CHAPTER VII

_PLANETS AND SATELLITES_

Johann Hieronymus Schröter was the Herschel of Germany. He did not, it

is true, possess the more brilliant gifts of his rival. Herschel's

piercing discernment, comprehensive intelligence, and inventive

splendour were wanting to him. He was, nevertheless, the founder of

descriptive astronomy in Germany, as Herschel was in England.

Born at Erfurt in 1745, he prosecuted legal studies at Göttingen, and

there imbibed from Kästner a life-long devotion to science. From the

law, however, he got the means of living, and, what was to the full as

precious to him, the means of observing. Entering the sphere of

Hanoverian officialism in 1788, he settled a few years later at

Lilienthal, near Bremen, as "Oberamtmann," or chief magistrate. Here he

built a small observatory, enriched in 1785 with a seven-foot reflector

by Herschel, then one of the most powerful instruments to be found

anywhere out of England. It was soon surpassed, through his exertions,

by the first-fruits of native industry in that branch. Schrader of Kiel

transferred his workshops to Lilienthal in 1792, and constructed there,

under the superintendence and at the cost of the astronomical

Oberamtmann, a thirteen-foot reflector, declared by Lalande to be the

finest telescope in existence, and one twenty-seven feet in focal

length, probably as inferior to its predecessor in real efficiency as it

was superior in size.

Thus, with instruments of gradually increasing power, Schröter studied

during thirty-four years the topography of the moon and planets. The

field was then almost untrodden; he had but few and casual predecessors,

and has since had no equal in the sustained and concentrated patience of

his hourly watchings. Both their prolixity and their enthusiasm are

faithfully reflected in his various treatises. Yet the one may be

pardoned for the sake of the other, especially when it is remembered

that he struck out a substantially new line, and that one of the main

lines of future advance. Moreover, his infectious zeal communicated

itself; he set the example of observing when there was scarcely an

observer in Germany; and under his roof Harding and Bessel received

their training as practical astronomers.

But he was reserved to see evil days. Early in 1813 the French under

Vandamme occupied Bremen. On the night of April 20, the Vale of Lilies

was, by their wanton destructiveness, laid waste with fire; the

Government offices were destroyed, and with them the chief part of

Schröter's property, including the whole stock of his books and

writings. There was worse behind. A few days later, his observatory,

which had escaped the conflagration, was broken into, pillaged, and

ruined. His life was wrecked with it. He survived the catastrophe three

years without the means to repair, or the power to forget it, and

gradually sank from disappointment into decay, terminated by death,

August 29, 1816. He had, indeed, done all the work he was capable of;

and though not of the first quality, it was far from contemptible. He

laid the foundation of the _comparative_ study of the moon's surface,

and the descriptive particulars of the planets laboriously collected by

him constituted a store of more or less reliable information hardly

added to during the ensuing half century. They rested, it is true, under

some shadow of doubt; but the most recent observations have tended on

several points to rehabilitate the discredited authority of the

Lilienthal astronomer. We may now briefly resume, and pursue in its

further progress, the course of his studies, taking the planets in the

order of their distances from the sun.

In April, 1792, Schröter saw reason to conclude, from the gradual

degradation of light on its partially illuminated disc, that Mercury

possesses a tolerably dense atmosphere.[796] During the transit of May

7, 1799, he was, moreover, struck with the appearance of a ring of

softened luminosity encircling the planet to an apparent height of three

seconds, or about a quarter of its own diameter.[797] Although a "mere

thought" in texture, it remained persistently visible both with the

seven-foot and the thirteen-foot reflectors, armed with powers up to

288. It had a well-marked grayish boundary, and reminded him, though

indefinitely fainter, of the penumbra of a sun-spot. A similar appendage

had been noticed by De Plantade at Montpellier, November 11, 1736, and

again in 1786 and 1789 by Prosperin and Flaugergues; but Herschel, on

November 9, 1802, saw the preceding limb of the planet projected on the

sun cut the luminous solar clouds with the most perfect sharpness.[798]

The presence, however, of a "halo" was unmistakable in 1832, when

Professor Moll, of Utrecht, described it as a "nebulous ring of a darker

tinge approaching to the violet colour."[799] Again, to Huggins and

Stone, November 5, 1868, it showed as lucid and most distinct. No change

in the colour of the glasses used, or the powers applied, could get rid

of it, and it lasted throughout the transit.[800] It was next seen by

Christie and Dunkin at Greenwich, May 6, 1878,[801] and with much

precision of detail by Trouvelot at Cambridge (U.S.).[802] Professor

Holden, on the other hand, noted at Hastings-on-Hudson the total absence

of all anomalous appearances.[803] Nor could any vestige of them be

perceived by Barnard at Lick on November 10, 1894.[804] Various effects

of irradiation and diffraction were, however, observed by Lowell and W.

H. Pickering at Flagstaff;[805] and Davidson was favoured at San

Francisco with glimpses of the historic aureola,[806] as well as of a

central whitish spot, which often accompanies it. That both are somehow

of optical production can scarcely be doubted.

Nothing can be learned from them regarding the planet's physical

condition. Airy showed that refraction in a Mercurian atmosphere could

not possibly originate the noted aureola, which must accordingly be set

down as "strictly an ocular nervous phenomenon."[807] It is the less

easy to escape from this conclusion that we find the virtually airless

moon capable of exhibiting a like appendage. Professor Stephen

Alexander, of the United States Survey, with two other observers,

perceived, during the eclipse of the sun of July 18, 1860, the advancing

lunar limb to be bordered with a bright band;[808] and photographic

effects of the same kind appear in pictures of transits of Venus and

partial solar eclipses.

The spectroscope affords little information as to the constitution of

Mercury. Its light is of course that of the sun reflected, and its

spectrum is consequently a faint echo of the Fraunhofer spectrum. Dr. H.

C. Vogel, who first examined it in April, 1871, _suspected_ traces of

the action of an atmosphere like ours,[809] but, it would seem, on

slight grounds. It is, however, certainly very poor in blue rays. More

definite conclusions were, in 1874,[810] derived by Zöllner from

photometric observations of Mercurian phases. A similar study of the

waxing and waning moon had afforded him the curious discovery that

light-changes dependent upon phase vary with the nature of the

reflecting surface, following a totally different law on a smooth

homogeneous globe and on a rugged and mountainous one. Now the phases of

Mercury--so far as could be determined from only two sets of

observations--correspond with the latter kind of structure. Strictly

analogous to those of the moon, they seem to indicate an analogous mode

of surface-formation. This conclusion was fully borne out by Müller's

more extended observations at Potsdam during the years 1885-1893.[811]

Practical assurance was gained from them that the innermost planet has a

rough rind of dusky rock, absorbing all but 17 per cent. of the light

poured upon it by the fierce adjacent sun. Its "albedo," in other words,

is 0·17,[812] which is precisely that ascribed to the moon. The absence

of any appreciable Mercurian atmosphere followed almost necessarily from

these results.

On March 26, 1800, Schröter, observing with his 13-foot reflector in a

peculiarly clear sky, perceived the southern horn of Mercury's crescent

to be quite distinctly blunted.[813] Interception of sunlight by a

Mercurian mountain rather more than eleven English miles high explained

the effect to his satisfaction. By carefully timing its recurrence, he

concluded rotation on an axis in a period of 24 hours 4 minutes. The

first determination of the kind rewarded twenty years of unceasing

vigilance. It received ostensible confirmation from the successive

appearances of a dusky streak and blotch in May and June, 1801.[814]

These, however, were inferred to be no permanent markings on the body of

the planet, but atmospheric formations, the streak at times drifting

forwards (it was thought) under the fluctuating influence of Mercurian

breezes. From a rediscussion of these somewhat doubtful observations

Bessel inferred that Mercury rotates on an axis inclined 70° to the

plane of its orbit in 24 hours 53 seconds.

The rounded appearance of the southern horn seen by Schröter was more or

less doubtfully caught by Noble (1864), Burton, and Franks (1877);[815]

but was obvious to Mr. W. F. Denning at Bristol on the morning of

November 5, 1882.[816] That the southern polar regions are usually less

bright than the northern is well ascertained; but the cause of the

deficiency remains dubious. If inequalities of surface are in question,

they must be on a considerable scale; and a similar explanation might be

given of the deformations of the "terminator"--or dividing-line between

darkness and light in the planet's phases--first remarked by Schröter,

and again clearly seen by Trouvelot in 1878 and 1881.[817] The

displacement, during four days, of certain brilliant and dusky spaces on

the disc indicated to Mr. Denning in 1882 rotation in about twenty-five

hours; while the general aspect of the planet reminded him of that of

Mars.[818] But the difficulties in the way of its observation are

enormously enhanced by its constant close attendance on the sun.

In his sustained study of the features of Mercury, Schröter had no

imitator until Schiaparelli took up the task at Milan in 1882. His

observations were made in daylight. It was found that much more could be

seen, and higher magnifying powers used, high up in the sky near the

sun, than at low altitudes, through the agitated air of morning or

evening twilight. A notable discovery ensued.[819] Following the planet

hour by hour, instead of making necessarily brief inspections at

intervals of about a day, as previous observers had done, it was found

that the markings faintly visible remained sensibly fixed, hence, that

there was no rotation in a period at all comparable with that of the

earth. And after long and patient watching, the conclusion was at last

reached that Mercury turns on his axis in the same time needed to

complete a revolution in his orbit. One of his hemispheres, then, is

always averted from the sun, as one of the moon's hemispheres from the

earth, while the other never shifts from beneath his torrid rays. The

"librations," however, of Mercury are on a larger scale than those of

the moon, because he travels in a more eccentric path. The temporary

inequalities arising between his "even pacing" on an axis and his

alternately accelerated and retarded elliptical movement occasion, in

fact, an oscillation to and fro of the boundaries of light and darkness

on his globe over an arc of 47° 22', in the course of his year of 88

days. Thus the regions of perpetual day and perpetual night are

separated by two segments, amounting to one-fourth of the entire

surface, where the sun rises and sets once in 88 days. Else there is no

variation from the intense glare on one side of the globe, and the

nocturnal blackness on the other.

