther to one of the very earliest records,
viz., that of the year 934, we find the date has receded to
October 14th. This change of the day on which the
shower occurs is of profound theoretical importance in con-
nection with the discovery of the orbit which these meteors
pursue. The advance of the date is, however, so slow that
for the past few generations as well as for the next, we may
sufficiently define this particular shower by the meteors
which enliven the skies between the 14th and the 16th of
November. In fact, the poetaster has parodied the well-
known lines for the days of the month by a similar effort,
which will serve to remind us also of another periodic shower
of shooting stars which occurs in August. He writes : —

" If you November's stars would see,
About the fifteenth watching be.
In August too stars shine from heaven,
On nights between nine and eleven."

These lines are intended to imply that the day named
will usually bring, in every November, a few meteors at

238 IN STARR Y REALMS.

all events belonging to the grand shower. These are
stragglers, as it were, from the mighty host which visits
us from time to time.

Astronomers have a special name for this group of
November meteors. They are called the " Leonids." To
explain why this name has been given, and why it is
appropriate, we must dwell on an important part of the
phenomena of the shower, to which we have not yet
alluded ; and this we shall now do.

Among the constellations there is a fine sickle-shaped
group, forming a part of Leo, one of the signs of the
Zodiac. That part of the sky defined by Lee is curiously
related to the meteors of the 12th to the 14th of Novem-
ber. Every shooting star truly belonging to that great
shower pursued a track across the heavens, the direction
of which, if carried back far enough, was always found
to pierce through the sickle of Leo. Indeed, the path*
of all the meteors formed a set of rays spreading away
from that one point in the constellation. An invari-
able characteristic of this particular shower is its connec-
tion with the constellation of Leo, hence the appropriate-
ness of the name of Leonids.

And now lor the explanation — and it is a remarkable
one — which astronomers have been able to give of the
annual appearance to some extent of the Leonids, and of
their occasional majestic displays.

Picture to yourself a mighty host of small particles out
in space. They are organized into a great shoal, but they
are so sparsely distributed that each particle is perhaps a
dozen miles away from its neighbours on each side. How
large each of these particles is we cannot indeed say. No-

SHOWERS OF SHOOTING STARS. 3J9

doubt they vary in size, but they are probably not larger
than the pebbles on an ordinary gravel walk, and possibly
much less. The shape in which this host is marshalled is
remarkable. It is a long column, of which the width is
small in comparison with the length. The dimension of
this celestial host is indeed portentous. We sometimes
judge of the length of a procession of carriages by stating
how long they take to pass a certain point. We can
express the length of this great procession of meteors by
saying that, if we were to stand still and watch them file
past, not less than a year must elapse before the mighty
host would have passed by. This is, of course, an imper-
fect conception until we realise the velocity with which
the meteoric procession is moving. Their speed is cer-
tainly short of that which flashes through the electric
wire, but none the less does it utterly transcend any speed
which we can produce mechanically. The velocity of the
Leonids sometimes exceeds twenty-six miles a second, a
pace which is more than fifteen hundred times swifter
than the swiftest express train.

The width of this column as it passes along is prodi-
gious when measured by ordinary standards. A cord
that would go four times round our earth would barely
suffice to stretch across the meteoric current, which is
100,000 miles from side to side. But this dimension
shrinks into insignificance when compared with the length.
To realise the true shape of the mighty host, we may use
Mr. Stoney's admirable illustration. He says, take a
piece of the finest sewing silk, about a foot and a half
long. Then imagine the silk to be magnified, while still
preserving its proportions, until its width is about one

24o JN STARRY REALMS.

hundred thousand miles and its length about fifteen thou-
sand times as great. Such is the shape of the mighty
shoal which has, for the past thousand years, given us
perennial displays of Leonids. It is at this moment and
at every moment pursuing a mighty path through space.

The matter is so important that I venture to repeat here
an illustration which I have already given in " Star Land."
Think of a racecourse which is oval-shaped, or elliptical,
as it should be more properly called. Think of a number
of men who started together on that course to run a race.
Let us further suppose that the number of competitors is
a large one, and that they have to complete a considerable
number of rounds ere the winning tape will be stretched
across the track. We should then find that the shape of
the group of athletes was continually being extended. We
should also find that some few exceptionally good runners
were able to draw ahead in advance of the main body. No
doubt a large field would also contain some tardy runners
who would inevitably be left far behind, so that as the
successive rounds were completed we should see that the
first were actually overtaking the last, and thus gaining
an entire circuit. After a time a condition of the field
would be reached in which the main body of average run-
ners was in a stream occupying a small fraction of the
entire course, while the rest of the track would be dotted
here and there with those competitors who were either
exceptionally swift or exceptionally slow.

Submit this illustration to an imaginary species of
transformation and of enlargement. Instead of the small
course a fraction of a mile in circumference, let us think
of a course still oval in shape, but many hundreds of mil-

SHOWERS OF SHOOTING STARS. 241

lions of miles round. Let the competitors be replaced by
the small objects which are suitable for the manufacture
of shooting stars. Let the number of these be magnified
until they attain to untold billions. Thus we obtain a
notion of the mighty celestial racecourse on which the
Leonids have for a thousand years at least been hurrying
along in a race, which is still in a comparatively early
stage of its progress. Many circuits have no doubt been
accomplished, the original host which started has been
drawn out into the long thin line which we have already
compared to a piece of silk. This contains the great
mass of the Leonids, but there are many of the meteors
which have been endowed with the gift of exceptional
fleetness. On the other hand, there are some which seem
not to have been able to keep up with the tremendous
speed of the main body. Thus it happens that all around
the mighty racecourse there are stragglers to be found,
each of which pursues its journey in its own fashion.

If I have succeeded in giving you a picture of the con-
dition of the Leonids, of enabling you to realise how the
greater portion of the meteors form a comparatively dense
shoal, while around the rest of the course the meteors are
few and far between, it will then be easy to understand
the laws of recurrence of the November showers.

It must be borne in mind that we can never see the
meteors until the fatal moment when they dive into our
atmosphere. We could, indeed, at any time point our
telescope to the spot in the heavens where we know the
great shoal must certainly be located. But the mightiest
telescope in the world does not disclose the shoal to us.
In fact we would never have seen these Leonids at all, we

14* IN STARRY REALMS.

would never have become conscious that such a shoal of
meteors existed, had it not been for a certain circum-
stance, which, for want of a better expression, I must
speak of as accidental.

Our globe pursues a certain definite track around the
sun. Year after year, with undeviating regularity, the
earth periorms the stages of its journey. If it reaches
certain points on the 1st of January and the 12th of Octo-
ber in one year, then it reaches the same points on the
1st of January and the 12th of October respectively 011
next year, or any other year. The same may be asserted
with regard to any other dates, so that when a date is
given the station at which the earth will then have arrived
is at once indicated.

The Leonids and the earth have each a certain track.
It might of course have happened that one of these tracks
lay quite outside or quite inside the other. It might also
have been the case that one of these tracks passed through
the other, so that the two orbits were related in the
manner of a pair of consecutive links of a chain. Had
any of these conditions prevailed, the two tracks would
have been quite independent, and the meteors could never
have become known to us. It might, however, have
happened that the two tracks did actually intersect, and
had therefore the point of crossing common to both orbits.
This would, generally speaking, be an unlikely circum-
stance, but it is an indispensable condition if the meteors
are to be visible from the earth. In the case of the
Leonids, it has chanced that their orbit does intersect
the orbit of the earth, and to this circumstance we are
indebted for the glorious displays every thirty-three years.

SHOWERS OF SHOOTING STARS. 243

We shall now be able to explain the chief features
exhibited by the famous showers. In the first plac« a
shower can only occur in that precise locality where the
earth in its path crosses the path of the meteors. This
junction is of course at a definite part of the earth's path.
We may conveniently mark such a point in the path by
the date at which the earth is to be found there in the
progress of its annual voyage. This particular point, or
rather region, of crossing happens to be in the place
through which the earth passes each year between the
14th and the 16th of November. Hence it follows that if
we are to see any Leonids at all it can only be at these
dates, and thus we at once explain that peculiar feature of
the shower, which is expressed in the fact that it can only
recur on certain special days.

Scattered along the great meteoric highway run those
irregular meteors that have forsaken the main host, either
by rushing on too fast or by delaying too much. As the
earth swoops across the highway it will be very likely,
indeed it will be certain, to capture some of these strag-
glers. They will appear to us who stand on the surface far
below to dart in from the constellation of Leo. Thus it is
from the 14th to the 16th November we often witness some
of the shooting stars belonging to this particular system.
As these stragglers are but few in number, we shall not
usually be gratified by any striking spectacle. A diligent
observer may note on such an occasion a dozen or twenty
Leonids, or sometimes even more, but they are neither
brilliant enough nor numerous enough to attract special
notice.

Sometimes, however, it will happen that the earth

244 IN STARRY REALMS.

arrives at the critical point of its path on the 15th of
November, during the time the mighty procession of
the long meteor stream is filing past. At once our globe
plunges into the current, and for a few hours must forge
its way across the stream, while all the time it is exposed
to a perfect hurricane of meteors. In their untold myriads
the little missiles dash themselves with unutterable pace
into the comparatively stagnant air-case in which this
world is enclosed. Destruction swift and complete is the
doom of every one of the meteors that has the misfortune
to graze our atmosphere. We on the earth's surface,
unmindful of the rapid voyage of our globe, only become
apprised of the singular cosmical event that is in progress
by seeing a beautiful shower of shooting stars.

For a few hours the grand display lasts, that is, until
the earth has traversed the stream and once more resumed
its way through space comparatively void. Nor can any
more Leonids be seen until the following 15th of Novem-
ber, when again in its annual course the earth reaches the
locality of the last display.

It will generally have happened that the whole of the
great shoal will have completely passed by the critical
point during the lapse of a twelvemonth, so that the earth
will at the next return only capture a few of the strag-
glers. Sometimes, however, it is found that the long
shoal has not had time, even in a year, to completely
hurry past the critical point. Accordingly the earth must
take another rush across the stream, and will again
encounter a meteoric tempest. We shall thus have two
grand displays of shooting stars in two consecutive years.
By the time the earth has once again fulfilled its course,

SHOWERS OF SHOOTING STARS. 245

and now for a third time approaches the spot so specially
pertaining to this shower, the great shoal must certainly
have passed, and only a few of the occasional Leonids will
this time enter the net. Many years must elapse before
we can again encounter the great host. They pursue
without interruption their appointed journey. For six-
teen or seventeen years they gradually retreat farther and
farther away from the neighbourhood of this world, then
they begin to turn round, and after the lapse of sixteen
or seventeen years more they regain our vicinity, when
great showers of Leonids are again to be anticipated.
Now we see how the great showers sometimes appear at
intervals of about thirty-three years. This is the period
which the swarm of little bodies require for the one
circuit. They did not appear in 1899.

There are many other periodic showers of shooting stars
besides those notable Leonids on which we have dwelt so
long. None of the other showers, however, possess the same
importance as the Leonids, nor do they ever manifest celes-
tial splendour comparable with that of those of the 13th
of November. The Perseids, for example, which appear
from the 9th to the llth of August, are tolerably constant
in their appearance, but have little spectacular interest.
There is also another shower called the Andromedes,
which occurs on the 27th of November. It has produced
certain displays, one of the most remarkable of which took
place in 1872. The meteors were excessively numerous
on that occasion, but they were so short in their paths,
and so insignificant as to brilliance, that the spectacle^
though of great scientific interest, could not be compared
as to splendour with that of the Leonids in 1866.

246 IN STARRY REALMS.

There are also several other showers which appear with
greater or less regularity. Each of these possesses two
distinct characteristics by which its meteors can be iden-
tified. One of these characters is the date on which the
shower appears. The other is the constellation or point
on the heavens from which all the meteors appear to
radiate. Thus when we speak of the Andromedes on the
27th of November, we express that the shower on the 27th
November comes from the part of the heavens marked by
the constellation of Andromeda.

A striking discovery has been made which points to a
curious connection between comets and shooting stars.
We have seen how a shoal of meteors pursues a definite
orbit through space. It has been found that the track
followed by a great shower of meteors is often identical
with the track pursued by a comet. It is wholly beyond
the province of mere chance that an orbit such as that
of the Leonids should, both as to its size and its position
in space, be likewise that of a comet, unless the comet
and the meteor swarm were objects related to each other.

In drawing this chapter to its close, I would remind
my readers that though we have been occupied in treating
of bodies which are often quite insignificent as to dimen-
sions, yet we have been really following one of the most
interesting and instructive branches of modern astrono-
mical research.

The great sun guides our world through its long annual
journey. The mighty mass of the earth yields compliance
to the potent sway of the ruler of our system. But the
sun does not merely exercise a control over the vast
planets which circulate around him. The supreme law

SHOWERS OF SHOOTING STARS. 24-)

of gravitation constrains the veriest mote that ever floated
in a sunbeam, with the same unremitting care that it does
the mightiest of planets. Thus it is that each little meteor
is guided in its journeys for untold ages. Each of these
little objects hurries along, deflected at every moment,
to follow its beautifully curved path by the incessant
attraction of the sun. At last, however, the fatal plunge
is taken. The long wanderings of the meteor have come
to an end and it vanishes in a streak of splendour.

CHAPTER XYTTI

l-HB NUMBER OF THE STAKS.

OF all the discoveries that have ever been made in science
there are two which especially baffle our powers of com-
prehension. They lie at the opposite extremes of nature.
One relates to objects which are infinitely small, the other
relates to objects which are almost infinitely great. The
microscope teaches us that there are animals so minute
that if a thousand of them were ranged abreast they
would easily swim without being thrown out of line
through the eye of the finest cambric needle. Each of
those minute creatures is a highly organized number of
particles, capable of moving about, of finding and devour-
ing its food, and of behaving in all other respects as
becomes an animal as distinguished from an unorganized
piece of matter. The mind is incapable of realising the
structure of these little creatures, and of fully appreciating
their marvellous adaptation to the life they are destined to
lead. If these animals excite our astonishment by reason
of their extreme minuteness, there is an appeal made to
conceptions of an entirely different character when we
learn the lessons which the telescope teaches. As tha
microscope reveals the excessively minute so does the tele-

THE NUMBER OF THE STARS. 249

scope disclose the sublimely great. In each case myriads
of objects are submitted to our astonished view, but while
the microscope brings before us creatures of which count-
less millions could swim about freely in a thimbleful of
water, the telescope conducts our vision to uncounted
legions of stars, many of them millions of times larger
than the earth.

The grandest truth in the whole of nature is conveyed
in that first lesson in astronomy which answers the ques-
tion— What are the stars ? This is a question that a child
will ask, and I have heard of a child's pretty idea that
the stars were little holes in the sky to let the glory of
Heaven shine through. The philosopher will replace this
explanation by another hardly less poetical, which will
enable us to form some more adequate notion of the real
magnificence of the universe. Each star that we see is,
it is true, only a glittering little point of light, but that
is merely because we are a long way from it. An electric
light which will dazzle your eye when quite close will be
reduced to an agreeable illumination if it is at a little
distance, will become a faint light a mile away, and at no
great distance will become altogether invisible. We must
remember that out in space there is plenty of room —
there are no bounds ; and therefore when we see light
glistening in the far distant depths we cannot at once con-
clude that the light is a faint one because it appears to us
to be faint. It may be that the light is only faint because
it comes from such a tremendous distance. In fact the
brightest light conceivable could be reduced to the in-
significance of a small star if only it were removed suffi.
ciently far.

350 IN STARRY REALMS.

The most intense light we know of comes, of course,
from the light which rules by day, from our sun himself.
The sun pours his unrivalled beams around us in all di-
rections with prodigal abundance, notwithstanding his
enormous distance of ninety-three millions of miles. Let
me describe an experiment with respect to our sun, an
experiment, it is needless to saj7, which could never be
performed, but the results to which it leads us are none
the less certain. Astronomers have demonstrated them in
many other ways.

Suppose that the sun were gradually to be moved away
farther and farther into space ; suppose that by this time
to-morrow the great luminary should be twice as far as
it is now, and the next day should be three times as far,
and the day after that four times, and so on until in a
year's time we should find that the sun was 365 times the
distance from us that it is at present. Let us now trace
the changes which we should see in the brilliancy of our
orb of day. When he had reached double his distance
from us we should find that the light had decreased to a
quarter of its present amount, and the heat which we
derived from his beams would have decreased in the same
proportion. In ten days we should find that the light
had become so feeble as to be only one-hundredth part of
that which we enjoy now. The apparent size of the sun
would also be steadily decreasing, for as the distance of a
body increases its apparent dimensions diminish. Some-
times the diminution of apparent size with distance is well
illustrated on a clock tower. You would hardly believe
that the hands and face of a clock like that at West-
minster were so large until you happen to see a man

THE NUMBER OF THE STARS. 251

cleaning or repairing it, when he appears a mere pigmy
in comparison with the mighty dial which points out the
hours. In a similar way with every increase of distance,
the apparent size of the sun would decline, and in the
lapse of a year the sunlight would be reduced to a feeble
twilight. The sun itself would remain visible for many
years, even if it were steadily moving away, though its
lustre would continually decline, and its size would con-
tinually diminish, until at last it would have shrunk to
the insignificance of a small point of light, still visible
as a glittering object, but too minute to enable any de-
finite form to be perceived. Further still, the sun might
recede until it passed beyond the reach of vision of the
unaided eye; the telescope would, however, be able to
pursue the retreating luminary until at last it sank into
the depths of space beyond the reach of any instrument
whatever.

This little argument will prepare us for an explanation
of the stars. They merely appear to us to be points of
light of varying degrees of brightness, but we have seen
that our own sun might be reduced in lustre to that of
the very dimmest of the stars if only it were removed suffi-
ciently far. If, therefore, the stars are at a great enough
distance from our system, it may indeed be that they also
are suns, possibly equalling, or possibly even surpassing,
our own sun in magnificence.

Here is indeed an imposing suggestion. Can it be that
the host of stars which adorn our midnight sky are ac-
tually suns themselves of an importance comparable with
that of our own ? This is a great thought, and we desire
to test it by every means in our power. You will see

*5* fN STARRY REALMS.

from the reasoning I have given that the whole question
turns simply on one point, and that is : How far off are
the stars ? The tiniest point of light that is just seen as
a glimmer in the mightiest of telescopes may be indeed a
sun as great, or indeed a million times greater, than our
sun, if only that star be sufficiently far off. To find th^
distance of a star is a problem which taxes the utmost
powers of the painstaking astronomer ; every refinement
of skill in making his measurements and of care in the
calculation of his observations has to be lavished on the
operation. Alas ! it but too often happens that the astro-
nomer's labours prove to be futile. The surveying navi-
gator often has to mark on his chart that no bottom could
be found in the depths of the sea. His appliances would
not work, or work reliably in those ocean abysses ; so too,
the astronomer, when he tries to sound the depths of
space to the distances of the stars, has also to mark,
generally speaking, "no bottom here/' as the result of most
of his investigations. When this is the case we know
for certain that the star on which his calculations have
been made must be a gorgeous sun, because we are assured
of the greatness of its distance, even though we have not
been able to find out what that distance was. There are,
however, some few places through the sky where the
astronomer's sounding line can, so to speak, touch bottom ;
there are a few stars of which we do know the distance,
and the result is not a little significant. Were our sun
to be withdrawn from us to a distance so great as
that of the very nearest of the stars, our magnificent
ruler and benefactor would certainly have lost all his
splendour ; he would, in fact, have shrunk to the simili-

THE NUMBER OF THE STARS. 253

tude of a little star not nearly so bright as many of those
which we see over our heads every night. Imagine the
fun's light subdivided into two hundred thousand parts,
each of which would give us only a feeble illumination,
and then imagine that each of these parts was again
divided into two hundred thousand parts more, and it is
one of these last fragments that would represent the
miserable lustre which the sun would then display.

From these considerations we can enunciate the magni-
ficent truth which astronomy discloses to us. I do not
think that in the whole range of nature there is any
thought so magnificent or so imposing as that which teaches
us to regard every star of every constellation as a sun.
We cannot indeed assert that they are all so great as our
sun, but we can affirm with certainty that many of them
are far greater and far more splendid. Considering that
our sun presides over a system of worlds of which the
earth is one, that it gives light and heat to those worlds,
and guides them in their movements, it would greatly
enlarge our conceptions of the universe if we were as-
sured that there was even one more sun as large and as
splendidly attended as is our own. But now we find that
not only is there one additional sun, but that they teem in
uncounted thousands through space. Look, for example, on
the next fine night at the Great Bear, the best known of
•all our northern constellations, and there you see seven
stars forming the well-known feature. Figure in your
mind's eye each one of those stars in the likeness of a
majestic sun as big, warm, and bright as our sun, and look
at other parts of the sky and repeat the process with the
'Other constellations, and your conception of the magni-

«54 W STARRY REALMS.

ficence of the starry system will begin to assume proper
proportions. But this is only the first step, you must
next look at the smaller stars, and reflect that they, too,
are also suns, only much farther off, as a general rule,
than the brighter stars, though this is by no means in-
variably the case. Thus your estimate of the number of
suns in the universe will rise to thousands, but you will
not stop there, you will get a telescope to help you, and
to your extreme delight and wonder you will find that
there are hosts of stars — too faint to be visible to the
eye, but which the telescope will immediately disclose.
You will get a more powerful instrument, and then you
will perceive that the stars are to be numbered by tens of
thousands, and even by millions, and with every fresh acces-
sion of power in your telescope fresh troops and myriads of
suns are revealed. Suns in clusters, suns strewn thickly
here and sparsely there, so as to give us the notion that
the only limit to the number we can see is the power of
the telescopes we are using. Attempts at actual numera-
tion are futile, for who can tell the number of the stars ?

We can, however, form an estimate, and by taking
samples, so to speak, of the sky here and other samples
there, we have been enabled to learn the overwhelming
fact that our universe does contain at the very least one
hundred millions of suna.

CHAPTER XIX.

THE EXTENT OF THE SIDEREAL HEAVENS.

