The “long-period hypothesis” has, however, almost ceased to need such

support. Schiaparelli’s facts are inconsistent with any other; and they

are scarcely controvertible. They have besides, as in the case of

Mercury, been verified by Mr. Lowell’s recent observations. Assuming,

then, its truth, we may consider what it implies. Since the rotation and

revolution of Venus synchronise, she always looks inwards toward the

sun, perpetual day reigning on one hemisphere, perpetual night on the

other. And these regulations are much more strictly conformed to than on

Mercury. For the orbital motion of Venus is so nearly uniform that

libratory effects count for very little. The equatorial breadth of the

libration-zones, where light alternates with darkness, is only

thirty-three miles. On the other hand, the atmospheric diffusion of

sunshine is a powerful illuminating agency. The meteorology of the

planet presents great difficulties. Its conditions are so remote from

our experience that we can barely sketch out their results. The most

obvious of these is the vehement aerial circulation which must proceed

without ceasing between the hemisphere upon which the sun never rises

and the hemisphere upon which the sun never sets. We should expect it to

be accompanied by agitated conflicts of winds, and surgings of the

atmosphere from its lowest to its highest strata, betrayed by rendings

of the brilliant condensation-canopy, by the rapid transport of torn

scuds, and wheeling vortices of clouds. But nothing of all this is

telescopically visible. The aspect of the morning star suggests serenity

rather than interior tumult.

One of the most remarkable instances of persistent optical illusion

refers to a supposed satellite of Venus. It was first seen by Fontana at

Naples in 1645; it was last seen by Horrebow at Copenhagen in 1768; and

the intermediate observations were numerous, usually careful, and

apparently authentic. Yet the body, of which they affirmed the

existence, was purely fictitious; and it is a suggestive circumstance

that it never ventured into the field of view of an achromatic lens.

Comparing the two planets nearest to the sun, the first spontaneous

impression is of astonishment at their unlikeness. One travels in an

almost circular, the other in a highly eccentric orbit. One possesses a

dense and extensive atmosphere; the other is barely gauze-clad, and is

hence exposed to almost unmitigated extremes of temperature, while the

conformation of its solid surface is left open to telescopic scrutiny,

impeded only by the inconvenient glare of the sun. That surface is of a

reddish hue, and absorbs more than four-fifths of the light with which

it is flooded; the disc of Venus being, on the contrary, of a dazzling

whiteness, and little less reflective than a summer cloud. Yet these two

globes, so dissimilar individually, have apparently had the same destiny

prepared for them. Deprived of all but a remnant of their rotation by

the frictional resistance of sun-raised tides, they were debarred from

the production of satellites, and subjected to what we, in our

ignorance, might be apt to call fantastic climatal conditions. With due

reserve it may be added that they have thus apparently been rendered

unfit to be the abodes of highly developed organisms. Why this has been

so ordained we are unable to conjecture; we must wait to know.

CHAPTER V.

THE EARTH AND MOON.

The earth occupies a critical position in the solar system. Its greater

distance from the sun preserved it from the fate of Mercury and Venus.

The influence of solar tidal friction fell short of predominance over

the terrestrial future. All that it could do was to defer to the latest

possible moment (so to speak) the separation of the moon, the

comparatively large size of which was doubtless due to this

postponement. For a viscous body, such as the earth must then have been,

can bear much more rotational strain than a less coherent mass; but when

the strain comes to be relieved, the needful sacrifice of material is

proportionally greater. The process of fission, instead of being a mere

incident, becomes a catastrophe. The most violent explosions are

precisely those which are longest delayed.

Had the earth then been situated a few millions of miles nearer to the

sun there would have been, so far as we can see, no moon; and the

terrestrial day and year would have been of equal length. This

equalisation was rendered impossible by lunar influence.[28] We are

indebted to our satellite for the alternations of day and night which

make life possible. How this came about is quite clear upon some brief

consideration. Lunar tides are now about three times more effective than

solar tides, and at their origin the disproportion was enormous. Their

power might be called exclusive. Now, how was that power exercised?

Primarily, in compelling an agreement between the duration of the month

and day—that duration, to begin with, being of only a few hours. The day

might, and in the long run did, fall short, but it could not possibly

get ahead of the month. Hence the earth’s rotation was for ages

protected against the destructive agency of solar tidal friction. By the

time that the moon left it, as it were, to take care of itself, the

plastic stage, during which alone rapid change could take place, had

passed, and the earth was solid and secure.

Thus, the axial rotation of our planet in twenty-four sidereal hours is

the outcome of a delicate balance of relations established in the “deep

backward and abysm of time.” Its shape matches, or has accommodated

itself to the period, which has perhaps not varied much since the epoch

when interior fires were first banked in by the formation of a rigid

crust. The compression of rotating globes is so connected with the

quickness of their spinning that one can be calculated from the other;

and the earth’s theoretical compression, or ellipticity, is found to be

practically identical with its measured ellipticity of about ¹⁄₂₉₃. Its

mean diameter is 7,927 miles; the equatorial is 26 miles longer than the

polar diameter; so that the globe is belted with a protuberance, 13

miles high, corresponding to the excess of centrifugal force at the

Equator.

The heat by which it was originally maintained in a liquid condition is

still in process of dissipation. A small part escapes year by year, but

enough remains to keep the earth _alive_ for ages to come. Were the

supply exhausted, the oxygen of our air, and the water forming our

oceans, would be rapidly absorbed, chemically and mechanically, and with

them, vitality should disappear. Volcanic action, in some of its many

forms, is accordingly a condition of existence. One unmistakable symptom

of central fires still glowing is the increase of subterranean

temperature. It averages one degree Fahrenheit for fifty-five feet of

descent. Below two miles then, water can only remain liquid through the

compulsion of the overlying strata, the slightest relaxation of which

occasions it to flash explosively into steam; the devastating power of

“super-heated” water being one of the chief causes of volcanic

outbreaks. The growth of temperature downward cannot be supposed to

proceed indefinitely; otherwise, a fabulous thermal state would be

reached long before we got near the core of the globe; but the region of

maximum heat depends upon an unknown quantity—that is, the lapse of time

since the antique lava-globe began to crust over. Assuming it to be

fifty million years, Lord Kelvin showed that the limiting temperature of

about 5,400° F. is located not more than fifty miles from the surface.

But 5,400° approaches the temperature of the electric arc, at which

there is an all but universal vaporisation of material substances, and

rocks liquefy while comparatively cool. Diabase, for instance, a typical

basalt, is completely fluid at 2,200° F. On the other hand, the pressure

at 50 miles beneath the earth’s surface is of inconceivable power; and

it is employed in resisting the expansive tendency of heat. The

condition of matter subjected to these opposing and potent influences we

are unable to divine, and have no means of ascertaining. We do, however,

know from the results of various astronomical lines of enquiry that the

earth is effectively as rigid as steel. Its mean density is about five

and a half times that of water, the entire globe being more than twice

as heavy as if made of the ordinary surface rocks. This, however, is not

surprising, since oxygen enters largely into the composition of the

exterior strata, while the subjacent materials are likely to be in large

measure metallic.

The epoch of the earth’s superficial solidification has again, quite

lately, been under discussion. “The subject,” Lord Kelvin wrote, “is

intensely interesting. I would rather know the date of the

_Consistentior Status_ than of the Norman Conquest; but it can bring no

comfort in respect to the demand for time in palæontological geology.

Helmholtz, Newcomb, and another (Kelvin) are inexorable in refusing

sunlight for more than a score, or a very few scores of millions of

years.”[29]

Improved data having been substituted, the problem was solved anew, with

the result of very notably diminishing the “age of the earth.” It is for

the present fixed at twenty-four million years, and upon such strong

evidence as to “throw the burden of proof upon those who hold to the

vaguely vast age derived from sedimentary geology.”[30]

The earth is the largest of the terrestrial planets; and it is

specifically the heaviest of all the planets. Its compactness is more

likely to be a consequence of a particular relation between internal

temperature and pressure, than of a difference in chemical constitution.

The mass of its atmosphere can be directly determined. We have only to

look at a barometer in order to gain the information that our “cloud of

all-sustaining air” weighs as much as a universal ocean of mercury

thirty inches in depth. The corresponding depth of air, were it of the

same density throughout, would be nearly five miles. But it is _not_ of

the same density throughout. With each three and a half miles of ascent,

atmospheric pressure is halved; and the interval is lessened by making

due allowance for decrease of temperature upwards. To the succession of

these tenuous strata, no definite end can be assigned. The duration of

twilight shows that, above forty-five miles, they cease to reflect

light; yet meteors can be set ablaze at heights up to 120 miles, through

the resistance offered to their motion by air reduced to

1/250,000,000,000th its density at sea-level!

The cloud-bearing capability of the atmosphere has only of late been

fully recognised. Ordinary cirrus float about five miles high. On

December 4, 1894, an aeronaut, Dr. A. Berson, passed right through a

bank of them at an altitude of five and a half miles, and was able to

verify by actual contact their composition out of snow-flakelets.[31]

But since 1885, a still more delicate kind of floating formation has

come within our acquaintanceship. “Luminous night-clouds” were first

noticed by Ceraski; they have been systematically studied by O. Jesse of

Berlin.[32] They appear long after sunset, between May and July, and

derive their silvery radiance from the sun-rays which their elevated

situation enables them to intercept, while all below is wrapt in

darkness. Their height has been determined, from the comparison of

photographs taken simultaneously at different places, to average

fifty-one miles, and to range from fifty to fifty-four miles. They are

an entirely new order of phenomenon.

This globe upon which we dwell is a great magnet. Its directive action

upon the compass sufficiently proves the fact. But it is a magnet

probably only by virtue of the electric currents which course round it.

And since these currents originate from diverse interacting causes, the

laws of terrestrial magnetism are necessarily complex. They are

conditioned, yet not prescribed by the earth’s rotation. The magnetic

and geographical systems of co-ordinates approximate, but by no means

coincide. The former is, indeed, both complex and variable.[33] The

inclination, or “dip,” of the needle does not vary in the same way as

the declination, or horizontal position. There are two points on the

earth’s surface, called “poles of verticity,” where a magnetic needle,

freely swung, points vertically downward. One is situated in the arctic

peninsula Boothia, the other on the antarctic continent within a few

hundred miles of Mount Erebus. An intermediate line where the needle

poises itself horizontally, corresponds roughly with the geographical

equator. Each hemisphere contains besides two centres of maximum force,

by the joint action of which magnetic deviations from true north and

south are determined. Their mutual relations are highly intricate. The

North American focus is stationary, the Siberian focus oscillates. Their

relative and absolute intensity is probably also subject to

fluctuations. Hence the inconstancy of magnetic directive influences.

The variation of the compass varies.

It varies hour by hour, as well as year by year. The needle performs a

diurnal oscillation, reaching an eastward maximum about eight A.M., and

a corresponding westward maximum towards four P.M. Moreover, the range

of this vibration increases concordantly with the growth of spotted area

upon the sun, and falls off again as spots diminish (see Fig. 2). The

cosmical relations of terrestrial magnetism are emphasised by the

obvious connexion between a disturbed state of the sun and the

occurrence of “magnetic storms.” During these crises, the smooth

progression and regression of the needle are superseded by violent and

irregular movements. The photographic tracing in which they are recorded

presents only a series of lawless zigzags; earth-currents are set up;

telegraph-wires transmit messages without batteries; and the skies are

at night draped with auroral streamers.

Auroræ are possibly a survival of our planet’s original self-luminosity.

If so, their dependence upon the terrestrial magnetic system is highly

significant. They obey the magnetic period, they accompany magnetic

disturbances, they illuminate magnetic lines of force. That they are

immediately caused by electrical discharges in the high vacua of our

upper air is no longer doubtful. In these latitudes, the auroral arch

and crown are formed at a height of ninety to one hundred miles, in

(about) 1/1,000,000,000th of an atmosphere; but in the polar regions

they approach much nearer to the earth. There, indeed, they more usually

assume the form of a curtain, undulating in luminous folds, and

traversed by vertical electric currents. That they are so traversed is

demonstrated by the behaviour of the magnetic needle, the deviations of

which change their sign as the auroral drapery crosses the zenith.[34]

Auroræ seem to be confined to two zones of the earth, which, like the

sun-spot zones, approach the equator as the solar cycle advances. Their

frequency in temperate regions corresponds, accordingly, to a scarcity

in high latitudes. The auroral spectrum consists of a number of bright

rays, one of which is invariably present, and seems to be essential and

fundamental. Its origin is unexplained.

