EXPEDITION TO THE CAPE.
The voyage was prosperous, but long. Nine weeks and two days passed
before the welcome cry of “Land” was heard; and it was in the dawn
of January 15, 1835, that Table Mountain at last stood full in view,
“with all its attendant range down to the farthest point of South
Africa,” outlined, ghost-like, in clear blue. The disembarkation of the
instruments and luggage took several days. They filled fifteen large
boats, and a single onslaught of the south-easterly gale, by which at
that time of the year Cape Town is harried, might easily have marred
the projected campaign. All, however, went well.
The travellers were welcomed by Dr. Stewart, one of Lady Herschel’s
brothers, and enthusiastically greeted by the Royal Astronomer, Sir
Thomas Maclear. They made no delay in fixing their headquarters.
“For the last two or three days,” Herschel wrote to his aunt, January
21, “we have been looking for houses, and have all but agreed for one,
a most beautiful place four or five miles out of town, called ‘The
Grove.’ In point of situation it is a perfect paradise in rich and
magnificent mountain scenery, and sheltered from all winds, even the
fierce south-easter, by thick surrounding woods. I must reserve for my
next all description of the gorgeous display of flowers which adorns
this splendid country, as well as the astonishing brilliancy of the
constellations.”
“The Grove” resumed its old Dutch name of “Feldhausen” during
Herschel’s occupation of it; and as “Feldhausen” it will always
be memorable in astronomical history as the scene of the first
effective exploration of the southern heavens. The place is
essentially unchanged. Only an avenue of fir-trees has been planted
by way of approach to the house, a solid Dutch structure, with a
disconsolate-looking garden in front; while in an adjacent field,
carpeted with yellow lupins every spring, and redolent of their
perfume, an obelisk has been erected on the former site of the great
reflector. Above, to the west, towers the _gable-end_ of Table
Mountain, and an exuberant growth of oaks and pines softens the
sternness of its “mural precipices.”
The neighbourhood was, in those days, lonely in the human sense,
although otherwise over- and ill-populated. Wolves and jackals abounded
in the forests; venomous snakes slid through the grass; baboons had the
run of the country; even the lion and the hippopotamus were scarcely
yet extinct in the Cape Peninsula. Many a wild hyæna-shriek startled
the astronomer at his nightly toil; and Dr. Whewell reported that he
had “spent one night in tiger-hunting, but seemed to think it poor
sport compared with the _chasse aux étoiles doubles_.” _Tiger_, it
should be explained, is a local name for a species of leopard: no true
tigers have ever been encountered in Africa.
His twenty-foot began its activity February 22nd, and the refractor,
which was equatorially mounted in a revolving dome, was ready early
in June. “But I am sorry to say,” he told Miss Herschel, “that the
nights in which it can be used to advantage are rare.” And he lamented
to his brother-in-law and intimate friend, Mr. James C. Stewart,
that, during the hot season, “the stars tremble, swell, and waver most
formidably.” The Cape heavens are indeed often exasperating. On nights
meteorologically quite fine, the dismayed astronomer not uncommonly
sees the stars “walking about” in the field of view; and a mere handful
of cloud will, at other times, with incredible swiftness, spread over
the whole face of the sky. Still, compensation is, sooner or later,
sure to come in a run of magnificent observing weather. This was
Herschel’s experience. He informed Francis Baily, October 23rd, 1834,
that “the definition was far beyond anything experienced in England.”
After rain especially, superb opportunities were afforded, when
“The starry sequence of nocturnal hours”[G]
[G] R. Garnett, “Iphigenia in Delphi.”
might be unbroken, perhaps for a week together, by a single adverse
incident of climate.
Herschel took three specula with him to the Cape; one made by his
father, another by himself with his father’s aid, and a third, of his
own exclusive manufacture. Their rapid tarnishing kept them in constant
circulation from the tube to the polisher. After half a dozen nights
they had lost all brilliancy; at the end of three months, they were
more than purblind. He acquired, however, such facility and skill in
the use of his polishing machine, that he was able, in 1835, to report
his mirrors as “more perfect than at any former time.”
He made astonishingly quick progress in observation. On October 24th,
1835, Miss Herschel was informed, “I have now very nearly gone over
the whole southern heavens, and over much of it often. In short, I
have, to use a homely phrase, broken the neck of the work, and my main
object now is to secure and perfect what is done.”