IN discussing the extent of the visible universe, it must
always be borne in mind that the farther a source of light
is from us the fainter is the illumination which we receive
from it. Suppose that a star which just lies on the limits
of naked-eye visibility were somehow to be transported to
a distance which is twice as great, then the lustre of that
star would be diminished to one-fourth of its original
amount. It would, therefore, be of course invisible to the
unaided eye, but could still be easily perceived by a tele-
scope. Indeed, the very word telescope means an instru-
ment for looking at objects a long way off, and the effect
of the telescope is to reduce the apparent distance of the
object. Thus the binocular glass that the mariner uses at
sea has the seeming effect of reducing distances to about
one-third of their amount, so that if a star were carried off
to three times its actual distance from us it would still be
shown as clearly in a binocular as it would be seen by the
unaided eye when placed at its original distance. A star
just on the verge of naked- eye visibility would still remain
within the grasp of such an instrument even were it three

*56 IN STARRY REALMS.

times as far away. The instrument the astronomer uses
must, however, be much more powerful than that which
suffices for the mariner. A telescope small enough to be
held in the hands would be competent to retain a star
within its ken even though the star were removed to a dis-
tance ten times as great as when it was just visible with
the unaided eye. The larger instruments would be suffi-
ciently powerful to follow a star even though it were
removed to a distance one hundred times as groat as that
at which it could be just glimpsed by the naked eye, while
the greatest instruments of which our observatories can
boast would still hold every lucid star in the heavens
within view even were those stars wafted off one thousand
times as far as they are at present.

These facts give us some notion of the extraordinary
extent to which great telescopes increase the range of our
vision. We are permitted by their aid to sound to a depth
in space one thousand times as great as that to which the
unaided eye would enable us to penetrate. Above and on
each side, and around us in every direction the range of
our vision is increased a thousand- fold. At first it might
appear that the extent of space accessible to our telescopes
would also be increased to one thousand times the extent
of naked-eye vision. This would, however, give a very
inadequate conception of the truth. "With our unaided
eyes we are able to see all the objects around us which lie
within a certain mighty globe of which our earth is the
centre. When we employ great telescopes we can explore
a globe of which the radius has been enlarged one
thousand-fold, therefore the quantity of space that we can
examine in the two cases must be represented by two

THE EXTENT OF THE SIDEREAL HEAVENS. 257

globes, one of which has a diameter a thousand times as
great as the other. The volumes of two such spheres have
a ratio so great that perhaps we do not readily compre-
hend it. The point I now wish to illustrate is one which
very frequently arises in the consideration of problems in
astronomy, and, indeed, in other subjects as well.

Let us take a very simple example. Suppose a cow is
tethered by chain in a field ; she will, of course, be only
able to graze over the grass which grows within the circle
to which her movements are confined. If we desire to
give the cow double as much grass as she had before,
would that end be accomplished by doubling the length of
the chain ? It would be more than accomplished ; in fact,
she would then have four times as much pasture as she
had before. The circle over which she can graze has
double the diameter, no doubt, but then its area is four
times as great. Thus we see that the area increases in a
far more rapid ratio than that in which the radius
increases. Think next of two bird cages. We may sup-
pose them to be both shaped like globes, the diameter of
the larger being double that of the smaller. What will
be the freedom enjoyed by the bird in the larger cage as
compared witji that' of the less fortunate bird in the other ?
The volume of space over which the bird in the big globe
oan fly is not twice nor even four times as great as that
allotted to the other bird ; it is no less than eight times as
great.

The bulk of a grain of sand as compared with the bulk
of a football may illustrate the space accessible to our eyes
when compared with the space accessible to one of the
great telescopes. The larger of these spaces has a thousand

258 IN STARRY REALMS.

times the diameter of the others ; therefore, the relative
quantities of these spaces are to be obtained by multiplying
1,000 by 1,000 and by 1,000 again. Thus we finally
learn that the amplitude of our vision is augmented ta
one thousand million times its original extent by the
use of our greatest telescopes. It need, therefore, be no
matter for surprise that the number of stars visible through
our great telescopes or recorded on the sensitive films of
photographic plates should number scores of millions. In
fact, it would sometimes seem surprising that the number
of telescopic stars is not even greater than it actually
appears to be. If we are able to explore one thousand
million times as much space we might expect that the
number of objects disclosed would be also increased about
a thousand million fold, but this is certainly not the case.
The truth seems to be that our sun is but one star of a
mighty cluster of stars ; we happen to lie near the middle
of the cluster, and the rest of the stars belonging to it
form what we know as the Milky Way. There are, of
course, other clusters scattered through the heavens, some
of them, perhaps, as great as that body of stars which forms
the Milky Way. Owing to our residence in this cluster
we see the neighbouring suns in multitudes, .and thus we
receive the impression that the solar system lies in an
exceptionally rich part of the universe in as far as the
distribution of stars is concerned.

On the outskirts of the universe lie those faintest and
dimmest of objects which we can just perceive through our
greatest telescopes. We know that many of the stars
around us would still remain visible in great instruments,
even though they were removed a thousand times as far

THE EXTENT OF THE SIDEREAL HEAVENS. 25?

off. Among the myriads of faint stars which we see from
our observatories there may be many, indeed there must
be many, which are fully a thousand times as distant as
the bright stars which twinkle in our comparative neigh-
bourhood. We thus obtain some conception of the stupen-
dous distance at which the outskirts of the universe are
situated. There are different ways of illustrating this
point, but I think the simplest, as well as the most striking,
is that which is founded on the velocity of light. It is a
remarkable fact that the beautiful star known as Yega has
a distance from us so tremendous that its light must have
taken somewhere about eighteen years to travel hither
from thence. Notwithstanding that the light dashes
along with such inconceivable speed that it will cover
185,000 miles in every second, notwithstanding that a
journey at this pace will complete the entire circuit of this
globe seven or eight times between two successive ticks of
a clock, the light will, nevertheless, take eighteen years to
reach our eye from the time it leaves Vega. We do not,
therefore, see the star as it is at present ; we see it as it
was eighteen years ago. For the light which this evening
enters our eyes has been all that time on its journey. In-
deed, if Vega were actually to be blotted out from exist-
ence it would still continue to shine out as vividly as ever
for eighteen years before all the light on its way had
reached us.

We have been led to the belief that among the more
distant stars in the universe there must be many which
are fully a thousand times as far from us as is Vega, hence
we arrive at the startling conception that the light they
emit has been on its journey for 18,000 years before it

260 IN STARRY REALMS.

reached us. When we look at those lights to-night we are
actually viewing them as they were 18,000 years ago. In
fact, those stars might have totally vanished 17,000 years
ago, though we and our descendants may still see them
glittering for yet another thousand years.

We shall realise a little more fully what this reasoning
involves if we suppose that astronomers dwelt on such a
star, and that they had eyes and telescopes sufficiently
keen not only to discern our little earth, but even to
scrutinise its surface with attention. Let us suppose that
the stellar astronomers looked at England : do you think
they would see a network of railways joining mighty and
populous cities, furnished with immense manufactories and
with countless institutions. Such would be the England
of to-day. But from the distance at which these astro-
nomers are situated light takes 18,000 years for its
journey, and, therefore, what they would see would be
England as it was 18,000 years ago. To them England
would even now appear as a country mainly covered with
forests inhabited by bears and wolves, and totally void of
any trace of civilisation. This illustration will at all events
serve to convey some conception of the distance at which
the outskirts of our visible universe are plunged in the
depths of space.

CHAPTER XX.

THE MOVEMENTS OF THE STARS.*

I SHALL follow the precedent set by my illustrious pre-
decessors in this chair, and endeavour to lay before you
some of the recent acquisitions of knowledge in those
branches of science with which I am myself most particu-
larly connected.

It has been my privilege on not a few occasions to
discourse to the members of the Midland Institute on
various astronomical matters, but our science is ever grow-
ing, it has shown quite unexpected vitality within the last
year or two, and consequently I find myself in posses-
sion of abundant material. Discoveries are just now
being made with ever increasing volume ; results we
despaired of attaining yesterday are familiar to us to-day,
while to-morrow will see the van of the army of research
advancing into some wholly new territory. I could not
attempt to give any comprehensive insight into the
marvellous accumulations of facts that have recently been
garnered by students of the heavens. I will rather endea-
vour to develop with fitting detail that particular line of
* Presidential address at the Midland Institute, Birmingham.

262 IN STARRY REALMS.

research which of all others seems to have yielded the most
striking results. The subject which I chiefly propose to
discuss is the application of the spectroscope to the study
of the movements of the heavenly bodies, a subject with
which the name of Huggins is primarily associated.

We have been long accustomed to receive with interest
the tidings which the spectroscope conveys as to the
constitution of the bodies throughout space. It was the
primary function of that beautiful instrument to analyse
the light which came from afar ; and by decomposing the
composite beam into its constituents, the spectroscope
declared to us the actual materials present in the bodies
from which the light emanated. It was astonishing to
learn that thirty-five of the elements known on this earth
were present on the sun. Somewhat similar announce-
ments were made with respect to the stars and various
other heavenly bodies. The information thus won was a
most important accession to our knowledge of the material
construction of the universe. Certain philosophers had
even formulated the doctrine that to discover the elements
present in the stars must necessarily lie beyond the pro-
vince of man's powers. The events, however, entirely
disconcerted these rash assertions. We have learned much
as to the elementary bodies present in various celestial
orbs, and quite recently we have come to believe not only
that the elements in the sun have much in common
with those in the earth, but that the two bodies are so
similar in constitution that, in the striking words of
Sir William Huggins in his address to the British Associa-
tion (1891), it almost seems that, if the earth were heated
to the same temperature as the sun, it would emit a spec-

THE MOVEMENTS OF THE STARS. 263

trum of the same character. To have demonstrated the
material unity of the universe is indeed a notable achieve-
ment for that little instrument, the spectroscope. But it
seems possible that the services of the prism to science are
now taking a direction in which the results to be achieved
will outshine in importance and interest even that grand
discovery of material unity, to which we have referred.
Within the last year or two the spectroscope has made
revelations with regard to the movements of certain of the
heavenly bodies of a character which have startled the
scientific world quite as much as did the fundamental
discovery made some thirty years ago of terrestrial ele-
ments in the sun. Let me unfold the matter.

Look at a star such as Sirius or Aldebaran. Bring to
bear upon it the meridian circle of the observatory, with
all its refinements for exact measurement, and we can
determine the exact place of the star in the heavens. We
can do so with an accuracy which makes the fixed star a
landmark of the universe. Years roll on, and by a
repetition of the observations, the place of that star is
again determined, and the two places do not agree. They
are purged from every source of error, but still a dis-
crepancy remains, the place of the star as now observed
is not the same as its place was years ago ; it has what we
call a proper motion.

Tested in this way almost all the stars will probably be
found to be in motion. We talk of these objects often as
fixed, but the word must be used in a relative sense. To
our ephemeral glance the stars seem fixed ; but what is
indeed fixed when sufficient time is allowed ? The cloud
that now caps a mountain summit is obviously unstable ;

264 IN STARRY REALMS.

in an hour it may have dissolved into the transparent
vapour from which it had been originally condensed. We
are apt to contrast the evanescent cloud with the great
mountain by which it was gathered. The mountain itself
seems the ideal of stability. For thousands of years it has
lasted, for thousands of years it seems destined to endure ;
but when sufficient time is allowed, the mountain is no
more permanent than the cloud. The avalanche that
thunders down its side is only a somewhat vigorous indi-
cation of the agencies that are incessantly tending to bear
down the mighty mass. The Matterhorn of to-day will
assuredly vanish in time just as many other mighty
mountains have done in the course of geological history.

Indeed it may be remarked that nature on a grand scale
exhibits no qualities of permanence. The Alps may last
longer than the ruins of Palmyra, but everything that we
know of geology teaches us that the sea has rolled where
Mount Blanc now rears its head, and that for anything we
can tell it may do so again. For permanence in nature we
must look not to great planets ; we find it, if at all, only
in the atoms of matter. The little pulsating molecule,
too small to be visible in the most powerful microscope,
even if the object were a thousand times larger than it is,
seems to possess attributes qualifying it for indefinite
duration.

The constellations are no doubt so far fixed that in the
course of a lifetime, or even in the lifetime of a nation,
they undergo but little change. The Orion at which Job
looked was almost the Orion of our skies to-night. I say
almost, because when close examination is made it will be
found that the stars in the constellations are gradually

THE MOVEMENTS OF THE STARS. 265

changing. We sometimes think that those groups of stars
to which from all antiquity certain names have been
assigned have bonds of affinity, and that their proximity
on the heavens is not to be attributed to a mere casual
arrangement, but is to be taken as indicating a community
of origin. In some cases there can be no doubt that this
is so. In the great group of Orion, for instance, to which
I have already referred, modern researches demonstrate
that the several stars of that grand constellation possess a
structure which may be described as peculiar to themselves,
inasmuch as a similar structure has only been observed
in one other star in the sky except those of Orion. In
this case we have the evidence not only of juxtaposition in
the heavens, but also of an allied material composition.
Under these circumstances it seems almost impossible to
doubt that the glorious assemblage of stars forming the
constellation of Orion does really represent portions of a
mighty system. If any further corroboration of this view
be required, it may be obtained from recent discoveries
with respect to the peerless nebula by which Orion is most
familiar to astronomers (p. 126). The beautiful photographs
which have been obtained by Common and other astrono-
mers have tended to disclose ever widening boundaries
to the great nebula when sufficiently long exposure has
been given. We thus see that the glowing gas encroaches
on the surrounding space to an extent much wider than
mere eye observation would have indicated. Several of
the bright stars are already seen to be invested with
whatever glory residence in the interior of a glowing
nebula may confer. Adding this circumstance to those
we have already mentioned of juxtaposition and of mate-

266 IN STARRY REALMS.

rial congruity, it seems impossible to doubt that Orion, the
finest constellation in the heavens, is not a mere fortuitous
concourse of stars, but is a system possessing indications of
a common origin.

In a similar way we are entitled to infer that many other
remarkable groups of stars give evidence of a certain
physical connection which corroborates the presumption
obtained from the fact that the stars happen to be close
neighbours. I do not suppose that anyone ever could
have doubted that so striking a group as the Pleiades had
some natural connection. But if there were such doubts
they must be dispelled when the photographs of the Messrs.
Henry and of Mr. Roberts show the seven stars of the
Pleiades to be immersed in a single nebula, invisible to the
eye, and perceptible only to the delicacy of the photo-
graphic plate. In other famous groups also there are
indications of relationship drawn from their common
movements. If seven fish were seen near together in the
sea there would be a certain presumption that they formed
a related group, and this presumption would be greatly
strengthened if it should appear that all the fish were
swimming in parallel directions. We can sometimes apply
a similar principle to the study of a constellation. If
seven bright stars lie comparatively near each other in the
sky, and if it be found that they participate in a common
motion so far as direction is concerned, we may not un-
naturally conclude that those stars belong to an organized
system, and that they are not merely a number of discrete
objects scattered promiscuously on the sky.

Picture this vast firmament of stars, some in associated
groups, some more sporadic and isolated, and all more or

THE MOVEMENTS OF THE STARS. 267

less in motion. For centuries, for untold thousands of years,
these stars have been moving, fresh stars have come in
from the abyss of space, and have gradually sunk away
again to invisibility. There is no permanence in the
heavens. Even in the lapse of geological time the heavens
must have worn a succession of aspects, each wholly
different from the other. The sky as we see it to-night
presents us with an arrangement of stars different from
the stars which were visible to the eye of the first man
that trod this earth, and the stars which the first man
saw must have been widely different from those on which
in far earlier times the ichthyosaurus may have looked,
while these again must have been totally different from
the stars which spangled the sky in those excessively
remote times when life began to dawn on the earth.

In our attempt to understand the nature of things
celestial there is nothing of more importance than a clear
comprehension of the mode in which the universe of stars
is gradually transforming itself. "We want to learn how
each star is moving, and whither it is moving ; and when
we have obtained such knowledge, we are in a position to
learn more truly the disposition of that vast array in which
our sun is only one of the units. Astronomers had well-
nigh despaired of attaining any comprehensive knowledge
on this subject within the lifetime of the present genera-
tion. Centuries must elapse before the majority of the
stars have advanced by proper motion to a sufficient
distance from their present places to render the change
conspicuous. We want some readier way of determining
the pace at which a star is going — we want an instrument
which shall tell us at once the speed of the body without

268 IN STARRY REALMS.

having to deduce that speed from a tardy comparison
between the places of the stars obtained after the lapse of
a sufficient interval between them. This the spectroscope
supplies, it tells us while we are looking at the star the
speed at which it is winging its flight.

To determine the rate at which a star is moving towards
or from us requires no nice comparison and discussion
of observations made after an interval of a century by
different observers using different methods and obtaining
determinations which are entitled to very different degrees
of confidence. The modern spectroscopic observer takes
his seat at his instrument, adjusts his spectroscope on
the star. He fixes his attention on a certain line in
the spectrum, a line, for instance, known to belong to
the element hydrogen ; he sees it shifted to the right
or to the left of a standard hydrogen line, introduced
for the purpose of comparison. If the shift be one
way, the star is urging its course towards us, if the
shift be the other way the star is retreating from us,
while by measuring the amount of the shift the actual
velocity of the motion is determined. There is a certain
indicator sometimes put on a locomotive engine which
shows by a dial the speed at which the engine is running.
By a glance at this dial the engine-driver learns at once
the pace he is making. The spectroscope is like this little
indicator ; it shows at once the pace of the body to or from
the observer without the necessity for awaiting the time
necessary to pass over a certain interval. Thus the
spectroscope offers to us a means of obtaining promptly
valuable information with regard to the proper motions of
the stars.

THE MOVEMENTS OF THE STARS. 269

I must, however, be careful to explain that though I have
thus drawn an antithesis between the spectroscopic method
of observing proper motions and the telescopic method,
yet that from another point of view the two methods
are to be regarded rather as complementary to each other.
Each purports to tell us something which the other is wholly
incompetent to tell. This will be plain if we endeavour
to realise the character that the motion of a star will
usually present. Let us suppose that the only movement
possessed by the star was directly towards the observer, or
directly from him, that there was, in fact, no movement
across the line of vision. In this case the star would never
appear to change its place with respect to the stars in its
neighbourhood ; in fact, the only conceivable alteration in
the telescopic appearance of the star would be, that in the
course of time it would become brighter if approaching or
fainter if receding. But any change of this kind would
be so insignificant that we can afford to disregard it. On
the other hand, it might happen that the movement of the
star was such as to lie entirely across the celestial sphere,
so that it was neither approaching to nor receding from
the observer. In this case the entire proper motion would
disclose itself by the displacement of the star relatively to
the other stars in the heavens, and these are precisely
the circumstances in which the old telescopic method is
available. In fact, the spectroscope will only show motion
directed along the line of sight, while the telescope
only shows the motion which is perpendicular to the line
of sight. Thus each of the methods exhibits that par-
ticular movement which the other is incompetent to
perceive. Of course it will not often happen that the

270 IN STARRY REALMS.

movement of a star is so adjusted as to be either entirely
in the line of sight or entirely perpendicular to the line of
sight. The movement will generally be of such a character
that, to use the well-known language of mechanics, it will
have one component along the line of vision and another
perpendicular thereto. For the complete determination of
the movement of the star it is necessary for us to know
both these components, and it will be plain in what sense
the spectroscopic method and the telescopic method are
complementary ; the latter determines the movement per-
pendicular to the line of vision, and the former gives us
the movement along the line of vision. Each method just
gives what the other is unable to give ; there remains,
however, the important difference, that the spectroscope
indicates instantly, if I may so speak, the velocity of the
body along the line of sight, while the telescope only
indicates the portion of the velocity perpendicular to the
line of sight by a comparison of observations separated by
a long interval of time.

There is another essential distinction to be noticed
between the information conveyed by the spectroscope and
that yielded by the older method of observing proper
motions. It is a distinct advantage of the spectroscopic
method that, so far as its indications are concerned, the
distance of the body from the observer is immaterial ; the
shift of the line which is observed has a magnitude which
depends solely on the relative speed with which the body
emitting the light is approaching to or receding from the
observer. A star which shows a certain shift of spectral
lines due to its velocity would exhibit precisely the same
shift were its distance ten times or a hundred times that

THE MOVEMENTS OF THE STARS. 271

which it actually happens to be, but the matter is quite
otherwise when proper motions are being measured by the
ordinary telescopic method. The displacement which the
telescope observes depends partly on the velocity with
which the body is moving and partly upon the distance at
which it is situated. If two stars had equal velocities per-
pendicular to the line of sight, but if one of those stars
was ten times as far as the other, then the apparent move -
ment of the more distant star, as measured by its dis-
placement on the heavens, would only seem to be one- tenth
part that of the nearer star. In the case of bodies which are
extremely distant from us, the circumstance that I have
just mentioned causes the determination of its movement
on the heavens to be impossible, owing to its minuteness.
A star might be so far off that even if it moved as quickly
as the most rapidly moving body that we know of,
still its apparent displacement on the heavens might be
quite inappreciable after the lapse of a year or many years,
and consequently we could hardly obtain any information
as to its movements across the face of the heavens. On the
other hand, the excessive distance of the star would be no
bar to the application of the spectroscopic method, so long
as the spectrum was bright enough to enable the lines to
be seen. The form, too, in which the spectroscope offers
to us its results is especially satisfactory ; it announces
definitely that the star is moving towards us or moving
from us, and it determines the velocity of that motion.
The telescopic method merely indicates that the star
traverses so many seconds of arc, or often mere fractions
of a second, in the course of a twelvemonth. We cannot
express the result in speed of miles per second unless we

272 IN STARRY REALMS.

happen to know the distance of the star from the earth,
and with this important element we are, in the majority of
cases, quite unacquainted.

Quite otherwise is it in the spectroscopic method of
measuring proper motions along the line of vision ; what
we now obtain is not a measurement of arc on the sky, it
is the velocity in miles per second that is given to us, and
this, be it observed, not after the lapse of a long period of
time between the first observation and the last, but directly,
so that by turning the spectroscope on one star we may,
for example, say that it is approaching at the rate of ten
miles a second, while the same method applied to another
star may show a recession at the rate of ten, fifteen, or any
other number of miles per second. To obtain a complete
knowledge of the movement of the stars we require both
the spectroscopic method and the old-fashioned telescopic
method. By the union of the two classes of measurement
a complete knowledge is obtained of the way in which each
star is moving.