The velocity of the earth in its orbit exceeds more than sixty times

that of a cannon ball just leaving the muzzle of an eighty-ton gun. In

other terms, the third planet from the sun travels at an average rate of

18½ miles per second. Its albedo has been estimated—probably

under-estimated—at 0·30. This would leave 70 per cent. of the solar

emanations striking the upper surface of its atmosphere available for

interior consumption. Most of this supply is absorbed or scattered in

the atmosphere. The proportion sent back to space after reflection from

the actual terrestrial surface must be extremely small. Very little

topographical detail could be made out by telescopic scrutiny from the

moon or Venus. At the most, the trend of some great mountain ranges,

such as the Andes and Himalayas, and a dozen snow-clad peaks, could be

visible. No sign of the teeming organic life brought forth by mother

earth could be detected from without.

The more we know of the moon, the less inviting, from our point of view

as animated beings, it appears. It is a harsh and inhospitable world,

from which vital possibilities, if they were ever present, have plainly

long ago departed. The diameter of our satellite is 2,162 miles. Its

disc, so far as the most exact measurements tell, is perfectly round.

This in itself indicates a slow rotation; and even casual observations

suffice to show that they relate to only one lunar hemisphere. Rotation

and revolution here again synchronise. In 27 days 8 hours (nearly), the

moon executes one circuit of the earth, and one gyration on its axis.

The coincidence was brought about in remote ages by the power of

terrestrial tidal friction. The averted hemisphere does not, however,

remain wholly invisible. Two-elevenths of it are, by the effect of

librations, both in longitude and latitude, brought piecemeal into view.

But the additional “lunes,” thus thrown open to glimpses round the

corner, are greatly foreshortened.

The area of the moon is somewhat less than one-thirteenth that of the

earth. Yet room could be found there for the entire British Empire, with

six million square miles to spare. Its volume is ¹⁄₄₉th, its mass

¹⁄₈₂th, the volume and mass of the earth. Hence the lunar materials are

less dense than the terrestrial in the proportion of about three to

five. But this may be because they are under comparatively slight

pressure.

At the moon’s surface, gravity possesses only one-sixth its power here,

so that a stone thrown upward with equal force would reach a six-fold

height. Further, a projectile shot straight from our satellite with a

velocity of one and a half miles a second would never return, while a

speed of seven miles a second is just controllable by the earth, to say

nothing of the immense efficacy of her dense atmosphere in hindering

escape from her precincts. No terrestrial bomb, it may therefore be

safely asserted, has ever been hurled into space, although volcanic

ejecta may very well, in past ages, have made their way hither from the

moon.

But lunar volcanoes are no longer active. Only their remains stand as

records of a fiery past. In guiding a telescope across the scarred face

of our satellite we seem to traverse a volcanic charnel-house. The

evidence of ancient seismic action on the moon is overwhelming. Its

surface is pitted all over with cones and craters. Nearly 33,000 are

marked on Schmidt’s map, and the list is very far from being exhaustive.

The resulting chiaroscuro is obvious to the naked eye. Dante tried to

explain it in the “Divina Commedia”; Galileo detected its cause and

manner of composition. The chief facts about it are these.

[Illustration:

FIG. 10.—_Map of the Moon._ (From Fowler’s “Telescopic Astronomy.”)

]

1. Furnerius

2. Petavius

3. Langrenus

4. Macrobius

5. Cleomedes

6. Endymion

7. Altas

8. Hercules

9. Römer

10. Posidonius

11. Fracastorius

12. Theophilus

13. Piccolomini

14. Albategnius

15. Hipparchus

16. Manilius

17. Eudoxus

18. Aristotle

19. Cassini

20. Aristillus

21. Plato

22. Archimedes

23. Eratosthenes

24. Copernicus

25. Ptolemy

26. Alphonsus

27. Arzachel

28. Walter

29. Clavius

30. Tycho

31. Bullialdus

32. Schiller

33. Schickard

34. Gassendi

35. Kepler

36. Grimaldi

37. Aristarchus

A. Mare Crisum

B. Mare Fecunditatis

C. Mare Nectaris

D. Mare Tranquilitatis

E. Mare Serenitatis

F. Mare Imbrium

G. Sinus Iridum

H. Oceanus Procellarum

I. Mare Humorum

K. Mare Nubium

V. Altai Mountains

W. Mare Vaporum

X. Apennine Mountains

Y. Caucasus Mountains

Z. Alps

The general albedo of the lunar surface is 0·17; but portions of the

disc are as obscure as basalt or obsidian, while isolated spots glitter

like snow-peaks. The former are usually admitted to be the oldest of

conspicuous lunar formations, the latter to be comparatively recent. The

dusky spaces too, are dead levels, if not depressions; they were

formerly taken for seas, and retain the name of “Maria.” One “ocean,”

extending over two million square miles, is included amongst them. This

is the “Oceanus Procellarum” (see Fig. 10), which is five times larger

than its nearest rival, the “Mare Nubium.” The late Mr. Gwyn Elger

regarded the lunar “seas” as lava outflows, by which certain earlier

formations were all but obliterated. M. Suess explains them as areas

where the primitive thin “slag-crust” re-melted. To the same category

belong the vast “bulwark plains,” the ramparts enclosing which are of so

wide a sweep as to be, not merely “hull-down,” but completely invisible

to an imaginary spectator placed at their centres. Yet Pelions by the

dozen are tumbled upon Ossas for their construction, with here and there

an Olympus flung on the top. Typical examples are Ptolemæus, 115 miles

across; and Plato (near the Northern Pole), “sixty miles in diameter,

with its bright border and dark steel-grey floor.”[35]

The bottoms of lunar craters and “circuses” are nearly always

depressed—sometimes thousands of feet—below the general level. Thus, the

central peak of the great crater Copernicus towers to 11,300 feet above

the depressed plain from which it rises, but surmounts by only 2,600

feet the average level of the moon.

Successive stages of activity have left ineffaceable marks upon this now

stereotyped page. Groups of immense craters mutually encroach, and seem

to have been scooped out of each other’s flanks, like Kilauea from Mauna

Loa; craters occur within craters, as Vesuvius inside the broken rampart

of Somma; and the most recent are invariably the deepest and steepest.

Cup-shaped depressions or “crater-pits” are innumerable; they result,

according to Suess’s theory,[36] each from a single explosion, the

bursting of a “big bubble” of gas in a cooling lava-field. Mountain

ranges are profusely strewn with them. These lunar Alps and Apennines

appear to be as unmistakably igneous in their origin as Tycho or

Aristarchus. They are colossal slag-walls. There are apparently no

sedimentary deposits upon the moon. Aqueous action had no concern with

its geological history. Yet on the earth water is essential to the

production of volcanic phenomena. If they are to be developed without

it, M. Angelot concludes, it must be by explosive escapes from

solidifying materials, of gases absorbed by them when in a state of

fusion.

The mountains of the moon are much higher, proportionally, than the

summits of the Hindu-Kush, or of the Himalayas. Mount Everest, reduced

to the lunar scale, would be a modest elevation of 8,200 feet; while

pinnacles in the lunar Apennines spring up to 22,000 feet, and

crater-peaks of eighteen or twenty thousand abound. The disparity is

scarcely surprising when it is remembered that there the convulsive

throes of cooling were restrained by gravity reduced to one-sixth the

power it exerts here.

Among the puzzles of selenography are the objects termed respectively

“rills” and “rays” The former are very numerous. Considerably more than

a thousand of them have been mapped or photographed. They resemble the

cañons of Colorado. Some few run to 150 miles; most are a couple of

miles wide, and above a quarter of a mile deep. Their volcanic origin

cannot be doubted. The “rays” diverge in extensive systems from such

huge ring-craters as Tycho and Copernicus. They cast no shadows, and

come out best at full moon, circumstances suggestive of their being

immemorial lava-streams bleached by the chemical action of fumes from

the interior. The whiteness of Aristarchus has been similarly explained;

but accumulations of pumice and snow-like volcanic ashes perhaps enhance

the effect. The flashing back by this wonderful peak, of earthshine at

determinate angles of illumination, has often counterfeited the vivid

glow of actual eruptions. Their possibility, however, belongs to the

past. Nor have any of the rumoured alterations in lunar topography,

which from time to time excited interest and raised controversy, made

good their footing as solid facts. Agencies of change are certainly

there, in tidal strains and alternations of temperature, but they work

very slowly. There is no erosion by air or water; no grinding by ice; no

transport of materials. Repose reigns apparently undisturbed. Lunar

landscapes exhibit abrupt transitions from the blinding glare of crude

sunlight to the blackness of absolute shadow. Their aspect excludes any

but the thinnest possible atmospheric remnant To all intents and

purposes, the moon is an airless globe. Occultations of stars afford a

very refined test of this condition; and their instantaneousness alone

suffices to demonstrate its reality. Spectroscopic evidence is to the

same effect. Dr. Huggins watched, January 4, 1865, a _prismatic_

occultation of the small star, ε Piscium. Had there been the slightest

inequality of dispersion or absorption at the moon’s limb, it could not

have failed to be perceived. There was none. The spectrum remained

unaffected, and vanished abruptly, all the colours together. And

moonlight, analysed by the most powerful apparatus, varies not an iota

from sunlight. It is reflected without the smallest selective change.

The absence of water is equally well attested. There are no river-beds

to be seen, no rounded surfaces, no alluvial plains. A mosquito could

not find a moist corner to lay its eggs in. There is nothing to show

that this was otherwise in any past age, although it is not improbable

that the lunar rocks contain large volumes of oxygen once free. As

regards the earth, we can entertain no doubt that a goodly proportion of

its original atmosphere and oceans is now permanently lodged in its

bedded crust. But the geological histories of the earth and moon

probably diverged from the first.

Indeed water, as such, could probably not exist upon the moon’s surface.

It would promptly take the form of ice. Professor Langley has shown that

the temperature prevailing there, under vertical sunshine, is about that

of frost; while it sinks, during the moon’s long night of fourteen days,

almost to absolute zero. This frigid state is due to the absence of

atmospheric protection, leaving heat free to depart into space as fast

as it is received. Thus, of the small quantity of heat contained in

moonlight, nearly the entire comes to us by mere superficial reflection;

a minute residuum only is absorbed previously to being emitted. The

distinction is brought into view by comparing the solar and lunar

heat-spectra, when moonlight is found to contain longer invisible heat

waves than can be detected in sunlight Moreover, Professor Frank Very,

through his experimental demonstration that the equatorial are slightly

hotter than the polar regions, has established the fact of a slight

retention of heat by the moon’s substance. How slight the retention is,

has been proved by Dr. Boeddicker’s observations with the Rosse

three-foot speculum, showing that, during total eclipses, moon-heat

vanishes almost completely. Less than 1 per cent, survives. The thermal

phases are not, however, identical with the luminous phases.

The eclipsed moon, on June 10, 1816, is said to have been utterly lost

to sight; but, as a rule, with very few exceptions, our satellite

traverses visibly the densest part of the earth’s shadow. Even during

“black eclipses,” such as that of October 4, 1884, a dusky spot remains

as an index to its locality; while in “red eclipses,” the great craters

and bulwark plains can be easily distinguished with an opera-glass.

Occasionally, the moon seems turned to blood, and the people cry out in

the streets with fear. Such a phenomenon was witnessed by the writer at

Florence, February 27, 1877. Its explanation is not difficult The

refractive power of the earth’s atmosphere suffices to bring

illumination to the lunar disc at the very middle of the shadow-cone. It

is shut off from direct solar rays, not from those that are bent into

convergence by the lens of our air. That they must be reddened by the

process, sunset-effects on the earth tell plainly enough. But when the

air is vapour, or dust-laden, and consequently opaque, little light is

transmitted, and a scarcely mitigated eclipse ensues. That of 1884 is

believed to have been darkened by the outpourings from Krakatoa. A

photograph by Professor Barnard, of the totally eclipsed moon, September

3, 1895, is reproduced in Fig. 11. It was one of a _search-series_ for a

lunar satellite. None was found: but the question of its possible

existence was set at rest.

De la Rue’s and Rutherfurd’s plan of photographing the moon as a whole

is no longer followed. Bit by bit photography, on a large scale, has

superseded it. Splendid pictures of individual formations and separate

regions have in this way been obtained, both at the Paris and the Lick

Observatories; and their microscopic study has given some interesting

results; yet it is undeniable that the “chemical retina” cannot here

claim its usual superiority. “The best photograph of the moon ever

taken,” Professor W. H. Pickering avers,[37] “will not show what can be

seen with a six-inch telescope, under favourable atmospheric conditions.

For general outlines, for completeness of the coarser detail, and for

purposes of future testimony, the photograph evidently stands without a

rival; but as regards that which is really most interesting upon the

moon—the finer detail and more delicate features—the photograph does not

even hint at their existence.” One of the most successful specimens of

lunar photography forms the frontispiece to this volume. It was taken by

MM. Loewy and Puiseux, with the large Coudé equatorial, February 14th,

1894, at 7^h 27^m Paris time, and cannot easily be surpassed in

pictorial effect.

