admit the existence of active volcanoes in the moon? I will explain this
singularity in a few words.
The various parts of our satellite are not all equally reflecting. Here,
it may depend on the form, elsewhere, on the nature of the materials.
Those persons who have examined the moon with telescopes, know how very
considerable the difference arising from these two causes may be, how
much brighter one point of the moon sometimes is than those around it.
Now, it is quite evident that the relations of intensity between the
faint parts and the brilliant parts must continue to exist, whatever be
the origin of the illuminating light. In the portion of the lunar globe
that is illuminated by the sun, there are, everybody knows, some points,
the brightness of which is extraordinary compared to those around them;
those same points, when they are seen in that portion of the moon that
is only lighted by the earth, or in the ash-coloured part, will still
predominate over the neighbouring regions by their comparative
intensity. Thus we may explain the observations of the Slough
astronomer, without recurring to volcanoes. Whilst the great observer
was studying in the non-illuminated portion of the moon, the supposed
volcano of the 20th of April, 1787, his nine-foot telescope showed him
in truth, by the aid of the secondary rays proceeding from the earth,
even the darkest spots.
Herschel did not recur to the discussion of the supposed actually
burning lunar volcanoes, until 1791. In the volume of the _Philosophical
Transactions_ for 1792, he relates that, in directing a twenty-foot
telescope, magnifying 360 times, to the entirely eclipsed moon on the
22d of October, 1790, there were visible, over the whole face of the
satellite, about a hundred and fifty very luminous red points. The
author declares that he will observe the greatest reserve relative to
the similarity of all these points, their great brightness, and their
remarkable colour.
Yet is not red the usual colour of the moon when eclipsed, and when it
has not entirely disappeared? Could the solar rays reaching our
satellite by the effect of refraction, and after an absorption
experienced in the lowest strata of the terrestrial atmosphere, receive
another tint? Are there not in the moon, when freely illuminated, and
opposite to the sun, from one to two hundred little points, remarkable
by the brightness of their light? Would it be possible for those little
points not to be also distinguishable in the moon, when it receives only
the portion of solar light which is refracted and coloured by our
atmosphere?
Herschel was more successful in his remarks on the absence of a lunar
atmosphere. During the solar eclipse of the 5th September, 1793, the
illustrious astronomer particularly directed his attention to the shape
of the acute horn resulting from the intersection of the limbs of the
moon and of the sun. He deduced from his observation that if towards the
point of the horn there had been a deviation of only one second,
occasioned by the refraction of the solar light in the lunar atmosphere,
it would not have escaped him.
Herschel made the planets the object of numerous researches. Mercury was
the one with which he least occupied himself; he found its disk
perfectly round on observing it during its projection, that is to say,
in astronomical language, during its transit over the sun on the 9th of
November, 1802. He sought to determine the time of the rotation of Venus
since the year 1777. He published two memoirs relative to Mars, the one
in 1781, the other in 1784, and the discovery of its being flattened at
the poles we owe to him. After the discovery of the small planets,
Ceres, Pallas, Juno, and Vesta, by Piazzi, Olbers, and Harding, Herschel
applied himself to measuring their angular diameter. He concluded from
his researches that those four new bodies did not deserve the name of
planets, and he proposed to call them asteroïds. This epithet was
subsequently adopted; though bitterly criticized by a historian of the
Royal Society of London, Dr. Thomson, who went so far as to suppose
that the learned astronomer "had wished to deprive the first observers
of those bodies, of all idea of rating themselves as high as him
(Herschel) in the scale of astronomical discoverers." I should require
nothing farther to annihilate such an imputation, than to put it by the
side of the following passage, extracted from a memoir by this
celebrated astronomer, published in the _Philosophical Transactions_,
for the year 1805: "The specific difference existing between planets and
asteroïds appears now, by the addition of a third individual of the
latter species, to be more completely established, and that
circumstance, in my opinion, has added more to the _ornament_ of our
system than the discovery of a new planet could have done."