Let me now elucidate with sufficient detail the principle
of the very beautiful process on which the spectroscopic
method of determining movements on the line of sight is
based. Light is known to be caused by waves in that
mysterious fluid, the ether. The ether fills all space, at
least, so far as space contains objects visible to us, and the
light from a star is caused by ethereal waves which have
trembled along an appalling distance of many billions of
miles from their origin in the glowing star to the retina,
where they set the nerves in vibration.

Everyone who has stood on a sea-beach and watched the
sea rolling in can find no difficulty in comprehending

THE MOVEMENTS OF THE STARS. 273

what we mean by the wave length. We do not here refer
to the length of the wave in the same sense as we could
speak of the length of the beach ; we simply mean the
interval between one wave and the next, or, to speak more
definitely, the number of feet between the crest of one wave
and the crest of the wave following it ; nor will there
be any difficulty in understanding from the analogy of the
beach the difference between long waves and short waves.
It is true that in many respects the analogy of waves
on the beach is defective when we would seek to under-
stand the waves in the ether ; we must rather think of
the latter as a succession of throbs rapidly succeeding each
other throughout this invisible and intangible fluid. Tf
the throbs succeed each other at comparatively long
intervals, then the light of which they convey the impres-
sion is red ; the waves that are the shortest, so far as visual
rays are concerned, correspond to light of a violet hue.

We thus assign a length to each wave, and we can
regard that length as in direct correspondence with th
colour of the light ; this may be submitted to strict
numerical expression, in fact, we know the wave lengths
of the principal rays of light with an accuracy which is
one of the most marvellous results of modern physical
research. These waves are very minute; take the veiy
longest we can see, which corresponds to extremely deep
red at the very end of the spectrum, the wave length of
the ray identified by the Fraunhofer line known as A is
expressed as 7604'04 tenth-metres. Let me attempt to
illustrate what this length really . corresponds to ; where
are we to find an object small enough for the comparison ?
If we divide an inch into 1,000 equal parts, yet each of

T

274 IN STARRY REALMS.

these would be 30 times as long as this tiny wave of light.
At the other end of the visual spectrum is the extreme
violet, the wave lengths are only about half as long, while
in that ultra-violet region occupied by rays incompetent
to excite any nerves possessed by our retina, but acutely
perceptible by the peculiar sensibility of the photographic
plate, the waves are shorter still.

Light travels across space with a tremendous speed of
185,000 miles per second. It gives us a wonderful im-
pression of the subtlety of the ether to think that waves,
of which tens of thousands are comprised within the
length of a single inch, can oscillate with such astonish-
ing rapidity that in a single second of time 185,000 miles
shall be travelled, each mile having 1,760 yards, each yard
having three feet, each foot twelve inches, and each inch
some thirty or forty thousand wave lengths.

A remarkable circumstance must now be mentioned ; it
has been discovered that rays of every colour travel with
the same speed. If a ray of blue and a ray of red be
started at the same moment they will require precisely the
same time to travel a million miles or any other distance.
If in the depths of space a blue star and a red star simulta-
neously sprang into existence, we should become aware of
the two creations at the same instant, provided the stars
were equally distant from us. Hence we are able to con-
clude the time of vibration of a wave of any given colour.
The vibrations of the ether as it conveys a red ray of light
are not so rapid as those which convey a ray from the
violet end of the spectrum ; or, to speak a little more cor-
rectly, when certain nerves on the retina are tingling with
so many vibrations a second, the brain is conscious of a

THE MOVEMENTS OF THE STARS. 275

red light, while vibrations of other numbers per second
correspond to the orange and blue rays respectively. We
must, therefore, understand that our sense of vision dis-
tinguishes between one colour and another solely by the
different number of vibrations of the ether to which the
rays correspond. On the correct apprehension of this
principle depends the right understanding of the spectro-
scopic method for the detection of motion along the line
of sight.

We have been hitherto supposing that the body from
which the light emanates is at rest. Now let us suppose
that it is in motion to or from the observer ; we shall see
that this would have the effect of modifying the inter-
pretation which the brain puts on the sensations which
it receives from the retina. The subject is one of some
difficulty, but I hope by the help of a few illustrations to
make it intelligible.

I remember some time ago hearing a question pro-
pounded something of this kind. Suppose that it takes a
week for a steamer to cross from Liverpool to New York,
and that one steamer starts from each end every day. Of
course we may suppose that, on an average, one arrives
every day, so that in the course of a week seven steamers
reach Liverpool. But now let us suppose that the ob-
server, instead of being stationed at Liverpool, takes his
berth on one of the outgoing steamers. Then the question
may be asked as to how many returning steamers he will
pass in the course of his week's voyage to New York.
One might hastily answer seven, that is the same number,
no doubt, that would be counted in a week from the
landing- stage at Liverpool, but the case is very different

276 IN STARRY REALMS.

as far as the outward bound ship is concerned. When the
observer started on his voyage there were already seven
steamers at different situations in their homeward passage ;
all of these the outward-going steamer must pass, but
during the week that he is on his voyage seven more
steamers must have left New York, and these, too, must
also be passed by the outward bound ship before she
reaches harbour on the other side of the Atlantic. Thus
you see that our passenger outward bound will, in the
course of a week, pass fourteen vessels coming the other
way. In fact, he will pass two vessels per day, whereas,
if he remained at Liverpool, he would only find one per
day arriving, on the supposition which we have made.
Suppose that the sensations caused by the moving of the
vessel were overlooked, so that, in fact, the observer was
unconscious of the fact that he was in rapid motion ; he
would not unnaturally seek for an explanation of the
phenomena he had observed in the movements of the
other vessels. In other words, he would conclude that the
vessels were leaving New York twice every day, the fact
that he was himself in motion having had the effect of
making the other vessels seem to arrive twice as often as
they would otherwise have done.

Instead of ships crossing the Atlantic, let us now return
to the conception of waves of light radiating from a star
in the depth of space. If the star is voyaging towards
the earth, then the number of waves that arrive every
second will depend not alone on the light itself, but also
on the relative velocity with which the bodies are ap-
proaching or receding. If the two bodies are approach-
ing, then more waves will reach the eye in the same

THE MOVEMENTS OF THE STARS. 277

period. If, however, the two bodies are retreating, then
fewer waves per second will enter the eye than in the
former case. We have, however, already seen that the
colour of the light depends on the number of undulations
in a second which strike the retina ; if this number be
altered, then the colour of the light, at least as interpreted
by our senses, will be altered also ; this principle leads to
some curious consequences, and in order to explain the
matter fully it will be necessary for me to take a some-
what simpler case than any that actually occurs in nature.

Generally speaking, the light radiated from a star or
from any other luminous source is of a highly composite
character, consisting of a number of undulations of very
varied wave lengths — in fact, of a multitude of hues
blended together. Thus, in the case of the sun, as well as
in most of the stars, the light received from them consists
of hues of all the colours of the rainbow, as well as of many
rays which are invisible, because they vibrate too quickly
in regions beyond the violet, or because they vibrate too
slowly for our eyes to recognise in regions beyond the
red. Let us, however, for the moment assume that a
star existed which dispensed rays of a perfectly uniform
type, which it will be convenient to take at a central
part of the spectrum, say in the position of the green.
Let our supposed star shed forth a flood of undulations all
of precisely similar wave length, and let us study the
modifications that would be caused in the appearance of
that star if it moved rapidly towards the earth.

According to the principle we have already explained,
as the two bodies come together, more undulations
would enter the eye in a second than would be the

278 IN STARRY REALMS.

case if the two bodies were relatively stationary. But
we have already seen that our senses have no means
of interpreting the hue of any beam that enters the eye
except by the number of undulations per second that it
causes on the retina. It follows that in the case we have
supposed, when the eye received more vibrations per
second than was appropriate to that particular shade of
green, the appearance presented to us would be that the
star had a different hue from that which it actually pos-
sessed— it would, in fact, shine with a shade of colour
nearer the blue end of the spectrum. Indeed, if the speed
of approach were sufficiently great the star would cease
to appear green at all, and would manifest a bluish hue —
and if we allow sufficient scope to our imagination in the
matter of velocity, we could conceive a star approaching
so fast that its colour, though originally green, appeared
to be indigo or even violet, while by a still farther stretch
we might suppose that the star is dashing towards us
with a pace even greater than that which would trans-
form its original green to an apparent violet. The
vibrations would then be poured into the eye with such
rapidity that no nerves on the retina, constructed as it is,
would be adapted to respond to them, and consequently
the star would become actually invisible to us.

On the other hand, let us suppose that this green-coloured
monochromatic star was dashing away from the earth with
sufficient speed, or, what comes to the same thing, that we
were dashing away from it — then a transformation in the
apparent character of the light would take place in the
opposite direction. The shift of the green colour would be,
in this case, not towards the blue end of the spectrum but

THE MOVEMENTS OF THE STARS. 279

towards the red end. If the star retreated sufficiently fast,
its green colour would assume a yellowish hue ; were its
motion faster still, the yellow would be transformed into
a red, while if the speed be still further accelerated the red
would continually deepen corresponding to the lessened
number of waves that entered the eye in a second, until at
last the vibrations would become so slow that invisibility
would again result from the want of nerves on the retina
competent to perceive the vibrations of the character thus
presented.

It was once supposed that the colours actually observed
in the stars might be, as a matter of fact, explained as
consequences of their varied movements to us and from us ;
the suggestion was an attractive one, but it will not bear
the light of examination. It must be particularly observed
that in the case we have supposed the light of the star is
strictly monochromatic, but this is a condition of things
which certainly does not exist in any ordinary star. In
fact, in this respect the radiation from the stars may be
said to resemble the radiations from the sun. There is an
immense variety of different coloured light blended to-
gether— not only all the hues of the visible spectrum, but
abundance of ultra-violet rays and abundance of rays
beyond the red. If, therefore, as in our first case, the star
be so hurrying towards us that the green light from the
central parts of the spectrum is carried on towards the
blue, the place of the green is itself filled by other rays
advancing from the red end. The rays initially red have
their places taken by rays from the invisible ultra-red
region, which, by a quickening in the number of vibra-
tions perceived per second in consequence of the rapid

28o IN STARRY REALMS.

approach, pass from the invisible into the visible region.
Thus, the general character presented by the appearance
of the star is not altered. No doubt there is a conceivable
velocity of approach so great that all the rays except a few
should be hurried into the ultra-violet, and then, of course,
the star would only have a violet or bluish colour ; and
there is a conceivable velocity of retreat from the earth by
which all the rays except a few should have their periods
of vibrations so lengthened that the only appeal to the eye
would be the glimmer of a ruddy object. The velocities,
however, that would be required for the display of such
phenomena, so enormously transcend those actually pos-
sessed by any known body in space, that we may at once
dismiss the supposition that the observed colours of the
stars are to be explained in this manner. In fact, we
have evidence presented by the spectroscope itself, which
shows that we must look for an explanation of the colours
of the stars in. quite a different direction. It was thus
known that the ingenious supposition that some stars
seemed red merely because they were hurrying away from
us, and that some stars seemed blue merely because they
were hurrying towards us, must be set aside.

But, though the notion of Doppler was, we now know,
in a large measure incorrect, yet we are indebted to it for
a suggestion which has already borne much fruit in
modern astronomical research, and which bids fair to
extend our knowledge of the heavens to an extent that
never would have been dreamed of until the last few years.
It is quite true that the celestial velocities are insufficient
to cause any grand transmutation in the hues of celestial
objects. We must not look for their effects on so coarse a

THE MOVEMENTS OF THE STARS. 281

canvas as that which suffices for our mere visual impres-
sions of colour. When we study the refinements of spec-
troscopic analysis, the indications of celestial movement
become visible enough, even though they are insufficient
to turn a green into a blue, or a yellow into a red. The
development of this subject is one of immense interest,
but it also possesses some little difficulty ; and I therefore
entreat your most particular attention.

I shall revert again to the supposition of a green star, che
radiation from which is monochromatic. If the light from
such an object were examined through a spectroscope the
appearance presented would not be that of a band of varied
colours which form the glories of the ordinary spectrum ; it
would be rather a single line of vivid green light, and occu-
pying a particular position in the spectrum as defined by
its wave length. If the body were moving towards us the
line would shift towards the blue end. If the body were
moving from us the lines would shift towards the red end.
Let us suppose that our spectroscope is furnished with a
measuring apparatus by which we are enabled to deter-
mine the amount of the shift with the necessary accuracy.
Suppose, in fact, that the wave lengths of the light, when
the object is relatively at rest, be the 30,000th of an inch,
but that when the object is moving towards us it is found
that the vibrations are 30,003 in the inch; it then follows
that the fact of this body's movement towards us has caused
us to receive one ten -thousandth part more vibrations per
second than when the body was at rest. It therefore fol-
lows that the velocity with which the body is dashing to-
wards us must be a ten- thousandth part of the velocity of
light — in other words, about eighteen miles a second. It is

282 IN STARRY REALMS.

a velocity of a magnitude comparable with this that we
generally meet in the celestial regions. It may perhaps be
two or three times as great, or in some cases possibly as much
as ten times as great — in any case, however, the velocities of
approach or recession are very small in comparison with
the velocity of light. Rarely, indeed, will the actual
velocity of a celestial body be so much as a thousandth
part of the velocity of light, and consequently no altera-
tions in the apparent quality of a ray arising from move-
ment can at the utmost do more than affect its wave length
by one-thousandth part of its total amount.

It would be useful to obtain some concrete notion as to
the nature of the magnitudes with which we have to deal
when discussing the problem of celestial movements in the
line of sight. Let us look therefore at that ever-beautiful
object the solar spectrum, especially at the double line D,
which is caused by the presence of the element sodium.
The wave lengths of these lines expressed in ten-millionths
of a millimetre are, according to Professor Rowland,
5896-08, 5890-12 ; the difference, amounting as it does to
5*96, is about one- thousandth part of the whole amount.
We are thus able to form the general conception that the
greatest possible shift that could be caused in the spectrum
is not likely to exceed the length of the interval between
the two lines of sodium. It should be observed that in
this calculation I have taken an extreme value for the
velocity, and many of the velocities met with in space
will not be one-tenth as great. It therefore follows that
we must be prepared to make very exact measurements
when the phenomena with which we have to deal are of
such a delicate character that they do not extend to a

THE MOVEMENTS OF THE STARS. 283

shift of the lines of the spectrum through more than a
fractional part of the interval between the components of
the sodium line.

To understand the method of procedure it is necessary
to recall the appearance presented by the spectrum of a
star. It gives us generally the red, orange, yellow, green,
blue, indigo, violet, forming the spectrum band so familiar
to every one who has ever looked through a prism. From
one end of this band to the other transverse lines vari-
ously grouped — often double, sometimes exquisitely fine,
sometimes more or less ill-defined — are to be seen. The
interpretation of these lines is now well understood. They
are due to the presence of metallic or other vapours in the
atmosphere surrounding the star. Such is at least the
general description of the origin of the majority of the
lines. Some are, however, caused by the presence in the
atmosphere of the star of other substances besides metals ;
there are, for example, numbers of stars of which the
brilliant Sirius may be taken as the type, the characteristic
feature of which is the presence of a number of lines due
to the element hydrogen.

For our present purpose, however, it is especially essen-
tial to observe that the lines are caused by something per-
taining to the star itself. We have to invoke their aid
for the measurement of the movements of the body in the
line of sight. For the purpose of description, I shall take
the line known as F in the solar spectrum. This line is
also found in a good many stars, and it is known to be an
indication of the presence of the element hydrogen. The
wave length of this line in air is represented by the
number 4860*72, the unit being, as usual, the ten-millionth

284 /W STARRY REALMS.

of a millimetre. Now if the star is coming towards us,
the line F will be displaced, with the rest of the spectrum,
towards the blue end, and we want to measure the amount
of that displacement. For this purpose we require a com-
parison spectrum which shall show where the line ought
to be, if the body were at rest, relatively to the earth. We
obtain this by availing ourselves of the wonderful principle
discovered a quarter of a century ago, by which the
interpretation of the dark lines in the spectrum is
obtained.

Suppose that the element hydrogen be heated to incan-
descence by the electric spark, or in any other way, it will
of course radiate light, and when that light is viewed
through the spectrum certain brilliant lines will be per-
ceived; these lines are characteristic of that particular
element ; where these lines are found hydrogen is present,
where they are absent, hydrogen is absent ; provided the
conditions as to incandescence have been attended to. But
these are bright lines on a dark background, whereas the
lines in the spectrum of a star are often black lines on a bril-
liant background ; how then are the two to be connected
together ? It is the unveiling of this connection that con-
stitutes the discovery to which I have referred. The
brilliant lines, it is to be observed, are produced by highly
heated hydrogen ; the dark lines in the spectrum of a star
are due, not to heated materials, but to comparatively cool
materials. In the upper regions of the star's atmosphere,
the actual glowing surface sheds forth in copious abund-
ance rays of every hue, but before these gain the outer
space they have to pierce through the comparatively cold
atmosphere with which each star may be surrounded.

THE MOVEMENTS OF THE STARS. 285

Now, though that atmosphere may be sensibly transparent,
it nevertheless levies a certain toll on the light passing
through it ; in fact, it stops some of the light.

Let us suppose, for the sake of argument, that the atmo-
sphere surrounding the star was solely composed of hydro-
gen gas. This gas would be almost perfectly transparent, so
tar as most of the rays are concerned. It is essential, in
this point, to pay attention to the wave lengths ; most
rays will be permitted to fly through unimpeded, but the
passage of certain particular rays will be opposed, if not
altogether prohibited, by the action of the hydrogen.
Take, for instance, the ray whose wave-length is 4860*72,
that particular ray will be more or less stopped ; if its wave
length had been a few units greater, or were a few units less,
it would have been allowed to pass, but being what j't is, its
passport is refused. The same may be said of certain
other wave lengths, The wave length of 6562*1 units,
also will be stopped ; while most rays more refrangible or
most rays less refrangible are allowed to make their out-
ward journey without hindrance. Here is indeed a
remarkable property possessed by a so-called transparent
gas ; it picks out here and there certain wave lengths, and
refuses all passage to rays which radiate in these for-
bidden periods, while rays of all other wave lengths pursue
their way uninterfered with. At first it might seem that
the rays which were stopped were selected apparently by
<japrice. Hydrogen, for instance, of which we are now
particularly speaking, stops certain rays in all parts of the
spectrum. But now comes the important discovery to
which we have referred — the discovery which in fact
renders spectrum analysis so important to the astronomer.

286 IN STARR* REALMS.

We have already seen that when hydrogen gas is heated,
it radiates forth light which the spectroscope resolves into
a certain number of bright lines in various parts of the spec-
trum, each with one particular wave length. We have also
seen that when a cold atmosphere of hydrogen surrounds
a star, the obstruction to the passage of the light is not
indiscriminate, but only operates on rays of certain par-
ticular wave lengths.

And now for the extraordinary fact which has provided
us with a key to the interpretation of spectroscopic
phenomena. It has been discovered that the particular
wave lengths which cold hydrogen is capable of stopping
are precisely those wave lengths which characterize the
light that heated hydrogen is capable of emitting. This
is a very singular fact, and to understand it fully we must
go a little further into the mechanism of the subject.
For this problem, like many another problem in nature,
resolves itself as a matter of last analysis into a question
of dynamics.

Hydrogen is an invisible gas, the lightest body in
nature, and we know that a gas consists of a number
of molecules darting about in all directions ; these mole-
cules are so small and so close together, that many
billions of them would fit into a lady's thimble. When
the gas is heated the activities of the molecules are in-
creased, each molecule in fact consists of a number of
portions which are themselves moving with almost in-
conceivable rapidity, and with a complexity which we can
only very partially understand. Fixing our attention for
the moment on one particular wave length, corresponding
to the ray F emitted by hydrogen, let us suppose that

THE MOVEMENTS OF THE STARS. 287

the hydrogen is heated to incandescence. We are to sup-
pose that one of the modes of vibration in the molecule of
the gas is so timed, that the oscillations it imparts to the
ether are those competent to produce light of the character
indicated by F. Let us now suppose that a light is radiated
through a mass of cold hydrogen. The majority of the wave
lengths of the light are such that no mode of vibration in
the molecule corresponds to them, and consequently they
are allowed to pass. Those particular rays, however,
which vibrate with the period belonging to F, find the
molecules in tune with them, so to speak ; the molecules
accordingly commence to vibrate, and thus absorb the
particular energy belonging to that particular wave length.

Let me illustrate the matter from other branches of
science. There are many analogous phenomena in the
theory of sound ; open a piano for instance, each string
corresponds to a particular note. Sound a note from some
other instrument, such as the human voice close by, and
the piano resounds, but this resounding is not an indis-
criminate return of the sound like an echo, for each note in
the piano remains silent unless it has been appealed to by
a precisely similar note from the other instrument. Thus
the vibrations caused by the other instrument pass with-
out effect over those notes with which they are not in
tune, while the energy that they possess is absorbed by
the strings which resound to them.

We may pursue this illustration a little farther. Let
us suppose a multitude of piano wires stretched from
floor to ceiling of an apartment, all tuned to one par-
ticular note; now of course if all these strings were
struck, and if suitable sounding boards could be pro-

288 IN STARRY REALMS.

vided, then a vast volume of sound in this particular note
would be the result. In other words, this apparatus
would radiate, if I may use the expression, a note of one
particular wave length. But suppose that the sounding
boards were removed, and that the forest of strings was
placed between an auditor and an orchestra, all the music
from the orchestra would have to come through the strings
before the auditor could hear it ; the great majority of
notes would pass through and between the strings, no
doubt with some loss, but without any special interference.
All those notes, however, from whatever instrument in the
orchestra they might have come, which harmonized with a
particular note to which the strings resounded, would be
specially affected in their transit ; as the music breathed
past the wires they would commence to vibrate, because
certain notes were tuned with the vibration that the
strings were able to perform. The energy of the per-
formers in the orchestra, so far as it was expended on the
production of music of this particular note, would be em-
ployed in setting the forest of strings into a quiver, and
would thus lose its power to affect the ears of the auditors
at the end of the room.