[Illustration:

FIG. 11.—_Photograph of the Totally Eclipsed Moon. By Professor

Barnard. Exposure, 3 Minutes._

]

Atmospheric agitations are one cause of imperfection in lunar

photographs. The eye can seize the instant of exquisite definition; the

camera must take what comes. Then the disparities of actinic intensity

in the various lunar formations are so wide that, in order to get an

ideal picture, a different length of exposure should be given to each.

What is enough for a plain—to take an example—is too much for the crater

rising from it, or for the rampart enclosing it. Minute irregularities

in the following motion of the telescope during the few seconds of

exposure occasion further difficulties. A momentary shifting, by half a

millimetre, of the image upon the sensitive plate, would suffice to blur

the negative seriously, if not fatally. For this, as for several other

lines of work, the instrument of the future may be of a type with which

the equatorial has little in common. Professor Pickering considers it

probable that “a horizontal telescope of three or four hundred feet

focus, and twelve to fifteen inches aperture, would give the most

satisfactory results. In such a case, it might be found best that the

mirror should remain fixed during the exposure, while the plate was

given an uniform motion by clock-work.”

The suggestion is one among many signs that a revolution in the mounting

of telescopes is at hand.

CHAPTER VI.

THE PLANET MARS.

The furthest terrestrial planet from the sun is Mars, the “star of

strength.” No other heavenly body, except the moon, is so well placed

for observation from our position in space. As a superior planet, it

does not merely, like Mercury and Venus, oscillate about the sun, but is

best seen when in opposition. It is then “full”; it crosses the meridian

at midnight, and is at its least distance from the earth. These

occasions recur every 780 days; but they are not all equally favourable.

The opposition distance of the planet varies, owing to the eccentricity

of its orbit, from thirty-five to sixty-one million miles; so that the

area of the disc is three times larger when a perihelion than when an

aphelion passage coincides with a midnight culmination. Under the best

circumstances it is of the apparent dimensions of a half-sovereign 2,000

yards from the spectator.

The diameter of Mars is 4,200 miles; its surface is equal to

two-sevenths, its volume to one-seventh those of the earth. But, in

consequence of its inferior mean density, nine such spheres would go to

make up the mass of our world. The superficial force of gravity on Mars,

compared with its terrestrial value, is as thirty-eight to a hundred. A

man could leap there a wall eight feet four inches in height with no

more effort than it would cost him here to spring over a two-foot fence.

The planet’s rotation is performed in 24 hours 37 minutes on an axis

deviating from the vertical by 240° 50′. Hence its seasons resemble our

own, except in being nearly twice as long, for the Martian year is of

687 days. They are modified, too, by the considerable elongation of the

ellipse traversed by Mars, causing a difference of 26½ millions of miles

in its greatest and least distances from the sun. These are respectively

155 and 128½ millions of miles, the mean distance being 141½ millions. A

polar compression of ¹⁄₂₂₀ is just what should be expected from its

rotatory speed. When at quadrature, it is plainly gibbous; but our

interior position with regard to it makes it impossible that it should

ever take the crescent form. Its albedo, according to Zöllner, is 0·26—a

figure intimating that sunlight is reflected from no cloud-canopy, but

by the soil itself. This atmospheric transparency leaves the door open

for researches into the condition of a very curious little world.

The disc of Mars is diversified with three shades of colour—reddish, or

dull orange, dark greyish-green, and pure white. The last shows mainly

in two diametrically opposite patches. Each pole is surrounded by a

brilliant cap, suggesting the deposition of ice or snow over the chilly

spaces corresponding to our arctic and antarctic regions. Nor is this

all. Each of the polar hoods shrinks to a mere remnant as the local

summer advances, but regains its original size when wintry influences

are again in the ascendant. Here, and nowhere else in the planetary

system, we meet evidence of seasonal change; and seasonal change is

associated with vital possibilities. Again, a globe upon which snow

visibly melts must contain water; hence the green markings cannot but

image to our minds seas and inlets sub-dividing continents, the blond

complexion of which may be caused by some native peculiarity of the

soil. It is in no way connected with vegetation, since it neither fades

nor flushes with the advent of spring; and an atmospheric origin is

excluded by the circumstance that it becomes effaced by a whitish haze

near the limb, just where the densest atmospheric strata are traversed

by the line of sight.

The spots on Mars are by no means so sharply defined as lunar craters

and _maria_; yet they are fundamentally permanent. Some can be

recognised from drawings made over two hundred years ago; and these

antique records have served modern astronomers to determine with minute

accuracy the rotation-period of the planet. There is accordingly no

doubt that “areography” has assured facts to deal with, although the

facts are not quite as “hard” as they might be. Continents are somewhat

vaguely outlined. Great tracts of them are of an uncertain and variable

hue, as if subject to inundations. This peculiarity, thoroughly

certified during the favourable opposition of 1892, makes a strong

distinction between Mars and the Earth. Terrestrial oceans keep within

the limits assigned to them. On the neighbouring planet—as M. Faye

observed in 1892—“Water seems to march about at its ease,” flooding,

from time to time, regions as wide as France. The imperfect separation

of the two elements recalls the conditions prevailing during the

terrestrial carboniferous era.

[Illustration:

FIG. 12.—_Chart of Mars on Mercator’s Projection._

(From Proctor’s “Old and New Astronomy.”)

]

The main part of the land of Mars is situated in the northern

hemisphere. It covers two-thirds of the entire globular surface. Rather

than land, indeed, it should be called a network of land and water. Fig.

12, from a chart by Schiaparelli, illustrates the remarkable fashion of

their intermixture. The great continental block—so its orange tint

declares it to be—is cut up in all possible directions by an intricate

system of what appear to be waterways, running in perfectly straight

lines—that is, along great circles of the globe—for distances varying

from 350 to upwards of 4,000 miles. They are frequently seen in

duplicate, strictly parallel companions developing thirty to three

hundred miles apart from the original formations. This mysterious

phenomenon is evanescent, or rather periodical. Canal-duplication is a

recurrent change, depending upon the Martian seasons, and becoming

obvious, according to Schiaparelli, chiefly near the equinoxes.

The canals invariably connect two bodies of water; hence they need no

locks or hydraulic machinery; their course is on a dead level. The

broadest of them are comparable with the Adriatic; those at the limit of

visibility, stretching like the finest spider-threads across the disc,

have a width of eighteen miles. “The canals,” Schiaparelli says, “may

intersect among themselves at all possible angles, but by preference

they converge towards the small spots to which we have given the name of

lakes. For example, seven are seen to converge in Lacus Phoenicis, eight

in Trivium Charontis, six in Lunae Lacus, and six in Ismenius

Lacus.”[38]

These “lakes” evidently form an integral part of the canal system. They

resemble huge railway-junctions; and the largest of them—the “Eye of

Mars” (Schiaparelli’s Lacus Solis)—seems, in Mr. Lowell’s phrase, like

the hub of a five-spoked wheel. It is depicted in Fig. 13 from a drawing

made by Professor Barnard with the great Lick refractor, September 3,

1894. Mr. W. H. Pickering in 1892, and Mr. Percival Lowell in 1894, were

amazed at their extraordinary abundance.

“Scattered over the orange-ochre groundwork of the continental regions

of the planet,” the latter wrote, “are any number of dark, round spots.

How many there may be it is not possible to state, as the better the

seeing, the more of them there seem to be. In spite, however, of their

great number, there is no instance of one occurring unconnected with a

canal. What is more, there is apparently none which does not lie at the

junction of several canals. Reversely, all the junctions appear to be

provided with spots.”

[Illustration:

FIG. 13.—_The “Eye of Mars,” drawn by Prof. Barnard with the great

Lick Refractor. The southern snow-cap is visible much shrunken by

melting._

]

Most of these foci are about 120 miles in diameter, and appear most

precisely circular when most clearly seen. “Plotted upon a globe,” Mr.

Lowell continues, “they and their connecting canals make a most curious

network over all the orange-ochre equatorial parts of the planet, a mass

of lines and knots, the one marking being as omnipresent as the other.

Indeed, the spots are as peculiar and distinctive a feature of Mars as

the canals themselves.”

Like the canals, too, they emerge periodically, and in the same but a

retarded succession. They “are therefore, in the first place, seasonal

phenomena, and, in the second place, phenomena that depend for their

existence upon the prior existence of the canals.”[39]

Mr. Lowell terms them “oases” (see Fig. 14), and does not shrink from

the full implication of the term.

The most important result of the numerous observations of Mars, made

during the oppositions of 1892 and 1894, was the recognition of a

regular course of change dependent upon the succession of its seasons.

Schiaparelli had long anticipated this result; he is commonly in advance

of his time. These changes, moreover, when closely watched, are really

self-explanatory. The alternate melting of the northern and southern

snow-caps initiates, and to some extent determines them. As summer

advances in either hemisphere, the wasting of the corresponding white

calotte can be followed in every minute particular. “The snowy regions

are then seen to be successively notched at their edges; black holes and

huge fissures are formed in their interiors; great isolated fragments

many miles in extent stand out from the principal mass, dissolve, and

disappear a little later. In short, the same divisions and movements of

these icy fields present themselves to us at a glance that occur during

the summer of our own arctic regions.”[40]

Indeed, glaciation on Mars is much less durable than on the earth. In

1894, the southern snow-cap vanished to the last speck 59 days after the

solstice; and the remnant usually left looks scarcely enough to make a

comfortable cap for Ben Nevis. An immense quantity of water is thus set

free. The polar seas overflow; gigantic inundations reinforced,

doubtless, from other sources, spread to the tropics; Syrtis regions of

marsh or bog deepen in hue, and become distinctly aqueous; canals dawn

on the sight, and grow into undeniable realities. We seem driven to

believe that they discharge the function of flood-emissaries.

Mr. Lowell does not hesitate to pronounce them of artificial formation,

and, on that large assumption, the purpose of their connexion with his

“oases” becomes transparently clear. They bring to these Tadmors in the

wilderness the water supply by which they are made to “blossom as the

rose.” The junction-spots, we are told, do not enlarge when the vernal

freshet reaches them; they only darken through the sudden development of

vegetation. These circular “districts, artificially fertilised by the

canal system,” are strewn broadcast over vast desert areas, the

orange-ochreous sections of Mars, covering the greater part of its

surface, but deep buried in the millennial dust of disintegrated red

sandstone strata.

“Here, then,” Mr. Lowell remarks,[41] “we have an end and reason for the

existence of canals, and the most natural conceivable—namely, that the

canals are constructed for the express purpose of fertilising the oases.

When we consider the amazing system of the canal lines, we are carried

to this conclusion as forth-right as is the water itself; what we see

being not the canal itself, indeed, but the vegetation along its banks.”

[Illustration:

FIG. 14.—_The Oases of Mars. Drawn by Percival Lowell._

(From “Popular Astronomy,” April, 1895.)

]

The idea that we see the water only by its effects along the shores of

these prodigious troughs, originated with Professor W. H. Pickering. It

is strikingly illustrated by the aspect of rivers from a balloon. Thus

the Rhine, as M. Flammarion attests,[42] seen from a perpendicular

altitude of 8,000 feet, shows like a green thread drawn in the midst of

a ribbon of meadow. The Martian canals, it is suggested, correspond to

the “ribbon of meadow.”

The hypothesis is seductive, but should not be hastily adopted. It gives

no account of the doubling of the canals, yet the process takes place on

a grand scale, at determinate epochs, and under fairly well ascertained

conditions. It undoubtedly belongs to the series of vernal changes going

forward upon the planet, and is accomplished with amazing rapidity. A

single canal may be transformed into a double canal within twenty-four

hours, and that simultaneously along its whole course. The two stripes,

so curiously substituted for one, “run straight and equal with the exact

geometrical precision of the two rails of a railroad.”[43] The tendency

is shared by the lakes or “oases.” “One of these,” we learn from the

same authority, “is often seen transformed into two short, broad dark

lines parallel to one another, and traversed by a yellow line.”

This singular principle of subdivision offers at present no hold for

profitable speculation. Schiaparelli trusts to the “courtesy of nature”

for some ray of light by which, in the future, to penetrate the mystery;

but wisely deprecates recourse being had to the intervention of

intelligent beings. Such arbitrary modes of dealing with perplexing

problems constitute, as he says, a grave obstacle to the acquisition of

just notions concerning them. They raise prepossessions by which the

progress of genuine research is impeded.