Although much has not resulted from Herschel's having occupied himself
with the physical constitution of Jupiter, astronomy is indebted to him
for several important results relative to the duration of that planet's
rotation. He also made numerous observations on the intensities and
comparative magnitudes of its satellites.
The compression of Saturn, the duration of its rotation, the physical
constitution of this planet and that of its ring, were, on the part of
Herschel, the object of numerous researches which have much contributed
to the progress of planetary astronomy. But on this subject two
important discoveries especially added new glory to the great
astronomer.
Of the five known satellites of Saturn at the close of the 17th century,
Huygens had discovered the fourth; Cassini the others.
The subject seemed to be exhausted, when news from Slough showed what a
mistake this was.
On the 28th of August, 1789, the great forty-foot telescope revealed to
Herschel a satellite still nearer to the ring than the other five
already observed. According to the principles of the nomenclature
previously adopted, the small body of the 28th August ought to have been
called the first satellite of Saturn, the numbers indicating the places
of the other five would then have been each increased by a unity. But
the fear of introducing confusion into science by these continual
changes of denomination, induced a preference for calling the new
satellite the sixth.
Thanks to the prodigious powers of the forty-foot telescope, a last
satellite, the seventh, showed itself on the 17th of September, 1789,
between the sixth and the ring.
This seventh satellite is extremely faint. Herschel, however, succeeding
in seeing it whenever circumstances were very favourable, even by the
aid of the twenty-foot telescope.
The discovery of the planet Uranus, the detection of its satellites,
will always occupy one of the highest places among those by which modern
astronomy is honoured.
On the 13th of March, 1781, between ten and eleven o'clock at night,
Herschel was examining the small stars near H Geminorum with a
seven-foot telescope, bearing a magnifying power of 227 times. One of
these stars seemed to him to have an unusual diameter. The celebrated
astronomer, therefore, thought it was a comet. It was under this
denomination that it was then discussed at the Royal Society of London.
But the researches of Herschel and of Laplace showed later that the
orbit of the new body was nearly circular, and Uranus was elevated to
the rank of a planet.
The immense distance of Uranus, its small angular diameter, the
feebleness of its light, did not allow the hope, that if that body had
satellites, the magnitudes of which were, relatively to its own size,
what the satellites of Jupiter, of Saturn are, compared to those two
large planets, any observer could perceive them, from the earth.
Herschel was not a man to be deterred by such discouraging conjectures.
Therefore, since powerful telescopes of the ordinary construction, that
is to say, with two mirrors conjugated, had not enabled him to discover
any thing, he substituted, in the beginning of January, 1787, _front
view_ telescopes, that is, telescopes throwing much more light on the
objects, the small mirror being then suppressed, and with it one of the
causes of loss of light is got rid of.
By patient labour, by observations requiring a rare perseverance,
Herschel attained (from the 11th of January, 1787, to the 28th of
February, 1794,) to the discovery of the six satellites of his planet,
and thus to complete the _world_ of a system that belongs entirely to
himself.
There are several of Herschel's memoirs on comets. In analyzing them, we
shall see that this great observer could not touch any thing without
making further discoveries in the subject.
Herschel applied some of his fine instruments to the study of the
physical constitution of a comet discovered by Mr. Pigott, on the 28th
September, 1807.
The nucleus was round and well determined. Some measures taken on the
day when the nucleus subtended only an angle of a single second, gave as
its real angle 6/100 of the diameter of the earth.
Herschel saw no phase at an epoch when only 7/10 of the nucleus could
be illuminated by the sun. The nucleus then must shine by its own light.
This is a legitimate inference in the opinion of every one who will
allow, on one hand, that the nucleus is a solid body, and on the other,
that it would have been possible to observe a phase of 8/10 on a disk
whose apparent total diameter did not exceed one or two seconds of a
degree.
Very small stars seemed to grow much paler when they were seen through
the coma or through the tail of the comet.