In this case the forest of strings would have really
acted as a sort of filter applied to the music diffused
from the orchestra; they would have stopped the pas-
sage of one particular note, while they would have per-
mitted notes of every other description to pass almost
unmolested. The forest of strings is in fact opaque to one
note, if I may use the expression, and transparent to every
other note. But now observe that when the sounding
boards are restored to the forest of strings, and when the

THE MOVEMENTS OF THE STARS. 289

orchestra is absent and the strings are set in vibration by
fiddle bows or otherwise, they shed forth a volume of
sound all of that one particular note, which alone they are
competent to produce. In other words, we see that when
the strings are used as a radiating source of music, the
character of the music they emit is absolutely identical
with those particular notes which they refuse to allow to
pass when they are interposed between the orchestra and
auditorium. In other words, the particular notes which
are absorbed in the one case are identically the notes which
are radiated in the other.

This illustration from sound will facilitate our ex-
planation of the dark lines in the solar spectrum. Hy-
drogen in the sun's atmosphere, being cold compared
with the deeper seated portions of the great luminary,
stops certain light on its way outwards from the heated
regions, and the light which the hydrogen stops is exactly
the same which that gas will itself give out if heated
to incandescence. We might make the illustration a
little more complete by supposing that the forest of strings
comprised not merely a single note, but that two, three,
or even more notes each have many strings belong-
ing to it. In that case the filtration from the music
produced by the orchestra would remove from it every
note of music to which there was a group of strings
oorresponding ; the auditors might therefore find not only
one note, but perhaps two, three, or even more wanting in
different parts. Now we come very close indeed to our
explanation of a system of dark lines corresponding to any
particular element in the solar spectrum. The molecules
of hydrogen, for instance, respond not merely to the

290 IN STARRY REALMS.

particular wave length of light corresponding to the
line F, but they also respond to the line C, and to several
other lines as well. As the solar light, therefore, streams
through the mighty encompassing atmosphere of the sun,
in which hydrogen is a very important ingredient, it has
filtered from it those rays which are of the same kind, as
incandescent hydrogen is itself competent to produce.
Accordingly, the dark lines belonging to hydrogen in the
solar spectrum occupy the same positions as do the series
of bright lines, which are generated when hydrogen is
rendered incandescent in a vacuum tube.

I am aware that the illustrations I have given only
render a very imperfect account of a great scientific doc-
trine, but they may suffice, at all events, for my present ob-
ject, which is to show the use of the dark lines when we are
studying the movements of the body from which the light
radiates. Let us take a star, surrounded with hydrogen,
and therefore showing in its spectrum the F lines as well
as the other lines characteristic of this gas. Let us further
suppose that the star is hurrying towards us, then, as we
have already seen, the effect of this movement is to impart to
the vibrations of each ray from the star, as interpreted by
the eye, a somewhat greater rapidity than they actually
possess. The F line of hydrogen coming from the star will
pour more vibrations into the eye in a second than would
have been the case if the star had no relative movement.

By the aid of the electric spark we are able to produce an
artificial spectrum of hydrogen, and by suitable appliances
we can introduce into the spectroscope the actual glowing
spectrum of hydrogen side by side with the dark line
spectrum of the same gas as it has come to us from the

THE MOVEMENTS OF THE STARS. 291

star. Here comes in the important principle on which
I have laid so much stress, that the dark line spec-
trum produced by the cold hydrogen would be identi-
cally situated line for line with the bright line spectrum
generated in the incandescent gas. We arrange, in fact,
that two spectra shall be produced by the same system of
prisms, so as to give every facility for the comparison. If
the star were at rest relatively to the earth, each delicately
bright line should be strictly continuous with each delicate
dark line. If, however, the star be in relative movement
to or from the earth, then the system of lines which
it radiates will exhibit a shift relative to the system of
lines produced by the incandescent hydrogen which is at
rest with respect to the apparatus. Here, then, we have
the means of measuring the velocity of the star along the
line of sight. Take any one of the lines, the F line, for
instance, its displacement relatively to the adjacent bright
line can be measured. It can be measured with accuracy
too, when the apparatus has been constructed with a
delicacy that optical skill has now rendered possible.

We are gradually learning the full significance of the
word light. We are now aware that the undulations which
we see are only a couple of octaves, so to speak, out of an
immense volume of undulations, which we have no senses
for perceiving. We are thus at last attaining some concep-
tion of the majesty of these words, " Let there be light."

CHAPTEK XXI.

PHOTOGRAPHING THE STARS.

WE have quite recently experienced one of the greatest
revolutions which the art of practical astronomy has ever
undergone. Professor Young, in an admirable article on
the subject, which has appeared in the Princeton Review,
has indeed regarded the impending metamorphosis as
parallel in importance to that which followed from the
invention of the telescope. Perhaps we should hardly
speak of the new departure as impending: we might
rather say that it nas already been in some degree realised.
We may fairly derive an illustration from the somewhat
similar change that our methods of illumination seem
likely to undergo. It will be generally admitted that the
present state of electric lighting is still in the initial and
tentative stages ; yet the overwhelming advantages of
electricity for many purposes is no longer disputed. Some-
what similar to the invasion of electricity on old-fashioned
sources of light is the invasion of photography into the
time-honoured methods of conducting astronomical ob-
servations. We cannot indeed assert that the application
of the photographic camera to the telescope is exactly

PHOTOGRAPHING THE STARS. 293

novel. Nor can we for that matter deny that the electric
light was invented half a century ago. But just as a few
brilliant inventions have transformed electrical lighting
from a scientific curiosity into an eminently practical
reality, so the recent improvements in photography have
rendered that art an indispensable auxiliary in the ob-
servatory of the future.

The applications of photography to astronomy are of
the most widely diverse kind. We may employ it, in
the first place, as an auxiliary in the production of accu-
rate pictorial representations of particular objects in the
universe, or in obtaining views of groups of such bodies ;
we may also employ it to aid the process of exact mea-
surement. There are still other and more delicate
branches of practical astronomy where the photograph
is not merely a rapid or a convenient means of doing
what could otherwise not be done so conveniently. Photo-
graphy is a process for actually observing phenomena in
such cases as entirely elude ordinary vision, and are only
perceptible by the peculiar sensibility of the salts of silver
contained in the film.

We shall first say a few words with regard to the suita-
bility of the new method for the purpose of recording
the appearances of the different celestial bodies. In all
the applications of this process to the heavens we must
bear in mind how widely different are the conditions
under which a celestial photograph is to be procured from
those which are met with in the more familiar pursuit of
the art. In taking a portrait with the camera, there is of
course only a few feet between the plate and the setter.
In the application of photography to the representation

294 IN STARRY REALMS.

of landscape the distance between the objects and the
camera is greatly increased ; but even here the length
of air which the rays have to traverse is generally
much less than in the case where we attempt the portrait
of a heavenly body. The atmosphere extends above our
heads to an altitude which is still very uncertain. We
learn, however, from the phenomena of shooting stars
that the summit of the air is at least a couple of hundred
miles aloft, and perhaps much more. The upper regions
are so highly rarefied that they are incapable of ex-
ercising much deleterious influence on the rays of light ;
it is in the lower and the denser portions that the atmo-
sphere is chiefly inimical to the photographer. Now,
though to the portrait- taker the atmosphere signifies but
little, except in so far as questions of light are involved,
yet it is well known that the state of the atmosphere is
very significant in landscape- photography ; while in the
case of the celestial photographer the behaviour of the
atmosphere is of paramount importance. Even if the
object be immediately over his head, the rays would have
to make their way through two hundred miles of air
before they entered his apparatus ; while if the body lay
far away from his zenith, as of course it usually does, the
air-journey of the rays of light would be considerably
longer. Any imperfections which the atmosphere is
capable of producing must therefore be felt much more
keenly by the celestial photographer than by the brothers
in the craft who confine their attention to mere terrestrial
objects. The qualities which characterize a suitable sky
are steadiness, though wind is not necessarily objectionable,
and photographic transparency, which is a very different

PHOTOGRAPHING THE STARS. 295

property from visual transparency. By steadiness is
meant such a regularity in the variations of density that
each ray of light is persistently refracted along the same
course throughout the duration of the exposure. By
transparency the celestial photographer will mean a state
of the air which will permit the particular rays of light
which he wants to pass through. It will often happen
that two nights which to the unaided eye, or even in the
ordinary telescope, seem equally clear, may be of widely
different clearness in so far as the photographic light is
concerned.

To illustrate the opacity of the atmosphere to photo-
graphic rays I may mention a fact told me by the Rev.
H. Swanzy, who accompanied the Rev. W. Green on his
recent exploration of the Selkirk range in British Colum-
bia. The plates they used required an exposure of three
seconds or more in the valleys, while similar plates
exposed at a height of ten thousand feet were found to
be destroyed if the exposure was more than a small
fraction of a second.

In the application of photography to celestial por-
traiture, we naturally first allude to the photographs of
the sun, by far the most exquisite of which are those taken
by Janssen at Meudon. His photographs, obtained by
an extremely short exposure of a fraction of a second,
display in a marvellous manner the actual texture of the
sun under the conditions of its surface at the moment.
They prove that the luminous parts are brilliant granules
or cloudlets, floating, so to speak, in a less bright medium
which is visible in the interstices between the cloudlets.
Occasionally the openings between the small luminous

206 IN STARRY REALMS.

portions are large enough to form dark spots. The
photographic examination of the sun certainly bears out
the view that the luminous surface is far from being
continuous, even setting aside the presence of large spots.
I must, however, say, that in none of the photographs
that I have seen are the cloudlets at all of the willow-leaf
or the rice-grain structure : they do not seem characte-
rized by any specially elongated shape, except round the
sun spots.

Photography has also been applied with success to the
representation of the phenomena seen during the occur-
rence of a total eclipse of the sun. The total eclipses of
recent years have been most assiduously observed by
parties of astronomers who have visited whatever parts
'of the globe offered exceptional opportunities for the
purpose. Special attention has been directed to the
corona, and large photographic apparatus has been con-
structed with the object of obtaining coronal pictures.
The result has been to produce most beautiful pictures.

Numerous photographs of the moon in very various
phases have been taken. Among the earliest of them
may be mentioned that of E/utherford, on March 6th,
1865, which is excellent, though of course now surpassed.
Admirable pictures of the moon have been obtained at
the Lick Observatory, in California, one of which is
represented in the frontispiece.

Though the lunar photographs are interesting, and
make beautiful transparencies to show on the screen, yet
it will, I think, be admitted that, so far as the represen-
tation of lunar details is concerned, they are disappointing.
Even the best of them will not bear much magnifying

PHOTOGRAPHING THE STARS. 297

without becoming blurred and indistinct. The view that
a photograph presents of any lunar mountain or crater,
cannot be compared either in beauty or in sharpness
with the picture that a telescope of adequate power will
give the eye. In fact we may certainly say that no
material addition to our knowledge of lunar topography
has been contributed by photography. We may, how-
ever, hope for better things ; for, with the extremely
sensitive plates now procurable, a picture of the moon
obtained under favourable atmospheric conditions with
an extremely short exposure might prove much more
capable of being magnified than any of the photographs
that have heretofore been taken.

In the delineation of the planets, photography has
comparatively been little applied, though the attempts
which have been made are full of interest. The pictures
of Jupiter by Henry at Paris, as well as those from the
Lick Observatory, are excellent. The bands and other
markings on the planet come out distinctly, and the
renowned red spot is a very conspicuous object. In a few
photographs, taken at intervals of half an hour, the gradual
shifting of the features shows in an interesting manner
the rotation of the planet on its axis. This very rota-
tion is, however, one of the difficulties which impede
successful photography of the planet. In the course of
a long exposure the gradual displacement of the features
by the rotation precludes the possibility of a sharp and
well-defined picture. Here, again, very brief exposures
and highly sensitive plates become the desideratum of
the astronomer.

T cannot but think that photography will have a

298 IN STARRY REALMS.

considerable share in our further study of this the
most gigantic of our planets. The marks on Jupiter
are so incessantly varying that the photograph seems
obviously the true method for recording its ever-fleeting
details. It will be noticed that the circumstances
are here quite different from those which attend the ap-
plication of photography to the moon. In the latter the
features are permanent, and the efforts of the eye and of
artistic sketching can be persistently accumulated, with
the result of giving us a delineation of the lunar surface
as faithful as the powers of our telescope will permit.
But there is no permanency in Jupiter, and our only
means of becoming acquainted with the marvellous mete-
orology of that planet must be derived from the bringing
together of as many accurate pictures of its disc as can
be obtained in its ever-varying moods. For this object
photography seems most admirably adapted. There is,
however, a point which should be mentioned, and it has
been brought before us very strongly while examining
the beautiful Jovian photographs taken by the Henrys. It
is that the photographic Jupiter and the visual Jupiter
are different pictures. This is no doubt largely due to the
atmosphere of the planet, which exercises a different
degree of absorption on the photograph rays from that
to which the visual rays are exposed. Here again the
difference between the problem of photographing the air-
less moon and photographing a planet becomes significant.
In the case of the moon the visual picture and the
photographic picture tend to coincidence in proportion as
they both approach perfection.

Pleasing pictures have also been taken of Saturn, es-

PHOTOGRAPHING THE STARS. 299

pecially by the Messrs. Henry. Not only does the broad
division of the ring, usually known as Cassini's line,
appear very distinctly, but many of the more delicate
features are also perceptible. But the point which has
struck me very forcibly about this picture of Saturn is
the remarkable amount of shading which it gives to the
Saturnian globe. As is well known to every practical
astronomer, this globe usually possesses no very striking
varieties of shade or of colouration in the telescope, and
the extraordinary darkness about the poles of Saturn in
the photograph will arrest the curiosity of every one
who is familiar with the ordinary telescopic spectacle.
The cause of this phenomenon appears to lie not in the
actual colouration of the planet's globe, but in the atmo-
spheric shell within which it is contained. It would seem
that the Saturnian atmosphere, whatever be its character
in other respects, must at all events possess the power
of largely absorbing the photographic rays of light.

At the present time the question of the application of
photography to the stellar regions is especially engrossing
attention, and for this purpose it would seem that the
new process is destined to effect a revolution in the art
of astronomical observation. "We must therefore con-
sider the question of sidereal photography in some detail.

When a telescope is directed towards a star, it brings
all the rays of that star to a focus ; and the more excel-
lent the construction of the optical part of the telescope,
the more accurately will the image of the star approximate
to that of a mathematical point. In the ordinary use
of a telescope for visual purposes, all the rays of light
collected by the aperture of the telescope are condensed

300 IN STARRY REALMS.

to a point on the retina, and if the image there produced
be sufficiently intense the sense of vision is excited, and
the star is seen. If, however, the star be not perceived
at the first glance, there is but little object in prolonging
the gaze. It is true that expert practical astronomers
know that a star which they fail to see when directly
looking at it can sometimes be " glimpsed " when the eye
is moved slightly away ; the explanation apparently being
that some fresher and more sensitive part of the retina is
by this act brought into use. But by merely steadily
staring at a faint star which is not bright enough to be
detected at the first glance, there is little success to be
expected. The fact is that the retina can only retain
an impression for a small time — perhaps about one-seventh
of a second — consequently there is no cumulative effect
of the luminous impression to be obtained by prolonged
watching.

But the case is very different when we place in the
focus of the telescope a highly sensitive photographic
plate, and permit the instrument to depict thereon an
image of the star. The vibrations of the rays of light throw
themselves assiduously on the plate, and steadily apply
themselves to the task of shaking asunder the molecules of
silver salts in the gelatine film. Just as the waves of
ocean by incessantly beating against a shore will gradually
wear away the mightiest cliffs of the toughest rock, so the
innumerable millions of waves of light persistently im-
pinging upon a single point of the plate will at length
effect the necessary decomposition, and so engrave the
image of the star. It will be obvious that this process
will be the more complete the longer the exposure which

PHOTOGRAPHING THE STARS. 301

is permitted, and thus we see one of the reasons why
photography forms such an admirable method for depicting
the stars. We can give exposures of many minutes, or of
one, two, three, or even four hours ; and all the time the
effect is being gradually accumulated. Hence it is that
a star which is altogether too feeble to produce an im-
pression upon the most acute eye fortified by a telescope
of the utmost power may yet be competent, when a
sufficient exposure has been allowed, to leave its record on
the plate. Thus it is that photographs of the heavens
disclose to us the existence of myriads of stars which
could never have been detected except for this cumulative
method of observation which photography is competent
to give.

There is another peculiarity about the photographic
methods of observation which gives them an importance
from quite a distinct point of view. The radiation from
a star consists of a number of rays of very varied hues
all blended together. If they were separated out, we
should find that they were divisible into two great groups
— namely, the visible and the invisible. As to the former,
they characterize the well-known hues of the rainbow :
the red, the orange, and the yellow, the green, blue, indigo,
and violet. It is to these rays in varying degrees of
combination that we are indebted for visibility in the star,
either to our unaided eye, or even to the eye aided by a
telescope. But it is conceivable that a star might dispense
a rich stream of rays, and yet be totally invisible from
the fact that none of those rays belonged to the special
group which can alone excite vision. These invisible rays
may be of different types. Some of them might be rays

302 IN STARRY REALMS.

of heat, for the greater part of the rays of Tieat are of
the invisible type ; though no doubt some of them are also
visible, as the red portions of the spectrum. I must also
add that within the last few months wondrous possibilities
nave been opened up as to the discovery of innumerable
other rays of much greater length, which do not directly
appeal to any senses that we have been provided with.
But with such extraordinary rays as those which can pass
through a stone wall, and be refracted by a prism of
pitch, we have not at this moment to do ; though they
are of the most intense interest, and possibly will admit
of remarkable astronomical applications. The rays with
which photography is concerned are mainly or largely
of the invisible type, but they are rays of high refran-
gibility : they lie out beyond the violet.

Thus it happens that the rays from the star which are
competent to excite an impression on the plate are partly
in the visual portion, but chiefly in the invisible part
of the total radiation. Now we can see another reason
why the photograph may, and indeed must, largely extend
our conceptions of the extent of the universe. It will
grasp and depict light which would be utterly wasted so
far as vision is concerned, for even were these rays poured
in torrents into our eyes they could excite no sense of
vision ; and consequently all stars whose radiation did
not contain a sufficient admixture of visual rays, no matter
how copiously they diffused these ultra-violet rays, would,
to the eye, be invisible in the most powerful telescope,
though capable of being recorded by a photograph. It
will thus be manifest that the grounds which the new
method furnishes of increased powers to the astronomer

PHOTOGRAPHING THE STARS. 303

are twofold. There is first the advantage of prolonged
exposure ; there is secondly the possibility of utilising
invisible rays.

The late Dr. Koberts, whose experience and marvellous
success in celestial photography entitle him to speak with
confidence on the matter, gives us striking evidence of
the detection of faint stars by the action of photography.
With an exposure of an hour he has shown on a plate of
about four square degrees a number of stars that he
estimates at more than sixteen thousand, of which the
brightest is less than the fifth magnitude. The circum-
stances appear to have been very favourable, for other
photographs have been obtained of the same region and
with exposures of equal duration. To all appearances the
nights on the three occasions were equally clear ; but
clearness for visual purposes and clearness for photo-
graphic purposes involve different conditions ; and this is
remarkably illustrated by the three photographs referred
to. One of them, by Messrs. Henry, showed three thou-
sand stars ; the next, by Mr. Roberts, showed five thou-
sand stars ; while the third, by the same operator and on
the same region of the sky, disclosed more than three
times the number.

It is of interest to attempt to estimate the total number
of stars visible to the photographic eye over the entire
surface of the heavens, assuming that the plate we have
just referred to may be taken as an average specimen of
the stellar richness of the entire firmament. The number
of square degrees in the heavens is about forty-one thou-
sand four hundred, and as the plate occupies four square
degrees, it will follow that upwards of ten thousand plates

304 IN STARRY REALMS.

of this size would be required to cover completely the
whole vault above the horizon and below. If, then, there
be over sixteen thousand stars on one of these plates, it
follows that the total number over the sky capable of
being disclosed by photography cannot be less than one
hundred and sixty millions. It will be instructive to
compare these figures with the stellar statistics afforded
by other methods.

If we take a position on the equator, from whence, oi
course, all the heavens can be completely seen in the lapse
of six months, the number of stars that can be reckoned
with the unaided eye will, according to Houzeau, amount
to about six thousand. If we augment our unaided vision by
a telescope of even small dimensions, such as three inches
in diameter, the number of stars in the northern hemi-
sphere alone is upwards of three hundred thousand. We
may assume that the southern hemisphere has an equally
numerous star- population, so that the entire multitude
visible with this optical aid is about six hundred thousand.
Thus we see that the use of a telescope small enough to
be carried in the hands suffices to multiply the lucid stars
one hundredfold. Great telescopes no doubt soon show
us that the hundreds of thousands are only the brighter
members of a host of millions, and now we receive the
assurance of photography that the telescopic stars are
only the more conspicuous members of that vast universe.
Mr. Roberts indeed declares that the multitudes of stars
on the photographic plate grow with each increase of
exposure to such a degree that it would almost seem as if
the plate would be a well-nigh continuous mass of stars if
the operations could be sufficiently protracted.

PHOTOGRAPHING THE STARS. 305

The long exposures necessary for celestial photography
have introduced a new class of requirements into the con-
struction of astronomical instruments. The questions here
involved are of much practical importance, and are ex-
citing a good deal of discussion at present.

There are, as is well known, two different classes of
astronomical instruments — namely, the reflectors and the
refractors ; and it is still a matter of debate as to which
class of instrument is the more suitable for the purposes
cf celestial photography.

In the reflector the rays from the star fall on the bril-
liant surface of a mirror carefully wrought into a special
form. Formerly mirrors were made of speculum metal,
consisting of two parts of copper to one of tin. This
material was difficult to cast and tedious to shape. Its
great weight was also a drawback, while the reflecting
power, though very considerable, was still short of that
possessed by silver. At present most of the reflectors are
made of glass, and, after being accurately ground and
polished to the true form, are chemically coated with silver.