The proportion of water to land is much smaller on Mars than on the

earth. Only two-sevenths of the disc are covered by the dusky areas, and

of late the aqueous nature of some, if not all of these, has been

seriously called in question. Professor Pickering was convinced by his

observations, in 1892 and 1894, “that the permanent water area upon

Mars, if it exist at all, is extremely limited in its dimensions.”[44]

He estimated it at about half the size of the Mediterranean. Professor

Schaeberle is similarly incredulous. If the dark markings are seas, he

asks, how explain the irregular gradations of shade in them?[45] How,

above all, explain their apparent intersection by well-marked canals?

Professor Barnard, observing with the Lick thirty-six inch in 1894,

discerned on the Martian surface an astonishing wealth of detail, “so

intricate, small, and abundant, that it baffled all attempts to properly

delineate it.”[46] It was embarrassing to find these minute features

belonging more characteristically to the “seas” than to the

“continents.” Under the best conditions, the dark regions lost all trace

of uniformity. Their appearance resembled that of a mountainous country,

broken by cañon, rift, and ridge, seen from a great elevation. These

effects were especially marked in the “ocean” area of the hour-glass

sea.

Evidently the relations of solid and liquid in that remote orb are

abnormal; they cannot be completely explained by terrestrial analogies.

Yet a series of well-attested phenomena are intelligible only on the

supposition that Mars is, in some real sense, a terraqueous globe. Where

snows melt there must be water; and the origin of the Rhone from a great

glacier is scarcely more evident to our senses than the dissolution of

Martian ice-caps into pools and streams.

The testimony of the spectroscope is to the same effect. Dr. Huggins

found, in 1867, the spectrum of Mars impressed with distinct traces of

aqueous absorption, and the fact, although called in question by

Professor Campbell of Lick, in 1894, has been re-affirmed both at Tulse

Hill and at Potsdam. That clouds form and mists rise in the thin Martian

air, admits of no doubt. During the latter half of October, 1894, an

area much larger than Europe remained densely obscured. Whether or no

actual rain was at that time falling over the Maraldi Sea and the

adjacent continent, it would be useless to conjecture. We only know that

with the low barometric pressure at the surface of Mars, the boiling

point of water must be proportionately depressed (Flammarion puts it at

115° Fahrenheit), which implies that it evaporates rapidly, and can be

transported easily.

If the Martian atmosphere be of the same proportionate mass as that of

our earth, it can possess no more than one-seventh its superficial

density. That is to say, it is more than twice as tenuous as the air at

the summits of the Himalayas.[47] The corresponding height of a

terrestrial barometer would be four and a half inches. Owing, however,

to the reduced strength of gravity on Mars, this slender envelope is

exceedingly extensive. In the pure sky scarcely veiled by it, the sun,

diminished to less than half his size at our horizons, probably exhibits

his coronal streamers and prominences as a regular part of his noontide

glory; atmospheric circulation proceeds so tranquilly as not to trouble

the repose of a land “In which it seemeth always afternoon”; no cyclones

traverse its surface, only mild trade-winds flow towards the equator to

supply for the volumes of air gently lifted by the power of the sun, to

carry reinforcements of water-vapour north and south. Aerial movements

are, in fact, by a very strong presumption, of the terrestrial type, but

executed with greatly abated vigour.

Brilliant projections above the terminator of Mars were first distinctly

perceived at the Lick Observatory in 1890. They have been re-observed at

Nice, Arequipa, and Flagstaff (Mr. Lowell’s Observatory), coming into

view, as a rule, when circumstances concur to favour their visibility.

They strictly resemble lunar peaks and craters, catching the first rays

of the sun, while the ground about them is still immersed in

darkness;[48] and Professor Campbell[49] connects them with “mountain

chains lying _across_ the terminator of the planet,” and in some cases

possibly snow-covered. He calculates their height at about ten thousand

feet. Their presence was unlooked-for, since a flat expanse is a

condition _sine quâ non_ for the minute intersection of land by water,

which seems to prevail on Mars.

Although the sun is less than half as powerful on Mars as it is here,

the Martian climate, to outward appearance, compares favourably with our

own. Polar glaciation is less extensive and more evanescent, and little

snow falls outside the arctic and antarctic regions. Yet the theoretical

mean temperature is minus 4°C., or 61° of Fahrenheit below freezing.

This means a tremendous ice-grip. The coldest spot on the earth’s

surface is considerably warmer than this cruel average. Fortunately, it

exists only on paper. Some compensatory store of warmth must then be

possessed by Mars, and it can scarcely be provided by its attenuated

air. Possibly, internal heat may still be effective, and we see

exemplified in Mars the geological period when vines and magnolias

flourished in Greenland, and date-palms ripened their fruit on the coast

of Hampshire.

The climate of Mars, according to Schiaparelli,[50] “must resemble that

of a clear day upon a high mountain. By day a very strong solar

radiation hardly at all mitigated by mist or vapour; by night a copious

radiation from the soil towards celestial space, and hence a very marked

refrigeration; consequently, a climate of extremes, and great changes of

temperature from day to night, and from one season to another. And as on

the earth, at altitudes of from 17,000 to 20,000 feet, the vapour of the

atmosphere is condensed only into the solid form, producing those

whitish masses of suspended crystals which we call cirrus-clouds, so in

the atmosphere of Mars it would be rarely possible to find collections

of cloud capable of producing rain of any consequence. The variation of

temperature from one season to another would be notably increased by

their long duration, and thus we can understand the great freezing and

melting of the snow, renewed in turn at the poles at each complete

revolution of the planet round the sun.”

But the anomalies in the Martian domestic economy cannot thus easily be

removed, and the only safe conclusion is Flammarion’s, that “the general

order of things is very different on Mars and on the earth.”

The German astronomer, Mädler, searched in 1830 for a Martian satellite,

and although his telescope was of less than four inches aperture, he

satisfied himself that none with a diameter of as much as twenty-three

miles could be in existence. As it happened, he was right. The pair of

moons detected by Professor Asaph Hall with the Washington twenty-six

refractor, August 11 and 17, 1877, are unquestionably below that limit

of size. Neither of them can well be more than ten miles across. Their

names, “Deimos” and “Phobos,” are taken from the _Iliad_, where Fear and

Panic are introduced as attendants upon the God of War. Deimos revolves

in 30 hours and 18 minutes at a distance of 14,600 miles from the centre

of Mars. And, since the planet rotates in 24 hours 37 minutes, the

diurnal motion of the sphere from east to west is so nearly neutralised

by the orbital circulation of the satellite from west to east that

nearly 132 hours elapse between its rising and its setting. During the

interval, it changes four times from new to full, and _vice versâ_.

Professor Young estimates that Mars receives from it when full only

¹⁄₁₂₀₀th of full moonlight.

Phobos is more effective in illumination, both because it is larger, and

because it is less distant. At the Martian equator, its brightness is

equal to ¹⁄₆₀th that of our moon, but beyond 69° of latitude it is

permanently shut out from view by the curvature of the globe. This

exclusion is an effect of its uncommon closeness to its surface, the

interspace being only 3,700 miles, while its distance from the centre is

5,800. Moreover, the period of Phobos being only 7 hours 39 minutes, or

less than ⅓ the time of rotation of its primary, it rises in the west,

sets in the east, and courses across the heavens in 11 hours, during

which interval it accomplishes one entire cycle of its phases, and gets

through half another. This is an unique phenomenon, and points to an

unique origin for the little moon. No other known satellite revolves

more quickly than its primary rotates, and the discovery of the fact has

dealt a fatal blow to Laplace’s method of planetary evolution. Were

Phobos capable of raising any appreciable tide on Mars, its frictional

effects would hence be of an opposite character to those of other tidal

waves; and instead of being pushed outward, it would be drawn inward,

and finally precipitated upon the planet. But it derives safety, on the

one hand, from its small mass; on the other, from the insensibility of

Mars to tidal action. The satellite is incapable of exerting the

required influence; the planet is not in a state to respond to it, were

it exerted. For the configuration of land and water upon its surface is

such as effectually to prevent the flow of tides, were the compulsive

power a thousand-fold that possessed by its pair of diminutive

satellites.

CHAPTER VII.

THE ASTEROIDS.

Between the orbits of Mars and Jupiter is interposed a huge gap. On one

side of it lie the terrestrial planets; on the other, the “major

planets”—orbs belonging to a different order, both as to magnitude and

as to constitution. The hiatus marks a change of front in planetary

development, and its existence gravely compromises the symmetry of the

solar system. Its inconsistency with Bode’s law of planetary distances

long troubled investigators. A member of the series had somehow dropped

out; it was sought for under the form of a planet, and found,

apparently, as its disintegrated constituents. The discovery of Uranus

nearly at the distance indicated for it by the law roused astronomers to

the necessity for a systematic chase; but before their organisation had

got into full working order, the missing occupant of the vacant zone

presented itself spontaneously. This was Ceres, the first asteroid,

discovered by Piazzi at Palermo, January 1, 1801, the opening day of the

present century.

A series of surprises followed. While watching its path, Dr. Olbers,

March 28, 1802, came across an associated body. He named it Pallas, and

it was at once proved by the calculations of Gauss to revolve

practically at the same distance from the sun as Ceres. _Both_ occupied

nearly the position required by Bode’s law. This double fulfilment was

more than was bargained for; it was unprecedented and perplexing; but

the anomaly was temporarily removed by Olbers’ daring hypothesis of an

exploded planet. The prediction based upon it that the acquaintance made

with two specimen-products of the catastrophe would be followed by an

introduction to many more, was strikingly verified by Harding’s

discovery of Juno, September 1, 1804, and by Olbers’ of Vesta, March 29,

1807. By a further coincidence, both were at the time situated in the

positions suggested as the most promising for a successful search—that

is, near the line of intersection which should necessarily be common to

orbits described by fragments of a single original mass.

The four asteroids received for many years no accession to their

numbers. They were found to deviate, in several respects, from the

example set them by the planets, properly so-called. They revolve,

indeed, from west to east, thus following the current of systemic

movement; but their paths are considerably eccentric and highly tilted.

Each one of the quartette transgresses the zodiacal limits; and Pallas

travels at an angle of no less than thirty-five degrees to the plane of

the ecliptic.

Vesta, the brightest asteroid, can occasionally be seen with the naked

eye; but the natural inference that it is the largest has lately been

disproved. No trustworthy measurements of the real _discs_ of the

asteroids had been made until Professor Barnard in 1894 successfully

performed the feat with a power of 1000 on the Lick refractor. The

upshot has been to substitute Ceres for Vesta as the leading member of

the group. Its diameter proved to be 485 miles, Pallas coming next with

304, while those of Vesta and Juno are respectively 243 and 118 miles.

Now, Professor Edward Pickering, by comparing the brightness of the same

bodies, and assuming for all indiscriminately an albedo equal to that of

Mars, had arrived at a diameter for Vesta of 319, for Pallas of 169

miles. The disparity between his results and Barnard’s can be reconciled

only on the supposition of marked differences in reflective power. Their

reality was established by G. Müller’s photometric observations at

Potsdam.[51] Thus Ceres is large and dull, Vesta comparatively small,

but exceedingly bright—almost incredibly bright, indeed, since its

albedo is estimated at 0·72, which represents a lustre midway between

those of white paper and fresh-fallen snow. Ceres, on the other hand, is

as obscure as Mercury, while Pallas throws back proportionately somewhat

less, and Juno considerably more light than Mars.[52] The phases of

these last two bodies progress besides in such a manner as to show that

they are superficially uneven, and at quadratures flecked with profound

shadows.

The facts thus arrived at are disconcerting to the views previously

entertained. Few expected to meet with so much individuality in the

asteroids. They were looked upon rather as loaves from the same batch.

But now we find among them bodies as physically unlike as Venus and the

moon. Ceres must be composed of rugged and sombre rock, unclothed

probably by any vestige of air. Vesta displays a brilliant shell of

clouds. And from Vesta alone among the asteroids, Vogel derived in 1873

some uncertain indications of atmospheric action upon the sun-rays

reflected by it. There is, nevertheless, great difficulty in supposing a

body of no more than one-thousandth the mass of Mars endowed with a

dense atmosphere. Yet it must be dense and extensive in order to

maintain the heavy cloud-layer implied, so far as our present knowledge

goes, by an unusually high albedo. The difficulty is this. All gases

tend, by their nature, to become indefinitely diffused through space.

They can be restrained within a sphere of finite radius only through the

exertion of some force capable of holding their elasticity in check.

This force is gravity; none other suitable for the purpose is known. It

acts as a counterpull to the translational velocities of the gaseous

particles which, according to the dynamical theory of gases, constitute

their elasticity. But if the confining power be insufficient, the roving

particles will dart away, each on its own account, and will cease to

form an atmosphere. This condition was adverted to some years ago by Dr.