His sweeps yielded a harvest of 1,202 double stars, and 1,708 nebulæ
and clusters, only 439 of which had been previously registered. Among
the novelties were a faint, delicate miniature of the ring-nebula
in Lyra, and five planetaries. One of these he described as “of a
beautiful greenish-blue colour, a full and intense tint.” This lovely
object, situated in Centaur, is sometimes distinguished as “_the_ blue
planetary”; although its hue is shared by all the members of its class.
The nature of their spectrum, in fact, obliges them to be more or less
green.
Sir John Herschel applied the term “falcated” to two curious nebulæ
belonging, undoubtedly, to the later recognised “spiral” class. He
perceived besides in oval nebulæ the annular lines of structure
emphasised in Dr. Roberts’s photographs. He remarked, further, that “as
the condensation increases toward the middle, the ellipticity of the
strata diminishes.”
His study of the Magellanic Clouds gave the first idea of their
composition. He showed them to be aggregations on a vast scale of
every variety of cosmical product. “When examined through powerful
telescopes, the constitution of the Nubeculæ, and especially of the
Nubecula Major, is found to be of astonishing complexity.” He drew up
a preliminary catalogue of 1,163 stars, nebulæ and clusters included
in them, the conjunction of which was really decisive as to nebular
status. For he showed, from the elementary principles of trigonometry,
that, taking the Greater Cloud to be roughly spherical in shape, its
nearest and remotest parts could differ in distance from ourselves
by little more than one-tenth the distance of its centre. The fact
was thus demonstrated that seventh and eighth-magnitude stars and
irresolvable nebulæ co-exist within those limits. He stopped short,
however, of the conclusion drawn by Whewell and Spencer, that the
stellar and nebular sub-kingdoms are not only locally intermixed, but
inseparably united.
The Magellanic Clouds are the most conspicuous features of the barren
south polar heavens. Round the Lesser Cloud especially, the sky,
Herschel said, “is most oppressively desolate.” And again: “The access
to the Nubecula Minor on all sides is through a desert.” One of the
separate inmates of the Larger Cloud is the “great looped nebula,”
compared by Herschel to “an assemblage of loops,” the complicated
windings of which make it “one of the most extraordinary objects which
the heavens present.” To the eye of the present writer it resembled
a shining strip of cellular tissue hung up against the sky. The
“lace-work nebula” in Cygnus is of the same type; but here the tracery
of nebula is closely followed by a tracery of stars. Truly, “A most
wonderful phenomenon!” as Herschel exclaimed in contemplating it.
The first photographs of the Magellanic Clouds were taken in 1890–91
by Mr. Russell of Sydney. They contained an extraordinary revelation.
Both objects came out in them as gigantic spirals. Their miscellaneous
contents are then arranged according to the dictates of a prevalent,
though unexplained cosmical law. The Nubecula Major is a double vortex,
and the extent of its outlying portions, invisible except to the
camera, is at least eight times that of the central mass; but they
conform to the same helical lines.
Herschel catalogued 1,203 stars strewn over the surface of the famous
Argo nebula, and devoted several months to its delineation. This he
found “a work of great labour and difficulty.” While at the telescope
he often half surrendered to despair “of ever being able to transfer to
paper, with even tolerable correctness, its endless details.” “Language
cannot easily convey,” he said, “a full impression of the beauty and
sublimity of the spectacle this nebula offers when viewed in a sweep,
ushered in by so glorious and innumerable a procession of stars, to
which it forms a sort of climax.” Only the Orion nebula may be thought
to surpass it in “magnitude, complexity, and brightness.” Its most
characteristic feature is an abrupt vacuity, of a “lemniscate oval”
shape, from which it derives the name of the “Keyhole Nebula.” The
value of Herschel’s drawing of this grand object has been accentuated
by its photographic portrayal. Their comparison betrays, in fact,
the occurrence in the interval of what appears to be a vast change.
Already, in 1871, Mr. Russell missed with surprise a prominent feature
in the Feldhausen picture; and its failure to appear on photographic
plates exposed for eight hours, yet impressed with innumerable stars
previously unseen, raised something more than a presumption that, as
Mr. Russell said, “a well-defined and brilliant portion of this nebula
vanished between 1837 and 1871.” Its disappearance was independently
verified by Dr. Gill, Royal Astronomer at the Cape. With a total
exposure of more than twelve hours, in March, 1892, he secured a
magnificent representation of this wonderful object, fundamentally
agreeing with Herschel’s, save only as regards the mass of bright
nebulosity vainly looked for by Mr. Russell. The “swan-shaped” or
“trident-like” structure was clean gone! That is to say, the matter
composing it had ceased to be luminous. It should be added that Mr.