To Schiaparelli's scrutiny, Mercury appeared as a "spotty globe,"

enveloped in a tolerably dense atmosphere. The brownish stripes and

streaks, discerned on his rose-tinged disc, and judged to be permanent,

were made the basis of a chart. They were not indeed always equally well

seen. They disappeared regularly near the limb, and were at times veiled

even when centrally situated. Some of them had been clearly perceived by

De Ball at Bothkamp in 1882.[820]

Mr. Lowell followed Schiaparelli's example by observing Mercury in the

full glare of noon. "The best time to study him," he remarked, "is when

planetary almanacs state 'Mercury invisible.'" A remarkable series of

drawings executed, some at Flagstaff in 1896, the remainder at Mexico in

1897, supplied grounds for the following, among other, conclusions.[821]

Mercury rotates synchronously with its revolution--that is, once in 88

days--on an axis sensibly perpendicular to its orbital plane. No certain

signs of a Mercurian atmosphere are visible. The globe is seamed and

furrowed with long narrow markings, explicable as cracks in cooling. It

is, and always was, a dead world. From micrometrical measures, moreover,

the inferences were drawn that the planet's mass has a probable value

about 1/20 that of the earth, while its mean density falls considerably

short of the terrestrial standard.

The theory of Mercury's movements has always given trouble. In

Lalande's,[822] as in Mästlin's time, the planet seemed to exist for no

other purpose than to throw discredit on astronomers; and even to

Leverrier's powerful analysis it long proved recalcitrant. On the 12th

of September, 1869, however, he was able to announce before the Academy

of Sciences[823] the terms of a compromise between observation and

calculation. They involved the addition of a new member to the solar

system. The hitherto unrecognised presence of a body about the size of

Mercury itself revolving at somewhat less than half its mean distance

from the sun (or, if farther, then of less mass, and _vice versâ_),

would, it was pointed out, produce exactly the effect required, of

displacing the perihelion of the former planet 38" a century more than

could otherwise be accounted for. The planes of the two orbits, however,

should not lie far apart, as otherwise a nodal disturbance would arise

not perceived to exist. It was added that a ring of asteroids similarly

placed would answer the purpose equally well, and was more likely to

have escaped notice.

Upon the heels of this forecast followed promptly a seeming

verification. Dr. Lescarbault, a physician residing at Orgères, whose

slender opportunities had not blunted his hopes of achievement, had,

ever since 1845, when he witnessed a transit of Mercury, cherished the

idea that an unknown planet might be caught thus projected on the solar

background. Unable to observe continuously until 1858, he, on March 26,

1859, saw what he had expected--a small perfectly round object slowly

traversing the sun's disc. The fruitless expectation of reobserving the

phenomenon, however, kept him silent, and it was not until December 22,

after the news of Leverrier's prediction had reached him, that he wrote

to acquaint him with his supposed discovery.[824] The Imperial

Astronomer thereupon hurried down to Orgères, and by personal inspection

of the simple apparatus used, by searching cross-examination and local

inquiry, convinced himself of the genuine character and substantial

accuracy of the reported observation. He named the new planet "Vulcan,"

and computed elements giving it a period of revolution slightly under

twenty days.[825] But it has never since been seen. M. Liais, director

of the Brazilian Coast Survey, thought himself justified in asserting

that it never had been seen. Observing the sun for twelve minutes after

the supposed ingress recorded at Orgères, he noted those particular

regions of its surface as "très uniformes d'intensité."[826] He

subsequently, however, admitted Lescarbault's good faith, at first

rashly questioned. The planet-seeking doctor was, in truth, only one

among many victims of similar illusions.

Waning interest in the subject was revived by a fresh announcement of a

transit witnessed, it was asserted, by Weber at Peckeloh, April 4,

1876.[827] The pseudo-planet, indeed, was detected shortly afterwards on

the Greenwich photographs, and was found to have been seen by M. Ventosa

at Madrid in its true character of a sun-spot without penumbra; but

Leverrier had meantime undertaken the investigation of a list of twenty

similar dubious appearances, collected by Haase, and republished by Wolf

in 1872.[828] From these, five were picked out as referring in all

likelihood to the same body, the reality of whose existence was now

confidently asserted, and of which more or less probable transits were

fixed for March 22, 1877, and October 15, 1882.[829] But, widespread

watchfulness notwithstanding, no suspicious object came into view at

either epoch.

The next announcement of the discovery of "Vulcan" was on the occasion

of the total solar eclipse of July 29, 1878.[830] This time it was

stated to have been seen at some distance south-west of the obscured

sun, as a ruddy star with a minute planetary disc; and its simultaneous

detection by two observers--the late Professor James C. Watson,

stationed at Rawlins (Wyoming Territory), and Professor Lewis Swift at

Denver (Colorado)--was at first readily admitted. But their separate

observations could, on a closer examination, by no possibility be

brought into harmony, and, if valid, certainly referred to two distinct

objects, if not to four; each astronomer eventually claiming a pair of

planets. Nor could any one of the four be identified with Lescarbault's

and Leverrier's Vulcan, which, if a substantial body revolving round the

sun, must then have been found on the _east_ side of that luminary.[831]

The most feasible explanation of the puzzle seems to be that Watson and

Swift merely saw each the same two stars in Cancer: haste and excitement

doing the rest.[832] Nevertheless, they strenuously maintained their

opposite conviction.[833]

Intra-Mercurian planets have since been diligently searched for when the

opportunity of a total eclipse offered, especially during the long

obscuration at Caroline Island. Not only did Professor Holden "sweep" in

the solar vicinity, but Palisa and Trouvelot agreed to divide the field

of exploration, and thus make sure of whatever planetary prey there

might be within reach; yet with only negative results. Photographic

explorations during recent eclipses have been equally fruitless. Belief

in the presence of any considerable body or bodies within the orbit of

Mercury is, accordingly, at a low ebb. Yet the existence of the anomaly

in the Mercurian movements indicated by Leverrier has been made only

surer by further research.[834] Its elucidation constitutes one of the

"pending problems" of astronomy.

* * * * *

From the observation at Bologna in 1666-67 of some very faint spots,

Domenico Cassini concluded a rotation or libration of Venus--he was not

sure which--in about twenty-three hours.[835] By Bianchini in 1726 the

period was augmented to twenty-four _days_ eight hours. J. J. Cassini,

however, in 1740, showed that the data collected by both observers were

consistent with rotation in twenty-three hours twenty minutes.[836] So

the matter rested until Schröter's time. After watching nine years in

vain, he at last, February 28, 1788, perceived the ordinarily uniform

brightness of the planet's disc to be marbled with a filmy streak, which

returned periodically to the same position in about twenty-three hours

twenty-eight minutes. This approximate estimate was corrected by the

application of a more definite criterion. On December 28, 1789, the

southern horn of the crescent Venus was seen truncated, an outlying

lucid point interrupting the darkness beyond. Precisely the same

appearance recurred two years later, giving for the planet's rotation a

period of 23h. 21m.[837] To this only twenty-two seconds were added by

De Vico, as the result of over 10,000 observations made with the

Cauchoix refractor of the Collegio Romano, 1839-41.[838] The axis of

rotation was found to be much more bowed towards the orbital plane than

that of the earth, the equator making with it an angle of 53° 11'.

These conclusions inspired, it is true, much distrust, consequently

there were no received ideas on the subject to be subverted.

Nevertheless, a shock of surprise was felt at Schiaparelli's

announcement, early in 1890,[839] that Venus most probably rotates after

the fashion just previously ascribed to Mercury. A continuous series of

observations, from November, 1877, to February, 1878, with their records

in above a hundred drawings, supplied the chief part of the data upon

which he rested his conclusions. They certainly appeared exceptionally

well-grounded; and the doubts at first qualifying them were removed by a

fresh set of determinations in July, 1895.[840] Most observers had

depended, in their attempts to ascertain the rotation-period of Venus,

upon evanescent shadings, most likely of atmospheric origin, and

scarcely recognisable from day to day. Schiaparelli fixed his attention

upon round, defined, lustrously white spots, the presence of which near

the cusps of the illuminated crescent has been attested for close upon

two centuries. His steady watch over them showed the invariability of

their position with regard to the terminator; and this is as much as to

say that the regions of day and night do not shift on the surface of the

planet. In other words, she keeps the same face always turned towards

the sun. Moreover, since her orbit is nearly circular, libratory effects

are very small. They amount in fact to only just one-thirtieth of those

serving to modify the severe contrasts of climate in Mercury.

Confirmatory evidence of Schiaparelli's result for Venus is not wanting.

Thus, observations irreconcilable with a swift rate of rotation were

made at Bothkamp in 1871 by Vogel and Lohse;[841] and a drawing executed

by Professor Holden with the great Washington reflector, December 15,

1877, showed the same markings in the positions recorded at Milan to

have been occupied by them eight hours previously. Further, a series of

observations, carried out by M. Perrotin at Nice, May 15 to October 4,

1890, and from Mount Mounier in 1895-6, with the special aim of testing

the inference of synchronous rotation and revolution, proved strongly

corroborative of it.[842] A remarkable collection of drawings made by

Mr. Lowell in 1896 appeared decisive in its favour;[843] Tacchini at

Rome,[844] Mascari at Catania and Etna,[845] Cerulli at Terano,[846]

obtained in 1892-6 evidence similar in purport. On the other hand,

Niesten of Brussels found reason to revert to Vico's discarded elements

for the planet's rotation;[847] and Trouvelot,[848] Stanley

Williams,[849] Villiger,[850] and Leo Brenner,[851] so far agreed with

him as to adopt a period of approximately twenty-four hours. Finally, E.

Von Oppolzer suggested an appeal to the spectroscope;[852] and

Bélopolsky secured in 1900[853] spectrograms apparently marked by the

minute displacements corresponding to a rapid rate of axial movement.

But they were avowedly taken only as an experiment, with unsuitable

apparatus; and the desirable verification of their supposed import is

not yet forthcoming. Until it is, Schiaparelli's period of 225 days must

be allowed to hold the field.

Effects attributed to great differences of level in the surface of Venus

have struck many observers. Francesco Fontana at Naples in 1643 noticed

irregularities along the inner edge of the crescent.[854] Lahire in 1700

considered them--regard being had to difference of distance--to be much

more strongly marked than those visible in the moon.[855] Schröter's

assertions to the same effect, though scouted with some unnecessary

vehemence by Herschel,[856] have since been repeatedly confirmed;

amongst others by Mädler, De Vico, Langdon, who in 1873 saw the broken

line of the terminator with peculiar distinctness through a veil of

auroral cloud;[857] by Denning,[858] March 30, 1881, despite preliminary

impressions to the contrary, as well as by C. V. Zenger at Prague,

January 8, 1883. The great mountain mass, presumed to occasion the

periodical blunting of the southern horn, was precariously estimated by

the Lilienthal observer to rise to the prodigious height of nearly

twenty-seven miles, or just five times the elevation of Mount Everest!