Johnstone Stoney, and he calculated the mass needed to secure to a

heavenly body the lasting possession of an aerial envelope. It differs

naturally for different gases; the lightest particles being affected by

the swiftest movements, and hence being the readiest to escape. The

earth, on this view, is impotent to retain hydrogen; since the critical

velocity at its surface is seven miles a second, and hydrogen-molecules

can, now and again, attain 7·4 miles, so that they would dribble away,

one after another, until the whole original supply was exhausted. Mars

(a projectile fired from which, with a speed exceeding three miles a

second, would depart irrevocably), can but just hold oxygen, nitrogen,

and water-vapour, all with more massive and sluggish molecules than

those of hydrogen; while the moon has long ago been forsaken by whatever

gaseous substances primitively belonged to it. The mass of Vesta,

however, is only ¹⁄₃₁₂ the lunar mass (supposing their mean densities

the same); hence, if the relation just described holds good under all

circumstances, its surface _ought_ to be as bare and dry as any lunar

volcano. The albedoes of the asteroids raise, then, questions of

fundamental importance in planetary physics.

Endeavours to add to the asteroidal group, after having been

relinquished for over a score of years, were resumed, in 1830, by a

retired Prussian post-master named Hencke. His watch was rewarded with

the discoveries of Astraea, December 8, 1845, and of Hebe eighteen

months later. Since then, every year has regularly brought its quota of

detections. About forty astronomers devoted themselves systematically to

the search, and some of them reckoned their trophies by the score. No

less than eighty-five were credited, in 1893, to Palisa of Vienna;

Peters of Clinton (N.Y.), whose career closed in 1890, owned

forty-eight; Watson, another American professor, made testamentary

provision for his twenty-two clients, lest, for lack of computational

care, they should relapse into their former outcast condition. The task

is, indeed, a heavy one of keeping guard over some hundreds of minute

objects threading their way through a maze of orbits, amid throngs of

stars, from which they are indistinguishable except by continuous

observation, and the question, _Cui bono?_ has been asked, and has only

with hesitation been answered. But the business has, up to the present,

been kept going; the registry and inquiry asteroidal office remains open

at Berlin, and the almost overwhelming mass of calculations, necessary

for identification, is punctually dealt with.

The work and responsibilities of this department have, of late, been

alarmingly augmented. Until five years ago the telescope was the sole

implement of research in connection with it, but on December 22, 1891,

Professor Max Wolf of Heidelberg, discovered No. 323, afterwards named

Brucia, on a sensitive plate exposed with a six-inch portrait lens, of

thirty inches focus, and a field of seventy square degrees. Before the

year 1892 had closed, his photographic discoveries of the same kind

numbered eighteen, and they had, in January, 1897, run up to fifty-six,

of which five were recorded on the same night. He picked up, besides,

several “lost” or strayed asteroids. M. Charlois of Nice immediately

adopted Wolf’s method, and emulated his success. About ninety of these

objects have already fallen to his share by telescopic and photographic

means. In either case they are discriminated from stars solely by their

motion; but on sensitive plates its effects are directly visible, fixed

objects being represented by round dots, travelling objects by lines,

the length of which is proportionate to the amount of displacement

during the hour, or hours, of exposure.

About 440 asteroids are now established members of the solar system. It

has long been thought that numerical identification is as much as they

can properly claim; but the old and inconvenient system of mythological

nomenclature is still pursued. Indeed, the supply of goddesses is

running out, and has to be reinforced by apotheosis or invention.

Already, to some extent, as Professor Holden remarks, the asteroidal

catalogue “reads like the Christian names at a girls’ school.” Needless

to say that the brightness of the objects annually registered is in

steady course of decline. Very few of those now drawn to shore in the

photographic net are likely to exceed twenty miles in diameter. Yet

although mere planetary shreds, they are probably large compared with

the grains of planetary dust, numberless as the sands of the seashore,

which indiscernably revolve round the sun under analogous conditions.

Their aggregate mass is very small. Leverrier assigned for its superior

limit one-fourth that of the earth, but the limit, we may rest assured,

is very far from being attained. M. Niesten of Brussels estimated that

the first 216 asteroids, including all the larger ones, amounted to

¹⁄₁₀₀₀th the earth’s volume, and we may add, since they are beyond doubt

specifically lighter, to about ¹⁄₈₀₀₀th the earth’s mass. Mr. Roszl

finds for the mass of 311 asteroids one-fortieth that of the moon.[53]

Still later, M. Gustave Ravené has attempted to account for the

superfluous movement of the perihelion of Mars by the gravitational

influence of these bodies.[54] He computes the required mass to be

two-thirds that of the moon. In other words, he assumes the group to be

fairly represented by 500 globes as large as Juno (124 miles in

diameter), and of terrestrial density. But he obviously puts some

constraint on nature in order to secure the desired agreement.

The distribution of these dwarfed globes is not without significant

features. It is such, at any rate, as absolutely to negative Olbers’s

hypothesis of their origin through the explosion of an already formed

planet. They represent, on the contrary, the materials of a planet that

never was, and never will be formed. They follow paths curiously

intertwined. D’Arrest noticed forty-five years ago, as a proof of the

intimate relation subsisting among the members of what was then a small

group, “that, if their orbits are figured under the form of material

rings, these rings will be found so entangled that it would be possible,

by means of one among them taken at hazard, to lift up all the rest.”

They are not, however, scattered at random over the wide zone

appropriated to them which, at its extreme limits, measures three times

the radius of the earth’s orbit. It includes blank spaces which seem as

if cleared by some expulsive agency. That agency, as Professor Kirkwood

divined in 1866, is the disturbing power of Jupiter. For the blank

spaces occur where there would be commensurability of periods, and

whence, accordingly, revolving particles should be ejected by

accumulated perturbations. The clearing power was not exerted once for

all; it is still active. But its effectiveness in modifying distribution

is now perceived to be less complete than it seemed when our

acquaintance with the bodies in question was more limited. It has

produced in general only partial vacancies. M. Parmentier[55] analysed

in 1895 the arrangement in space of 390 orbits, with the result of

finding that some of the originally noted gaps had ceased to exist. The

mean distances, for instance, corresponding to periods two-sevenths and

three-sevenths the Jovian period, are fairly well frequented; while, on

the other hand, there is an unmistakable thinning out where five

revolutions are performed while Jupiter accomplishes two. He found again

that no asteroid circulates either in half, or in one-third the same

dangerous period. Yet, even since he wrote, No. 401 has been detected

occupying the former of these prohibited spaces. But this apparent

breach of rule may turn out to result from a miscalculation, as in the

case of Menippe, which has in consequence never been recaptured since

she first presented herself in 1878, and was erroneously assigned a

period two-fifths that of Jupiter. There is no doubt that the asteroids

are collected most densely about the mean distance 2·8 of the earth’s,

just where conformity to Bode’s law would place them. Nor is it less

certain that Kirkwood’s “rule of commensurability” has fundamentally

influenced their distribution.

He further discerned among them groups of two or three moving in

closely-related orbits. Additional examples of this sort of connexion,

which is far too close to be casual, have been pointed out by M.

Tisserand and Mr. Monck, and eighty asteroids are at present known to

have companions, their actual ties with which indicate, as Kirkwood

held, original identity. Each group consists of fragments of a primitive

nebular mass torn asunder by the unequal attraction of Jupiter shortly

after its detachment from the great parent sphere eventually condensed

to form the sun. As an example, we may take Juno and its twin Clotho.

Both revolve at a mean distance from the sun 2·67 times that of the

earth, in orbits of sensibly the same eccentricity, and of nearly the

same inclination to the ecliptic, their major axes diverging, however,

to the extent of ten degrees, obviously through unequal perturbations.

As surely as corresponding scars on opposite cliffs vouch for their

antique disruption, do these concurrent paths attest the primitive unity

of the pair of planetules traversing them. And bodies similarly

connected occur not in pairs only, but in triplets as well.

From whatever point of view the “planetary cluster” composed by the

asteroids is regarded, the influence of Jupiter is perceived as dominant

in the background. The manner of planetary production underwent a marked

change subsequently to the separation of his mighty mass. No interval of

repose followed; but a constant shredding off of chips and shavings.

This may safely be attributed (in accordance with Professor Kirkwood’s

surmise) to the tide-raising power of Jupiter at close quarters, by

which strain in the central rotating mass was almost prevented, through

the facility with which it was relieved. Hence the parent nebula long

remained incapable of parting with any appreciable portion of its

substance, and never resumed planet-making on the ancient scale. The

asteroids then came into existence under Jupiter’s auspices; they were,

while still in an inchoate state, subdivided, or even pulverised by his

disruptive influence, and scattered over the zone allotted to them under

the compulsion of his perturbing power.

CHAPTER VIII.

THE PLANET JUPITER.

Jupiter is by far the most important member of the solar family. The

aggregate mass of all the other planets is only two-fifths of his, which

316 earths would be needed to counter-balance. His size is on a still

more colossal scale than his weight, since in volume he exceeds our

globe 1,380 times. His polar and equatorial diameters measure

respectively 84,570 and 90,190 miles,[56] giving a mean diameter of

88,250 miles, and a polar compression of ¹⁄₁₆th. The corresponding

equatorial protuberance rises to 2,000 miles, so that the elliptical

figure of the planet strikes an observer at the first glance. This at

once indicates rapid axial movement; and Jupiter’s rotation is

accordingly performed in nine hours and fifty-five minutes, with an

uncertainty of a couple of minutes. The cause of this uncertainty will

presently appear.

The numbers just given imply that this great planet is of somewhat

slight consistence, and its mean density is in fact, a little less than

that of the sun. The sun is heavier than an equal bulk of water in the

proportion 1·4 to 1, Jupiter in the proportion of 1·33 to 1. The earth

is thus more than four times specifically heavier than the latter globe.

Three Jupiters would keep in equipoise four equal globes of water, while

the earth would turn the scale against five and a half aqueous models of

itself. This low density, an unfailing characteristic of all the giant

planets, is charged with meaning. It at once gives us to understand

that, in crossing the zone of asteroids, we enter upon a different

planetary region from that left behind. The bodies revolving there are

on an immensely larger scale of magnitude than those on the hither side;

they are of solar, rather than terrestrial, density; they rotate much

more rapidly, and are in consequence of a more elliptical shape; they

display, and most likely possess, no solid surface; they are attended by

retinues of satellites.

Jupiter circulates round the sun in 11·86 years, in an orbit deviating

by less than one and a half degrees from the plane of the ecliptic, but

of thrice the eccentricity of the ellipse traced out by the earth. With

a mean distance from the sun of 483 millions of miles, it accordingly

approaches within 462 at perihelion, and withdraws to 504 millions of

miles at aphelion. And since the heat and light received from the sun

are inversely as the squares of these numbers, it follows that Jupiter

is better warmed and illuminated when at the near than when at the far

extremity of its orbit, in the proportion of 109 to 100. Seasons it has

none worth mentioning; nor could they be of much effect even if they

were better marked. At its mean distance of 5·2 “astronomical

units”—that is, radii of the earth’s orbit—the sun’s potency is reduced

to ¹⁄₂₇th what it is here; we might accordingly have expected to meet in

this planet the conditions of a frozen world. But this anticipation has

been singularly falsified.

Under propitious circumstances Jupiter comes within 369 million miles of

the earth. These occur when he is in opposition nearly at the epoch of

his perihelion passage. His maximum opposition distance, on the other

hand, is 411 million miles. He is then at aphelion. Thus, at the most

favourable opposition, he is 42 million miles nearer to us than at the

least favourable. The effect on his brightness is evident to the eye.

When his midnight culmination takes place in October, he in fact sends

us one and a half times more light than when the event comes round to

April. We need only recall the unusual splendour of his appearance in

September and October, 1892, when his lustre was double that of Sirius.

His opposition period, as we may call it, is 399 days.

The intrinsic brilliancy of his surface is surprising, especially when

we consider that it is somewhat deeply tinged with colour. According to

Müller’s determination (relative to Mars), it actually returns 78 per

cent. of the incident light. But this would imply self-luminosity, the

presence of which is negatived by trustworthy evidence. Hence Zöllner’s

absolute albedo of 0·62 seems preferable. In either case, Jupiter does

not fall far short of being as reflective as white paper.

The minimum diameter of the visible disc considerably exceeds the

maximum of that of Mars. The latter never measures more than 25″;

Jupiter at conjunction, when (in round numbers), 600 million miles

distant from us, presents a surface 32″ in diameter, widened at a

favourable opposition to 50″. Even with a low power it thus makes a

beautiful and interesting telescopic object Its distinctive aspect is

that of a belted planet, the belts varying greatly in number and

arrangement. As many as thirty have, on occasions, been counted,

delicately ruling the disc from pole to pole. They are always parallel

to the equator, but are otherwise highly changeable, and cannot be too

closely studied as an index to the planet’s physical constitution. Two

in particular are remarkable. They are called the north and south

equatorial belts, and enclose a lustrous equatorial zone. The poles are

shaded by dusky hoods.