Ranyard, whose special experience lent weight to his opinion, thought
it unsafe to trust much to comparisons of drawings of such baffling
objects, either among themselves or with photographs.
Before leaving the Cape, Herschel witnessed an event testifying
surprisingly to the _vitality_ of this nebula. In a condensed tract
close to the dark “keyhole,” he was accustomed to see the bright star
Eta Argûs. It gave no sign of being variable until, on December 16,
1837, he perceived with amazement that it had, all at once, nearly
tripled in brightness. After this sudden leap, it mounted gradually
to the level of Alpha Centauri, then slowly declined. It just matched
Aldebaran when Herschel lost sight of it in March, 1838. A second, and
even more vigorous outburst was watched by Sir Thomas Maclear in 1843.
It then overtopped every star except Sirius, and for seven subsequent
years rivalled the splendour of Canopus. No notice was at first taken
of its colour; but it was redder than Mars in 1850, and reddish it
still remains, in its low estate of invisibility to the naked eye. But
since bright lines of hydrogen show in its photographed spectrum, we
may suspect that--
“Even in its ashes live its former fires,”
and that, consequently, its vicissitudes are not yet terminated. The
instability of its character was virtually discovered at Feldhausen.
Except by Burchell, the African traveller, no previous suspicion of
it had been entertained; the numerous facts denoting that the star’s
past behaviour had been abnormal were collected by Sir John Herschel
after it had been caught _in flagrante delicto_. In his belief, it
had no physical connection with, but was merely projected upon, the
nebula. But since then the nebular relations of blazing stars have
been strongly underlined. The mass of circumstantial evidence now
accumulated on the point fully warrants the assertion that Eta Argûs
makes an integral part of the formation it once illuminated.
A cluster in the constellation of the Cross, unique in the varied and
brilliant tints of its principal components, was compared by Herschel
to “a gorgeous piece of fancy jewellery.” Within the space of 1/48th
part of a square degree, he determined the places of no less than 110
of them, referred to Kappa Crucis, a rosy orb round which they are
irregularly scattered. The colour-effects in this beautiful ornament of
the sky need large apertures for their full display.
An object showing to the eye as a hazy star of the fourth magnitude
was entitled by Bayer in 1603 Omega Centauri. Herschel’s twenty-foot
disclosed it as “a noble globular cluster, beyond all comparison the
richest and largest in the heavens.” Dr. Gill obtained an admirable
photograph of it May 25, 1892. The stars composing it are literally
countless. On a plate exposed for two hours at Arequipa, Mr. Solon I.
Bailey reckoned nearly 6,400; yet he made no allowance for those “too
faint and closely packed” to be perceptible except as a “mottled grey
background between the distinct images.”
Somewhat inferior to Omega Centauri in size, though not at all in
beauty, is 47 Toucani. So obvious is it to the naked eye that, for
several nights after his arrival in Peru, Humboldt took it for a comet.
Central condensation in this cluster appeared to Herschel as if marked
off into three distinct stages; and to his delighted perception the
whole interior offered, by its roseate hue, an exquisite contrast to
the silvery radiance of the outer portions. No other observer has,
however, noticed this chromatic peculiarity. The structure of 47
Toucani is almost perfectly uniform. It is broken by none of the “dark
lanes,” rifts, or tunnels which so curiously diversify many globular
clusters. The usual hirsute aspect lent by the spreading abroad of
_tentacles_, or radiating stellar streams, is likewise scarcely
distinguishable either in 47 Toucani or Omega Centauri. Indeed, Mr.
Bailey noticed that the photographic images of both were all but
perfectly circular. In a future age this may be otherwise. Streams of
stars will, perhaps, set outward from these grand assemblages, leaving
vacancies behind. Thus, if it be permissible to judge of the relative
antiquity of clusters by their advance towards disruption, 47 Toucani
and Omega Centauri may be reckoned among the youngest of the globular
kind existing in the heavens.
The mechanism of clusters has received little attention from any
astronomer beside Herschel. And a solution of an ideal case of the
problem it presented was the utmost he could achieve.