The mirror, when used for celestial photography, is at
the lower end of a tube, and the rays falling upon it from
the star travel again up the tube to a focus on the plate,
which is exposed with its face towards the mirror at the
upper end. The plate is supported by slight arms from
the side of the tube, and it offers of course an impene-
trable obstacle to some of the rays from the star, and so
far diminishes the effective size of the mirror. As how-
ever the diameter of the plate will not be more than
perhaps one-fifth that of the mirror, it follows that only
about four per cent of light is lost by this cause. The

X

306 IN STARRY REALMS.

chief recommendation of the reflecting telescope is found
in the circumstance that the rays of light of every de-
scription are all brought to the same focus. Thus if the
plate be placed at the correct point for visual purposes, it
is also correctly placed for the photographic rays. There
is here no troublesome question as to the difficulty of
securing a confluence of all the rays at a single point
where their united action shall be devoted to engraving a
mark on the plate. On the other hand, it has been
customary to believe that the support of the mirror, and
the precautions necessary to prevent the distortion of its
figure by flexure, went far to neutralize the advantage of
the useful indiscriminateness with which all rays were
conducted to the same focus. The remarkable achieve-
ments of Mr. Roberts and of Mr. Common have, however,
been accomplished by reflectors, in a way which proves
that the difficulties attendant on this form can be sur-
mounted.

For most of the great photographic enterprises which
are now proposed to be undertaken refractors are being
erected, and here a difficulty of a peculiar kind is en-
countered. A glass lens of accurate figure, when it
receives a parallel beam of any homogeneous light, will
direct all the rays of the beam to concentration at a focal
point. To this extent the action of the lens and the
action of the reflector are identical. By homogeneous
light we mean light which we may with sufficient
accuracy describe as being one of the prismatic colours.
Thus a beam of pure red falling in parallel rays on the
lens are all brought to the same focus. So also are the
rays of a blue beam ; but the point to which the blue rays

PHOTOGRAPHING THE STARS. 307

are brought by a single lens is different from that in which
a beam of red would be concentrated. The blue focus is
nearer to the lens than the red focus. There is here a
radical difference between the action of the lens upon
light and the action of the mirror. In the latter case,
every hue, of whatever colour, if in the visible part, or
indeed whether the rays belong to the visible portion of
the spectrum or not, is brought to coincidence at the
same point. The glass lens, however, has a different
focus for every different quality of light which can fall
upon it. Hence, when a beam from the sun or from a
star, or indeed from almost any celestial source, falls upon
a lens of glass, the composite nature of the light gives
rise to the difficulty that the reds, the yellows, and the
blues are all brought to different foci. It is therefore
impossible for this reason to obtain a distinct and
definite image of any celestial object with a single
glass lens; for if the lens be focussed truly for some
of the rays it is necessarily out of focus for others.
This difficulty is well known, and was for a long time
regarded as presenting an insuperable difficulty in the
way of constructing efficient refracting telescopes with
glass lenses. Indeed, it was the perception of this diffi-
culty that led Newton to turn his attention to the con-
struction of reflectors, the best known form of which still
bears his name. By the admirable discovery that what a
single lens could not do a pair of glasses could certainly
accomplish, the refracting telescope was made the valuable
instrument we now find. The achromatic objective is
formed of a lens of crown glass and a lens of flint glass.
A beam of composite light, on passing through a powerful

308 IN STARRY REALMS.

convex lens of crown, tends towards different foci. But
in contact with, or very close to, the crown lens is a con-
cave lens of flint glass, which proceeds to undo the bend-
ing which the beam has received from the crown. It is
possible by a combination of two lenses to produce a single
objective which shall bring the foci for any two desired
hues into coincidence. If, for example, we arrange
the proportions of the lenses appropriately, the red rays
and the blue rays will be conducted to a common focus,
and all the other visual rays of the intermediate hues will
be brought to foci so close to the main focus that the
telescope will be practically perfect for optical purposes.
Such is the modern achromatic object-glass.

When an objective is to be employed for photography a
new class of considerations arises. The rays most specially
potent in their action on the salts of silver are not visual
rays. The focus to which they would be brought by a
single lens is much nearer the glass than is the focus of
the extreme violet. In the ordinary adjustment of the
achromatic objective for visual purposes the photographic
rays are, as the optician says, allowed to go wild, for there
would be no object in leading them to the common focus,
and the attempt to do so would seriously impair the visual
performance of the telescope. Hence we see the important
fact that an achromatic telescope, however perfect for the
ordinary purposes of astronomy, would be unsuited for the
photographer, If a plate be placed at the ordinary
optical focus of such an instrument, the visible rays from
a star are no doubt brought to a point on that plate, but
the photographic rays, not having the same focus, will be
spread over a little circle instead of a point, and the

PHOTOGRAPHING THE STARS. yx>

resulting photograph will be entirely wanting in delicacy
Xor will a mere alteration of the place of the plate suffice
to give precision to the image, for there are so many dif-
ferent shades of photographic light that an ordinary ob-
jective when focussed for one kind of invisible light will
be out of focus for another.

For photographic purposes we must therefore entirely
reject the familiar objective of the observatory, and con-
struct a different one. All the reds and yellows may with
safety be permitted to run wild, inasmuch as their photo-
graphic capacities are insensible. But the true chemical
rays, beginning in the blues and the violets and extending
far off into the invisible portion of the spectrum, must be
carefully gathered into one point. A pair of flint and
crown lenses must thus be so wrought that the two ends
of the chemical parts of the spectrum shall be practically
brought to a common focus, in which, of course, the
photographic plate is to be placed.

We thus obtain an objective which is utterly unsuited
for visual purposes, but which will give an exquisitely de-
fined photographic image of a star. But now comes one
of the practical difficulties of the optician. In forming
the visual objective it is easy for him to test the successive
approaches to the perfect form of the lenses, but how is he
to test the performance of the photographic objective?
Here the eye cannot directly appreciate the degree of
success which has been obtained.

At the request of Signer Anguiano, the present writer
has tested the large photographic objective constructed by
Sir Howard Grrubb for the Observatory of Tacubaya be-
longing to the Mexican Government. A description of

3io IN STARRY REALMS.

the test employed will show the peculiarities of a photo-
graphic objective. The instrument was directed on an
artificial star a couple of hundred feet distant. The star
was merely a reflecting bead, illuminated by a spectrum
obtained by passing a beam from a small incandescent
electric light through a prism ; any part of the spectrum
could be cast upon the bead, and thus stars of varied hues
could be observed in the telescope. Were the objective
designed for visual purposes, the focus of a star near the
extreme red should coincide with the focus of a blue star,
while the foci of all the other stars would be in the im-
mediate vicinity. For the photographic telescope, how-
ever, the essential point is that all the bluish stars shall
be brought practically to the same focus, and this being
so for the visible stars, the invisible foci of photographic
light will be all sufficiently concentrated. It should, how-
ever, be understood that the only final test of an objective
consists in the nature of the photographs it produces.

Supposing that the photographic telescope, either
reflector or refractor, has been prepared, the practical
conduct of the work demands a few words of explanation.
It is of course essential that the telescope be presented to
the same part of the sky throughout the entire duration
of the exposure. This condition is complied with by a
simultaneous observation of the heavens through a visual
telescope rigidly attached to the photographic tube with
the axes of the two instruments parallel. The clock
motion of the equatorial must be of the highest order of
excellence, but notwithstanding the exquisite refinement
obtained by the electrical control of the driving clock,
it is impossible to dispense with simultaneous watching

PHOTOGRAPHING THE STARS. 311

through the guiding telescope. A star is chosen, and
this star is brought on the intersection of a pair of spider
webs in the guiding telescope. During the entire expo-
sure this star must remain in the same position, and this
the attending astronomer will secure by gently correcting
the speed of the driving clock. When this fiducial star
has been kept carefully on one point throughout the
exposure, then, assuming that other obvious conditions
are fulfilled, each star will have been constantly brought
to a focus on the same point of the photographic plate.
The condition is a somewhat trying one, when we remem-
ber that the image of the star is an extremely small point,
and that the duration of the exposure is in some cases as
long as four hours, or even more.

A combined effort has been made to secure a repre-
sentation of the entire surface of the heavens by photo-
graphy. A congress met in Paris, under the presidency
of Admiral Mouchez, consisting of astronomers from all
parts of the world, and the conditions under which this
stupendous survey of the universe was to be undertaken
were then decided on. The operations are divided among
a number of observatories situated over the world, and
each of them undertakes to photograph on plates of a
uniform size a certain region of the heavens. The work
has been entered upon with the heartiest enthusiasm, and
ere many years have elapsed we may anticipate being in
possession of what will practically be a photograph of the
entire heavens. This great piece of work will provide us
with the means of making a reasonably complete inven-
tory of the entire contents of that small portion of the
universe which lies within the reach of our instruments.

312 IN STARRY REALMS.

That all the stars which can be exhibited on long exposed
plates shall ever be completely catalogued is a task as much
beyond our power to complete as it would be to obtain
a descriptive list of the several pebbles on a sea beach or
of the several leaves in an ample forest. The more modest
scheme has, however, been suggested of taking the two
million brightest stars and forming a complete catalogue
of them, in which their brightness and their absolute
positions in the heavens shall be given with all attainable
precision. Even this is a sufficiently magnificent under-
taking, but it is within the practical limits of scientific
enterprise, and it ought to be done — it must be done.
Not alone is it our manifest duty to obtain a comprehen-
sive survey of that universe around us, but there are many
other special astronomical problems that will be largely
forwarded by its accomplishment. There are some pro-
blems indeed which must remain unsolved so long as this
task remains unfulfilled. To mention only a single one of
the questions for which the great survey is imperatively
demanded, I may refer to the interstellar motion of our
solar system. It is well known that our sun, accompanied
by the whole system of planets, is at present bound on a
voyage through space. Astronomy presents no grander
problem than the discovery of the circumstances of this
voyage. Whence has our system come, whither is it
bound, and with what speed ? We can never learn such
particulars as these without the information that the great
survey would be capable of giving us. It is impossible to
allude to the present favourable aspect of this great under-
taking without mentioning the name of His Majesty's
Astronomer at the Cape of Good Hope, Sir David

PHOTOGRAPHING THE STARS. 315

Gill, K.C.B., to whose zeal in the pursuit of his science we
are so much indebted for the initiative of the great survey.
Dr. Koberts propounded and proved to astronomers
the practicability of making and engraving a chart of the
heavens on which many more stars shall be depicted. He
has devised a very ingenious and accurate instrument, by
which a copy of the stars on the photographic plate can b^
faithfully engraved on copper. I have had the privilege
of seeing and using this apparatus, and hardly know
whether to admire most the accuracy of the measurements
that can be made by it, or the celerity with which the
copper-plate facsimile of the heavens can be obtained. The
measures of the distances between stars that can be made
with this instrument, either on the photographic plate
itself, or on the copper engraved plate, or almost on the
impressions taken from that plate on paper, may favourably
compare with the most exact and laborious measurements
that can be obtained with the heliometer or the micrometer
on the actual stars in the heavens. The taking of the photo-
graphs being a comparatively simple matter, since an hour
with a single telescope will book many thousands of stars,
the practicability of the completion of the entire chart of
the sky depends on the rapidity with which the plates can
be transferred to the copper. Dr. Koberts found that he
could easily engrave fifty stars in an hour; so that if
twenty engraving instruments were steadily employed for
ten years of reasonable working hours a magnificent
celestial chart could be completely engraved, consisting of
twenty-three millions of stars. This superb undertaking is
quite feasible, and every one interested in astronomy will
recognise its utility. Here is a splendid opportunity for

1 14 IN STARRY REALMS.

some wealthy Englishman to accomplish a work which
could be worthily mentioned beside the magnificent
Draper Memorial now being reared by Professor Pickering
in America.

Hitherto I have spoken of photography merely as an
appliance for the simple purpose of charting or of mapping
the stars. It remains to mention some of the numerous
other applications of which it is susceptible. One of the
most delicate problems of celestial measurement is the
determination of the distance of a fixed star. This is
derived from a series of measures made at varying seasons
of the year between the star under examination and some
more distant star which happens to lie nearly in the same
direction of vision. If, therefore, a series of photographs
at different seasons be obtained, the measurements made
on these photographs will disclose the star's distance, if it
be sufficiently near to admit of the application of the
process. The late Dr. Pritchard, who had been a diligent
cultivator of the photographic methods, had already made
several successful attempts of this description by measures
on photographs which he had obtained in the University
Observatory at Oxford.

The applications of photography to the stars which
I have already mentioned are mainly improvements on
methods formerly used, except indeed in so far as they dis-
close to us stars which are not visibly perceptible, but we
have now to speak of the manner in which photography
has laid open to us discoveries of the most remarkable
character in a province peculiarly its own. I can only
mention the two most remarkable instances.

The great nebula in Andromeda is a familiar telescopic

PHOTOGRAPHING THE STARS. 315

•object. It is indeed a unique spectacle in many respects,
one of which is that it alone of all the thousands of nebulae
is visible to the unaided eye. Many drawings of the
nebula in Andromeda have been made, and since the era
of powerful telescopes it was perceived that the spindle-
shaped nebulosity was marked by two remarkable dark
" lanes," parallel, or nearly so, to the length of the spindle.
These lanes are well shown in the later drawings of the
nebula, but they seemed devoid of significance till quite
lately.

Dr. Isaac Roberts, on a favourable night, exposed to
the nebula for four hours one of the highly sensitive
plates that he uses ; and on developing and enlarging,
a picture was obtained which struck me at the time
I saw it, and which still appears to me, to be perhaps
the most instructive portrait of any celestial object I have
ever beheld. At once the significance of the mysteri-
ous lanes becomes apparent, and the structure of the
mighty nebula is for the first time disclosed. It is obvi-
ously a somewhat disc-shaped or rather lens-shaped mass,
tilted nearly edgeways towards us. The central portion
is especially brilliant and greatly condensed, and it is
surrounded by two or three rings of nebulous material.
The lanes are thus shown to be merely the better marked
portions of the divisions between these rings. They can
be traced nearly the whole way round in the photograph,
though, owing to the foreshortening, and the want of out-
line which is characteristic of nebulae, they become a little
confused at the extremities. The two other well-known
nebulae in the neighbourhood are also shown : they are
obviously parts of the same system.

JI6 IN STARRY REALMS.

This marvellous structure will naturally suggest that
Laplace could have no more appropriate picture to illus-
trate his nebular theory than the photograph of the
nebula in Andromeda. There seems no doubt, indeed,
that this nebula is condensing down, but the magni-
tudes involved show us that the solar system bears but
little resemblance to the gigantic, rings of the vast nebula.
Look at the facts of the case. It happens that we have
in the case of Andromeda a partial hint as to its actual
dimensions of which we are usually destitute in objects of
this description. A few years ago a variable star broke
out in Andromeda under circumstances which rendered it
in the highest degree , probable that the star was actually
in the nebula, and not merely accidentally on the line of
sight. The parallax of this star was sought for by
astronomers — myself among the number — and we came
to the unanimous conclusion that the star, and therefore
presumably the nebula, was too remote for our methods of
survey to be successful. The diameter of the earth's orbit
cannot subtend an angle at the very most of more than a
couple of seconds at the nebula, which is itself more than a
couple of degrees in length. We are hence assured that
the diameter of the system which is being evolved in
Andromeda, whatever it may be, is at the very least three
thousand six hundred times as great as that of the earth's
orbit round the sun.

Another superb achievement in the exclusive depart-
ment of photography is the discovery of the nebulae which
surround some of tiie stars in the Pleiades. We may
look in vain for them with the ordinary telescope, but the
exquisite pictures of Koberts demonstrate their exist-

PHOTOGRAPHING THE STARS. 317

«nce, and show that the stars of the Pleiades seem to have
resulted from the condensation of a mighty nebula, some
portions of which are still in the vicinity of the group. It
seems clear that the results obtained in the case of the
nebula in Andromeda, and of the Pleiades, would be alone
sufficient to justify all the expenditure of time and trouble
made on behalf of celestial photography.

Several photographs of the great nebula in Orion have
also been taken, those of the Lick and Yerkes Observatories
being especially successful. It would seem, however, as
if the bluish nebulae, such as Orion and the Dumb-bell,
did not admit of such good photographic portraits as the
nebula in Andromeda which is of a whiter hue. The draw-
back to all nebular photographs is that, to give sufficient
exposure for the faint parts, the bright parts must be over-
exposed, while the stars are of course burnt into dis-
figuring blotches.

It does not enter into the scheme of this chapter to discuss
with any detail the splendid applications of photography
to the spectroscopic study of the heavens. Here, indeed,
the pre-eminent utility of photography comes out most
distinctly. I must, however, give a few concluding lines
to the subject. In this department of celestial spectro-
scopy Sir William Huggins, K.C.B., is the most renowned
discoverer, and he has obtained exquisite photographs of the
spectra of stars. The white stars, such as Sirius and Vega,
show a truly marvellous spectrum ; there are a few lines
in the visible part, and a great number of lines in the
photographic part, due to hydrogen. The spectra of comets
and of nebulae have also been obtained, and are replete
with truly marvellous interest and instruction.

3i8 IN STARRY REALMS.

But perhaps the most comprehensive piece of astro-
nomical speetroscopy which has yet been undertaken is the
great Draper Memorial, at which Professor Pickering \&
labouring with such consummate skill at Harvard College,
Massachusetts.

Mrs. Draper, in memory of her accomplished husband,
provided the means by which Professor Pickering might
carry on his work. The photographs of the stellar
spectra which have been obtained present a magnificent
display of lines. His operations have been conducted on
such a comprehensive scale that a complete spectj oscopic
review of all the stars in the heavens to the ninth
magnitude has been obtained. One who has not visited
Professor Pickering's observatory, and so has not seen the
yast astronomical research that is there carried on, can
have hardly any idea of the magnificence of the great
task. Many other observations, both in the old world
and the new, are also daily adding to our knowledge, until
in these later days we have learned how full of meaning
are the words, " One star differeth from another star in
glory."

CHAPTER
\N ASTRONOMER'S THOUGHTS ABOUT KRAKATOA.

AN event like the great eruption of Krakatoa can only
be studied properly when placed in suitable perspective.
Accordingly years have been required before sufficient
data could be collected to enable us to take an ade-
quate view of the several incidents of the explosion.
The eruption of Krakatoa in August, 1883, was not only a
mighty and appalling incident in the neighbourhood of the
Straits of Sunda. It was there no doubt that the fatal aspects
of the disaster were exclusively developed. It was along
the shores of Sumatra and Java that the inundations took
place in which 36,380 lives are said to have been lost.
But the phenomena of Krakatoa, which give it a peculiar
interest, are of an innocuous type, and have had a far
wider range than those of a tragical character. The shock
given to our globe was such that the influence of the
explosion has extended in some degree to almost every
part. To appreciate all that Krakatoa implies it is there-
fore not sufficient merely to gather the information which
can be procured at the seat of the volcano itself ; we must
extend our inquiries much farther afield. We have to-

320 IN STARRY REALMS.

learn what observers within many hundreds of miles
tell us. Ships' logs have to be examined. The records of
barometers and of magnetic instruments all over the globe,
even to the very antipodes of Krakatoa, have to be
brought together. The descriptions of extraordinary optical
phenomena, such as wonderful ruddy glows at sunset and
sunrise, or strange hues in which the sun and the moon
were occasionally decked, have to be collected and scru-
tinized from numerous places scattered over both hemi-
spheres. Need it be said that such a task as this must be
a protracted one, but it has been accomplished, and now
those interested in the matter have the opportunity of
studying a unique chapter in the history of the earth.

It is to the Royal Society that we are indebted for the
inception and the carrying out of this laborious undertak-
ing. They appointed a Krakatoa Committee, under the
chairmanship of the late Mr. Symons. So multitudinous
were the phenomena to be investigated that the committee
was divided into sections. To examine the eruption itself
and the volcanic phenomena generally, a geological section
was necessary. To study the air-waves and the sounds, as
well as the distribution of dust and pumice by wind and
water, required the aid of meteorologists. On the border
territory, between the sciences of meteorology and of astro-
nomy, must be placed the investigation of the twilight
effects and the strange coronas and weird colours of the
sun and moon. The great sea- waves must clearly be
studied by hydrographers, and there were also some
groups of facts connected with terrestrial magnetism and
electricity. Immense numbers of letters and reports from
all parts of the globe had to be brought to a focus, and

THOUGHTS ABOUT KRAKATOA. 321

the extensive printed literature relating to Krakatoa had
to be ransacked. At length, however, by the spring of
1887, the manuscript was completed, and, in the autumn
of 1888, a superb quarto volume of nearly 500 pages,
copiously illustrated both by artistic drawings and by
charts and maps, was issued.

Midway between Sumatra and Java lies a group of
small islands, which, prior to 1883, were beautified by the
dense forests and glorious vegetation of the tropics. Of
these islands Krakatoa was the chief, though even of it
but little was known. Its appearance from the sea must,
indeed, have been familiar to the crews of the many
vessels that navigated the Straits of Sunda, but it was not
regularly inhabited. Glowing with tropical verdure, such
an island seemed an unlikely theatre for the display of an
unparalleled effect of plutonic energy, but yet there were
certain circumstances which may tend to lessen our sur-
prise at the outbreak. In the first place, as Professor
Judd has so clearly pointed out, not only is Krakatoa
situated in a region famous, or perhaps infamous, for
volcanoes and earthquakes, but it actually happens to lie
at the intersection of two main lines, along which volcanic
phenomena are, in some degree, perennial. In the second
place, history records that there have been previous erup-
tions at Krakatoa. The last of these appears to have
occurred in May, 1680, but unfortunately only imperfect
accounts of it have been preserved. It seems, however,
to have annihilated the forests on the island, and to have
ejected vast quantities of pumice, which cumbered the
seas around. Krakatoa then remained active for a year
and a half, after which the mighty fires subsided. The

322 IN STARRY REALMS.

irrepressible tropical vegetation again resumed possession*
The desolated islet again became clothed with beauty, and
for a couple of centuries reposed in peace:

A few significant warnings were given before the
recent tremendous outbreak. Admonitory earthquakes
began to be felt in the vicinity some years before, and
for a period of three months Krakatoa was gradually
preparing for the majestic performance with which the
world was astounded on August 26-27. The inhabi-
tants of those regions were so accustomed to be
threatened by volcanic phenomena that the early stages
of the outbreak, which began on May 20, do not seem to
have created any alarm; quite the reverse, indeed, for
a pleasant excursion was organized from Batavia, and
a trip made to Krakatoa in a steamer, to see what wa»
going on. The party landed on the island, and found a
large basin-shaped crater, more than half a mile across at
the top, and almost 150 feet deep. In the centre of this
was an aperture 150 feet in diameter, from which a column
of steam issued with a terrific noise. Even at this early
stage of the eruption the volcanic dust was projected aloft
in quantities sufficient to be wafted to the adjoining shores
of Sumatra and Java.