This general scheme of markings, however, when viewed with one of the

great telescopes of the world, is so overlaid with minor particulars as

sometimes to be scarcely recognisable. One cannot see the wood for the

trees. Lovely colour-effects, too, come out under the best circumstances

of definition and aerial transparency. The tropical belts may be

summarily described as red; but they are of complex structure, and their

subordinate features and formations are marked out, under the sway of a

ternating and tumultuous activities, by strips and patches of vermilion,

pink, purple, drab and brown. The intermediate space is divided into two

bands by a line, or narrow riband, pretty nearly coinciding with the

equator, and rosy, or vivid scarlet in hue. The polar caps are sometimes

of a delicate wine-colour, sometimes pale grey.

Professor Keeler made an elaborate study of the planet with the Lick

36-inch in 1889, and executed a series of valuable drawings, one of

which we are privileged to reproduce (Fig. 15). With a power of 320, the

disc, he tells us, “was a most beautiful object, covered with a wealth

of detail which could not possibly be accurately represented in a

drawing.” Most of the surface was then “mottled with flocculent and

irregular cloud-masses. The edges of the equatorial zone were

brilliantly white, and were formed of rounded, cloud-like masses, which,

at certain places, extended into the red belt as long streamers. These

formed the most remarkable and curious feature of the equatorial

regions. They are the cause of the double or triple aspect which the red

belts present in small telescopes.”[57]

Near their starting-points the streamers were white and sharply defined,

but became gradually diffused over the ruddy surface of the belts. When

at all elongated, they invariably flowed backward _against_ the

rotational drift, and were inferred to be cloud-like masses expelled

from the equatorial region, and progressively left behind by its

advance. This hypothesis was confirmed by the motion of some bright

points, or knots, on the streamers. “The portions of the equatorial zone

surrounding the roots of well-marked streamers were somewhat brighter,”

Professor Keeler continues, “than at other places, and it is a curious

circumstance that they were almost invariably suffused with a pale

olive-green colour, which seemed to be associated with great

disturbance, and was rarely seen elsewhere.”

[Illustration:

FIG. 15.—_Jupiter, October 3, 1890. Drawn by Professor Keeler with the

great Lick Refractor. The Red Spot is visible._

]

Now, if the material of the streamers had been simply a superficial

overflow, it should have carried with it into higher latitudes an excess

of linear rotational speed, and should hence have pushed its way onwards

as it proceeded north and south. But, instead, it fell behind; its

velocity was less, not greater than that of the belts with which it

eventually became incorporated. What are we to gather from this fact?

Evidently that the currents issuing north and south were of eruptive

origin. Their motion, in miles per second, was slow, because they

belonged to profound strata of the planet’s interior. Their backward

drift measured the depth from which they had been flung upward.

The spots, red, white, and black, constantly visible on the Jovian

surface, excite the highest curiosity. They are of all kinds and

qualities, and their histories and adventures are as diverse as they are

in themselves. Some are quite evanescent; others last for years. At

times they come in undistinguished crowds, like flocks of sheep, then a

solitary spot will acquire notoriety on its own account. White spots

appear in both ways; black spots more often in communities; and it is

remarkable that the former frequent distinctively, though not

exclusively, the southern, the latter the northern hemisphere. Red

spots, too, develop pretty freely; but the attention due to them has

been mainly absorbed by one striking specimen.

The Great Red Spot has been present with us for at least nineteen years;

and it is a moot point whether its beginnings were not watched by

Cassini more than two centuries ago. Its modern conspicuousness,

however, dates from 1878. Then of a full brick-red hue, and

strongly-marked contour, it measured 30,000 by nearly 7,000 miles, and

might easily have enclosed three such bodies as the earth. It has since

faded several times to the verge of extinction, and partially recovered;

but there has never been a time when it ceased to dominate the planet’s

surface-configuration. More than once it has been replaced by a bare

elliptical outline, as if through an effusion of white matter into a

mould previously filled with red matter; and just such a sketch was

observed by Gledhill in 1870. The red spot is attached, on the polar

side, to the southern equatorial belt. It might almost be described as

jammed down upon it; for a huge gulf, bounded at one end by a jutting

promontory, appears as if scooped out of the chocolate-coloured material

of the belt to make room for it. Absolute contact, nevertheless, seems

impossible. The spot is surrounded by a shining aureola, which seemingly

defends it against encroachments, and acts as a _chevaux de frise_ to

preserve its integrity. The formation thus constituted behaves like an

irremovable obstacle in a strong current. The belt-stuff encounters its

resistance, and rears itself up into a promontory or “shoulder,”

testifying to the solid presence of the spot, even though it be

temporarily submerged. The great red spot, the white aureola, and the

brownish shoulder are indissolubly connected.

The spot is then no mere cloudy condensation. Yet it has no real fixity.

Its period of rotation is inconstant. In 1879–80, it was of 9 hours, 55

minutes, 34 seconds; in 1885–86, it was longer by 7 seconds. The object

had retrograded at a rate corresponding to one complete circuit of

Jupiter in six years, or of the earth in seven months.[58] It is not

then fast moored, but floats at the mercy of the currents and breezes

predominant in the strange region it navigates. A quiescent condition is

implied by the approximate constancy of its rotation-period during the

last ten years. With the paling of its colour, its “proper motion”

slackens or ceases. This must mean that, at its maxima of agitation, it

is the scene of uprushes from great depths, which, bringing with them a

slower linear velocity, occasion the observed laggings. It is not

self-luminous, and shows no symptom of being depressed below the general

level of the Jovian surface. A promising opportunity was offered in 1891

of determining its altitude relative to a small dark spot on the same

parallel, by which, after months of pursuit, it was finally overtaken.

An occultation appeared to be the only alternative from a transit; yet

neither occurred. The dark spot chose a third. It coasted round the

obstacle in its way, and got damaged beyond recognition in the process.

Its material, as Mr. Stanley Williams observed, “was diverted and forced

bodily southwards, and obliged to pass round the southern side of the

red spot as if it were an island projecting above a stream.”

Jupiter has no certain and single period of rotation. Nearly all the

spots that from time to time come into view on its disc are in relative

motion, and thus give only individual results. The great red spot has

the slowest drift of all (with the rarest exceptions), while the black

cohorts of the northern hemisphere outmarch all competitors. Mr. Stanley

Williams,[59] as the upshot of long study, has delimitated nine

atmospheric surfaces with definite periods. They are well marked, and

evidently have some degree of permanence, yet the velocities severally

belonging to them are distributed with extreme irregularity. Thus, two

narrow, adjacent zones differ in movement by 400 miles an hour. This

state of things must obviously be maintained by some constantly acting

force, since friction, if unchecked, would very quickly abolish such

enormous discrepancies. The rotational zones are unsymmetrically placed;

there is no correspondence between those north and south of the Jovian

equator; and, although the equatorial drift is quicker than that of

either tropic, it is outdone in 20° to 24° north latitude. The stability

of this anomalous mode of rotation was remarkably illustrated by Dr.

Rambaud’s measurements of the “Garnet Spot” of October, 1895. Its

movement proved to be strictly conformable to that of the zone in which

it was situated (10° to 20° north latitude), and to agree, moreover,

within a fifth of a second with the value deduced by Schröter in 1787

for that of a spot in the same “zenographical” district.[60]

Jupiter’s equatorial rotation, as indicated by observations of spots, is

accomplished in 9 hours 50 minutes; but Bélopolsky’s and Deslandres’

spectrographic determinations gave rates of approach and recession

falling somewhat short of the corresponding velocity.[61] Possibly the

spots forge ahead in the medium that sustains them; or it may be, as M.

Bélopolsky suggests, that the planetary sphere itself has been measured

too large, owing to refraction in its atmosphere.

However this be, the rotation of the great planet, albeit ill-regulated

(if the expression be permissible), is distinctly of the solar type. It

is itself a “semi-sun,” showing no trace of a solid surface, but a

continual succession of cloud-like masses belched forth from within.

Each series, in fact, of certain classes of markings, such as the

equatorial “port-holes,” plainly owes its origin to the rhythmical

activity of a solitary, deep-buried focus.[62] Jupiter’s low mean

density, considered apart from every other circumstance, suffices to

demonstrate the primitive nature of his state. Under the enormous

pressure reigning in his interior, the same materials should be vastly

more massive, specifically, than within our own small globe; their

fourfold expansion gives us to understand the intensity of that heat by

which pressure has been so much more than neutralised. Moreover, the

agitations due to the cooling of a fluid globe make their mark on its

turbulent surface. On a solidified body like the earth, circulation is

kept up by heat received from without, and is purely atmospheric, and

essentially horizontal. In a sun-like body, the circulation is bodily

and vertical. That the processes going on in Jupiter are of this kind is

beyond question. Exchanges of hot and colder substances are effected,

not by surface-flows, but by up and down rushes. The parallelism of his

belts to his equator makes this visible to the eye. An occasional

oblique streak[63] betokens a current in latitude, but it is

exceptional, and might be called out of character.

Jupiter’s true atmosphere encompasses the disturbed shell of vapours

observed telescopically. Its general absorptive action upon light is

betrayed by the darkening of the planet’s limb—another point of

resemblance to the sun; while its special, or selective, absorption can

only be detected with the spectroscope. The arresting effect of

water-vapour was early noticed by Huggins and Vogel, and they measured a

strong line in the red of unknown origin, but contained in banded star

spectra. Atmospheric absorption is strongest above the ruddy equatorial

belts, which are hence concluded to be placed at a lower level than the

white surface.

Planetary photography was set on foot by Dr. Gould of Boston, in 1879,

when he obtained some promise of success with Mars, Jupiter, and Saturn;

and Dr. Lohse prosecuted the subject in 1883. The actinic power of

Jupiter’s light is very remarkable. It surpasses that of moonlight nine

times, and that of Mars twenty-four times. Dr. Lohse further ascertained

that the southern hemisphere is twice as chemically effective as the

northern.[64] This superiority is doubtless connected with the greater

physical agitation of the same region. A series of photographs of

Jupiter, taken in 1891 with the great Lick refractor, were the first of

any value for purposes of investigation. Each is one inch in diameter;

the image of the planet having been enlarged eight times before being

received upon the plate. Mr. Stanley Williams found them full of

interesting detail. Figure 16 shows an enlargement of a striking

photograph taken by Professor E. C. Pickering.

Jupiter’s satellites were the first trophies of telescopic observation.

They are, indeed, bright enough for naked eye perception, could they be

removed from the disc which obscures them with its excessive splendour;

and the first and third have actually been seen, in despite of the

glare, by a few persons with phenomenally good eyesight. The

mythological titles of the Galilean group—Io, Europa, Ganymede, and

Calypso (proceeding from within outward) have been superseded by prosaic

numbers. The change was unlucky, but is now probably irremediable.

The Jovian family presents an animated and attractive spectacle. The

smallest of its original members (No. II.) is almost exactly the size of

our moon; the largest (No. III.), with its diameter of 3,550 miles,

considerably exceeds the modest proportions of Mercury. Satellite I.

revolves in 42½ hours at the same average distance from Jupiter’s

surface that our moon does from that of the earth. No. II. has a period

of 3 days 13 hours, and its distance from Jupiter’s centre is 415,000

miles. Both these orbits are sensibly circular; and Nos. III. and IV.

travel in ellipses of very small eccentricity, the one at a mean

distance of 664,000, the other at 1,167,000 miles, in periods

respectively of 7 days 4 hours, and 16 days 16½ hours. All four revolve

strictly in the plane of Jupiter’s equator.

[Illustration:

FIG. 16.—_Photograph of Jupiter. Exposure, 87 seconds._

(From _Knowledge_, November, 1889.)

]

They constitute a system bound together by peculiar dynamical relations,

in consequence of which they can never be all either eclipsed, or seen

aligned at one side of their primary, at the same time. They can all,

however, be simultaneously hidden behind it, or in its shadow; although

this moonless condition is looked out for as a telescopic rarity.

The varied phenomena of eclipses, occultations, and transits, offer the

interest, not only of predictions fulfilled, but sometimes of

discrepancies detected. The three inner satellites plunge through the

huge neighbouring shadow-cone at every revolution; the fourth, owing to

its greater distance, escapes eclipse when the shadow makes an

appreciable angle with the plane of its orbit. When Jupiter is in

opposition or conjunction, occultations, but no eclipses, of his moons

take place; at other periods, the two kinds of obscuration merge into,

or succeed each other. “Time cannot stale their infinite variety.”