This faintness may have only been apparent, and might arise from the
circumstance of the stars being then projected on a luminous background.
Such is, indeed, the explanation adopted by Herschel. A gaseous medium,
capable of reflecting sufficient solar light to efface that of some
stars, would appear to him to possess in each stratum a sensible
quantity of matter, and to be, for that reason, a cause of real
diminution of the light transmitted, though nothing reveals the
existence of such a cause.
This argument, offered by Herschel in favour of the system which
transforms comets into self-luminous bodies, has not, as we may
perceive, much force. I might venture to say as much of many other
remarks by this great observer. He tells us that the comet was very
visible in the telescope on the 21st of February, 1808; now, on that
day, its distance from the sun amounted to 2.7 times the mean radius of
the terrestrial orbit; its distance from the observer was 2.9: "What
probability would there be that rays going to such distances, from the
sun to the comet, could, after their reflection, be seen by an eye
nearly three times more distant from the comet than from the sun?"
It is only numerical determinations that could give value to such an
argument. By satisfying himself with vague reasoning, Herschel did not
even perceive that he was committing a great mistake by making the
comet's distance from the observer appear to be an element of
visibility. If the comet be self-luminous, its intrinsic splendour (its
brightness for unity of surface) will remain constant at any distance,
as long as the subtended angle remains sensible. If the body shines by
borrowed light, its brightness will vary only according to its change of
distance from the sun; nor will the distance of the observer occasion
any change in the visibility; always, let it be understood, with the
restriction that the apparent diameter shall not be diminished below
certain limits.
Herschel finished his observations of a comet that was visible in
January, 1807, with the following remark:--
"Of the sixteen telescopic comets that I have examined, fourteen had no
solid body visible at their centre; the other two exhibited a central
light, very ill defined, that might be termed a nucleus, but a light
that certainly could not deserve the name of a disk."
The beautiful comet of 1811 became the object of that celebrated
astronomer's conscientious labour. Large telescopes showed him, in the
midst of the gazeous head, a rather reddish body of planetary
appearance, which bore strong magnifying powers, and showed no sign of
phase. Hence Herschel concluded that it was self-luminous. Yet if we
reflect that the planetary body under consideration was not a second in
diameter, the absence of a phase does not appear a demonstrative
argument.
The light of the head had a blueish-green tint. Was this a real tint, or
did the central reddish body, only through contrast, make the
surrounding vapour appear to be coloured? Herschel did not examine the
question in this point of view.
The head of the comet appeared to be enveloped at a certain distance, on
the side towards the sun, by a brilliant narrow zone, embracing about a
semicircle, and of a yellowish colour. From the two extremities of the
semicircle there arose, towards the region away from the sun, two long
luminous streaks which limited the tail. Between the brilliant circular
semi-ring and the head, the cometary substance seemed dark, very rare,
and very diaphanous.
The luminous semi-ring always presented similar appearances in all the
positions of the comet; it was not then possible to attribute to it
really the annular form, the shape of Saturn's ring, for example.
Herschel sought whether a spherical demi-envelop of luminous matter, and
yet diaphanous, would not lead to a natural explanation of the
phenomenon. In this hypothesis, the visual rays, which on the 6th of
October, 1811, made a section of the envelop, or bore almost
tangentially, traversed a thickness of matter of about 399,000
kilometres, (248,000 English miles,) whilst the visual rays near the
head of the comet did not meet above 80,000 kilometres (50,000 miles) of
it. As the brightness must be proportional to the quantity of matter
traversed, there could not fail to be an appearance around the comet, of
a semi-ring five times more luminous than the central regions. This
semi-ring, then, was an effect of projection, and it has revealed a
circumstance to us truly remarkable in the physical constitution of
comets.
The two luminous streaks that outlined the tail at its two limits, may
be explained in a similar manner; the tail was not flat as it appeared
to be; it had the form of a conoid, with its sides of a certain
thickness. The visual lines which traversed those sides almost
tangentially, evidently met much more matter than the visual lines
passing across. This maximum of matter could not fail of being
represented by a maximum of light.