Yet the phenomenon persists, whatever may be thought of the explanation.

Moreover, the speck of light beyond, interpreted as the visible sign of

a detached peak rising high enough above the encircling shadow to catch

the first and last rays of the sun, was frequently discerned by Baron

Van Ertborn in 1876;[859] while an object near the northern horn of the

crescent, strongly resembling a lunar ring-mountain, was delineated both

by De Vico in 1841 and by Denning forty years later.

We are almost equally sure that Venus, as that the earth is encompassed

with an atmosphere. Yet, notwithstanding luminous appearances plainly

due to refraction during the transits both of 1761 and 1769, Schröter,

in 1792, took the initiative in coming to a definite conclusion on the

subject.[860] It was founded, first, on the rapid diminution of

brilliancy towards the terminator, attributed to atmospheric absorption;

next, on the extension beyond a semicircle of the horns of the crescent;

lastly, on the presence of a bluish gleam illuminating the early hours

of the Cytherean night with what was taken to be genuine twilight. Even

Herschel admitted that sunlight, by the same effect through which the

heavenly bodies show _visibly above_ our horizons while still

_geometrically below_ them, appeared to be bent round the shoulder of

the globe of Venus. Ample confirmation of the fact has since been

afforded. At Dorpat in May, 1849, the planet being within 3° 26' of

inferior conjunction, Mädler found the arms of waning light upon the

disc to embrace no less than 240° of its extent;[861] and in December,

1842, Mr. Guthrie, of Bervie, N.B., actually observed, under similar

conditions, the whole circumference to be lit up with a faint nebulous

glow.[862] The same curious phenomenon was intermittently seen by Mr.

Leeson Prince at Uckfield in September, 1861;[863] but with more

satisfactory distinctness by Mr. C. S. Lyman of Yale College,[864]

before and after the conjunction of December 11, 1866, and during nearly

five hours previous to the transit of 1874, when the yellowish ring of

refracted light showed at one point an approach to interruption,

possibly through the intervention of a bank of clouds. Again, on

December 2, 1898, Venus being 1° 45' from the sun's centre, Mr. H. N.

Russell, of the Halsted Observatory, descried the coalescence of the

cusps, and founded on the observation a valuable discussion of such

effects.[865] Taking account of certain features in the case left

unnoticed by Neison[866] and Proctor,[867] he inferred from them the

presence of a Cytherean atmosphere considerably less refractive than our

own, although possibly, in its lower strata, encumbered with dust or

haze.

Similar appearances are conspicuous during transits. But while the

Mercurian halo is characteristically seen on the sun, the "silver

thread" round the limb of Venus commonly shows on the part _off_ the

sun. There are, however, instances of each description in both cases.

Mr. Grant, in collecting the records of physical phenomena accompanying

the transits of 1761 and 1769, remarks that no one person saw both kinds

of annulus, and argues a dissimilarity in their respective modes of

production.[868] Such a dissimilarity probably exists, in the sense that

the inner section of the ring is illusory, the outer, a genuine result

of the bending of light in a gaseous envelope; but the distinction of

separate visibility has not been borne out by recent experience. Several

of the Australian observers during the transit of 1874 witnessed the

complete phenomenon. Mr. J. Macdonnell, at Eden, saw a "shadowy nebulous

ring" surround the whole disc when ingress was two-thirds accomplished;

Mr. Tornaghi, at Goulburn, perceived a halo, entire and unmistakable, at

half egress.[869] Similar observations were made at Sydney,[870] and

were renewed in 1882 by Lescarbault at Orgères, by Metzger in Java, and

by Barnard at Vanderbilt University.[871]

Spectroscopic indications of aqueous vapour as present in the atmosphere

of Venus, were obtained in 1874 and 1882, by Tacchini and Riccò in

Italy, and by Young in New Jersey.[872] Janssen, however, who made a

special study of the point subsequently to the transit of 1882, found

them much less certain than he had anticipated;[873] and Vogel, by

repeated examinations, 1871-73, could detect only the very slightest

variations from the pattern of the solar spectrum. Some additions there

indeed seem to be in the thickening of a few water and oxygen-lines; but

so nearly evanescent as to induce the persuasion that most of the light

we receive from Venus has traversed only the tenuous upper portion of

its atmosphere.[874] It is reflected, at any rate, with comparatively

slight diminution. On the 26th and 27th of September, 1878, a close

conjunction gave Mr. James Nasmyth the rare opportunity of watching

Venus and Mercury for several hours side by side in the field of his

reflector; when the former appeared to him like clean silver, the latter

as dull as lead or zinc.[875] Yet the light _incident_ upon Mercury is,

on an average, three and a half times as strong as the light reaching

Venus. Thus, the reflective power of Venus must be singularly strong.

And we find, accordingly, from a combination of Zöllner's with Müller's

results, that its albedo is but little inferior to that of new-fallen

snow; in other words, it gives back 77 per cent. of the luminous rays

impinging upon it.

This extraordinary brilliancy would be intelligible were it permissible

to suppose that we see nothing of the planet but a dense canopy of

clouds. But the hypothesis is discountenanced by the Flagstaff

observations, and is irreconcilable with the visibility of mountainous

elevations, and permanent surface-markings. To Mr. Lowell these were so

distinct and unchanging as to furnish data for a chart of the Cytherean

globe, and the peculiar arrangement of divergent shading exhibited in it

cannot off-hand be set down as unreal, in view of Perrotin's earlier

discernment of analogous linear traces. Gruithuisen's "snow-caps,"[876]

however--it is safe to say--do not exist as such; although shining

regions near the poles form a well-attested trait of the strange

Cytherean landscape.

The "secondary," or "ashen light," of Venus was first noticed by

Riccioli in 1643; it was seen by Derham about 1715, by Kirch in 1721, by

Schröter and Harding in 1806;[877] and the reality of the appearance has

since been authenticated by numerous and trustworthy observations. It is

precisely similar to that of the "old moon in the new moon's arms"; and

Zenger, who witnessed it with unusual distinctness, January 8,

1883,[878] supposes it due to the same cause--namely, to the faint gleam

of reflected earth-light from the night-side of the planet. When we

remember, however, that "full earth-light" on Venus, at its nearest, has

little more than 1/12000 its intensity on the moon, we see at once that

the explanation is inadequate. Nor can Professor Safarik's,[879] by

phosphorescence of the warm and teeming oceans with which Zöllner[880]

regarded the globe of Venus as mainly covered, be seriously entertained.

Vogel's suggestion is more plausible. He and O. Lohse, at Bothkamp,

November 3 to 11, 1871, saw the dark hemisphere _partially_ illuminated

by secondary light, extending 30° from the terminator, and thought the

effect might be produced by a very extensive twilight.[881] Others have

had recourse to the analogy of our auroræ, and J. Lamp suggested that

the grayish gleam, visible to him at Bothkamp, October 21 and 26,

1887,[882] might be an accompaniment of electrical processes connected

with the planet's meteorology. Whatever the origin of the phenomenon, it

may serve, on a night-enwrapt hemisphere, to dissipate some of the thick

darkness otherwise encroached upon only by "the pale light of stars."

Venus was once supposed to possess a satellite. But belief in its

existence has died out. No one, indeed, has caught even a deceptive

glimpse of such an object during the last 125 years. Yet it was

repeatedly and, one might have thought, well observed in the seventeenth

and eighteenth centuries. Fontana "discovered" it in 1645; Cassini--an

adept in the art of seeing--recognised it in 1672, and again in 1686;

Short watched it for a full hour in 1740 with varied instrumental means;

Tobias Mayer in 1759, Montaigne in 1761; several astronomers at

Copenhagen in March, 1764, noted what they considered its unmistakable

presence; as did Horrebow in 1768. But M. Paul Stroobant,[883] who in

1887 submitted all the available data on the subject to a searching

examination, identified Horrebow's satellite with Theta Libræ, a

fifth-magnitude star; and a few other apparitions were, by his industry,

similarly explained away. Nevertheless, several withstood all efforts to

account for them, and together form a most curious case of illusion. For

it is quite certain that Venus has no such conspicuous attendant.

* * * * *

The third planet encountered in travelling outward from the sun is the

abode of man. He has in consequence opportunities for studying its

physical habitudes altogether different from the baffling glimpse

afforded to him of the other members of the solar family.

Regarding the earth, then, a mass of knowledge so varied and

comprehensive has been accumulated as to form a science--or rather

several sciences--apart. But underneath all lie astronomical relations,

the recognition and investigation of which constitute one of the most

significant intellectual events of the present century.

It is indeed far from easy to draw a line of logical distinction between

items of knowledge which have their proper place here, and those which

should be left to the historian of geology. There are some, however, of

which the cosmical connections are so close that it is impossible to

overlook them. Among these is the ascertainment of the solidity of the

globe. At first sight it seems difficult to conceive what the apparent

positions of the stars can have to do with subterranean conditions; yet

it was from star measurements alone that Hopkins, in 1839, concluded the

earth to be solid to a depth of at least 800 or 1,000 miles.[884] His

argument was, that if it were a mere shell filled with liquid,

precession and nutation would be much larger than they are observed to

be. For the shell alone would follow the pull of the sun and moon on its

equatorial girdle, leaving the liquid behind; and being thus so much the

lighter, would move the more readily. There is, it is true, grave reason

to doubt whether this reasoning corresponds with the actual facts of the

case;[885] but the conclusion to which it led has been otherwise

affirmed and extended.

Indications of an identical purport have been derived from another kind

of external disturbance, affecting our globe through the same agencies.

Lord Kelvin (then Sir William Thomson) pointed out in 1862[886] that

tidal influences are brought to bear on land as well as on water,

although obedience to them is perceptible only in the mobile element.

Some bodily distortion of the earth's figure _must_, however, take

place, unless we suppose it of absolute or "preternatural" rigidity, and

the amount of such distortion can be determined from its effect in

diminishing oceanic tides below their calculated value. For if the earth

were perfectly plastic to the stresses of solar and lunar gravity,

tides--in the ordinary sense--would not exist. Continents and oceans

would swell and subside together. It is to the _difference_ in the

behaviour of solid and liquid terrestrial constituents that the ebb and

flow of the waters are due.