For the next fortnight or three weeks the intensity of
the eruptive phenomena seemed at first to decline, but
about the end of June other craters began to open on the
island, and the volcanic energy from that date increased
until the mighty climax. The actual nature of that awful
event can only be imperfectly known. The Straits of
Sunda were no longer a pleasant place for a steamboat
excursion They had become the theatre of an- appalling

THOUGHTS ABOUT KRAKATOA. 323

catastrophe. For many hours the adjacent shores were
wrapped in profound darkness, while the tremendous
agitation of the volcano originated great sea waves which
swept away entire towns and villages, and in a great
measure destroyed their populations.

It was one o'clock in the afternoon of Sunday, August
26, 1883, when Krakatoa commenced a series of gigantic
volcanic efforts. Detonations were heard which succeeded
each other at intervals of about ten minutes. These were
loud enough to penetrate as far as Batavia and Buitenzorg,
distant 96 and 100 miles respectively from the volcano.
A vast column of steam, smoke, and ashes ascended to a
prodigious elevation. It was measured at 2 P.M. from a
ship 76 miles away, and was then judged to be 17 miles
high — that is, three times the height of the loftiest moun-
tain in the world. As the Sunday afternoon wore on, the
volcanic manifestations became ever fiercer. At 3 P.M.
the sounds were loudly heard in a town 150 miles away.
At 5 P.M. every ear in the island of Java was engaged in
listening to volcanic explosions, which were considered to
be of quite unusual intensity even in that part of the
world. These phenomena were, however, only introduc-
tory. Krakatoa was gathering strength. Between 5 and
6 P.M. the British ship Charles Bal, commanded by Cap-
tain Watson, was about ten miles south of the volcano.
The ship had to shorten sail in the darkness, and a rain of
pumice, in large pieces and quite warm, fell upon her
decks. At 7 P.M. the mighty column of smoke is described
as having the shape of a pine-tree, and as being brilliantly
illuminated by electric flashes. The sulphurous air is
laden with fine dust, while the lead dropped from a ship

324 IN STARR Y REALMS.

in its anxious navigation astounds the leadsman by coming
ap hot from the bottom of the sea. From sunset on
Sunday till midnight the tremendous detonations followed
each other so quickly that a continuous roar may be said
to have issued from the island. The full terrors of the
eruption were now approaching. The distance of 96
miles between Krakatoa and Batavia was not sufficient to
permit the inhabitants of the town to enjoy their night's
sleep. All night long the thunders of the volcano sounded
like the discharges of artillery at their very doors, while
the windows rattled with aerial vibrations.

On Monday morning, August 27, the eruption cul-
minated in four terrific explosions, of which the third,
shortly after 10 A.M. Krakatoa time, was by far the most
violent. The quantity of material ejected was now so
great that darkness prevailed even as far as Batavia soon
after 11 A.M., and there was a rain of dust until three in
the afternoon. The explosions continued with more or
less intensity all the afternoon of Monday and throughout
Monday night. They finally ceased at about 2'30 A.M.
on Tuesday, August 28. The entire series of grand phe-
nomena thus occupied a little more than thirty-six hours.

We may imagine several different standards by which
the significance of a volcanic outbreak is to be estimated.
The most obvious standard of comparison is, of course,
that of the quantity of materials which are extruded.
Another would be the area covered by the clouds of
volcanic dust and the duration of the darkness thus caused.
Other standards would be sought in the incidental effects
of the outbreak, such as the great waves which are thereby
propagated in the sea, and the distances to which the

THOUGHTS ABOUT KRAKATOA. 325

sounds are carried. Other more subtle, but not less
interesting, phenomena are the waves in the atmospheric
ocean, which are neither seen nor heard, but of which the
barometer gives no uncertain indications. Among the
remaining effects of a volcanic explosion are the curious
sunset glows and the strange optical phenomena which are
sometimes witnessed. We have thus a number of distinct
points of view from which the significance of a volcano
can be estimated.

We had all heard so much about Krakatoa that at
first it is a little disappointing to read the assurances
of Professor .Tudd that, so far as the first two of these
standards are concerned, Krakatoa has been surpassed
by other volcanoes. He enumerates three distinct out-
breaks—viz., that of Papandayang, in Java, in 1772;
of Skaptar Jokull (Varmardalr), in Iceland, in 1783;
and of Tomboro, in Sumbawa, in 1815 — in all of which
the quantity of matter poured forth was considerably
greater than that from Krakatoa. However, even in this
respect the achievements of Krakatoa if second-rate are at
least respectable. The estimates made are necessarily
founded on precarious data, but it seems to be certain that
if all the materials poured forth from Krakatoa during the
critical period could be collected together, the mass they
would form would be considerably over a cubic mile in
volume. It is in the other standards of comparison that
the importance of the explosion at Krakatoa is to be
sought. The intensity of this outbreak in its last throes
was such that mighty sounds were heard and mighty
waves arose in the sea for which we can find no parallel.
Every part of our globe's surface felt the pulse of the air*

326 IN STARRY REALMS.

waves, and beautiful optical phenomena made the circuit
of the globe even more than once or twice. In these last
respects the eruption of Krakatoa is unique.

Professor Judd has satisfactorily accounted for the
enormous manufacture of dust during the eruption. It
appears to consist of comminuted pumice, and is produced
by the attrition of the pumice masses, as in successive out-
bursts they are hurled aloft, and then tumble back again
into the crater.

It appears to me that the most remarkable incident
connected with the eruption of Krakatoa was the pro-
duction of the great air-wave by that particular explosion
that occurred at ten o'clock on the morning of Monday,
August 27. The great air-wave was truly of cosmical
importance, affecting as it did every particle of the atmo-
sphere on our globe. This phenomenon alone extends the
study of Krakatoa beyond the province of vulcanology,
and gives to the subject a particular interest in physical
science.

A pebble tossed into a pond of unruffled water gives
rise to a beautiful series of circular waves that gradually
expand and ultimately become evanescent. A very large
body falling into the ocean would originate waves that
might diverge for miles from the centre of disturbance ere
they became inappreciable. "Waves can originate in air
as well as in water. We are not at this moment speaking
of those familiar air- waves by which sounds are conveyed.
The waves we now mean are inaudible and apparently
much longer undulations than those of sound.

Imagine a great globe, which for simplicity we may
think of as smooth all over. Let us suppose that this

THOUGHTS ABOUT KRAKATOA. 327

globe has the stupendous dimensions expressed by a dia-
meter of 8,000 miles, and imagine it to be enclosed in a
uniform shell of air. Now, suppose that all is quiet, till
at some point, which for the moment we may speak of as
the pole, a mighty disturbance is originated. Let us
regard this disturbance as produced by a sudden but local
pushing up of the atmosphere by a force directed from the
earth's surface outwards, and let us trace the effect thereby
produced on the atmosphere. Such a sudden impulse will
at once initiate a series of circular atmospheric waves,
which will speed away from the centre of disturbance
just like the waves caused by the pebble in the pond. If
the original atmospheric impulse be large enough we shall
find the circle growing larger and larger, its radius in-
creasing from hundreds of miles to thousands of miles,
until at last the wave reaches the equator. "What is to
happen when the diverging waves have attained the
equator, and are now confronted by the opposite hemi-
sphere ? This is one of those cases in which the mathe-
matician can guide us where the experimentalist would be
otherwise somewhat at fault. We know that as the wave
entered the opposite hemisphere it would at once move
through a similar series of changes to those through which
it had already gone, but in the inverse order. The wave
will thus, after leaving the equator, glide onwards into a
parallel small circle, ever decreasing in diameter, and con-
verging toward the anti-pole. Finally, just as the waves
all radiated from the original pole, so will they all con-
centrate towards the opposite one. But what is now to
happen ? Here, again, the mathematician will inform us.
He can follow the oscillations after their confluence, and

32S IN STARRY REALMS.

he finds that from the anti-pole they will again commence
to diverge. Again they will expand, again they will
reach the equator, and again will they gradually draw into
concentration at the original pole. Nor will the process
even here end. From the second confluence there will be
a new divergence, and thus the oscillations will be sent
quivering from one pole of the globe to the other, until
they gradually subside by friction.

This comprehensive series of phenomena wherein the
atmosphere of the entire globe participates in an organized
vibration has, so far as we know, only once been witnessed,
and that was after the greatest outbreak at Krakatoa, at
ten o'clock on the morning of August 27. But the ebb
and the flow of these mighty undulations are not imme-
diately appreciable to the senses. The great wave, for
instance, passed and re-passed and passed again over
London, and no inhabitant was conscious of the fact.
But the automatic records of the barometer at Greenwich
show that the vibration from Krakatoa to its antipodes,
and from the antipodes back to Krakatoa, was distinctly
perceptible over London, not less then six or seven times.
The instruments at the Kew Observatory confirm those at
Greenwich, and if further confirmation were required it
can be had from the barograms at many other places in
England. This is truly a memorable incident, and the
scientific value of the labours of those who so diligently
obtain automatic barometric records year after year would
be amply demonstrated, if demonstration were required,
by this single discovery of the great Krakatoa air- wave.

From all parts of Europe, from Berlin to Palermo, from
St. Petersburg to Valencia, we obtain the same indications.

THOUGHTS ABOUT KRAKATOA. 329

Fortunately self-recording barometric instruments are now
to be found all over the world. Almost all the instruments
show distinctly the first great wave from Krakatoa to its
antipodes in Central America, and the return wave from
the antipodes to Krakatoa. They also all show the second
great wave which sped from Krakatoa, as well as the
second great wave which returned from the antipodes.
Thus, the first four of the oscillations are depicted on up-
wards of forty of the barograms. The fifth and sixth
oscillations are also to be distinguished on several of the
curves, and even the seventh is certainly established at some
few places, of which Kew is one. Then the gradually
increasing faintness of the indications renders them un-
recognisable, from which we conclude that after seven
pulsations our atmosphere had sensibly regained its former
condition ere it was disturbed by Krakatoa.

Among the instruments which have yielded valuable
information about the air- wave, we have, curiously enough,
to mention the register of the recording gasometer-indica-
tor at Batavia. This apparatus, designed and employed
for a widely different purpose, shows that extraordinary
fluctuations in the barometric pressure occurred at the
time when the great wave passed over the town.

It is of particular interest, from a physical point of
view, to study the numerical facts with reference to the
speed at which this world-embracing wave was propagated.
We shall for this purpose select the records taken at
Greenwich. The phase of the wave found most conve-
nient for measurement was the depression following the
outbreak, and the moment at which this phase started
from Krakatoa was 3 hrs. 32 mins. P.M. on August 17,

330 IN STARRY REALMS.

Greenwich mean time. This is probably correct within
two or three minutes. Diverging from its source this
wave reached Greenwich after an interval of a little more
than ten hours. The interesting point is, however, the
determination of the period of a complete oscillation, that
is to say, the interval between the passage of the wave
over Greenwich and the next passage of the wave in the
same direction also over Greenwich. It has been found
convenient to designate the successive waves as i., ii., iii.,
iv., &c., the odd numbers being those from Krakatoa to its
antipodes, and the even numbers being the return waves
from the antipodes to Krakatoa. At Greenwich, for
example, we find the interval between i. and iii. to have
been 36*47 hours, between iii. and v. 36*82 hours, and
between v. and vii. 37'05 hours. For the return waves
the , intervals between ii. and iv. was 34*78 hours, and
between iv. and vi. 35*25 hours. The similar values vary
slightly when obtained at the several stations, but the
average results indicate that for its first circuit of the
earth the wave required 36 hrs. 24 mins., for the second
36 hrs. 30 mins., and for the third 36 hrs. 50 mins. The
similar periods for the waves travelling in the reverse way
were 34 hrs. 46 mins., and 35 hrs. 4 mins. respectively.
The average of all is very nearly a day and a half.

Before leaving this part of the subject, I must refer to
the approximate identity between the velocity of this
aerial disturbance and the velocity of ordinary sound.
This is well brought out by General Strachey . The speed of
the wave varied from 674 to 726 miles per hour. The
speed of sound propagation is 723 miles at zero Fahren-
heit, and is 781 miles at 80° Fahrenheit. Considering

THOUGHTS ABOUT KRAKATOA. 331

that the waves had, of course, to cross the poles in their
journeys, it would almost seem that within the limits of
probable error the speed of the great wave and the speed of
ordinary sound waves were identical. It would, I think,
have been an improvement on the plates containing the
barograms, if the scale had been given, so that it would
have been possible to obtain some definite notion of the
amplitudes of the oscillations at the different stations.
The only pressure-diagram contained in the plates which
does give any scale measures, is that of the gasholder at
Batavia ; from this it would appear that the barometric
fluctuation produced by the great wave was about four-
tenths of an inch of mercury at a distance of 100 miles
from the source of disturbance.

While the chapter on the air- waves is the most novel
scientific feature in the Report of the Krakatoa Com-
mittee, it will be admitted that the most amazing features
of the same work are those contained in the section on
" Sounds." Here we find a collection of statements so
marvellous that they would be well-nigh incredible were
it not for the ample body of excellent testimony by which
they are substantiated. In the whole annals of noise
there is nothing which can be compared to the records
set forth in a table which occupies not less than eight
pages of the volume. (See Fig. 19.) Let us select a few
instances, almost at random.

Lloyd's agent at Batavia, 94 miles distant, says that
on the morning of the 27th of August the reports and
concussions were simply deafening. At Carimon, Java
Island, reports were heard which led to the belief that
some vessel offshore was making signals of distress, and

332 IN STARRY REALMS.

boats were accordingly put out to render succour, but no
vessel was found, as the reports were from Krakatoa, at a
distance of 355 miles. At Macassar, in Celebes, explosion*
were heard all over the province. Two steamers were sent
out to discover the cause, for the authorities did not then
know that what they heard came from Krakatoa, 969
miles away. But mere hundreds of miles will not suffice
to exemplify the range of this stupendous siren. In St.
Lucia Bay, in Borneo, a number of natives, who had
been guilty of murder, thought they heard the sounds of
vengeance in the approach of an attacking force. They fled
from their village, little fancying that what alarmed them
really came from Krakatoa, 1116 miles distant. All over
the island of Timor alarming sounds were heard, and so
urgent did the situation appear that the Government was
aroused, and sent off a steamer to ascertain the cause. The
sounds had, however, come 1351 miles, all the way from
Krakatoa. In the Victoria Plains of West Australia the
inhabitants were startled by the discharge of artillery —
an unwonted noise in that peaceful district — but the artil-
lery was at Krakatoa, 1700 miles distant. The inhabi-
tants of Daly "Waters, in South Australia, were rudely
awakened at midnight on Sunday, August 26, by an
explosion resembling the blasting of a rock, which lasted
for a few minutes. The time and other circumstances
show that here again was Krakatoa heard this time at the
monstrous distance of 2023 miles. But there is undoubted
testimony that to distances even greater than 2023 miles
the waves of sound conveyed tidings of the mighty con-
vulsion. Diego Garcia, in the Chagos Islands, is 2267
miles from Krakatoa, but the thunders traversed even this

THOUGHTS ABOUT KRAKATOA.

333

distance, and created the belief that there must be some
ship in distress, for which a diligent but necessarily
ineffectual search was made. To pass at once to the most
remarkable case of all, we have a report from Mr. James

Fig. 19.— Krakatoa : Eruptions of August 26-27,

(The shaded portion indicates approximately the area over which the sounds
were heard.)

Wallis, chief of police in Rodriguez, that " several times
during the night of August 26-27, 1883, reports were
heard coming from the eastward, like the distant roar of
heavy guns. These reports continued at intervals of
between three and four hours." Were it not for the con-

334 IN STARRY REALMS.

tinuous chain of evidence from places at gradually increas-
ing distances from Krakatoa, we might well hesitate to
believe that the noises Mr. Wallis heard were really from
the great volcano, but a glance at the map, which shows
the several stations where the great sounds were heard,
leaves no room for doubt. We have thus the astounding
fact that almost across the whole wide extent of the
Indian Ocean, that is, to a distance of nearly 3000 miles
(2968), the sound of the throes of Krakatoa was pro-
pagated.

We appreciate this result more strikingly if we reflect
on the velocity of sound. Seconds or minutes may elapse
between the appearance of a flash of lightning and the
arrival of the thunder. But the volcanic sounds could not
have been heard at Rodriguez until four hours after they
had commenced to travel from Krakatoa. Were Vesuvius
now to break out as Krakatoa has done, every inhabitant
of Great Britain would apparently be quite near enough
to hear the awful detonation.

I shall content myself with the mention of three facts
in illustration of the great sea waves which accompanied
the eruption of Krakatoa. Of these, probably the most
unusual is the magnitude of the area over which the
undulations were perceived. Thus, to mention but a
single instance, and that not by any means an extreme
one, we find that the tide gauge at Table Bay reveals
waves which, notwithstanding that they have travelled
5100 miles from Krakatoa, have still a range of eighteen
inches when they arrive at the southern coast of Africa.
The second fact that I mention illustrates the magnitude
of the seismic waves by the extraordinary inundations

THOUGHTS ABOUT KRAKATOA. 335

that they produced on the shores of the Straits of Sunda.
Captain Wharton shows that the waves, as they deluged
the land, must have been fifty feet, or, in one well-
authenticated case, seventy- two feet high. It was, of
course, these vast floods which caused the fearful loss of
life. The third illustrative fact concerns the fate of a
man-of-war, the Berouw. This unhappy vessel was borne
from its normal element and left high and dry in Sumatra,
a mile and three-quarters inland, and thirty feet above
the level of the sea.

Such incidents are not so unusual as the exquisite series
of optical phenomena which has made most of the nations
on the earth spectators in some degree of the wonders of
Krakatoa. Resounding as were the crashes of the explo-
sions, they still subsided thousands of miles to the east of
Great Britain, and though the great aerial vibrations
tingled to and fro through the air over every part of
this globe, yet they were not perceptible to our unaided
senses. But now we are to consider a splendid series of
phenomena which scorned limitations of distance, and
which obtruded their glories on our notice for weeks and
even months together.

One of the most striking maps that the Report of the
Royal Society contains is that which illustrates the pro-
gress of the main sky-phenomena from August 26 (even-
ing) to September 9 1883. I doubt if the skies have ever
presented to our vision, within atmospheric limits, a more
singular series of phenomena than those which are most
clearly depicted within the modest limits of this little map.
(See Fig. 20,) Let me endeavour from the series of maps,
of which this is one, as well as from the abundant body

336

IN STARRY REALMS.

of matter so luminously set forth by the Hon. F. A.
Rollo Russell and Mr. E. Douglas Archibald, to present a
brief outline of this elaborately beautiful series of pheno-
mena and their cause.

During the crisis on August 26-27, the volume of
material blown into the air was sufficiently dense to
obscure the coasts of Sumatra to such a degree that at

PROGRESS OFTHE MAIN SKY PHENOMENA FROM AUG.26 (EVENING) toSEP.9. 1883.

120

60 60 40 20 O 20 40 60 60 U

0 120 140 IgQ 180

&

N

E^fi

•£

.•S

C^

¥* -----

160 I4O 120

80 60 40

Fig. 20.

10 A.M. the darkness there is stated to have been more
intense than it is even in the blackest of nights. The
fire-dust ascended to an elevation which, as we have
already mentioned, is estimated to have been as much as
seventeen miles. Borne aloft into these higher regions of
our atmosphere, the clouds of dust at once became the
sport of the winds and the currents which may be found
there. If we had not previously known the prevailing

THOUGHTS ABOUT KRAKA1OA. 337

tendency of the winds at these elevations and in these
latitudes, the journey of the Krakatoa dust would have
taught us. We shall confine our attention for the pre-
sent to the chief phenomena, and we begin with the
manifestation of these phenomena which were witnessed
in the tropics.

It seems certain that, having attained their lofty eleva-
tion, the mighty clouds of dust were seized by easterly
winds, and were swept along with a velocity which may
not improbably be normal at a height of twenty miles
above the earth's surface. It has been demonstrated by
Dr. Vettin, at Berlin, that the upper cirrus clouds in
winter at a height of only four or five miles have an aver-
age velocity of 44*5 miles an hour. The Rev. W. Clement
Ley has shown that the velocities of the upper cirrus
clouds often amount to 120 miles an hour. These facts
enable us without hesitation to attribute velocities to the
great clouds of Krakatoa dust which shall be quite suffi-
cient to account for the various phenomena.

It appears that this cloud of dust started immediately
from Krakatoa for a series of voyages round the world.
The highway which it at first pursued may, for our pre-
sent purpose, be sufficiently defined by the Tropic of
Cancer and the Tropic of Capricorn, though it hardly
approached these margins at first. Westward the dust
of Krakatoa takes its way. In three days it had crossed
the Indian Ocean and was rapidly flying over the heart of
Equatorial Africa ; for another couple of days it was
making a transatlantic journey ; and then it might be
found, for still a couple of days more, over the forests of
Brazil ere it commenced the great Pacific voyage, which

338 IN STARRY REALMS.

brought it back to the East Indies. The dust of Krakatoa
had put a girdle round the earth in thirteen days ! The
shape of the cloud appears to have been elongated, so that
it took two or three da}'8 to complete the passage over
any stated place.

When the dast- cloud had regained the Straits of
Sunda the great eruption was over, but the winds were
still the same as before, and again the comminuted
pumice sped on its impetuous career. The density of the
cloud had, however, lessened. Doubtless much of the
material was subsiding, and the remainder was becoming
diffused over a wider area. Accordingly, we find that the
track of the stream during this second revolution is some-
what wider than it was on the first, though still mainly
confined between the tropics. The speed with which the
dust revolved was, however, unabated. Continents and
oceans were again swept over with a velocity double that
of an express train, and again the earth was surrounded
within the fortnight. The dust-cloud had now further
widened its limits, ut was still distinguishable, and with
unlessened speed commenced for a third time to encircle
the earth. The limits of the stream had spread themselves
outside the tropics, though still falling short of Europe.
There is no reason to think that there was any decline in
the velocity of 76 miles per hour, but the gradual diffu-
sion of the dust begins to obliterate the indications by
which its movements could be perceived, so that during,
and after, the third circuit the phenomena became so dif-
fused that while their glory covered the earth, the distinc-
tion between the successive returns had vanished. In
November the area which contained the Krakatoa dust

THOUGHTS ABOUT KRAKATOA.