The luminous semi-ring floated; it appeared one day to be suspended in
the diaphanous atmosphere by which the head of the comet was surrounded,
at a distance of 518,000 kilometres (322,000 English miles) from the
nucleus.
This distance was not constant. The matter of the semi-annular envelop
seemed even to be precipitated by slow degrees through the diaphanous
atmosphere; finally it reached the nucleus; the earlier appearances
vanished; the comet was reduced to a globular nebula.
During its period of dissolution, the ring appeared sometimes to have
several branches.
The luminous shreds of the tail seemed to undergo rapid, frequent, and
considerable variations of length. Herschel discerned symptoms of a
movement of rotation both in the comet and in its tail. This rotatory
motion carried unequal shreds from the centre towards the border, and
reciprocally. On looking from time to time at the same region of the
tail, at the border, for example, sensible changes of length must have
been perceptible, which however had no reality in them. Herschel
thought, as I have already said, that the beautiful comet of 1811, and
that of 1807, were self-luminous. The second comet of 1811 appeared to
him to shine only by borrowed light. It must be acknowledged that these
conjectures did not rest on any thing demonstrative.
In attentively comparing the comet of 1807 with the beautiful comet of
1811, relative to the changes of distance from the sun, and the
modifications resulting thence, Herschel put it beyond doubt that these
modifications have something individual in them, something relative to a
special state of the nebulous matter. On one celestial body the changes
of distance produce an enormous effect, on another the modifications are
insignificant.
OPTICAL LABOURS.
I shall say very little on the discoveries that Herschel made in
physics. In short, everybody knows them. They have been inserted into
special treatises, into elementary works, into verbal instruction; they
must be considered as the starting-point of a multitude of important
labours with which the sciences have been enriched during several years.
The chief of these is that of the dark radiating heat which is found
mixed with light.
In studying the phenomena, no longer with the eye, like Newton, but with
a thermometer, Herschel discovered that the solar spectrum is prolonged
on the red side far beyond the visible limits. The thermometer sometimes
rose higher in that dark region, than in the midst of brilliant zones.
The light of the sun then, contains, besides the coloured rays so well
characterized by Newton, some invisible rays, still less refrangible
than the red, and whose warming power is very considerable. A world of
discoveries has arisen from this fundamental fact.
The dark heat emanating from terrestrial objects more or less heated,
became also subjects of Herschel's investigations. His work contained
the germs of a good number of beautiful experiments since erected upon
it in our own day.
By successively placing the same objects in all parts of the solar
spectrum Herschel determined the illuminating powers of the various
prismatic rays. The general result of these experiments may be thus
enunciated:
The illuminating power of the red rays is not very great; that of the
orange rays surpasses it, and is in its turn surpassed by the power of
the yellow rays. The maximum power of illumination is found between the
brightest yellow and the palest green. The yellow and the green possess
this power equally. A like assimilation may be laid down between the
blue and the red. Finally, the power of illumination in the indigo rays,
and above all in the violet, is very weak.
Yet the memoirs of Herschel on Newton's coloured rings, though
containing a multitude of exact experiments, have not much contributed
to advance the theory of those curious phenomena. I have learnt from
good authority, that the great astronomer held the same opinion on this
topic. He said that it was the only occasion on which he had reason to
regret having, according to his constant method, published his labours
immediately, as fast as they were performed.
LAPLACE.
Having been appointed to draw up the report of a committee of the
Chamber of Deputies which was nominated in 1842, for the purpose of
taking into consideration the expediency of a proposal submitted to the
Chamber by the Minister of Public Instruction, relative to the
publication of a new edition of the works of Laplace at the public
expense, I deemed it to be my duty to embody in the report a concise
analysis of the works of our illustrious countryman. Several persons,
influenced, perhaps, by too indulgent a feeling towards me, having
expressed a wish that this analysis should not remain buried amid a heap
of legislative documents, but that it should be published in the
_Annuaire du Bureau des Longitudes_, I took advantage of this
circumstance to develop it more fully so as to render it less unworthy
of public attention. The scientific part of the report presented to the
Chamber of Deputies will be found here entire. It has been considered
desirable to suppress the remainder. I shall merely retain a few
sentences containing an explanation of the object of the proposed law,
and an announcement of the resolutions which were adopted by the three
powers of the State.