Six years later, the distinguished Glasgow professor suggested that this

criterion might, by the aid of a prolonged series of exact tidal

observations, be practically applied to test the interior condition of

our planet.[887] In 1882, accordingly, suitable data extending over

thirty-three years having at length become available, Mr. G. H. Darwin

performed the laborious task of their analysis, with the general result

that the "effective rigidity" of the earth's mass must be _at least_ as

great as that of steel.[888]

Ratification from an unexpected quarter has lately been brought to this

conclusion. The question of a possible mobility in the earth's axis of

rotation has often been mooted. Now at last it has received an

affirmative reply. Dr. Küstner detected, in his observations of 1884-85,

effects apparently springing from a minute variation in the latitude of

Berlin. The matter having been brought before the International Geodetic

Association in 1888, special observations were set on foot at Berlin,

Potsdam, Prague, and Strasbourg, the upshot of which was to bring

plainly to view synchronous, and seemingly periodic fluctuations of

latitude to the extent of half a second of arc. The reality of these was

verified by an expedition to Honolulu in 1891-92, the variations there

corresponding inversely to those simultaneously determined in

Europe.[889] Their character was completely defined by Mr. S. C.

Chandler's discussion in October, 1891.[890] He showed that they could

be explained by supposing the pole of the earth to describe a circle

with a radius of thirty feet in a period of fourteen months.

Confirmation of this hypothesis was found by Dr. B. A. Gould in the

Cordoba observations,[891] and it was provided with a physical basis

through the able co-operation of Professor Newcomb.[892] The earth,

owing to its ellipsoidal shape, should, apart from disturbance, rotate

upon its "axis of figure," or shortest diameter; since thus alone can

the centrifugal forces generated by its spinning balance each other.

Temporary causes, however, such as heavy falls of snow or rain limited

to one continental area, the shifting of ice-masses, even the movements

of winds, may render the globe slightly lop-sided, and thus oblige it to

forsake its normal axis, and rotate on one somewhat divergent from it.

This "instantaneous axis" (for it is incessantly changing) must, by

mathematical theory, revolve round the axis of figure in a period of 306

days. Provided, that is to say, the earth were a perfectly rigid body.

But it is far from being so; it yields sensibly to every strain put upon

it; and this yielding tends to protract the time of circulation of the

displaced pole. The length of its period, then, serves as a kind of

measure of the plasticity of the globe; which, according to Newcomb's

and S. S. Hough's independent calculations,[893] seems to be a little

less than that of steel. In an earth compacted of steel, the

instantaneous axis would revolve in 441 days; in the actual earth, the

process is accomplished in 428 days. By this new path, accordingly,

astronomers have been led to an identical estimate of the consistence of

our globe with that derived from tidal investigations.

Variations of latitude are intrinsically complex. To produce them, an

incalculable interplay of causes must be at work, each with its proper

period and law of action.[894] All the elements of the phenomenon are

then in a perpetual state of flux,[895] and absorb for their continual

redetermination, the arduous and combined labours of many astronomers.

Nor is this trouble superfluous. Minute in extent though they be, the

shiftings of the pole menace the very foundations of exact celestial

science; their neglect would leave the entire fabric insecure. Just at

the beginning of the present century they reached a predicted minimum,

but are expected again to augment their range after the year 1902. The

interesting suggestion has been made by Mr. J. Halm that such

fluctuations are, in some obscure way, affected by changes in solar

activity, and conform like them to an eleven-year cycle.[896]

In a paper read before the Geological Society, December 15, 1830,[897]

Sir John Herschel threw out the idea that the perplexing changes of

climate revealed by the geological record might be explained through

certain slow fluctuations in the eccentricity of the earth's orbit,

produced by the disturbing action of the other planets. Shortly

afterwards, however, he abandoned the position as untenable;[898] and it

was left to the late Dr. James Croll, in 1864[899] and subsequent years,

to reoccupy and fortify it. Within restricted limits (as Lagrange and,

more certainly and definitely, Leverrier proved), the path pursued by

our planet round the sun alternately contracts, in the course of ages,

into a moderate ellipse, and expands almost to a circle, the major axis,

and consequently the mean distance, remaining invariable. Even at

present, when the eccentricity approaches a minimum, the sun is nearer

to us in January than in July by above three million miles, and some

850,000 years ago this difference was more than four times as great. Dr.

Croll brought together[900] a mass of evidence to support the view,

that, at epochs of considerable eccentricity, the hemisphere of which

the winter, occurring at aphelion, was both intensified and prolonged,

must have undergone extensive glaciation; while the opposite hemisphere,

with a short, mild winter, and long, cool summer, enjoyed an approach to

perennial spring. These conditions were exactly reversed at the end of

10,500 years, through the shifting of the perihelion combined with the

precession of the equinoxes, the frozen hemisphere blooming into a

luxuriant garden as its seasons came round to occur at the opposite

sites of the terrestrial orbit, and the vernal hemisphere subsiding

simultaneously into ice-bound rigour.[901] Thus a plausible explanation

was offered of the anomalous alternations of glacial and semi-tropical

periods, attested, on incontrovertible geological evidence, as having

succeeded each other in times past over what are now temperate regions.

They succeeded each other, it is true, with much less frequency and

regularity than the theory demanded; but the discrepancy was overlooked

or smoothed away. The most recent glacial epoch was placed by Dr. Croll

about 200,000 years ago, when the eccentricity of the earth's orbit was

3·4 times as great as it is now. At present a faint representation of

such a state of things is afforded by the southern hemisphere. One

condition of glaciation in the coincidence of winter with the maximum of

remoteness from the sun, is present; the other--a high eccentricity--is

deficient. Yet the ring of ice-bound territory hemming in the southern

pole is well known to be far more extensive than the corresponding

region in the north.

The verification of this ingenious hypothesis depends upon a variety of

intricate meteorological conditions, some of which have been adversely

interpreted by competent authorities.[902] What is still more serious,

its acceptance seems precluded by time-relations of a simple kind. Dr.

Wright[903] has established with some approach to certainty that glacial

conditions ceased in Canada and the United States about ten or twelve

thousand years ago. The erosive action of the Falls of Niagara qualifies

them to serve as a clepsydra, or water-clock on a grand scale; and their

chronological indications have been amply corroborated elsewhere and

otherwise on the same continent. The astronomical Ice Age, however,

should have been enormously more antique. No reconciliation of the facts

with the theory appears possible.

The first attempt at an experimental estimate of the "mean density" of

the earth was Maskelyne's observation in 1774 of the deflection of a

plumb-line through the attraction of Schehallien. The conclusion thence

derived, that our globe weighs 4-1/2 times as much as an equal bulk of

water,[904] was not very exact. It was considerably improved upon by

Cavendish, who, in 1798, brought into use the "torsion-balance"

constructed for the same purpose by John Michell. The resulting estimate

of 5·48 was raised to 5·66 by Francis Baily's elaborate repetition of

the process in 1838-42. From experiments on the subject made in 1872-73

by Cornu and Baille the slightly inferior value of 5·56 was derived; and

it was further shown that the data collected by Baily, when corrected

for a systematic error, gave practically the same result (5·55).[905] M.

Wilsing's of 5·58, obtained at Potsdam in 1889,[906] nearly agreed with

it; while Professor Poynting, by means of a common balance, arrived at a

terrestrial mean density of 5·49.[907] Professor Boys next entered the

field with an exquisite apparatus, in which a quartz fibre performed the

functions of a torsion-rod; and the figure 5·53 determined by him, and

exactly confirmed by Dr. Braun's research at Mariaschein, Bohemia, in

1896,[908] may be called the standard value of the required datum.

Newton's guess at the average weight of the earth as five or six times

that of water has thus been curiously verified.

Operations for determining the figure of the earth were carried out during

the last century on an unprecedented scale. The Russo-Scandinavian arc,

of which the measurement was completed under the direction of the elder

Struve in 1855, reached from Hammerfest to Ismailia on the Danube,

a length of 25° 20'. But little inferior to it was the Indian arc, begun

by Lambton in the first years of the century, continued by Everest,

revised and extended by Walker. Both were surpassed in compass by the

Anglo-French arc, which embraced 28°; and considerable segments of

meridians near the Atlantic and Pacific shores of North America were

measured under the auspices of the United States Coast Survey. But these

operations shrink into insignificance by comparison with Sir David

Gill's grandiose scheme for uniting two hemispheres by a continuous

network of triangulation. The history of geodesy in South Africa began

with Lacaille's measurements in 1752. They were repeated and enlarged in

scope by Sir Thomas Maclear in 1841-48; and his determinations prepared

the way for a complete survey of Cape Colony and Natal, executed during

the ten years 1883-92 by Colonel Morris, R.E., under the direction of

Sir David Gill.[909] Bechuanaland and Rhodesia were subsequently

included in the work; and the Royal Astronomer obtained, in 1900, the

support of the International Geodetic Association for its extension to

the mouth of the Nile. Nor was this the limit of his design. By carrying

the survey along the Levantine coast, connection can be established with

Struve's system, and the magnificent amplitude of 105° will be given to

the conjoined African and European arcs. Meantime, the French have

undertaken the remeasurement of Bouguer's Peruvian arc, and a

corresponding Russo-Swedish[910] enterprise is progressing in

Spitzbergen; so that abundant materials will ere long be provided for

fresh investigations of the shape and size of our planet. The smallness

of the outstanding uncertainty can be judged of by comparing J. B.

Listing's[911] with General Clarke's[912] results, published in the same

year (1878). Listing stated the dimensions of the terrestrial spheroid

as follows: Equatorial radius = 3,960 miles; polar radius = 3,947 miles;

ellipticity = 1/288·5. Clarke's corresponding figures were: 3,963 and

3,950 miles, giving an ellipticity of 1/293·5. The value of the latter

fraction at present generally adopted is 1/292; that is to say, the

thickness of the protuberant equatorial ring is held to be 1/292 of the

equatorial radius. From astronomical considerations, it is true, Newcomb

estimated the ratio at 1/308;[913] but for obtaining this particular

datum, geodetical methods are unquestionably to be preferred.