From observations of the eclipses of Jupiter’s satellites, Olaus Römer

gathered, in 1675, the first intimations of the finite velocity of

light. He noticed that their visibility was alternately retarded and

accelerated as the earth withdrew from, and approached the scene of

their occurrence; and he designated half the extreme difference, or the

time occupied by light in travelling from the earth to the sun, the

“equation of light.” Its value is 500 seconds; and until recently, no

other measure was available of that fundamental constant of nature—the

rate of luminous transmission.

The transits of the satellites across the Jovian disc present many

curious appearances, due to complicated and changeable effects of light

and shade both upon the planetary background, and upon the little

circular objects self-compared with it. These, in the ordinary course,

show bright while near the dusky limb, then vanish during the central

passage, and re-emerge again bright at the opposite side. But, instead

of duly vanishing, they now and then darken even to the point of

becoming indistinguishable from their own shadows, by which they are

preceded or followed. This difference of behaviour cannot be attributed

wholly to varieties of lustre in the sections of the disc transited;

otherwise, it could be predicted. But this has never been attempted;

“black transits” come when least expected. The third and fourth

satellites are those chiefly subject to these phases; the second has

never been known to exhibit them; and they but slightly affect the

first. A drawing by Professor Barnard of one of its bright transits with

an attendant shadow that Peter Schlemyl might have envied, is reproduced

in Figure 17. Its belted appearance, detected by that eminent observer,

will be noted. Indeed, all the satellites, except perhaps No. II. are

striped or spotted; and this leads to seeming deformations in their

shape, as well as fluctuations in their brightness, the markings being

evidently of atmospheric origin, and hence changeable. Their distinct

and accurate perception has been made possible by the excellence of the

Lick thirty-six inch refractor.

[Illustration:

FIG. 17.—_Transit of Jupiter’s first Satellite, with Shadow, drawn by

Prof. Barnard, November 19, 1893._ (From _Monthly Notices_, January,

1894.)

]

Jupiter’s moons seem to resemble him in constitution. The three first

possess the same high reflective power. No. II. is as bright as the

planet’s brightest parts, so that its albedo cannot fall short of 0·70.

And even No. IV. (formerly designated “Calypso” in reference to its

frequent obscurations) exactly matches, during its darkest phases, the

blue-grey polar hoods of its primary. On an average, too, the satellites

seem to be of about the same mean density as Jupiter, No. I. being

considerably the lightest for its bulk; and their spectra, according to

Vogel’s observations in 1873, are composed of solar rays modified in

precisely the same way as those reflected by the planet. Nothing is

known quite certainly about their rotation-periods. Sir William Herschel

concluded them to be of the same length with their periods of

revolution; but recent work throws some doubt upon the reality of this

agreement.

The discovery, September 9, 1892, of Jupiter’s “fifth satellite” was one

of the keenest astronomical surprises on record. An accession to a

system so symmetrically arranged, so complete, to our judgment, as it

stood, appeared superfluous, and, considering the eager scrutiny devoted

to it during 282 years, well-nigh incredible. But the extra member was

in truth out of reach until it was found; original discovery being, as

every one knows, a greatly more arduous feat than subsequent

verification. Nor could it have been casually detected. Professor

Barnard seized the opportunity, lent by the specially favourable

opposition of 1892, to rummage the system for novelties. Keeping the

telescopic field dark by means of a metallic bar placed so as to occult

the gorgeous planetary round, he sought, night after night, for what

might appear. At length, on September 9, he caught the glimmer he

wanted, and made sure, September 10, that it truly intimated the

presence of a new satellite.

This small body revolves in a period of 11 hours, 57 minutes, 23

seconds, at a mean distance of 112,160 miles from Jupiter’s centre, or

67,000 from his bulged equatorial surface. Hence, it should by right be

called “No. I.” instead of “No. V.” The major axis of the ellipse in

which it circulates advances so rapidly, owing to the disturbance caused

by Jupiter’s spheroidal figure, as to complete a revolution in five

months. The implied eccentricity of its orbit, as M. Tisserand has

shown,[65] very slightly exceeds that of the orbit of Venus, yet it has

been made obvious by Barnard’s observations of the differences between

its east and west elongations. Its orbital velocity of 16½ miles a

second far surpasses that of any other satellite in the solar system.

Close vicinity to a mass so vast as Jupiter’s demands counter-balancing

swiftness. Its period of revolution being, however, longer by one hour

than Jupiter’s period of rotation, it so far conducts itself normally as

to rise in the east and set in the west. On the other hand, since its

progress over the sphere is measured by the difference between the two

periods, it spends five Jovian days in journeying from one horizon to

the other, running, in the meantime, four times through all its phases.

Yet it never appears full. Jupiter’s voluminous shadow cuts off sunlight

from it during nearly one-fifth of each circuit.

It is an exceedingly elusive telescopic object. There is no chance of

catching a glimpse of it except with a powerful and perfect telescope at

its “elongations,” or furthest excursions of about eight seconds of arc

on either side of the planet For the most part, it lurks within the

blaze as closely as Teucer behind the shield of Ajax. It is far too

small to be discerned in projection upon the disc, which, viewed from it

in mid-transit, is _full_ with a diameter of 42° 2′, and an area 6,440

times that of our moon. Yet, since its intrinsic lustre is less in the

proportion of 2 to 15, the light shed by Jupiter upon the “fifth

satellite” equals the joint radiance of no more than 860 full moons.

The new satellite is indistinguishable in aspect from a star of the

thirteenth magnitude. And its neighbour No. I. being of 5·6 magnitude,

we receive from it 910 times more light than from the stranger. If both

be equally reflective, the diameter of the latter is ¹⁄₃₀th the diameter

of the former, or, approximately, 80 miles. But its albedo is unlikely

to exceed that of Mars. By a rough estimate, therefore, this interesting

object measures 120 miles across, and 9000 such miniature globes would

go to the making of one full-sized Jovian attendant. Instead of being a

late addition to the system, or, so to speak, an afterthought, it may be

presumed, from the perceptible eccentricity of its path, to be the

senior member of the family. But the subject of its origin is not yet

ripe for discussion.

CHAPTER IX.

THE SATURNIAN SYSTEM.

Nearly twice as far from the sun as Jupiter revolves a planet, the

spacious orbit of which was, until 1781, supposed to mark the uttermost

boundary of the solar system. The mean radius of that orbit is 886

millions of miles; but in consequence of its eccentricity, the sun is

displaced from its middle point to the extent of 50 million miles, and

Saturn is accordingly 100 million miles nearer to him at perihelion than

at aphelion. The immense round assigned to the “saturnine” planet is

traversed in 29½ years, at the tardy pace of six miles a second. His

seasons are thus twenty-nine times more protracted than ours, and are

nominally more accentuated, since his axis of rotation deviates from the

vertical by 27°. But solar heat, however distributed, plays an

insignificant part in his internal economy. In the first place, its

amount is only ¹⁄₉₁th its amount on the earth; in the second, Saturn,

like Jupiter—even more than Jupiter—is thermally self-supporting. The

bulk of his globe comparatively to its mass suffices in itself to make

this certain. The mean diameter of Saturn is 71,000 miles, or nine times

(very nearly) that of the earth; if of equal density, its mass should

then be nine cubed, or 729 times the same unit The actual proportion,

however, is 95; hence the planet has a mean density of only ⁹⁵⁄₇₂₉, or

between ⅐th and ⅛th the terrestrial, and being thus composed of matter

as light as cork, would float in water. Professor G. H. Darwin has

moreover demonstrated, from the movements of its largest satellite, that

its density gains markedly with descent into the interior, so that its

surface-materials must be lighter than any known solid or liquid.

When at its nearest to the earth, Saturn is as large as a sixpence held

up at a distance of 210 yards.[66] But instead of being round like a

sixpence, it is strongly compressed—more compressed even than Jupiter.

The spectra of the two planets are almost identical. Both are impressed

with traces of aqueous absorption, and include the “red star line.”

About the albedo of Saturn there is some uncertainty. Zöllner made it

0·50, a very probable value; Müller of Potsdam determined it at 3·3

times that of Mars, the unit of his scale. For the value of the unit,

the only authority is Zöllner, who found Mars to give back 0·26 of the

light dispensed to him. Multiplying then 0·26 by 3·3 we get for the

albedo of Saturn 0·86, an impossible number for a non-luminous body, the

albedo of “untrodden snow” being, as already stated, 0·78.

Saturn resembles to the eye a large, dull star; its rays are entirely

devoid of the sparkling quality which distinguishes those of Jupiter.

But it shows telescopically an analogous surface-structure. Its most

conspicuous markings are tropical dark belts of a greyish or greenish

hue; the equatorial region is light yellow, diversified by vague white

spots; while the poles carry extensive pale blue canopies. The apparent

tranquillity of the disc may be attributed in part to the vast distance

from which it is viewed; yet not wholly. For lack of fiducial points, no

attempt was made to determine the planet’s rotation until 1794, when the

elder Herschel, by following an identified irregularity in a complex

banded formation, arrived at a period of 10 hours 16 minutes. The first

possibility of checking this result offered itself to Professor Hall of

Washington, after fourteen years of vain expectation, in the emergence

of a white spot just north of the equator, the movement of which gave

for the length of the Saturnian day, 10 hours, 14 minutes, 24 seconds.

In 1891–2, Mr. Stanley Williams made observations upon a good many such

objects; and their discussion by Mr. Denning afforded a mean period two

seconds longer than Hall’s. Individual variations, however, to the

extent of 14 seconds were brought out by it, proving that Saturnian,

like Jovian, spots have “proper motions,” and cannot be depended upon to

give the true rotation of the planet. Its compound nature may be

suspected, but has not yet been proved.

From measures executed by Barnard in 1895, it appears that the

equatorial diameter of Saturn is 76,470, its polar diameter 69,770

miles, giving a mean diameter of 74,240, and a compression of about

¹⁄₁₂. Gravity, at its surface, is only one-fifth more powerful than on

the earth.

Thus, Saturn not only belongs to the same celestial species as Jupiter,

but is a closely-related individual of that species. There is no

probability that either is to any extent solid. Both exhibit the same

type of markings; both betray internal tumults by eruptions of spots

which, by their varying movements, supply a measure for the profundity

of their origin; both possess identically constituted atmospheres, and

are darkened marginally by atmospheric absorption.

[Illustration:

FIG. 18.—_Saturn and its Rings. Drawn by Prof. Barnard, July 2, 1894._

]

Saturn is, however, distinguished by the possession of an unique set of

appendages. Nothing like them is to be seen elsewhere in the heavens;

and when well opened (as in Fig. 18) they form, with the globe they

enclose, and the retinue of satellites in waiting outside, a strange and

wonderful telescopic object. The rings, since they lie in the plane of

Saturn’s equator, are inclined 27° to the Saturnian orbit, and 28° to

the ecliptic. The earth is, however, comparatively to Saturn, so near

the sun, that their variations in aspect, as viewed from it, may in a

rough way be considered the same as if seen from the sun. They

correspond exactly with the Saturnian seasons. At the Saturnian

equinoxes, the rings are illuminated edgewise, and disappear, totally or

approximately; at the Saturnian solstices, sunlight strikes them nearly

at the full angle of 27°, first from _below_, then from _above_. At

these epochs, we perceive the appendage expanded into an ellipse about

half as wide as it is long. Two concentric rings (generally called A and

B) are then very plainly distinguishable, the inner being the brighter.

The black fissure which separates them is called “Cassini’s division,”

because that eminent observer was, in 1675, the first to perceive it. A

chasm known as “Encke’s division,” in the outer ring (A), is a thinning

out rather than an empty space; and temporary gaps frequently appear in

A, while B is entirely exempt from them. There are then two definite and

permanent bright rings, and no more; but with them is associated the

dusky formation discovered by W. C. Bond, November 15, 1850, and

described by Lassell as “something like a crape veil covering a part of

the sky within the inner ring.” It is semi-transparent the limb of

Saturn showing distinctly through it.

The exterior diameter of the ring-system is 172,800, while its breadth

is 42,300 miles.[67] The rings A and C are each 11,000 miles wide; while

B measures 18,000, Cassini’s division 2,270, and the clear interval

between C and the planetary surface somewhat less than 6,000 miles. Each

ring, C included, is brightest at its outer edge; but there is no gap

between the shining and the dusky structures, B shading by insensible

gradations up to C, yet maintaining distinctness from it. The earliest

exact determinations of the former were made by Bradley in 1719, since

when they have been affected by no appreciable change.[68] The

theoretically inevitable subversion of the system is progressing with

extreme slowness.