33?

had sufficiently expanded from its original tropical limits
to include Europe and the greater part of North America.
During the winter months the suspended material gradu-
ally subsided or, at all events, became evanescent, and in
the following spring the earth regained its normal state
in so far as the Straits of Sunda were concerned (Fig. 21).
It remains to give some brief account of the optical

APPROXIMATE NORTHERN UMIToFTNE MAIN SKY PHENOMENA AT THE END OF NOVEMBER 1863.

/I MSf rct«

Fig. 21.

phenomena due to the presence of dust, unusual both in
quantity and in character, in the upper atmosphere. The
frontispiece of the volume shows some beautiful pictures
of the twilight and after-glow effects as seen by Mr. "W.
Ascroft on the bank of the Thames a little west of London,
on the evening of November 26, 1883. Analogous phe-
nomena to those here depicted were seen almost univer-
sally during November and December in the same year.

340 IN STARRY REALMS.

Who is there that does not remember the wondrous love-
liness of the twilights and the after-glows during that
remarkable winter ! These appearances at sunrise and
sunset are only the more generally recognised of a whole
system of strange optical phenomena. One of the most
striking indications of the presence of the dust-stream in
its first voyage round the earth was given by the strange
blue hue it imparted to the sun. The dust-stream was also
visible in its rapid voyages as a lofty haze or extensive
cloud of cirro-stratus. Then, too, strange haloes were
often seen, there were occasional blue or green moons,
and the sun was sometimes glorified by a corona that
had its origin in our atmosphere. Everywhere in the
world there were remarkable features in the sky that
winter : from Tierra del Fuego to Lake Superior ; from
China to the Gulf of Guinea ; from Panama to Aus-
tralia. Wherever on land there were inhabitants with
sufficient intelligence to note the unusual, wherever on
the sea there were mariners who kept a careful log, from
all such observers we learn that in the autumn and winter
months following the great eruption of Krakatoa, there
were extraordinary manifestations witnessed in the
heavens.

Just one point more in conclusion. We have recorded
the great volcanic outbreak of Krakatoa, and we have
recorded a wonderful series of optical phenomena. It
remains to say a word as to the proof that the latter were
indeed the consequence of the former. As the Committee
have begun their book with pictures of sun-glows, and as
they have occupied more than half of the work with
descriptions of the purely optical effects, it seems as if

THOUGHTS ABOUT KRAKATOA. 341

they, at all events, entertained but little doubt that the
dust of Krakatoa was responsible for the sunsets of
Chelsea. Still I notice that some members of the Com-
mittee seem to shrink from deliberately committing them-
selves to this view. Indeed, the very title of their book
exhibits a certain degree of caution on this point. They
have called it " The Eruption of Krakatoa and subsequent
Phenomena." The word I have italicized would not
improbably have been consequent had it not been for the
existence of some such reserve as that I have indicated.
But the magnificent body of information which their
labours have brought together will enable every one who
will carefully study the volume to form his own opinion
as to whether or not it was Krakatoa dust which painted
our sunsets with those glorious hues. In attempting to
decide this question we must first endeavour to conceive
the kind of evidence which would be necessary and suffi-
cient to establish the fact that the optical phenomena were
consequent upon, as well as subsequent to, the great
eruption.

First of all it would be natural to ask whether the
existence of volcanic dust in the air could have produced
the optical effects that have been observed. This must
be answered in the affirmative. Then it would be proper
to inquire whether other volcanic outbreaks in other parts
of the world, and on other occasions, had been known to
have been followed by similar results. Here, again, we
have page after page of carefully stated and striking
historical facts which answer this question also in the
affirmative. Next it would be right to see whether the
sequence in which the phenomena were produced at

j42 IN STARRY REALMS.

different places in the autumn of 1883 tallied with the
supposition that they all diverged from Krakatoa. The
instances that could be produced in support of the affir-
mative number many hundreds, though it must be
admitted that there are some few cases about which there
are difficulties. Surely we have here what is practically
a demonstration. It is certain that these optical pheno-
mena existed. No cause can be assigned for them except
the presence, at that particular time, of vast volumes of
dust in the air. What brought that dust into the air
except the explosion of Krakatoa ? Most people find
themselves unable to share the scruples of those who think
there can be a doubt on the matter. Would another
eruption of Krakatoa, followed by a repetition of all the
optical phenomena, convince them that in this case, at all
events, post hoc was propter hoc ? Perhaps not, if they
have already failed to be convinced by the fact that, when
Krakatoa exploded two centuries ago, blood- red skies
appear to have been seen shortly afterwards even as far
away as Denmark.

When we reflect that an explosion on an insignificant
islet in the Straits of Sunda has sufficed to set the whole
atmospheric covering of our globe trembling, when we
remember that the dust then poured forth in a few days
of volcanic activity was adequate to adorn the sunsets of
every country in the earth, we are reminded once again
of the old truth : " How small the world is after all I "

CHAPTER XXIII.

DARWINISM AND ITS RELATION TO OTHER BRANCHES OF
SCIENCE.

IN the year 1831 a naval expedition sailed from Devon-
port. That expedition consisted of a single vessel, His
Majesty's ship Beagle, a ten-gun brig under the command
of Captain Fitzroy, R.N. The Beagle was a stout old
wooden ship, destined on this occasion for a most pacific
enterprise. Her duty was to survey parts of the coast
of South America and some islands in the Pacific, and
to carry a chain of chronometrical measurements round
the world. Five years later the Beagle returned from
this cruise, and thus brought to a close one of the most
remarkable voyages that can be found in the annals
of the British navy. Now, why was the cruise of the
Beagle of such unparalleled importance ? There have
been many other surveying expeditions quite as success-
ful. No doubt the memorable voyage of the Challenger
accomplished much more surveying than the voyage of
the Beagle. But we are gradually learning that even
such achievements as those of the Challenger must sink
into insignificance when compared with the voyage of the
Beagle. I would rather liken the voyage of the Beagle to

344 IN STARRY REALMS.

the immortal voyage of Columbus. In each case 8 new
world was discovered.

When the voyage of the Beagk was planned, the
captain expressed a wish that some scientific observer
should join the expedition. A young naturalist, eager to
see the glories of the tropics, volunteered his services and
was accepted. He sailed in the ship. For the whole five
years he diligently sought every opportunity to gain a
knowledge of nature. He pondered on that knowledge
when he came home. He added to it by further observa-
tion and matured it by careful thought. After many
years of labour and of thought the naturalist of the Beagle
produced a book. The name of the book was the
" Origin of Species/' the name of the author was Charles
Darwin.

The " Origin of Species " appeared when I was a
student in college, and I can recall at this day the intense
delight with which I read it. I was an instantaneous con-
vert to the new doctrines, and I have felt their influence
during all my subsequent life. And here let me hasten to
anticipate an objection. It is in the domain of natural
history that the great achievements of Darwin have been
wrought. It might be urged that the discussion of such
a subject lay within the province of biologists or of geolo-
gists, but could hardly be considered a legitimate enter-
prise for those whose studies led them in other directions.
But this is a view from which I dissent. I cannot admit
the " Origin of Species " to be the exclusive property of
biologists. In a more extensive view of the subject it
will be seen that the great doctrine of Evolution is of the
very loftiest significance, and soars far above the distinc-

DARWINISM AND OTHER BRANCHES OF SCIENCE. 345

tion between one science and another to which we are
accustomed.

It is interesting to note the wondrous change that is
taking place, I might almost say that has taken place, in
the popular estimate of the Darwinian theory. It has
been well said that a new theory, if eventually proved
true, has often to run through three different phases.
In the first place, every one exclaims that the theory
is not true ; then it is urged that the theory is contrary
to religion; and, lastly, that everybody knew it long
ago. The great doctrine of natural selection promul-
gated by Darwin has run through these courses. At
its first publication it was received with an outburst of
incredulity among the unthinking part of the commu-
nity. Every one recollects the denunciations it re-
ceived and the ridicule which the new doctrine had to
encounter. But the theory of Darwin has survived that
stage. The truth inherent in the principles of Darwin
has quietly brushed aside opposition, and now we hear
but little of it. The funeral of Darwin at Westminster
Abbey must be regarded as marking a momentous epoch
in the history of thought. That the great doctrine would
some day be accepted was a necessary truth, but I do not
think that any one who recollects the publication of the
" Origin of Species " could then have anticipated the
enormous change in educated opinion which the next
quarter of a century was to disclose. Still less likely
would it have seemed that the whole nation would have
so far acknowledged Darwin as, with one voice, to de-
mand that his remains should be interred in the national
mausoleum.

$46 IN STARRY REALMS.

Darwin has worked out one of the most splendid details
in the history of the universe. His methods and his
theory have intimate connections with other branches of
science, and some of these it is our object to consider in
this chapter. In particular I propose to sketch the posi-
tion which the Darwinian theory occupies with reference
to a celebrated branch of astronomical speculation.

One hundred years ago the diameter of the sun was pos-
sibly four miles greater than it is at present. One thousand
years ago the diameter of the sun was forty miles greater
than it is at present. Ten thousand years ago the diameter
of the sun was 400 miles greater than it is now. The
advent of man upon the earth took place no doubt a long
time ago, but in the history of the earth the advent of
man is a comparatively recent phenomenon. Yet it seems
certain that when man first trod our planet, the diameter
of the sun must have been many hundreds, perhaps many
thousands, of miles greater than it is at present. We
must not, however, overestimate the significance of this
statement. The diameter of the sun is at present 860,000
miles, so that a diminution of 10,000 miles would be little
more than the hundredth part of its diameter. If the
diameter of the sun were to shrink to-morrow to the extent
of 10,000 miles, the change would not be appreciable to
common observation, though even a much smaller change
would not elude delicate astronomical measurement. The
world on which the primitive man trod was certainly
illuminated by a larger sun than that which now shines
upon us. It does not necessarily follow that the climates
must have been much hotter then than now. The ques-
tion of warmth, as we have seen in a previous chapter

DAR W1NISM AND OTHER BRANCHES OF SCIENCE. 347

(p. 32), depends upon other matters besides sunbeams, so
that we must be cautious in any inferences drawn in
this way, nor are any such inferences needed for our
present purpose.

But we must not stop in our retrospect at the epoch
even of primeval man. We must go back earlier and
earlier through the long ages of the geologists, and
back still further to the earliest epochs, when life first
began to dawn on the earth. Still we find no reason to
suppose that the law of the sun's decreasing heat is not
still maintained, and thus, as far as our present knowledge
goes, we are bound to suppose that the sun must have been
larger and larger the further our retrospect extends. I
do not say that the rate at which the sun changes its dia-
meter was then the same as the four miles per century
which is an approximation to its present rate. It is suffi-
cient for our purpose that the sun is larger and larger the
further we peer back into the remote abyss of the past.
There was a time when the sun must have been twice as
large as it is at present ; it must once have been three
times as large ; it must once have been ten times as large.
How long ago that was no one can venture to say ; it
would be rash to attempt any estimate. But we cannot
stop at the stage when the sun was even ten times as large
as it is at present ; the arguments we have used will still
apply with equal, if not greater, force. And, looking
back earlier yet, there was a time when the sun was once
swollen to such an extent that the mighty orbit of Nep-
tune itself would be merely a girdle around the stupendous
globe. At that time the sun must have been a gaseous
mass of almost inconceivable tenuity. We are not to

34» /AT STARRY REALMS.

suppose that the earth and the other planets were solid
bodies deeply buried in the vast bulk of the sun. It seems
evident that the planets were gaseous masses in those
ancient days and undistinguishable from the sun, which
gave them birth.

We are now able to make an attempt to trace the his-
tory of the solar system, and to indicate the share which
Darwin has had in the solution of the noble problem. We
do not inquire how the original nebula came into being ;
our history must commence with the actual existence of
this nebula. There is, let it be confessed, a great deal of
obscurity still clinging to the subject. Though we may
be sure that the great nebula once existed, we cannot with
much confidence trace out the method by which the
planets were actually formed. It seems to be generally
thought that the nebula must have been originally endowed
with a certain rotation. This may be regarded as certain ;
indeed, it would be infinitely improbable that the nebula
should not have had some rotation. As the nebula began
to radiate heat, so it must have begun to contract ; and as
it began to contract, it began to rotate more rapidly.
This is only the consequence of a well-known dynamical
principle. But as the nebula spins more and more rapidly,
the cohesion of its parts is lessened by centrifugal force.
The moment at length arrives when the centrifugal force
detaches a fragment of the nebula. The process of con-
densation still continues both in the fragment and in the
central mass ; the fragment changes from the gaseous
state to the liquid, perhaps even from the liquid to the
solid, and thus becomes a planet. Still the central mass
condenses, and spins more and more rapi/lly, until a

DAR W1NISM AND OTHER BRANCHES OF SCIENCE. 34^

rupture again takes place and a second planet is pro-
duced.

Again, and still again, the same process is repeated, un-
til at length we recognise the central mass as our great and
glorious sun, diminished by incessant contraction, though
still vast and brilliantly hot. One of the lesser fragments
which he cast oiF has consolidated into our earth, while
other fragments, greater and smaller, have formed the
rest of the host of planets. There are many features in
the planets which seem to corroborate this view of their
origin. They all revolve around the sun in the same
direction ; they all rotate on their own axes in the same
direction, that direction being also coincident with the
sun's rotation on its axis. Most astronomers are agreed
that the history of the solar system has been something of
the kind that I have ventured to describe. Astronomers
were thus the first evolutionists ; they had sketched out a
majestic scheme of evolution for the whole solar system,
and now they are rejoiced to find that the great doctrine
of Evolution has received an extension to the whole domain
of organic life by the splendid genius of Darwin.

At its first separation from the shrinking central nebula,
our earth was probably a mass of glowing gas, of far
greater volume than it is at present. Gradually the
earth parted with its heat by radiation, and commenced
to shrink also. The temperature was so high, that iron
and other still more refractory substances were actually in
a state of vapour, but, as the temperature fell, these sub-
stances could not remain in the gaseous form ; they con-
densed first into liquids, these liquids coalesced into a vast
central mass, and still that mass went on cooling until ita

350 IN STARRY REALMS.

surface, passing through the various stages of incandes-
cence, sank at length to a temperature comparatively
cool. Still the earth was swathed with a deep and dense
mantle of air, charged with an enormous load of watery
vapour ; but, as the temperature of the surface gradually
decreased, at length the watery vapours were condensed
and descended to form the oceans with which our earth is
so largely covered. At this point the functions of the
astronomer are at an end : he has traced in outline the
manufacture of the earth from the primeval nebula ; he
has accounted for its revolution round the sun, for its
rotation on its axis ; he has accounted for the shape of the
earth and for its internal heat. His work being done, he
now hands over the continuance of the history to the
biologist.

The lifeless earth is the canvas on which has been drawn
the noblest picture that modern science has produced.
It is Darwin who has drawn this picture. He has shown
that the evolution of the lifeless earth from the nebula is
but the prelude to an organic evolution of still greater
interest and complexity. He has taken up the history of
the earth at the point where the astronomer left it, and he
has made discoveries which have influenced thought and
opinion more than any other discoveries that have been
made for centuries. We here encounter a very celebrated
difficulty. The theory of Darwin requires life to begin
with, but how did that life originate? I need hardly
remind you of the celebrated controversy which has taken
place on this subject. It has been contended that life can
never be produced except from life ; but just as stoutly
has the opposite view been maintained. Can it be pos-

DAR WINISM AND OTHER BRANCHES OF SCIENCE. 351

•sible that the wondrous and complex phenomena known as
life are purely material ? Can a particle of matter which
consists only of a definite number of atoms of definite
chemical composition manifest any of those characters
which characterize life ? Take as an extreme instance the
brain of an ant, which is not larger than a quarter of a
good- sized pin's head. It would require a volume to
describe what we know of the powers of ants. Huber
showed this long ago, and Lord Avebnry has lately
reminded us of it, while adding further discoveries of
his own.

I here quote Darwin's vivid description ; but it is
only right to add that many different species of ants are
referred to, though included under the common designa-
tion : " Ants certainly communicate information to each
other, and several unite for the same work, or for games
of play. They recognise their fellow-ants after months of
absence, and feel sympathy for each other. They build
great edifices, keep them clean, close the door in the even-
ing, and post sentries. They make roads as well as tun-
nels under rivers, and temporary bridges over them by
clinging together. They collect food for the community,
and when an object too large for entrance is brought to
the nest, they enlarge the door, and afterwards build it up
again. They store up seeds of which they prevent the
germination, and which if damp are brought to the sur-
face to dry. They keep aphides and other insects as milch
cows. They go out to battle in regular bands, and freely
sacrifice their lives for the common weal. They emigrate
according to a peculiar plan. They capture slaves. They
move the eggs of their aphides, as well as their own eggs

352 IN STARRY REALMS.

and cocoons, into warm parts of the nest, in order that
they may he quickly hatched." *

Well may Darwin speak of the brain of an ant as one
of the most wondrous particles of matter in the world.
We are apt to think that it is impossible for so minute a
piece of matter to possess the necessary complexity re-
quired for the discharge of such elaborate functions. The
microscope will no doubt show some details in the ant's
brain, but these fall hopelessly short of revealing the
refinement which the ant's brain must really have. The
microscope is not adequate to show us the texture of
matter. It has been one of the greatest discoveries of
modern times to enable us to form some numerical esti-
mate of the exquisite delicacy of the fabric which we know
as inert matter. Water, or air, or iron may be divided
and subdivided, but the process cannot be carried on inde-
finitely. There is a well-defined limit. We are even able
to make some approximation to the number of molecules
in a given mass of matter. It has been estimated that the
number of atoms in a cubic inch of air may be approxi-
mately expressed by the number 3, followed by no fewer
than twenty ciphers. The brain of the ant doubtless contains
more atoms than an equal volume of air ; but even if we
suppose them to be the same, and if we take the size of an
ant's brain to be a little globe one- thousandth of an inch
in diameter, we are able to form some estimate of the
number of atoms it must contain. The number is to be
expressed by writing down 6, and following it by eleven
ciphers. We can imagine these atoms grouped in so many
various ways that even the complexity of the ant's brain

* Darwin, "Descent of Man," p. 147.

DAR W1NISM AND OTHER BRANCHES OF SCIENCE. 553

may be intelligible when we have so many units to deal
with.

An illustration will perhaps make the argument clearer.
Take a million and a half of little black marks, put
them in a certain order, and we have a wondrous re-
sult— Darwin's "Descent of Man." This book merely
consists of about a million and a half letters, placed one
after the other in a certain order. Whatever be the com-
plexity of the ant's brain, it is still hard to believe that it
could not be fully described in 400,000 volumes, each as
large as Darwin's work. Yet the number of molecules in
the ant's brain is at least 400,000 times as great as the
number of letters in the memorable volume in question.

It would seem that by merely studying the behaviour
of an infusion of hay or a tincture of turnips in a test
tube, we do not rise to the full magnificence of the problem
as to whether life can have originated on the globe from
the particles of inorganic matter.

Unusual, indeed, must be the circumstances which will
have brought about such a combination of atoms as to
form the first organic being. But great events are always
unusual. Because we cannot repeatedly make an organ-
ized being from inert matter in our test tubes, are we to
say that such an event can never once have occurred with
the infinite opportunities of nature ? We have in nature
the most varied conditions of temperature, of pressure,
and of chemical composition. Every corner of the earth
and of the ocean has been the laboratory in which these
experiments have been carried on. It is not necessary to
suppose that such an event as the formation of an organ-
ized being shall have occurred often. If in the whole

A A

354 /JV STARRY REALMS.

course of millions of years past it has once happened,
either on the land or in the depths of the ocean, that a
group of atoms, few or many, have been so segregated as
to have the power of assimilating outside material, and
the power of producing other groups more or less similar
to themselves, then we have but little more to demand from
the " Theory of Spontaneous Generation."

The more we study the actual nature of matter
the less improbable will it seem that organic beings
should have so originated. One of the most obvious
contrasts between organic and inorganic bodies seems to
be the power of motion often inherent in the organized
body, which is not possessed by the inorganic body ; but
this is really a superficial view of the question. Take
any mass of inorganic matter, a drop of water or a grain
of sand. Each of these bodies is composed of a cer-
tain number of ultimate atoms. We have no hope
that we shall ever have a microscope sufficiently power-
ful to detect these atoms ; but we nevertheless know
that they exist, and we know several of their properties.
We know, for instance, that even in solid bodies these
particles are not at rest, that they are in rapid and cease-
less motion, even though the body may be as rigid as a
diamond. In ultimate analysis we see that the atoms of
inorganic matter seem to have that mobility which is fre-
quently noticed as a characteristic of vital action. A
mere arrangement of the movements of the atoms of a
grain of sand could confer on the little object some of
the attributes of an organized body.

The method Darwin adopts is of the most captivating
simplicity. When the history of Science in the last

DAR WIN ISM AND OTHER BRANCHES OF SCIENCE. 355

century comes to be written, the interest will culminate
in the supreme discovery of Natural Selection.

There are so many modifying circumstances to be taken
into account that it is not often easy to trace the actual
course of natural selection ; but the leading idea is so
simple that, once it is properly stated, I do not see how
any reasonable person can refuse his assent. There is a
well-known proverb, "As like as two peas," and at a
superficial glance two peas are no doubt very like each
other. They are like in their size, shape, and colour;
they are like in their internal structure; but, when we
look closely into the subject, no two peas are exactly alike.
Take any two peas from a sack, and after a brief examina-
tion we can detect innumerable points of difference.
Weighed in a careful balance, they have not the same
actual weight ; gauged with a pair of callipers, they have
not the same size ; and these differences extend to every
minute part of the structure. One pea will have more
nourishment stored up for the benefit of the future plant.
Another will be better able to resist hurtful influences.
That two peas should be so absolutely identical in every
feature as to be indistinguishable is an impossibility, or,
as a mathematician would say, the chances are infinitely
against such an occurrence. If we find that two peas are
never really alike, we shall also find that no two organisms
of any kind are really alike when attention is directed to
minute points of distinction. A shepherd will laugh to scorn
the idea that any two of his flock are so alike that they could
be mistaken. Even his dog knows better than that. A
poultry fancier will see in his pets conspicuous marks of
difference which are barely apparent to the unskilled eye.