* * * * *

The moon possesses for us a unique interest. She in all probability

shared the origin of the earth; she perhaps prefigures its decay. She is

at present its minister and companion. Her existence, so far as we can

see, serves no other purpose than to illuminate the darkness of

terrestrial nights, and to measure, by swiftly-recurring and conspicuous

changes of aspect, the long span of terrestrial time. Inquiries

stimulated by visible dependence, and aided by relatively close

vicinity, have resulted in a wonderfully minute acquaintance with the

features of the single lunar hemisphere open to our inspection.

Selenography, in the modern sense, is little more than a hundred years

old. It originated with the publication in 1791 of Schröter's

_Selenotopographische Fragmente_.[914] Not but that the lunar surface

had already been diligently studied, chiefly by Hevelius, Cassini,

Riccioli, and Tobias Mayer; the idea, however, of investigating the

moon's physical condition, and detecting symptoms of the activity there

of natural forces through minute topographical inquiry, first obtained

effect at Lilienthal. Schröter's delineations, accordingly, imperfect

though they were, afforded a starting-point for a _comparative_ study of

the superficial features of our satellite.

The first of the curious objects which he named "rills" was noted by him

in 1787. Before 1801 he had found eleven; Lohrmann added 75; Mädler 55;

Schmidt published in 1866 a catalogue of 425, of which 278 had been

detected by himself;[915] and he eventually brought the number up to

nearly 1,000. They are, then, a very persistent lunar feature, though

wholly without terrestrial analogue. There is no difference of opinion

as to their nature. They are quite obviously clefts in a rocky surface,

100 to 500 yards deep, usually a couple of miles across, and pursuing

straight, curved, or branching tracks up to 150 miles in length. As

regards their origin, the most probable view is that they are fissures

produced in cooling; but Neison inclines to consider them rather as

dried watercourses.[916]

On February 24, 1792, Schröter perceived what he took to be distinct

traces of a lunar twilight, and continued to observe them during nine

consecutive years.[917] They indicated, he thought, the presence of a

shallow atmosphere, about 29 times more tenuous than our own. Bessel, on

the other hand, considered that the only way of "saving" a lunar

atmosphere was to deny it any refractive power, the sharpness and

suddenness of star-occultations negativing the possibility of gaseous

surroundings of greater density (admitting an extreme supposition) than

1/500 that of terrestrial air.[918] Newcomb places the maximum at 1/400.

Sir John Herschel concluded "the non-existence of any atmosphere at the

moon's edge having 1/1980 part of the density of the earth's

atmosphere."[919]

This decision was fully borne out by Sir William Huggins's spectroscopic

observation of the disappearance behind the moon's limb of the small

star Eta Piscium, January 4, 1865.[920] Not the slightest sign of

selective absorption or unequal refraction was discernible. The entire

spectrum went out at once, as if a slide had suddenly dropped over it.

The spectroscope has uniformly told the same tale; for M. Thollon's

observation during the total solar eclipse at Sohag of a supposed

thickening at the moon's rim, of certain dark lines in the solar

spectrum, is now acknowledged to have been illusory. Moonlight, analysed

with the prism, is found to be pure reflected sunlight, diminished in

_quantity_, owing to the low reflective capability of the lunar surface,

to less than one-fifth its incident intensity, but wholly unmodified in

_quality_.

Nevertheless, the diameter of the moon appeared from the Greenwich

observations discussed by Airy in 1865[921] to be 4" smaller than when

directly measured; and the effect would be explicable by refraction in a

lunar atmosphere 2,000 times thinner than our own at the sea-level. But

the difference was probably illusory. It resulted in part, if not

wholly, from the visual enlargement by irradiation of the bright disc of

the moon. Professor Comstock, employing the 16-inch Clark equatoreal of

the Washburn Observatory, found in 1897 the refractive displacements of

occulted stars so trifling as to preclude the existence of a permanent

lunar atmosphere of much more than 1/5000 the density of the terrestrial

envelope.[922] The possibility, however, was admitted that, on the

illuminated side of the moon, temporary exhalations of aqueous vapour

might arise from ice-strata evaporated by sun-heat. Meantime, some

renewed evidence of actual crepuscular gleams on the moon had been

gathered by MM. Paul and Prosper Henry of the Paris Observatory, as well

as by Mr. W. H. Pickering, in the pure air of Arequipa, at an altitude

of 8,000 feet above the sea.[923] An occultation of Jupiter, too,

observed by him August 12, 1892,[924] was attended with a slight

flattening of the planet's disc through the effect, it was supposed, of

lunar refraction--but of refraction in an atmosphere possessing, at the

most, 1/4000 the density at the sea-level of terrestrial air, and

capable of holding in equilibrium no more than 1/250 of an inch of

mercury. Yet this small barometric value corresponds, Mr. Pickering

remarks, "to a pressure of hundreds of tons per square mile of the lunar

surface." The compression downward of gaseous strata on the moon should,

in any case, proceed very gradually, owing to the slight power of lunar

gravity,[925] and they might hence play an important part in the economy

of our satellite while evading spectroscopic and other tests. Thus--as

Mr. Ranyard remarked[926]--the cliffs and pinnacles of the moon bear

witness, by their unworn condition, to the efficiency of atmospheric

protection against meteoric bombardment; and Mr. Pickering shows that it

could be afforded by such a tenuous envelope as that postulated by him.

The first to emulate Schröter's selenographical zeal was Wilhelm

Gotthelf Lohrmann, a land-surveyor of Dresden, who, in 1824, published

four out of twenty-five sections of the first scientifically executed

lunar chart, on a scale of 37-1/2 inches to a lunar diameter. His sight,

however, began to fail three years later, and he died in 1840, leaving

materials from which the work was completed and published in 1878 by Dr.

Julius Schmidt, late director of the Athens Observatory. Much had been

done in the interim. Beer and Mädler began at Berlin in 1830 their great

trigonometrical survey of the lunar surface, as yet neither revised nor

superseded. A map, issued in four parts, 1834-36, on nearly the same

scale as Lohrmann's, but more detailed and authoritative, embodied the

results. It was succeeded, in 1837, by a descriptive volume bearing the

imposing title, _Der Mond; oder allgemeine vergleichende Selenographie_.

This summation of knowledge in that branch, though in truth leaving many

questions open, had an air of finality which tended to discourage

further inquiry.[927] It gave form to a reaction against the sanguine

views entertained by Hevelius, Schröter, Herschel and Gruithuisen as to

the possibilities of agreeable residence on the moon, and relegated the

"Selenites," one of whose cities Schröter thought he had discovered, and

of whose festal processions Gruithuisen had not despaired of becoming a

spectator, to the shadowy land of the Ivory Gate. All examples of change

in lunar formations were, moreover, dismissed as illusory. The light

contained in the work was, in short, a "dry light," not stimulating to

the imagination. "A mixture of a lie," Bacon shrewdly remarks, "doth

ever add pleasure." For many years, accordingly, Schmidt had the field

of selenography almost to himself.

Reviving interest in the subject was at once excited and displayed by

the appointment, in 1864, of a Lunar Committee of the British

Association. The indirect were of greater value than the direct fruits

of its labours. An English school of selenography rose into importance.

Popularity was gained for the subject by the diffusion of works

conspicuous for ingenuity and research. Nasmyth's and Carpenter's

beautifully illustrated volume (1874) was succeeded, after two years, by

a still more weighty contribution to lunar science in Mr. Neison's

well-known book, accompanied by a map, based on the survey of Beer and

Mädler, but adding some 500 measures of positions, besides the

representation of several thousand new objects. With Schmidt's _Charte

der Gebirge der Mondes_, Germany once more took the lead. This splendid

delineation, built upon Lohrmann's foundation, embraced the detail

contained in upwards of 3,000 original drawings, representing the labour

of thirty-four years. No less than 32,856 craters are represented in it,

on a scale of seventy-five inches to a diameter. An additional help to

lunar inquiries was provided at the same time in this country by the

establishment, through the initiative of the late Mr. W. R. Birt, of the

Selenographical Society.

But the strongest incentive to diligence in studying the rugged features

of our celestial helpmate has been the idea of probable or actual

variation in them. A change always seems to the inquisitive intellect of

man like a breach in the defences of Nature's secrets, through which it

may hope to make its way to the citadel. What is desirable easily

becomes credible; and thus statements and rumours of lunar convulsions

have successively, during the last hundred years, obtained credence, and

successively, on closer investigation, been rejected. The subject is one

as to which illusion is peculiarly easy. Our view of the moon's surface

is a bird's-eye view. Its conformation reveals itself indirectly through

irregularities in the distribution of light and darkness. The forms of

its elevations and depressions can be inferred only from the shapes of

the black, unmitigated shadows cast by them. But these shapes are in a

state of perpetual and bewildering fluctuation, partly through changes

in the angle of illumination, partly through changes in our point of

view, caused by what are called the moon's "librations."[928] The result

is, that no single observation can be _exactly_ repeated by the same

observer, since identical conditions recur only after the lapse of a

great number of years.

Local peculiarities of surface, besides, are liable to produce

perplexing effects. The reflection of earth-light at a particular angle

from certain bright summits completely, though temporarily, deceived

Herschel into the belief that he had witnessed, in 1783 and 1787,

volcanic outbursts on the dark side of the moon. The persistent

recurrence, indeed, of similar appearances under circumstances less

amenable to explanation inclined Webb to the view that effusions of

native light actually occur.[929] More cogent proofs must, however, be

adduced before a fact so intrinsically improbable can be admitted as

true.

But from the publication of Beer and Mädler's work until 1866, the

received opinion was that no genuine sign of activity had ever been

seen, or was likely to be seen, on our satellite; that her face was a

stereotyped page, a fixed and irrevisable record of the past. A profound

sensation, accordingly, was produced by Schmidt's announcement, in

October, 1866, that the crater "Linné," in the Mare Serenitatis, had

disappeared,[930] effaced, as it was supposed, by an igneous outflow.

The case seemed undeniable, and is still dubious. Linné had been known

to Lohrmann and Mädler, 1822-32, as a deep crater, five or six miles in

diameter, the third largest in the dusky plain known as the "Mare

Serenitatis"; and Schmidt had observed and drawn it, 1840-43, under a

practically identical aspect. Now it appears under high light as a

whitish spot, in the centre of which, as the rays begin to fall

obliquely, a pit, scarcely two miles across, emerges into view.[931] The

crateral character of this comparatively minute depression was detected

by Father Secchi, February 11, 1867.