The thickness of the rings is quite inconsiderable. They are flat

sheets, without (so to speak) a third dimension. For this reason, they

disappear utterly in most telescopes, when their plane passes through

the earth, as it does twice in each Saturnian year. Only under

exceptional conditions, a narrow, knotted, often nebulous, streak

survives as an index to their whereabouts. On October 26, 1891,

Professor Barnard,[69] armed with the Lick refractor, found it

impossible to see them projected upon the sky, notwithstanding that

their shadow lay heavily on the planet It was not until three days

later, that “slender threads of light” came into view. The corresponding

thickness of the formation was estimated at less than fifty miles. The

phenomenon of the disappearance of the rings will not recur until July

29, 1907.

The constitution of this marvellous structure is no longer doubtful. It

represents what might be called the fixed form of a revolving multitude

of diminutive bodies. This was demonstrated by Clerk Maxwell in the

Adams Prize Essay of 1857. His conclusion proved irreversible. The

pulverulent composition of Saturn’s rings is one of the acquired truths

of science. An incalculable number of tiny satellites, revolving

independently in distinct orbits, in the precise periods prescribed by

their several distances from the planet, are aggregated into the

unmatched appendages of Galileo’s _tergeminus planeta_. The local

differences in their brightness depend upon the distribution of the

component satelloids. Where they are closely packed, as in the outer

margins of rings A and B, sunlight is copiously reflected; where the

interspaces are wide, the blackness of the sky is barely veiled by the

scanty rays thrown back from the thinly scattered cosmic dust. The

appearance of the crape ring as a _dark_ stripe on the planet results—as

M. Seeliger has pointed out—not from the transits of the objects

themselves, but from the flitting of their shadows in continual

procession across the disc.

The albedo of these particles is so high as to render it improbable that

they are of an earthy or rocky nature, such as the meteorites which

penetrate our atmosphere. The rings they form are, on the whole, more

lustrous than Saturn’s globe; but this superiority is held to be due to

the absence of atmospheric absorption. Their spectrum is that of

unmodified sunlight.

An eclipse of Japetus, the eighth Saturnian moon, by the globe and

rings, November 1, 1889, was highly instructive as to the nature of the

dusky appendage. The satellite was never lost sight of during its

passage behind it; but became more and more deeply obscured as it

travelled outward; then, at the moment of ingress into the shadow of

ring B, suddenly disappeared. Certainty was thus acquired that the

particles forming the crape ring are most sparsely strewn at its inner

edge—which is, nevertheless, perfectly definite—and gradually reach a

maximum of density at its outer edge. Yet, while there is not the

smallest clear interval, a sharp line of demarcation separates it from

the contiguous bright ring. Professor Barnard was the only observer of

these curious appearances. The distribution of the ring-constituents,

like that of the asteroids, was governed by the law of commensurable

periods, Saturn’s moons replacing Jupiter as the perturbing and

regulating power. Kirkwood showed in 1867, that Cassini’s division

represents a region of peculiarly strong disturbance; since a body

revolving there would have a period connected by a simple relation with

the periods of no less than _four_ satellites. Encke’s division, too, as

Dr. Meyer has indicated, and other lines of scanty occupation and

occasional vacancy, coincide with districts of space where similar

combinations occur.

The “satellite-theory” of Saturn’s rings has received confirmation from

apparently the least promising quarters. Professor Seeliger of Munich

showed, from photometric experiments in 1888, that their constant lustre

under angles of illumination ranging from 0° to 30° was proof positive

of their composition out of discrete small bodies.[70] And Professor

Keeler of Alleghany, by a beautiful and refined application of the

spectroscopic method, arrived at the same result in April, 1895.[71]

“Under the two different hypotheses,” he remarked, “that the ring is a

rigid body, and that it is a swarm of satellites, the relative motion of

its parts would be essentially different.” The former would necessarily

involve increasing velocity _outward_, the latter, increase of velocity

_inward_, just for the same reason that Mercury moves more swiftly than

the earth, and the earth than Saturn; while the sections of a solid

body, which could have but one period of rotation, should move faster,

_in miles per second_, the farther they were from the centre of

attraction. The line of sight test is then theoretically available; but

it was an arduous task to render it practically so. The difficulties

were, however, one by one overcome; and a successful photograph of the

spectra of Saturn and its rings gave the required information in

unmistakable shape. From measurements of the inclinations of five dusky

rays contained in it with reference to a standard horizontal line, rates

of movement were derived of 12½ miles per second for the inner edge of

ring B, and of 10 miles for the outer edge of ring A. The agreement with

theory was, as nearly as possible, exact; the components of the rings

were experimentally demonstrated to be moving, each independently of

every other, under the dominion of Kepler’s laws.

For the globe of Saturn, Professor Keeler obtained, by the same

exquisite method, a rotational period of 10 hours, 14 minutes, 24

seconds, in precise accordance with that indicated by the white spot of

1876, which thus seems to have had no proper motion, but to have floated

on the ochreous equatorial surface as tranquilly as a water-lily upon a

stagnant pool. The result, so far as it goes, hints that Saturn may be

really, as well as apparently, less ebullient than Jupiter.

Seers into the future of the heavenly bodies consider that the rings of

Saturn, like the gills of a tadpole, are symptomatic of an early stage

of development; and will be disposed of before he arrives at maturity.

They cannot be regarded otherwise than as abnormal excrescences. No

other planet retains matter circulating round it in such close relative

vicinity. It was proved by Roche of Montpellier that no secondary body

of importance can exist within less than 2·44 mean radii of its primary;

inside of that limit, it would be rent asunder by tidal strain. But the

entire ring-system lies within the assigned boundary; hence, being

_where_ it is, it can only exist _as_ it is—in flights of discrete

particles. Will it, however, always remain where it is?

“Clerk Maxwell,” wrote Mr. Cowper Ranyard,[72] “used to describe the

matter of the rings as a shower of brickbats, amongst which there would

inevitably be continual collisions. The theoretical results of such

impacts would be a spreading of the ring both inwards and outwards. The

outward spreading will in time carry the meteorites beyond Roche’s

limit, where, in all probability, they will, as Professor Darwin

suggests, slowly aggregate, and a minute satellite will be formed. The

inward spreading will in time carry the meteorites at the inner edge of

the ring into the atmosphere of the planet, where they will become

incandescent, and disappear as meteorites do in our atmosphere.”

Yet it may be that collisions are infrequent in this conglomeration of

“brickbats.” There is the strongest presumption that they all circulate

in the same direction, in orbits nearly circular, and scarcely deviating

from the plane of the Saturnian equator. Those pursuing markedly

eccentric tracks must long ago have been eliminated. Thus, encounters

can only occur through gravitational disturbances by Saturn’s moons, and

they must be of a mild character, depending upon very small differences

of velocity. The first sign of a “spreading outwards” should be the

formation of an exterior “crape ring,” of which no faintest trace has

yet been perceived.

Saturn’s rings are entirely invisible from its polar regions, but

occasion prolonged and complex eclipse-effects in its temperate and

equatorial zones. They have been fully treated of from the geometrical

point of view by Mr. Proctor in “Saturn and its System.”

Of this planet’s eight satellites, the largest, Titan (No. VI.), was

discovered first (by Huygens in 1655), and the smallest, Hyperion (No.

VII.), last (by Lassell and Bond in 1848). The five others were detected

by J. D. Cassini and William Herschel. Titan, alone of the entire group,

equals our moon in size. It measures, according to Professor Barnard,

2,720 miles across. Its period of revolution is nearly sixteen days, its

distance from Saturn’s centre, 771,000 miles. The orbit of Japetus (No.

VIII.) is the largest, and its period the longest of any secondary body

in the solar system. It circulates in 79⅓ days at a distance of

2,225,000 miles, equal to 59½ of Saturn’s equatorial radii. Hence its

path is of about the same _proportional_ dimensions as that of our moon.

Japetus is remarkable for its variability in light. It is capable of

tripling or quadrupling its minimum lustre. Sir William Herschel noticed

that these maxima coincided with a position on the western side of the

planet, and inferred rotation of the lunar kind. “From the changes in

this body,” he argued in 1792,[73] “we may conclude that some part of

its surface, and this by far the largest, reflects much less light than

the rest; and that neither the darkest nor the brightest side is turned

towards the planet, but partly one and partly the other, though probably

less of the bright side.”

This explanation, however, he admitted to be incomplete. There was, and

is, outstanding variability, which seems to intimate the presence of an

atmosphere and the formation of clouds. But no positive knowledge has

yet been gained regarding the physical state of Saturn’s moons. We may

nevertheless conjecture that, since tidal friction has destroyed the

rotation (as regards Saturn) of the remotest member of the family, it

has not spared those more exposed to its grinding-down action. All

presumably rotate in the same time that they revolve.

The five inner satellites move in approximately circular orbits; the

three outer in ellipses about twice as eccentric as the terrestrial

path. All, Japetus only excepted, keep strictly to the plane of the

rings. And since this makes an angle of 270 with the planet’s orbit,

eclipses are much less frequent here than in the Jovian system. They can

only occur when Saturn is within a certain distance (different for each)

from the node of the satellite-orbit. Even Mimas (No. I.), although it

wheels round the ring at an interval of only 34,000 miles, often slips

outside the obliquely-projected shadow-cone. Its distance from Saturn’s

centre is 118,000 miles, and it completes a circuit in 22½ hours.

Perpetually wrapped in the glare of its magnificent primary, it is a

very shy object, only to be caught sight of in its timid excursions by

the very finest telescopes. Like all the Saturnian moons, except Titan,

and, by a rare conjuncture, Japetus, it is far too much contracted to be

visible in transit across the disc.

The movements of these bodies have been carefully studied, and their

mutual perturbations to some extent unravelled. They have proved

exceedingly interesting to students of celestial mechanics. Titan has,

in this department, chiefly to be reckoned with. He exercises in the

Saturnian system a similar overpowering influence to that wielded by

Jupiter in the solar system. Mr. Stone finds its mass to be ¹⁄₇₆₀₀th

that of Saturn, showing that its density is nearly equal to that of our

moon. This seems to indicate an advanced stage of cooling. On the other

hand, its albedo is evidently very high. The other satellites appear in

the largest telescopes as mere stellar points.

CHAPTER X.

URANUS AND NEPTUNE.

The four giant planets, closely allied as they are, and strongly

distinguished in physical constitution from the terrestrial planets,

divide again of themselves into two sub-groups. Jupiter and Saturn have

much more in common than either has with Uranus or Neptune; while Uranus

and Neptune present peculiar analogies. Conclusions concerning one may

almost be said to apply to the other. Their enormous distance, it is

true, tends to efface minor differences; yet it is insufficient to

obliterate similarities of a peculiar kind.

Uranus is a globe 32,000 miles in mean diameter, and decidedly

elliptical in shape. Mädler and Schiaparelli agreed in assigning to it a

compression of ¹⁄₁₁; Barnard, in 1894, uninformed of their results,

noticed the disc to be more oval than Saturn’s. The indicated rotational

movement must be very swift; and a lucid spot watched by MM. Perrotin

and Thollon at Nice in 1884, seemed to fix it at about ten hours. This

was, however, only a vague estimate. Faint equatorial belts, too, have

with difficulty been seen. Remembering, indeed, that the object they

diversify is just large enough to be _annularly eclipsed_ by a cricket

ball two miles off, there is little cause for surprise at the

indistinctness of its surface-markings. They probably consist, like

those of Jupiter and Saturn, in dusky polar hoods, a brilliant

equatorial zone, and obscure intermediate bands. The last were seen as

“the merest shades on the planet’s surface,” and under a somewhat

deformed aspect, by the Lick observers in 1890 and 1891.[74] By

Professor Young in 1883, on the other hand, and by the MM. Henry at

Paris in 1884, they were observed to be symmetrically placed, parallel

one to the other, and of what might be called the normal type for great

planets. That they constitute, with the bright space they enclose, an

equatorial scheme of marking, was proved by Barnard’s comparison of the

trend (or position angle), determined for them by Young, with the

direction of the shortest axis of the little disc they traverse.[75]

Their considerable foreshortening in 1894 was, doubtless, the reason why

Barnard, with his acute vision, was compelled to rely upon earlier

observations, brought up to date by computation. Unless, indeed, the

markings are intrinsically variable.

This was suspected at Nice in 1889, when a thirty-inch refractor was

available for their scrutiny.[76] Dusky rulings were obvious on a

strongly compressed spheroid; and they ran parallel to the major axis of

the spheroid—that is, to the planet’s equator. But their appearance

varied, and their width seemed irregular. At the same establishment, but

with a fourteen-inch telescope, Uranus was observed, under particularly

favourable circumstances, March 18, 1884.[77] An unexpected resemblance

to Mars was apparent. The ordinarily sea-green disc was divided into a

sombre north-western and a bluish-white south-eastern hemisphere. Dark

spots were visible, and a conspicuous white one at the limb simulated a

snow-cap. But ulterior observations resolved the spots into belts, and

showed the shining patch to be, not polar, but equatorial. It was

presumably of an eruptive nature.