"Laplace has endowed France, Europe, the scientific world, with three
magnificent compositions: the _Traité de Mécanique Céleste_, the
_Exposition du Système du Monde_, and the _Théorie Analytique des
Probabilités_. In the present day (1842) there is no longer to be found
a single copy of this last work at any bookseller's establishment in
Paris. The edition of the _Mécanique Céleste_ itself will soon be
exhausted. It was painful then to reflect that the time was close at
hand when persons engaged in the study of the higher mathematics would
be compelled, for want of the original work, to inquire at Philadelphia,
at New York, or at Boston for the English translation of the _chef
d'oeuvre_ of our countryman by the excellent geometer Bowditch. These
fears, let us hasten to state, were not well founded. To republish the
_Mécanique Céleste_ was, on the part of the family of the illustrious
geometer, to perform a pious duty. Accordingly, Madame de Laplace, who
is so justly, so profoundly attentive to every circumstance calculated
to enhance the renown of the name which she bears, did not hesitate
about pecuniary considerations. A small property near Pont l'Evêque was
about to change hands, and the proceeds were to have been applied so
that Frenchmen should not be deprived of the satisfaction of exploring
the treasures of the _Mécanique Céleste_ through the medium of the
vernacular tongue.
"The republication of the complete works of Laplace rested upon an
equally sure guarantee. Yielding at once to filial affection, to a noble
feeling of patriotism, and to the enthusiasm for brilliant discoveries
which a course of severe study inspired, General Laplace had long since
qualified himself for becoming the editor of the seven volumes which are
destined to immortalize his father.
"There are glorious achievements of a character too elevated, of a
lustre too splendid, that they should continue to exist as objects of
private property. Upon the State devolves the duty of preserving them
from indifference and oblivion: of continually holding them up to
attention, of diffusing a knowledge of them through a thousand channels;
in a word, of rendering them subservient to the public interests.
"Doubtless the Minister of Public Instruction was influenced by these
considerations, when upon the occasion of a new edition of the works of
Laplace having become necessary, he demanded of you to substitute the
great French family for the personal family of the illustrious geometer.
We give our full and unreserved adhesion to this proposition. It springs
from a feeling of patriotism which will not be gainsayed by any one in
this assembly."
In fact, the Chamber of Deputies had only to examine and solve this
single question: "Are the works of Laplace of such transcendent, such
exceptional merit, that their republication ought to form the subject of
deliberation of the great powers of the State?" An opinion prevailed,
that it was not enough merely to appeal to public notoriety, but that it
was necessary to give an exact analysis of the brilliant discoveries of
Laplace in order to exhibit more fully the importance of the resolution
about to be adopted. Who could hereafter propose on any similar occasion
that the Chamber should declare itself without discussion, when a desire
was felt, previous to voting in favour of a resolution so honourable to
the memory of a great man, to fathom, to measure, to examine minutely
and from every point of view monuments such as the _Mécanique Céleste_
and the _Exposition du Système du Monde_? It has appeared to me that the
report drawn up in the name of a committee of one of the three great
powers of the State might worthily close this series of biographical
notices of eminent astronomers.[22]
The Marquis de Laplace, peer of France, one of the forty of the French
Academy, member of the Academy of Sciences and of the _Bureau des
Longitudes_, an associate of all the great Academies or Scientific
Societies of Europe, was born at Beaumont-en-Auge of parents belonging
to the class of small farmers, on the 28th of March, 1749; he died on
the 5th of March, 1827.