This is not all. Schröter's description of Linné, as seen by him

November 5, 1788, tallies quite closely with modern observation;[932]

while its inconspicuousness in 1797 is shown by its omission from

Russell's lunar globe and maps.[933] We are thus driven to adopt one of

two suppositions: either Lohrmann, Mädler, and Schmidt were entirely

mistaken in the size and importance of Linné, or a real change in its

outward semblance supervened during the first half of the century, and

has since passed away, perhaps again to recur. The latter hypothesis

seems the more probable: and its probability is strengthened by much

evidence of actual obscuration or variation of tint in other parts of

the lunar surface, more especially on the floor of the great "walled

plain" named "Plato."[934] From a re-examination with a 13-inch

refractor at Arequipa in 1891-92, of this region, and of the Mare

Serenitatis, Mr. W. H. Pickering inclines to the belief that lunar

volcanic action, once apparently so potent, is not yet wholly

extinct.[935]

An instance of an opposite kind of change was alleged by Dr. Hermann J.

Klein of Cologne in March, 1878.[936] In Linné the obliteration of an

old crater had been assumed; in "Hyginus N.," the formation of a new

crater was asserted. Yet, quite possibly, the same cause may have

produced the effects thought to be apparent in both. It is, however, far

from certain that any real change has affected the neighbourhood of

Hyginus. The novelty of Klein's observation of May 19, 1877, may have

consisted simply in the detection of a hitherto unrecognised feature.

The region is one of complex formation, consequently of more than

ordinary liability to deceptive variations in aspect under rapid and

entangled fluctuations of light and shade.[937] Moreover, it seems to be

certain, from Messrs. Pratt and Capron's attentive study, that "Hyginus

N." is no true crater, but a shallow, saucer-like depression, difficult

of clear discernment.[938] Under suitable illumination, nevertheless, it

contains, and is marked by, an ample shadow.[939]

In both these controverted instances of change, lunar photography was

invoked as a witness; but, notwithstanding the great advances made in

the art by De la Rue in this country, by Draper, and, above all, by

Rutherford in America, without decisive results. Investigations of the

kind began to assume a new aspect in 1890, when Professor Holden

organised them at the Lick Observatory.[940] Autographic moon-pictures

were no longer taken casually, but on system; and Dr. Weinek's elaborate

study, and skilful reproductions of them at Prague,[941] gave them

universal value. They were designed to provide materials for an atlas on

the scale of Beer and Mädler's, of which some beautiful specimen-plates

have been issued. At Paris, in 1894, with the aid of a large "equatoreal

coudé," a work of similar character was set on foot by MM. Loewy and

Puiseux. Its progress has been marked by the successive publication of

five instalments of a splendid atlas, on a scale of about eight feet to

the lunar diameter, accompanied by theoretical dissertations, designed

to establish a science of "selenology." The moon's formations are thus

not only delineated under every variety of light-incidence, but their

meaning is sought to be elicited, and their history and mutual relations

interpreted.[942] Henceforth, at any rate, the lunar volcanoes can

scarcely, without notice taken, breathe hard in their age-long sleep.

Melloni was the first to get undeniable heating effects from moonlight.

His experiments, made on Mount Vesuvius early in 1846,[943] were

repeated with like result by Zantedeschi at Venice four years later. A

rough measure of the intensity of those effects was arrived at by Piazzi

Smyth at Guajara, on the Peak of Teneriffe, in 1856. At a distance of

fifteen feet from the thermomultiplier, a Price's candle was found to

radiate just twice as much heat as the full moon.[944] Then, after

thirteen years, in 1869-72, an exact and extensive series of

observations on the subject were made by the present Earl of Rosse. The

lunar radiations, from the first to the last quarter, displayed, when

concentrated with the Parsonstown three-foot mirror, appreciable thermal

energy, increasing with the phase, and largely due to "dark heat,"

distinguished from the quicker-vibrating sort by inability to traverse a

plate of glass. This was supposed to indicate an actual heating of the

surface, during the long lunar day of 300 hours, to about 500° F.[945]

(corrected later to 197°),[946] the moon thus acting as a direct

radiator no less than as a reflector of heat. But the conclusion was

very imperfectly borne out by Dr. Boeddicker's observations with the

same instrument and apparatus during the total lunar eclipse of October

4, 1884.[947] This initial opportunity of measuring the heat phases of

an eclipsed moon was used with the remarkable result of showing that the

heat disappeared almost completely, though not quite simultaneously,

with the light. Confirmatory evidence of the extraordinary promptitude

with which our satellite parts with heat already to some extent

appropriated, was afforded by Professor Langley's bolometric

observations at Allegheny of the partial eclipse of September 23,

1885.[948] Yet it is certain that the moon sends us a perceptible

quantity of heat _on its own account_, besides simply throwing back

solar radiations. For in February, 1885, Professor Langley succeeded,

after many fruitless attempts, in getting measures of a "lunar

heat-spectrum." The incredible delicacy of the operation may be judged

of from the statement that the sum-total of the thermal energy dispersed

by his rock-salt prisms was insufficient to raise a thermometer fully

exposed to it one-thousandth of a degree Centigrade! The singular fact

was, however, elicited that this almost evanescent spectrum is made up

of two superposed spectra, one due to reflection, the other, with a

maximum far down in the infra-red, to radiation.[949] The corresponding

temperature of the moon's sunlit surface Professor Langley considers to

be about that of freezing water.[950] Repeated experiments having failed

to get any thermal effects from the dark part of the moon, it was

inferred that our satellite "has no internal heat sensible at the

surface"; so that the radiations from the lunar soil giving the low

maximum in the heat-spectrum, "must be due purely to solar heat which

has been absorbed and almost immediately re-radiated." Professor

Langley's explorations of the terra incognita of immensely long

wave-lengths where lie the unseen heat-emissions from the earth into

space, led him to the discovery that these, contrary to the received

opinion, are in good part transmissible by our atmosphere, although they

are completely intercepted by glass. Another important result of the

Allegheny work was the abolition of the anomalous notion of the

"temperature of space," fixed by Pouillet at -140° C. For space in

itself can have no temperature, and stellar radiation is a negligible

quantity. Thus, it is safe to assume "that a perfect thermometer

suspended in space at the distance of the earth or moon from the sun,

but shielded from its rays, would sensibly indicate the absolute

zero,"[951] ordinarily placed at -273° C.

A "Prize Essay on the Distribution of the Moon's Heat" (The Hague),

1891, by Mr. Frank W. Very, who had taken an active part in Professor

Langley's long-sustained inquiry, embodies the fruits of its

continuation. They show the lunar disc to be tolerably uniform in

thermal power. The brighter parts are also indeed hotter, but not much.

The traces perceived of a slight retention of heat by the substances

forming the lunar surface, agreed well with the Parsonstown observations

of the total eclipse of the moon, January 28, 1888.[952] For they

brought out an unmistakable divergence between the heat and light

phases. A curious decrease of heat previous to the first touch of the

earth's shadow upon the lunar globe remains unexplained, unless it be

admissible to suppose the terrestrial atmosphere capable of absorbing

heat at an elevation of 190 miles. The probable range of temperature on

the moon was discussed by Professor Very in 1898.[953] He concluded it

to be very wide. Hotter than boiling water under the sun's vertical

rays, the arid surface of our dependent globe must, he found, cool in

the 14-day lunar night to about the temperature of liquid air.

Although that fundamental part of astronomy known as "celestial

mechanics" lies outside the scope of this work, and we therefore pass

over in silence the immense labours of Plana, Damoiseau, Hansen,

Delaunay, G. W. Hill, and Airy in reconciling the observed and

calculated motions of the moon, there is one slight but significant

discrepancy which is of such importance to the physical history of the

solar system, that some brief mention must be made of it.

Halley discovered in 1693, by examining the records of ancient eclipses,

that the moon was going faster then than 2,000 years previously--so much

faster, as to have got ahead of the place in the sky she would otherwise

have occupied, by about two of her own diameters. It was one of

Laplace's highest triumphs to have found an explanation of this puzzling

fact. He showed, in 1787, that it was due to a very slow change in the

ovalness of the earth's orbit, tending, during the present age of the

world, to render it more nearly circular. The pull of the sun upon the

moon is thereby lessened; the counter-pull of the earth gets the upper

hand; and our satellite, drawn nearer to us by something less than an

inch each year,[954] proportionately quickens her pace. Many thousands

of years hence the process will be reversed; the terrestrial orbit will

close in at the sides, the lunar orbit will open out under the growing

stress of solar gravity, and our celestial chronometer will lose instead

of gaining time.

This is all quite true as Laplace put it; but it is not enough. Adams,

the virtual discoverer of Neptune, found with surprise in 1853 that the

received account of the matter was "essentially incomplete," and

explained, when the requisite correction was introduced, only half the

observed acceleration.[955] What was to be done with the remaining half?

Here Delaunay, the eminent French mathematical astronomer, unhappily

drowned at Cherbourg in 1872 by the capsizing of a pleasure-boat, came

to the rescue.[956]

It is obvious to anyone who considers the subject a little attentively,

that the tides must act to some extent as a friction-brake upon the

rotating earth. In other words, they must bring about an almost

infinitely slow lengthening of the day. For the two masses of water

piled up by lunar influence on the hither and farther sides of our

globe, strive, as it were, to detach themselves from the unity of the

terrestrial spheroid, and to follow the movements of the moon. The moon,

accordingly, holds them _against_ the whirling earth, which revolves

like a shaft in a fixed collar, slowly losing motion and gaining heat,

eventually dissipated through space.[957] This must go on (so far as we

can see) until the periods of the earth's rotation and of the moon's

revolution coincide. Nay, the process will be continued--should our

oceans survive so long--by the feebler tide-raising power of the sun,

ceasing only when day and night cease to alternate, when one side of our

planet is plunged in perpetual darkness and the other seared by

unchanging light.