“A quiescent spherical form,” he wrote in 1833, “may subsist as the
bounding outline of an immense number of equal stars, uniformly
distributed through its extent. In such a state of things each star
might describe an ellipse in any plane, and in any direction in that
plane, about the common centre without the possibility of collision.
If the form be not spherical, and the distribution of the stars not
homogeneous, the dynamical relations become too complicated to be
distinctly apprehended.”
But the more closely these aggregations are examined, the less likely
does it seem that they in any sense represent “quiescent forms.” The
arrangement of the stars composing them rather suggests their being
outward bound into the ocean of surrounding space, although the orders
that they carry are to us sealed.
Herschel subsequently altered his views regarding the composition
of clusters, and threw out in 1847 “the possibility of masses of
luminous matter--of whatever density or rarity, of whatever bulk or
minuteness--forming a connected system, and being prevented from
collapse or from mutual interference by the resistance of a transparent
and non-luminous medium.” For a “dynamical” he, in short, substituted
a “statical equilibrium,” the interposed medium lending unity to the
mixed aggregate, and enabling it to rotate, as a whole, upon an axis.
But the rotation is more than questionable. It seems to be precluded
by the ragged contours and indeterminate boundaries of all starry
collections. Photographic evidence, on the other hand, favours Sir John
Herschel’s surmise as to the composite nature of clusters. Some at
least evidently unite within themselves the “two sidereal principles.”
The stellar points they mainly consist of are immersed in, or linked
together by, shining nebulous stuff.
Herschel provided a southern sequel to his father’s star-gauging work
by counting 70,000 stars in 2,300 fields. Their distribution was in
complete accordance with the results of the earlier experiments.
“Nothing can be more striking,” Sir John wrote, “than the gradual,
but rapid increase of density on either side of the Milky Way as we
approach its course.” The existence of an “ecliptic of the stars” (in
Lambert’s almost prophetic phrase) was demonstrated. Or, as Herschel
himself put it, the plane of the Galaxy “is to sidereal, what the
ecliptic is to planetary astronomy, a plane of ultimate reference, the
ground-plan of the sidereal system.” He estimated, from the basis of
his gauge-reckonings, that his twenty-foot reflector was capable of
showing, in both hemispheres, about five and a half million stars. The
smallest of these would be of 14·5 magnitude, on the strict photometric
scale. But, unless his valuation was greatly too small, there must
be a conspicuous falling off in stellar density beyond the region of
tenth or eleventh magnitude. If this be so, scarcely one-quarter of
the expected stars will make their appearance on the plates of the
International Survey.
The grand feature of southern celestial scenery is the splendour of the
Milky Way. One of the galactic condensations in Sagittarius actually
seems to start out from the sky in a definite globular form; and the
darkness of the great rift beginning near the Cross is so intensified
by contrast with the strongly luminous branches it separates, as to
throw the blackness of the exterior heavens _into the shade_. This
part of the Milky Way may even be seen in southern latitudes--as it
was by the present writer--reflected from a glassy ocean-surface. The
section passing from Centaur through the Ship to Orion is, in some
respects, still more striking. Captain Jacob remarked at Madras that
“the general blaze from this portion of the sky is such as to render
a person immediately aware of its having risen above the horizon,
though he should not be at the time looking at the heavens.” Herschel
commented on the singular interruptions of the shining zone by obscure
spaces in Scorpio, near Alpha Centauri, and elsewhere; and admired
the enhancement afforded to its magnificence by “a marvellous fringe
of stars” attached pretty regularly to its southern border. “It is
impossible,” he wrote to Sir William Hamilton in June, 1836, “to resist
the conviction that the Milky Way is not a stratum, but a ring.”
His telescopic analysis disclosed in it a variety and complexity of
structure for which he was wholly unprepared. “Great cirrous masses
and streaks” of galactic light presented themselves in Sagittarius;
and, as the telescope moves, the appearance is that of clouds passing
in a _scud_.” “The Milky Way,” he continued, “is like sand, not strewn
evenly as with a sieve, but as if flung down by handfuls, and both
hands at once, leaving dark intervals, and all consisting of stars
of the lowest magnitudes,” down to nebulosity, in a most astonishing
manner.” As he proceeded, the stars became “inconceivably numerous and
minute. There must be millions on millions, and all most unequally
massed together; yet they nowhere run to nuclei, or clusters much
brighter in the middle.”