350 IN STARRY REALMS.

I need not multiply illustrations ; the innumerable variety
of roses and of geraniums, of apples, and of other fruits,
will show how universal is the law of variety among all
the productions of the organic world.

The great doctrine of Natural Selection is founded upon
this susceptibility to variation. Suppose that you wished
to improve the peas in your garden, it is quite possible to
do so in a few years in the following manner : Take 100
peas, sow them and preserve the seed. You will have
some thousands of seeds, but no two peas will be exactly
alike ; pick out the hundred heaviest seeds and sow them
again next season. You will have a crop of thousands,
from which you are again to pick out the heaviest hun-
dred. As this process is repeated year by year you will
find that within certain limits the peas are gradually
increased in size from one generation to another, and thus
it is that improved varieties can be artificially established.
The success of this process depends merely upon taking
judicious advantage of the variability inherent in the
organic world. This we may call an artificial selection as
opposed to the natural selection.

What we have here described as being produced artifi-
cially in the pea is going on everywhere on the grandest
scale in nature. Take an illustration this time from
animal life ; and I choose, as one of the most widely
known instances, some incidents in the history of the
common herring, which exists in such countless myriads
in our oceans. Those who frequent the sea are well
acquainted with certain features in the life of the herring.
The herring is a fish deservedly prized for food, but it is
not only mankind that are fond of devouring it ;

DARWINISM AND OTHER BRANCHES OF SCIENCE. 357

a similar taste is widely spread among the fowls of the
air and the fishes of the sea. The herring has no defence
from innumerable enemies but its agility and its caution.
Around the shoal swarm troops of porpoises, while pollack
and various other predatory fish devour the herrings in
myriads. The female herring lays a stupendous quantity
of eggs. It is perfectly certain that only a very minute
fraction of these eggs ever reach maturity. If only one
per cent, of the eggs grew to full size and reproduced
more herrings, the herring population of the sea would
multiply enormously every year. This cannot always, or
indeed often, be the case, and we are thus compelled to
believe that out of every million herring eggs only a
small fraction usually come to maturity. To those who
have ever observed the herring this appalling mortality
will not seem strange.

To begin with, when the eggs are laid the flat fishes
congregate and feast thereon to such an extent that fisher-
men repair to these spots and catch the flat fish in scores
with their stomachs filled with the eggs of the herring.
No doubt there are many other enemies at this stage,
so that vast multitudes of the herring eggs never become
hatched at all ; even those that are hatched have indeed
a bad time of it. Around our coasts we see in the
autumn shoals of the tiny herrings pursued and devoured
by hosts of young codfish and mackerel. Sometimes the
fish surround the shoal completely, and the miserable
prey cluster together near the top of the water in a vain
hope of safety ; but, alas ! here the enemies from the air
attack them. Sea-gulls crowd to the spot and swallow
the young herrings in mouthfuls, and once a shoal has

3S« IN STARRY REALMS.

been thus imprisoned between air and water, the slaughter
is truly prodigious.

The voracity of enemies is not the only danger to
which young herrings are exposed ; often they are left
on the beach by the falling tide, and may be seen lying
in hundreds along the sea margin. I purposely leave out
of account all mere human enemies. The efforts of man in
catching herrings are quite insignificant in comparison with
their more numerous and incessantly voracious destroyers.
Indeed Professor Huxley states that the codfish caught in
our seas each season would, if they had not been caught,
have eaten as many herrings during the next season as
those which have actually fallen to the nets of the fisher-
man. The survivors of this fearful massacre are natu-
rally objects of interest. How is it that they have
been spared when so many myriads of their brothers and
sisters have been annihilated ? No doubt their safety is
partly due to the chapter of accidents. They happened to
be out of the way when the mackerel made a fatal rush.
The sea-gull had eaten so many that when it came to
their turn , he positively could not eat any more. They
got into the middle of the shoal afterwards and escaped
the fish that preyed on its margin. But, making every
allowance for the benefit of the accidents, I think we must
credit the surviving herrings themselves with some share
in their success. The few that have survived were on the
whole certainly not the most stupid. They must have had
quick sight, they must have had nimble fins, they must have
had vigilance and activity. They must have been skilful
in procuring food as well as alert in avoiding danger.
They had no maternal solicitude to watch over them.

DAR WIN1SM AND OTHER BRANCHES OF SCIENCE. 359

Every little herring had to forage for himself, and to hide
from or elude his enemies as well as he could ; he had
no kind warning that the tide was falling and that he
would be left high and dry if he did not keep away from
the edge. I think we must admit that the few herrings
that survive out of a million eggs are above the average in
whatever qualities best adapt the herring for fighting his
battle of life. I will not say that they must be actually
the very best of the million, but I think we must admit
that they were among the best.

What we have here attempted to illustrate takes place
in the whole realm of organized life. The organic beings,
animal and vegetable, tend to increase faster than the
limit to the supply of food or the presence of enemies will
permit. Many must therefore perish. No two of these
organisms are exactly identical. There will be trifling
differences (sometimes, indeed, the differences are by no
means trifling). It thus happens that in the struggle for
life one individual will have a slight advantage over
another. It therefore may be anticipated that the more
favoured individuals will be those which survive ; their
peculiarities will be more or less inherited by their descen-
dants. Thus the variations which are useful to the animal
will in successive generations be gradually added to, and
in course of time the widest changes in organization can
thus arise.

It may at first seem hard to realise that so trifling a
change as that between one generation and the next can
ever by repetition amount up to so great a change as that
between one species of animal and another ; still less can
we imagine at first how animals so widely distinct as, for

300 IN STARRY REALMS.

instance, a bird and a fish can have originated by natural
selection from some common ancestor. The whole question
is, no doubt, complicated; but it is easy to show how
minute differences between one generation and the next,
all tending in one direction, speedily reach to an appre-
ciable aggregate.

Let me give an illustration. I know some tender
mothers who like to have their darlings photographed
every year in order to preserve a permanent record
of their development. No doubt the mother would have
no difficulty in distinguishing between the photographs
of her child at two years old or at three, or even
between those of her boy at thirteen and at fourteen.
But suppose that, instead of having the child photo-
graphed only once a year, he were to be photographed
every week from birth until he was full grown. This is
not at all an impracticable suggestion ; there would be
little more than a thousand photographs altogether. An
album could easily be made which would hold them all.
Of course the prudent mother would mark the dates on the
back ; but suppose this was not done, and the whole thou-
sand photographs got into confusion, would it be possible
to arrange them all in order again ? Certainly no out-
sider could do it ; he could sort them in a general way, so
as to have the babies at one end, and the young men at the
other, and the boys in the middle. But could he put the
whole thousand in regular order from one end to the
other? He could not. I doubt very much whether
even the mother herself could do it without numerous
mistakes,

Now if this be granted, the difficulty sometimes felt in

DAR WINISM AND OTHER BRANCHES OF SCIENCE. 361

believing natural selection to be the cause of species may
be lessened. Great as is the difference between a newborn
infant and a man of twenty, the one passes into the other
by such imperceptible gradations that the boy of this
Monday is hardly distinguishable from the boy of last
Monday or of next Monday. We thus see that if we
divide the growth of an individual man into one thousand
stages the passage from one stage to the next is almost
imperceptible. In the same way, if we subdivide the
growth of a species into a thousand parts or a million
parts, we shall have gradations quite comparable with
those we meet with in the ordinary variation from one
generation to the next.

Nor is it hard to see how the process of natural selec-
tion has gradually produced diverging branches from the
parent stem. The variations which occur may be of use
to the organism in various ways. Among the progeny of
A single pair there may be two individuals, A and B,
which are specially favoured ; but they may be favoured
in different ways. A may have some increased facility in
catching his prey ; B, by his peculiar colour, or greater
activity, may have superior success in eluding his enemies.
The descendants of A will gradually from one generation
to the next strengthen and reinforce the special feature
which characterized A. The descendants of B will grow
more and more adapted for eluding their enemies. The
influence of natural selection is in both cases promoting
the survival of the fittest, but each generation will see the
consins more and more widely separated. In no case, in-
deed, would the process be so simple as that here described
multitude of circumstances will occur to complicate it ;

362 IN STARRY REALMS.

but enough has been said to show that in the great prin-
ciple of natural selection we have a means of producing
animals and plants which in the course of time will differ
widely from other organisms from the same progenitors.

No one has ever seen a new species developed by
natural selection ; but this is because no one has ever
lived long enough for that purpose. The circumstantial
evidence in favour of natural selection is indeed so strong
that no unprejudiced person can refuse to accept it. That
evidence has of late years been poured out with a pro-
fusion which could hardly have been anticipated at the
time when the " Origin of Species " was published. En-
tombed within solid rocks we find fossil remains of the
former inhabitants of our earth. There lies in these rocks
a record of vast extent and of the utmost interest ; but
that record is to a great extent screened from our view.
Here and there fossils have been brought to light; but
the greater part of the earth has never been examined,
and we have as yet only the veriest fragments of the
geological record before us. But these fragments of the
record are of great importance ; they show us several
of the links which connect one class of animals with
another in the way the Darwinian theory suggests ; and
they encourage us to hope that, when the geological
record shall have been fully explored, we shall have
glimpses of a majestic panorama of the salient points in
the history of life on our globe.

Mathematicians are long accustomed to the use of what
is known as the infinitesimal calculus. It is indeed
chiefly the infinitesimal calculus which has raised the
science of mathematics to its present position, and which has

DAR W2NISM AND OTHER BRANCHES OF SCIENCE. 303

given to that science a potent grasp over some of the inmost
recesses of nature. Suppose, for instance, to take one of
the most profound problems, we proceed to investigate on
mathematical principles the movement of one of the
planets. The sun, in the first place, attracts the planet,
and in virtue of that attraction the planet would move in
a certain path which could he determined with compara-
tive ease. But the actual problem is by no means so
simple. The planet is acted on by other planets ; its orbit
is thus deflected slightly from the simple form it would
otherwise have ; and while the orbit preserves a general
resemblance to the ellipse, it is in reality a path of no
little complexity. But still the mathematician can follow
the planet with his figures he can point out with
accuracy where the planet was at any ancient date ; he
can show where it will be at any future date. It is the
infinitesimal calculus, the invention of Newton and Leib-
nitz, which enables this to be done. By this subtle and
exquisite contrivance we attack the problem in detail.
It is comparatively easy to find out the direction in which
the planet is moving at any instant, as well as its velocity.
This will enable us to ascertain where it will be in the next
moment of time. We then repeat the operation and
carry on this process as long as we like, and thus discover
where the planet will be at any future date. The success
of the process consists in attacking the question in detail.
IB there not in this a striking analogy to the great
principle of Darwin ? In each case great effects are pro-
duced by the constant addition of innumerable small
tendencies, all in the same direction. As the infini-
tesimal calculus of Newton has led us to a knowledge of

364 fff STARRY REALMS

the physical laws which regulate the universe, so the infini-
tesimal calculus of Darwin has afforded the solution of the
profound problem presented by organic life.

It must have been with prophetic insight that Cuvier
exclaimed, "Shall not natural history some day have its
Newton ?" At the time these words were uttered the
Newton of natural history had been born, and his immortai
work has revolutionized knowledge.

INDEX

INDEX.

AIR, great wave caused by Kra-

katoa explosion, 326-330
Andromeda, nebula in, 124
photograph of, 315-317
Andromedes, 245, 246
Ant, brain of, 351-353
Astrology, survivals of, 196
the number seven, 197
days of the week, 198-20
Atmosphere, of moon, 8} ,- ut
planets, 86

B

BBAGLB, voyage of they 343, 344

CARBON, formation of, in trees,

42-44

Chart of the heavens, 311
Climatic changes, 4
Coal, prehistoric sunbeams, 44
Colours of stars, Doppler's theory,

280

affected by motion, 278-281
Constellations, changes in forms of,

264, 267
Cnvier quoted, 364

DAT, length of, increasing, 64, 66
ancient five-hours' day, 66
future 1,400-hours' day, 72, 73

Days of week, names of, 198-200

Darwinism and astronomy, 343-364
" The Origin of Species," 344
natural selection, 345, 355, 356,

361, 362

the past of the sun, 346-348
history of solar system, 348
genesis of earth, 349
organic evolution, 350
spontaneous generation, 354
accumulations of variations,

359-361
Denning, Mr. W. F., observation

of a shooting star, 211-214
Distances of stars, 259, 260
Doppler, theory of star colours,
280

EABTH, attraction by the sun, 2, 5
elements common to it and sun,

19

tides and the moon, 63, 74
tides and the sun, 75
antiquity of, 65, 66
genesis of, 349
length of day increasing, 64,

66

ancient five-hours' day, 66
future 1,400-hours' day and

month, 72, 73
speed of rotation and the moon,

67

moon receding from, 67
ancient propinquity of moon,

68

will present a fixed face, 74
as seen from moon, 82, 95
accumulation of meteoric de-
bris on, 230

368

INDEX.

Extent of starry heavens, 255, 304
range of telescope, 256-258
distances, 260

FALLING stars, fire-balls (see Me-
teors)

G

GALILEO, view of Venus, 141

Georgium Sidus, 201

Great spiral nebula, 140

Green, N. E., drawings of Mars,

159, 161, 162
Gulliver's Travels, 41, 179

HALL, Prof. Asaph, discovery of

moons of Mars, 168-172
Heavens, chart of, 311, 312
Herring, 357, 358
Herschel, Sir W., as an observer,
54

and Saturn's rings, 121
Huggins, Sir William, discoveries
in nebula of Orion, 134, 136
work in spectrum analysis,
262

INFINITESIMAL calculus, 362, 363

JANSSEN'S photographs of the sun,

295
Jupiter, orbit, 180, 181

rotation, 182

belts, 184

phases, 185

solidity of, doubtful, 185, 187

great red spot in, 186, 187

bulk of, 188

mass, 188, 189

internal heat, 189

clouds, 190, 191, 194

solar heat on, 191-193

photograph of, 298

K

KNOBEL, observation of Mars, 164
Krakatoa, the eruption, 319-342

investigation by the Royal
Society, 320

description of locality, 321

previous eruptions, 321

warnings, 322

the catastrophe, 323, 324

compared with other eruptions.
*! 325

the great air wave, 326-329 •
its rate of motion, 330

area of the sound, 331-334

great sea-waves, 334

sky phenomena, 335

dust clouds, 336-338

after-glow, 339-341

LEONIDS, 238, 242

Lick Observatory, 103-106

the great refractor, 105
Light and colour rays, 277

of 18,000 years ago, 260

M

MAGNETIC needle and sunbeams I
Mars, discovery of satellites of, 75r

76, 151, 168-172
and Venus, 141
rotation of, 144, 153, 154
revolution and orbit, 151, 157,

158

position in solar system, 150
and the earth, 152, 156
atmosphere of, 162
through the telescope, 153, 16*
clouds on, 163
in opposition, 154-156
colours of surface, 163
south pole, pictures of, 1 59
white polar regions, 162
Mr. N. E. Green's drawings,

159, 161, 162
mapping of, 160
zero of longitudes, 161
Knobel's observations, 164
Schiaparelli's " canalb, " 166-

167
Swift's guesses, 179

INDEX.

3<>9

Material unity of the Universe, 263
Mercury, rotation and revolution,

145

Schiaparelli, 145-146
constant face to the sun, 146
tidal action on, 148
early discovery of, 197
Meteoric dust, 208-209

debris on the globe, 230
Meteors, inflinr of into the sun, 21-

23
»hooting-star, autobiography

203-209

constitution of, 203
flight, 203, 215
wanderings in space, 204
combustion, 207
height of, 209, 210, 213
Mr. W. F. Denning' s ob-
servations, 211-214
fireballs of 1877-8, 217

1879, at York, 218, 219
1876, in America, 220
brilliancy of, 218
detonation, 221
energy of, compared with

powder, 223, 224
energy of, compared with

steam, 224
luminous trails of, 225-

227

star showers, 228
periodicity of, 235, 237
early records of showers, 236
telescopic shooting stars, 229,

230
November showers of 1866,

231-234
November showers of 1698,

237

Leonids, 238, 242
the Great Shoal, 238-246
its width, 239
its intersection of earth's

orbit, 242, 245
Perseids and Andromedes,

245

Milky Way, 121

Months, future, of 1,400 hours, 73
Moon, smallest of visible orbs, 49
dimensions, 50
unchanging face, 52
dark side of, 53-55

Moon (continued) —

rotation on axis. 55-57

the rotation paradox, 56

revolution, 57

lunar history, 57

ancient heated condition, 58

cooling of, 61

active lunar volcanoes, 58

lunar tides, 58-62

their effect, 63
absence of water on the, 59
of life, 84
of ice, 85
of air, 86, 88
what became of lunar water.

85

and terrestrial tides, 63, 74
and earth's speed of rotation,

67
recession of, from the earth,

67
ancient propinquity to earth,

68

mountains on, 90
craters, 90

the crater Ptolemy, 75
why does the moon not fallf

78,79

non-luminous, 80
earth seen from, 82, 95
old, in new moon's arms, 82
whether inhabited, 82
through the telescope, 83, 84
indications of gaseous material

on, 87

weights on, 94
origin of, 69-71
future career, 71-73

N

NATURAL Selection, 345, 355-356
Nebula in Andromeda, 124, 315-
317

Great Spiral, 140

Orion, 127, 128, 133, 265, 317

Huggins' discoveries, 134, 136
Nebulae, 124, 125, 132

and spectrum analysis, 137

Newton's discoveries, 77, 79-80

Niagara, debt of, to the sun, 46

November star showers (tet

Meteors)

B

INDEX.

OBSEBVATORY, women's worlt in the,

97, 169

prosaic labours in, 98-99
conditions of observation, 100
impediments to work, 101
the Lick, 103-106
the Lick refractor, 105
meridian circle, 107-110
equatorial, 110
Orion, Great Nebula in, 127, 133-

134, 265, 317
composition of, 131
presence of hydrogen in, 136-

•138
unknown spectrum lines in,

137

magnitude of, 138
Orionis Theta, 129, 130

PEBSETTS, 121, 122

distance of cluster, 128

Perseids, 245

Photographing the stars, 292, 318
atmospheric difficulties, 294
Janssen's sun-pictures, 295
photographs of moon, 296
photographs of planets, 297
of Jupiter, 298
of Saturn, 299
of extra-telescopic, 300, 301
Mr. Roberta's work, 303, 315
instruments for photography,

305-310
charting the heavens, 311,

31 Ji

work at the Draper Memorial,
318

Planets, rotation of, 54
atmosphere of, 86
how to weigh, 115, 172-176
Mars, Venus, Jupiter, 142
compared with stars, 149-

150

and tiieir satellites, 151, 168
relative size of, and sun, 183
heated stage of, 193
names of, 195, 200, 201

Pleiades, 266, 316

REFRACTION, 102, 103

Roberts, Dr. Isaac, and oelestia)

photography, 303
charting the heavens, 313
photograph of the Andromeda

Nebula, 315, 317
photograph of the Pleiades,
316

SANDWICH Islands and lunar land-
scape, 91, 92
Saturn, period of revolution, 113,

114

bands, 115
weight, 116
rings, 116-118, 121
photograph of, 299
SchiapareUi, observation of Mer-
cury and Venus, 145-147
his "canals" onMars, 165-167
Sea-waves, great, caused by Kra-

katoa, 334
Senses, two, useless on the moon

92, 93

Shooting stars (see Meteors)
Sound, eruption of Krakatoa, 331-

334

detonation of fireballs, 221
Spectroscope and star-movements.

262

motion in line of sight, 268-272
explanation of, 272-289
musical illustration of , 287-289
light and colour rays, 277
colour affected by motion, 278,

279, 281

value of the dark lines, 283
Spectrum analysis, 19, 135-137,

262
elements common to earth and

sun, 19
material unity of the universe,

263

Spontaneous generation, 354
Stars, growth and decay of, 35, 37
dark, 37, 38
double, 129
colour of, 278-281

INDEX.

37i

Stars (continued)—
triple, 130
number of, 248, 254
suns, 251-253

extent of starry space, 255, 304
light of 18,000 years ago, 260
no "fixed," 264
change of groupings, 264
movements in space, 267
motion and spectroscope, 268-

272
photography of, 292-318

invisible, 300, 301
Star-worship, 200
Sun, attraction of, and the earth,

2,5
heat-supply constant, 3, 5, 6

expenditure of heat, 7-9,

11, 16
elements common to it and

earth, 19
the " fuel " problem, 6, 7, 24,

25

not explained by chemical ac-
tion, 17-20
nor by mechanical, 21
true explanation, 24-34
dimensions of, compared with

earth's, 12
density of, compared with

earth's, 25

prominence of, corona, 14
rotation of, 53
influx of meteors, 21-23
temperature and radiation,

30-32
future of, ultimate dark and

cold, 35, 36

nebular origin, 39, 46, 346-b
pre-nebular stage, 47, 48
what we owe to the, 40
tidal action, 75
heat of, on Jupiter, 191-193
Janssen's photograph* of, 295

Sunbeams and magnetic needle,
out of cucumbers, 41, 42
pre-historic in coal, 44
all artificial light due to, 45
and water power, 46

Swift, Dean, his guesses, 41, 179

TELESCOPE and the moon, 83, 84
range of vision, 256-258

Tidal action on earth, 63, 74-75
on moon, 58, 62, 63
on Venus and Mercury, 147-
148

Transit of Venus, 158

Tycho Brahe, 98

U
UNITY, material, of universe. 263

VEGA, light-distance of, 269

Venus and Mars, 141

Galileo's view of, 141
heat, light, atmosphere, 143
rotation, length of day, 144

145

Schiaparelli' s observations, 147
one face to the sun, 147
tidal action on, 148
transit of, 158

W

WATER in the moon, 85
"Weights on the moon, 94
Weighing planets, 115, 172-179
Women and astronomical work, 97
169

ZEBO of longitudes on Mars, 161

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(2190)

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