Here, then, we have the secret of the moon's turning always the same

face towards the earth. It is that in primeval times, when the moon was

liquid or plastic, an earth-raised tidal wave rapidly and forcibly

reduced her rotation to its present exact agreement with her period of

revolution. This was divined by Kant[958] nearly a century before the

necessity for such a mode of action presented itself to any other

thinker. In a weekly paper published at Königsberg in 1754, the modern

doctrine of "tidal friction" was clearly outlined by him, both as

regards its effects actually in progress on the rotation of the earth,

and as regards its effects already consummated on the rotation of the

moon--the whole forming a preliminary attempt at what he called a

"natural history" of the heavens. His sagacious suggestion, however,

remained entirely unnoticed until revived--it would seem

independently--by Julius Robert Mayer in 1848;[959] while similar, and

probably original, conclusions were reached by William Ferrel of

Allensville, Kentucky, in 1858.[960]

Delaunay was not then the inventor or discoverer of tidal friction; he

merely displayed it as an effective cause of change. He showed reason

for believing that its action in checking the earth's rotation, far from

being, as Ferrel had supposed, completely neutralised by the contraction

of the globe through cooling, was a fact to be reckoned with in

computing the movements, as well as in speculating on the history, of

the heavenly bodies. The outstanding acceleration of the moon was thus

at once explained. It was explained as apparent only--the reflection of

a real lengthening, by one second in 100,000 years, of the day. But on

this point the last word has not yet been spoken.

Professor Newcomb undertook in 1870 the onerous task of investigating

the errors of Hansen's Lunar Tables as compared with observations prior

to 1750. The results, published in 1878,[961] proved somewhat

perplexing. They tend, in general, to reduce the amount of acceleration

left unaccounted for by Laplace's gravitational theory, and

proportionately to diminish the importance of the part played by tidal

friction. But, in order to bring about this diminution, and at the same

time conciliate Alexandrian and Arabian observations, it is necessary to

reject _as total_ the ancient solar eclipses known as those of Thales

and Larissa. This may be a necessary, but it must be admitted to be a

hazardous expedient. Its upshot was to indicate a possibility that the

observed and calculated values of the moon's acceleration might after

all prove to be identical; and the small outstanding discrepancy was

still further diminished by Tisserand's investigation, differently

conducted, of the same Arabian eclipses discussed by Newcomb.[962] The

necessity of having recourse to a lengthening day is then less pressing

than it seemed some time ago; and the effect, if perceptible in the

moon's motion, should, M. Tisserand remarked, be proportionately so in

the motions of all the other heavenly bodies. The presence of the

apparent general acceleration that should ensue can be tested with most

promise of success, according to the same authority, by delicate

comparisons of past and future transits of Mercury.

Newcomb further showed that small residual irregularities are still

found in the movements of our satellite, inexplicable either by any

known gravitational influence, or by any _uniform_ value that could be

assigned to secular acceleration.[963] If set down to the account of

imperfections in the "time-keeping" of the earth, it could only be on

the arbitrary supposition of fluctuations in its rate of going

themselves needing explanation. This, it is true, might be found in very

slight changes of figure,[964] not altogether unlikely to occur. But

into this cloudy and speculative region astronomers for the present

decline to penetrate. They prefer, if possible, to deal only with

calculable causes, and thus to preserve for their "most perfect of

sciences" its special prerogative of assured prediction.

FOOTNOTES:

[Footnote 796: _Neueste Beyträge zur Erweiterung der Sternkunde_, Bd.

iii., p. 14 (1800).]

[Footnote 797: _Ibid._, p. 24.]

[Footnote 798: _Phil. Trans._, vol. xciii., p. 215.]

[Footnote 799: _Mem. Roy. Astr. Soc._, vol. vi., p. 116.]

[Footnote 800: _Month. Not._, vol. xix., pp. 11, 25.]

[Footnote 801: _Ibid._, vol. xxxviii., p. 398.]

[Footnote 802: _Am. Jour. of Sc._, vol. xvi., p. 124.]

[Footnote 803: _Wash. Obs._ for 1876, Part ii., p. 34.]

[Footnote 804: _Pop. Astr._, vol. ii., p. 168; _Astr. Jour._, No. 335.]

[Footnote 805: _Astr. and Astrophysics_, vol. xiii., p. 866.]

[Footnote 806: _Ibid._, p. 867.]

[Footnote 807: _Month. Not._, vol. xxiv., p. 18.]

[Footnote 808: _Ibid._, vol. xxiii., p. 234 (Challis).]

[Footnote 809: _Untersuchungen über die Spectra der Planeten_, p. 9.]

[Footnote 810: _Sirius_, vol. vii., p. 131.]

[Footnote 811: _Potsdam Publ._, No. 30; _Astr. Nach._, No. 3,171; Frost,

_Astr. and Astrophysics_, vol. xii., p. 619.]

[Footnote 812: Zöllner and Winnecke made it=O·13, _Astr. Nach._, No.

2,245.]

[Footnote 813: _Neueste Beyträge_, Bd. iii., p. 50.]

[Footnote 814: _Astr. Jahrbuch_, 1804, pp. 97-102.]

[Footnote 815: Webb, _Celestial Objects_, p. 46 (4th ed.).]

[Footnote 816: _L'Astronomie_, t. ii., p. 141.]

[Footnote 817: _Observations sur les Planètes Vénus et Mercure_, p. 87.]

[Footnote 818: _Observatory_, vol. vi., p. 40.]

[Footnote 819: _Atti dell' Accad. dei Lincei_, t. v. ii., p. 283, 1889;

_Astr. Nach._, No. 2,944.]

[Footnote 820: _Astr. Nach._ No. 2,479.]

[Footnote 821: _Memoirs Amer. Acad._, vol. xii., No. 4, p. 464.]

[Footnote 822: _Hist. de l'Astr._, p. 682.]

[Footnote 823: _Comptes Rendus_, t. xlix., p. 379.]

[Footnote 824: _Comptes Rendus_, t. l., p. 40.]

[Footnote 825: _Ibid._, p. 46.]

[Footnote 826: _Astr. Nach._, Nos. 1,248 and 1,281.]

[Footnote 827: _Comptes Rendus_, t. lxxxiii., pp. 510, 561.]

[Footnote 828: _Handbuch der Mathematik_, Bd. ii., p. 327.]

[Footnote 829: _Comptes Rendus_, t. lxxxiii., p. 721.]

[Footnote 830: _Nature_, vol. xviii., pp. 461, 495, 539.]

[Footnote 831: Oppolzer, _Astr. Nach._, No. 2,239.]

[Footnote 832: _Ibid._, Nos. 2,253-4 (C. H. F. Peters).]

[Footnote 833: _Ibid._, Nos. 2,263 and 2,277. See also Tisserand in

_Ann. Bur. des Long._, 1882, p. 729.]

[Footnote 834: See J. Bauschinger's _Untersuchungen_ (1884), summarised

in _Bull. Astr._, t. i., p. 506, and _Astr. Nach._, No. 2,594. Newcomb

finds the anomalous motion of the perihelion to be even larger (43"

instead of 38") than Leverrier made it. _Month. Not._, February, 1884,

p. 187. Harzer's attempt to account for it in _Astr. Nach._, No. 3,030,

is more ingenious than successful.]

[Footnote 835: _Jour. des Sçavans_, December, 1667, p. 122.]

[Footnote 836: _Élémens d'Astr._, p. 525. Cf. Chandler, _Pop. Astr._,

February, 1897, p. 393.]

[Footnote 837: _Beobachtungen über die sehr beträchtlichen Gebirge und

Rotation der Venus_, 1792, p. 35. Schröter's final result in 1811 was

23h. 21m. 7·977s. _Monat. Corr._, Bd. xxv., p. 367.]

[Footnote 838: _Astr. Nach._, No. 404.]

[Footnote 839: _Rendiconti del R. Istituto Lombardo_, t. xxiii., serie

ii.]

[Footnote 840: _Astr. Nach._, No. 3,304.]

[Footnote 841: _Bothkamp Beobachtungen_, Heft ii., p. 120.]

[Footnote 842: _Comptes Rendus_, t. cxi., p. 542; t. cxxii., p. 395.]

[Footnote 843: _Month. Not._, vol. lvii., p. 402; _Astr. Nach._, No.

3,406.]

[Footnote 844: _Mem. Spettroscopisti Italiani_, t. xxv., p. 93;

_Nature_, vol. liii., p. 306.]

[Footnote 845: _Astr. Nach._, No. 3,329.]

[Footnote 846: _Ibid._]

[Footnote 847: _Bull. de l'Acad. de Belgique_, t. xxi., p. 452, 1891.]

[Footnote 848: _Observations sur les Planètes Vénus et Mercure_, 1892.]

[Footnote 849: _Astr. Nach._, No. 3,300.]

[Footnote 850: _Ibid._, No. 3,332.]

[Footnote 851: _Ibid._, No. 3,314.]

[Footnote 852: _Ibid._, No. 3,170.]

[Footnote 853: _Ibid._, No. 3,641. The velocity of a point on the

equator of Venus, if Brenner's period of 23h. 57m. were exact, would be

0·28 miles per second; but the displacements due to this rate would be

doubled by reflection.]

[Footnote 854: _Novæ Observationes_, p. 92.]

[Footnote 855: _Mém. de l'Ac._, 1700, p. 296.]

[Footnote 856: _Phil. Trans._, vol. lxxxiii., p. 201.]

[Footnote 857: Webb, _Cel. Objects_, p. 58.]

[Footnote 858: _Month. Not._, vol. xlii., p. 111.]

[Footnote 859: _Bull. Ac. de Bruxelles_, t. xliii., p. 22.]

[Footnote 860: _Phil. Trans._, vol. lxxxii., p. 309;

_Aphroditographische Fragmente_, p. 85 (1796).]

[Footnote 861: _Astr. Nach._, No. 679.]

[Footnote 862: _Month. Not._, vol. xiv., p. 169.]

[Footnote 863: _Ibid._, vol. xxiv., p. 25.]

[Footnote 864: _Am. Jour. of Sc._, vol. xliii., p. 129 (2d ser.); vol.

ix., p. 47 (3d ser.).]

[Footnote 865: _Astroph. Jour._, vol. ix., p. 284.]

[Footnote 866: _Month. Not._, vol. xxxvi., p. 347.]

[Footnote 867: _Old and New Astronomy_, p. 448.]

[Footnote 868: _Hist. Phys. Astr._, p. 431.]

[Footnote 869: _Mem. Roy. Astr. Soc._, vol. xlvii., pp. 77, 84.]

[Footnote 870: _Astr. Reg._, vol. xiii., p. 132.]

[Footnote 871: _L'Astronomie_, t. ii., p. 27; _Astr. Nach._, No. 2,021;

_Am. Jour. of Sc._, vol. xxv., p. 430.]