In some regions, the formation proved unfathomable; all traces of
stellar _texture_ disappeared. In others, it was plainly perceived to
consist of portions differing exceedingly in distance, but brought by
projection into nearly the same visual line. Near the Trifid Nebula,
“we see foreshortened,” he said, “a vast and illimitable area scattered
over with discontinuous masses and aggregates of stars, in the manner
of the cumuli of a mackerel-sky, rather than of a stratum of regular
thickness and homogeneous formation.”
These varied observations compelled him to reject decisively Olbers’s
hypothesis of light-extinction in space. For, if the possible range of
ethereal messages be restricted in one direction, it must be equally
restricted in all. “We are not at liberty,” he reasoned, “to argue that
in one part of the circumference of the galaxy our view is limited by
this sort of cosmical veil which extinguishes the smaller magnitudes,
cuts off the nebulous light of distant masses, and closes our view in
impenetrable darkness; while, at another, we are compelled, by the
clearest evidence telescopes can afford, to believe that star-strewn
spaces _lie open_, exhausting their powers and stretching out beyond
their utmost reach.” These objections seem fatal to what we may
call the “agnostic” theory of the sidereal world--the theory that
investigations into its construction are for ever barred by failure
of the means of communication--that we can never see more than a
necessarily meaningless part of a possibly infinite, and, in any case,
absolutely inscrutable whole.
The general telescopic exploration of the Milky Way began and ended
with the Herschels. Their great reflectors have been superseded by
the photographic camera. This particular application of its versatile
powers encountered special difficulties; but they were happily
overcome by Professor Barnard in July, 1889. A six-inch portrait
lens afforded the two chief requisites of a powerful light-grasp and
an extensive field; and plates exposed with it for some three hours
showed accordingly, for the first time, “in all their delicacy and
beauty” (to quote Professor Barnard’s words), “the vast and wonderful
cloud-forms, with their remarkable structure of lanes, holes, and black
gaps, and sprays of stars, as no eye or telescope can ever hope to see
them.” The work has since been continued by him and others, notably
by Mr. Russell at Sydney, and by Professor Max Wolf at Heidelberg,
so that the complete round of the “circling zone” will, before long,
have its varied aspects permanently recorded. They frequently present
strange and significant forms. Branching, leaf-like, spiral, elliptical
structures abound; individual stars are disposed in circlets, streams,
parallel rows, curves of sundry kinds. A “clustering power” of unknown
nature is ubiquitously active; orderly development is in progress.
A creative purpose can be _felt_, although it cannot be distinctly
followed by the mind.
Herschel’s “sweeps” in southern skies were continued until January,
1838; but with frequent intermissions. He was ready for every
interesting object that came in his way--comets among the rest.
“Encke’s--_yours_,” he informed his aunt, October 24, 1835, “escaped me
owing to trees and the Table Mountain, though I cut away a good gap in
our principal oak avenue to get at it.” Four days later he caught sight
of Halley’s comet at its second predicted return. But for the stellar
aspect of this body his observations of it would have begun much
earlier; for, in the absence of an exact ephemeris, it was impossible
to pick it out from among the stars it long precisely counterfeited.
“I am sure,” he said, “that I must often have swept with a night-glass
over the very spot where it stood in the mornings before sunrise; and
never was surprise greater than mine at seeing it riding high in the
sky, broadly visible to the naked eye, when pointed out to me by a note
from Mr. Maclear, who saw it with no less amazement on the 24th.”
“This comet,” he wrote to Miss Herschel, March 8, 1836, “has been a
great interruption to my sweeps, and I _hope_ and _fear_ it may yet
be visible another month.” It lingered on just two. He watched with
astonishment the changes it underwent. “Within the well-defined head,”
he wrote in his “Cape Observations,” “and somewhat eccentrically
placed, was seen a vividly luminous nucleus, or rather, an object
which I know no better way to describe than by calling it a miniature
comet, having a nucleus, head, and tail of its own, perfectly distinct,
and considerably exceeding in intensity of light the nebulous disc or
envelope.”
This strangely organised body was a very Proteus for instability of
form. It alternately lost and recovered its tail. It contracted into
the likeness of a star, then dilated into a nebulous globe, which
at last vanished as if through indefinite diffusion. The whole mass
“seemed touched, seemed turned to finest air.” During one week at
the end of January--it had passed perihelion November 16--Sir John
estimated that the cometary Amœba had increased its bulk no less than
forty times!