[Footnote 872: _Mem. Spettr. Ital._, Dicembre, 1882; _Am. Jour. of Sc._,

vol. xxv., p. 328.]

[Footnote 873: _Comptes Rendus_, t. cxvi., p. 288.]

[Footnote 874: Vogel, _Spectra der Planeten_, p. 15.]

[Footnote 875: _Nature_, vol. xix., p. 23.]

[Footnote 876: _Nova Acta Acad. Naturæ Curiosorum_, Bd. x., 239.]

[Footnote 877: _Astr. Jahrbuch_, 1809, p. 164.]

[Footnote 878: _Month Not._, vol. xliii., p. 331.]

[Footnote 879: _Report Brit. Ass._, 1873, p. 407. The paper contains a

valuable record of observations of the phenomenon.]

[Footnote 880: _Photom. Untersuchungen_, p. 301.]

[Footnote 881: _Bothkamp Beobachtungen_, Heft ii., p. 126.]

[Footnote 882: _Astr. Nach._, No. 2,818.]

[Footnote 883: _Mémoires de l'Acad. de Bruxelles_, t. xlix., No. 5, 4to;

_Astr. Nach._, No. 2,809; _f._ Schorr, _Der Venusmond_, 1875.]

[Footnote 884: _Phil. Trans._, 1839, 1841, 1842.]

[Footnote 885: Delaunay objected (_Comptes Rendus_, t. lxvii., p. 65)

that the viscosity of the contained liquid (of which Hopkins took no

account) would, where the movements were so excessively slow as those of

the earth's axis, almost certainly cause it to behave like a solid. Lord

Kelvin, however (_Report Brit. Ass._, 1876, ii., p. 1), considered

Hopkins's argument valid as regards the comparatively quick solar

semi-annual and lunar fortnightly nutations.]

[Footnote 886: _Phil. Trans._, cliii., p. 573.]

[Footnote 887: _Report Brit. Ass._, 1868, p. 494.]

[Footnote 888: _Ibid._, 1882, p. 474.]

[Footnote 889: Albrecht, _Astr. Nach._, No. 3,131.]

[Footnote 890: _Astr. Jour._, Nos. 248, 249.]

[Footnote 891: _Ibid._, No. 258.]

[Footnote 892: _Month. Not._, vol. lii., p. 336.]

[Footnote 893: _Astr. Nach._, No. 3,097; _Phil. Trans._, vol. clxxxvi.,

A., p. 469; _Proc. Roy. Soc._, vol. lix.]

[Footnote 894: See Chandler's searching investigations, _Astr. Jour._,

Nos. 329, 344, 351, 392, 402, 406, 412, 446, 489, 490, 494, 495.]

[Footnote 895: Rees, _Pop. Astr._, No. 74, 1900.]

[Footnote 896: _Nature_, vol. lxi., p. 447; see also A. V. Bäcklund,

_Astr. Nach._, No. 3,787.]

[Footnote 897: _Trans. Geol. Soc._, vol. iii. (2d ser.), p. 293.]

[Footnote 898: See his _Treatise on Astronomy_, p. 199 (1833).]

[Footnote 899: _Phil. Mag._, vol. xxviii. (4th ser.), p. 121.]

[Footnote 900: _Climate and Time_, 1875; _Discussions on Climate and

Cosmology_, 1885.]

[Footnote 901: See for a popular account of the theory, Sir R. Ball's

_The Cause of an Ice Age_, 1892.]

[Footnote 902: See A. Woeikof, _Phil. Mag._, vol. xxi., p. 223.]

[Footnote 903: _The Ice Age in North America_, London, 1890.]

[Footnote 904: _Phil. Trans._, vol. lxviii., p. 783.]

[Footnote 905: _Comptes Rendus_, t. lxxvi., p. 954.]

[Footnote 906: _Potsdam Publ._, Nos. 22, 23.]

[Footnote 907: _Phil. Trans._, vol. clxxxii., p. 565; _Adams Prize Essay

for 1893._]

[Footnote 908: _Denkschriften Akad. der Wiss. Wien_, Bd. lxiv.; quoted

by Poynting. _Nature_, vol. lxii., p. 404.]

[Footnote 909: _Report on the Geodetic Survey of S. Africa_, 1894.]

[Footnote 910: _Nature_, vol. lxii., p. 622; Hollis, _Observatory_, vol.

xxiii., p. 337; Poincaré, _Comptes Rendus_, July 23, 1900.]

[Footnote 911: _Astr. Nach._, No. 2,228.]

[Footnote 912: Young's _Gen. Astr._, p. 601.]

[Footnote 913: _Astr. Constants_, p. 195.]

[Footnote 914: The second volume was published at Göttingen in 1802.]

[Footnote 915: _Ueber Rillen auf dem Monde_, p. 13. _Cf. The Moon_, by

T. Gwyn Elger, p. 20. W. H. Pickering, _Harvard Annals_, vol. xxxii., p.

249.]

[Footnote 916: _The Moon_, p. 73.]

[Footnote 917: _Selen. Fragm._, Th. ii., p. 399.]

[Footnote 918: _Astr. Nach._, No. 263 (1834); _Pop. Vorl._, pp. 615-620

(1838).]

[Footnote 919: _Outlines of Astr._, par. 431.]

[Footnote 920: _Month. Not._, vol. xxv., p. 61.]

[Footnote 921: _Month. Not._, vol. xxv., p. 264.]

[Footnote 922: _Astroph. Jour._, vol. vi., p. 422.]

[Footnote 923: _Harvard Annals_, vol. xxxii., p. 81.]

[Footnote 924: _Astr. and Astrophysics_, vol. xi., p. 778.]

[Footnote 925: Neison, _The Moon_, p. 25.]

[Footnote 926: _Knowledge_, vol. xvii., p. 85.]

[Footnote 927: Neison, _The Moon_, p. 104.]

[Footnote 928: The combination of a uniform rotational with an unequal

orbital movement causes a slight swaying of the moon's globe, now east,

now west, by which we are able to see round the edges of the averted

hemisphere. There is also a "parallactic" libration, depending on the

earth's rotation; and a species of nodding movement--the "libration in

latitude"--is produced by the inclination of the moon's axis to her

orbit, and by her changes of position with regard to the terrestrial

equator. Altogether, about 2/11 of the _invisible_ side come into view.]

[Footnote 929: _Cel. Objects_, p. 58 (4th ed.).]

[Footnote 930: _Astr. Nach._, No. 1,631.]

[Footnote 931: Cf. Leo Brenner, _Naturwiss. Wochenschrift_, January 13,

1895; _Jour. Brit. Astr. Ass._, vol. v., pp. 29, 222.]

[Footnote 932: Respighi, _Les Mondes_, t. xiv., p. 294; Huggins, _Month.

Not._, vol. xxvii., p. 298.]

[Footnote 933: Birt, _Ibid._, p. 95.]

[Footnote 934: _Report Brit. Ass._, 1872, p. 245.]

[Footnote 935: _Observatory_, vol. xv., p. 250.]

[Footnote 936: _Astr. Reg._, vol. xvi., p. 265; _Astr. Nach._, No.

2,275.]

[Footnote 937: Lindsay and Copeland, _Month. Not._, vol. xxxix., p.

195.]

[Footnote 938: _Observatory_, vols. ii., p. 296; iv., p. 373. N. E.

Green (_Astr. Reg._, vol. xvii., p. 144) concluded the object a mere

"spot of colour," dark under oblique light.]

[Footnote 939: Webb, _Cel. Objects_, p. 101.]

[Footnote 940: _Publ. Lick Observatory_, vol. iii., p. 7.]

[Footnote 941: _Ibid._, p. 21; Mee, _Knowledge_, vol. xviii., p. 135.]

[Footnote 942: _Comptes Rendus_, t. cxxii., p. 967; _Bull. Astr._,

August, 1899; _Ann. Bureau des Long._, 1898; _Nature_, vols. lii., p.

439; lvi., p. 280; lix., p. 304; lx., p. 491; _Astroph. Jour._ vol. vi.,

p. 51.]

[Footnote 943: _Comptes Rendus_, t. xxii., p. 541.]

[Footnote 944: _Phil. Trans._, vol. cxlviii., p. 502.]

[Footnote 945: _Proc. Roy. Soc._, vol. xvii., p. 443.]

[Footnote 946: _Phil. Trans._, vol. clxiii., p. 623.]

[Footnote 947: _Trans. R. Dublin Soc._, vol. iii., p. 321.]

[Footnote 948: _Science_, vol. vii., p. 9.]

[Footnote 949: _Amer. Jour. of Science_, vol. xxxviii., p. 428.]

[Footnote 950: "The Temperature of the Moon," _Memoirs National Acad. of

Sciences_, vol. iv., p. 193, 1889.]

[Footnote 951: _Temperature of the Moon_, p. iii.; see also App. ii., p.

206.]

[Footnote 952: _Trans. R. Dublin Soc._, vol. iv., p. 481, 1891; Rosse,

_Proc. Roy. Institution_, May 31, 1895.]

[Footnote 953: _Astroph. Jour._, vol. viii., pp. 199, 265.]

[Footnote 954: Airy, _Observatory_, vol. iii., p. 420.]

[Footnote 955: _Phil. Trans._, vol. cxliii., p. 397; _Proc. Roy. Soc._,

vol. vi., p. 321.]

[Footnote 956: _Comptes Rendus_, t. lxi., p. 1023.]

[Footnote 957: Professor Darwin calculated that the heat generated by

tidal friction in the course of lengthening the earth's period of

rotation from 23 to 24 hours, equalled 23 million times the amount of

its present annual loss by cooling. _Nature_, vol. xxxiv., p. 422.]

[Footnote 958: _Sämmtl. Werke_ (ed. 1839), Th. vi., pp. 5-12. See also

C. J. Monro's useful indications in _Nature_, vol. vii., p. 241.]

[Footnote 959: _Dynamik des Himmels_, p. 40.]

[Footnote 960: Gould's _Astr. Jour._, vol. iii., p. 138.]

[Footnote 961: _Wash. Obs._ for 1875, vol. xxii., App. ii.]

[Footnote 962: _Comptes Rendus_, t. cxiii., p. 669; _Annuaire_, Paris,

1892.]

[Footnote 963: Newcomb, _Pop. Astr._ (4th ed.), p. 101.]

[Footnote 964: Sir W. Thomson, _Report Brit. Ass._, 1876, p. 12.]