The paraboloidal form characteristic of this comet and many others, was
to him “inconceivable,” apart from the play of repulsive, in addition
to attractive forces; and he suggested that high electrical excitement
due to vaporisation, if of the same kind with a permanent charge on
the sun, would plausibly account for the enigmatical appearances he
had witnessed. From their close study at Königsberg, Bessel had
already concluded “the emission of the tail to be a purely electrical
phenomenon.”
In March, 1836, Herschel attacked the subject of southern stellar
photometry. Carrying further the “method of sequences,” he determined
the relative brightness of nearly five hundred stars, which he disposed
in order on a single descending scale, and linked on by careful
comparisons to the northern stars, as they “lightened into view” on the
homeward voyage. By the device of an “artificial standard star,” he was
besides enabled to obtain numerical values for the lustre of each star
examined, in terms of that of Alpha Centauri. Most important of all, he
rectified the current system of magnitudes, and introduced a definite
“light ratio,” which has since been extended, and more strictly
defined, but not altered.
His “astrometer” gave Herschel the means of _balancing_ the lustre of
Alpha Centauri against full moonlight. The latter proved to be 27,500
times more powerful. And Wollaston having determined the ratio of
moonlight to sunlight at 1/800000 (corrected by Zöllner to 1/600000),
it became feasible to compare the brightness of any particular star,
_as we see it_, with the brightness of the sun. Alpha Centauri, for
example, sends us, according to Herschel, 1/22 thousand millionth of
the light we receive from our domestic luminary. Moreover, when the
distance of the star came to be measured (it amounts to twenty-five
billions of miles), _light received_ could at once be translated into
_light emitted_. And the result has been to show that the components of
this splendid binary are, taken together, four times more luminous than
the sun. Through Sir John Herschel’s photometric researches, then, the
real light-power of stars at known distances became an ascertainable
quantity; and it is an element of great importance to astrophysical
inquiries.
On January 10, 1837, he wrote from Feldhausen to his brother-in-law:
“I am now at work on the spots in the sun, and the general subject
of solar radiation.” The sun was just then at an exceptionally high
maximum of disturbance. Spots of enormous size frequently obscured
its disc. One was estimated by Herschel, March 29, 1837, to cover,
independently of others, an area of 3,780 millions of square miles. So
that it considerably exceeded in dimensions the great spot-group of
February, 1892, the largest ever photographed at Greenwich. The study
of a series of such phenomena led him to propound the “cyclone-theory”
of their origin. It marked a decided advance in solar physics, if
only because it rested upon the fact--until then unaccountably
overlooked--that spot-production is intimately connected with the
sun’s rotation. He regarded it as a kind of disturbance incidental to
a system of fluid circulation analogous to the terrestrial trade- and
anti-trade winds. “The spots,” he said, “in this view of the subject
would come to be assimilated to those regions on the earth’s surface
where, for the moment, hurricanes and tornadoes prevail; the upper
stratum being temporarily carried downwards, displacing by its impetus
the two strata of luminous matter beneath, the upper of course to a
greater extent than the lower, and thus wholly or partially denuding
the opaque surface of the sun below.”
But the fundamental cause of our atmosphere’s flow and counter-flow
is absent in the sun. The earth is heated from the outside, and
therefore unequally; hence the air rushes along, turning westward
as it goes, from the chilly poles to the torrid zone of vertical
sunshine. No reason is, however, apparent why the solar equator should
be hotter than the solar poles. That adduced by Herschel is certainly
inadequate. He supposed that, by a retention of heat at the equator due
to the accumulation there, consequent upon his rotation, of the sun’s
absorbing atmosphere, a difference of temperature might be maintained
sufficient to keep the solar trade-winds blowing. But the effect is too
slight to be detected. And, in fact, the main drift of the photospheric
layers is along parallels of latitude. Polar and equatorial currents
are insignificant and uncertain.
Herschel and Pouillet contemporaneously, although at opposite sides
of the globe, succeeded in 1837 in measuring the intensity of solar
radiation. They were the first to apprehend the true bearings of the
question, which in principle are simple enough. All that is required
is to determine the heating effects, in a given time, of direct
sunshine. Its despoilment by our air has, indeed, to be allowed for.
Here the chief element of uncertainty comes in. Herschel put the loss
at one-third the original thermal power of vertical rays; Pouillet
pronounced it nearly one-half; Langley, using the most refined
appliances, concludes it to be four-tenths. Striking an average between
his own and the French results, Herschel calculated that, at the sun’s
surface, a shell of ice forty feet thick would melt in one minute, the
rate being reduced, at the distance of the earth, to an inch in two
hours and twelve minutes. And it is now practically certain that this
estimate was too small by about half its amount.
By way of illustrating the effects obtained with his philosophical
apparatus, he constructed a popular kind of actinometer, in the shape
of an “American dispatch,” made of a few pieces of wood and two panes
of glass, in which eggs were roasted, and beef-steaks broiled, by
sun-heat alone. The viands thus _cosmically cooked_ were “eaten with no
small relish by the entertained bystanders.”
Mimas and Enceladus, Saturn’s innermost moons, had persistently eluded
Herschel’s search for them in England; but, to his great delight, both
favoured him at the Cape. His observations of them in 1835–6 were the
first since his father’s time. The next detection of Mimas was by Mr.
Lassell in 1846.
The extent, variety, and completeness of the work done at Feldhausen
strike one with ever-fresh admiration. It seems scarcely credible that
so much was accomplished in four years by a single unaided individual.
Herschel’s only assistant was an honest mechanic named John Stone,
faithful, serviceable, in his way skilful, but not a “being” of the
“quick as lightning” sort, imagined and realised by Caroline Herschel.
It is related that during his observations of Halley’s comet, Sir John
on one occasion fell asleep, and while he remained in this condition
of peril (owing to the elevation and insecurity of his perch), Stone
kept dutifully turning the telescope. At last the astronomer awoke,
rubbed his eyes, looked down the great tube, saw nothing, rubbed his
eyes again, and exclaimed, “Why, John, where’s the comet?” The comet
had meantime set, and the telescope was duly directed towards its place
behind Table Mountain!
The splendid fulfilment of his astronomical tasks did not represent
the whole of Herschel’s activity at the Cape. He collected a large
store of tidal data for Dr. Whewell; started scientific meteorology;
established a system of national education still working beneficially,
and presided over the South African Literary and Scientific
Institution, the members of which presented him with a gold medal on
his departure. His visit made an epoch in the development of the Colony.
To himself personally it was a time of intense enjoyment. His labours,
arduous though they were, proceeded calmly, disembarrassed from
jostling claims and counter-claims. They were carried on with absorbed
enthusiasm, inspired in part by their sublime nature, in part by the
excitement of novelty. His family throve and multiplied at Feldhausen.
Sir Thomas Maclear’s friendship supplied unfailing social pleasure. An
exhilarating climate, moreover, enchanting scenery, translucent skies,
blossoming glens and hillsides worthy of Maeldune’s Isle of Flowers,
contributed to render his southern sojourn a radiant episode. He wrote
of it to Mr. Stewart as “the sunny spot in my whole life, where my
memory will always love to bask.” But “the dream,” he added, “was too
sweet not to be dashed by the dread of awakening.” The spell was broken
when in the middle of March, 1838, he sailed in the _Windsor Castle_
for England.
The interest created by his romantic expedition spread to the other
side of the Atlantic. A grotesque narrative, published in the _New
York Sun_ for September, 1835, of lunar discoveries made at the Cape
with the combined aid of the twenty-foot reflector and the Drummond
limelight, was eagerly read and believed by thousands, was reprinted,
re-circulated, and re-read. Nor were common gulls the only victims
to the hoax. The truth of the story was gravely debated by the Paris
Academy of Sciences.
Herschel’s home-coming was a triumph. He was overwhelmed with applause
and gratulation. His fellow-countrymen offered him what compensation
they could for the disappearance from his horizon of the Southern
Cross. He was created a baronet at the Queen’s Coronation, received
an honorary degree of D.C.L. at Oxford in 1839, and was offered, but
declined, reimbursement from the Treasury for the entire cost of his
trip. He peremptorily refused as well to represent the University of
Cambridge in Parliament, or to be nominated for the Presidentship of
the Royal Society. His utmost desire was for a quiet and laborious
life. A banquet, however, given in honour of his return, June 15,
1838, could not be shunned; the less so that the celebration had a
typical character. “In honouring a man,” Sir William Hamilton said, in
proposing his health, “we honour science too.” For “the cultivators and
lovers of Science have chosen Herschel for their chief--say, rather,
have as such received him by inheritance.”