indicating the dimensions of the celestial regions, the magnitudes and
periodical disturbances of the heavenly bodies, the triple elements of
terrestrial magnetism, the mean pressure of the atmosphere, and the quantity
of heat which the sun imparts in each year, and in every season of the year,
to all points of the solid and liquid surface of our planet. These sources
of enjoyment do not, however, satisfy the poet of Nature, or the mind of the
inquiring many. To both of these the present state of science appears as a
blank, now that she answers doubtingly, or wholly rejects as unanswerable,
questions to which former ages deemed they could furnish satisfactory
replies. In her severer aspect, and clothed with less luxuriance, she shows
herself deprived of that seductive charm with which a dogmatizing and
symbolizing physical philosophy knew how to deceive the understanding and
give the rein to imagination. Long before the discovery of the New World,
it was believed that new lands in the Far West might be seen from the shores
of the Canaries and the Azores. These illusive images were owing, not to
any extraordinary refraction of the rays of light, but produced by an eager
longing for the distant and the unattained. The philosophy of the Greeks,
the physical views of the Middle Ages, and even those of a more recent
period, have been eminently imbued with the charm springing from similar
illusive phantoms of the imagination. At the limits of circumscribed
knowledge, as from some lofty island shore, the eye delights to penetrate
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to distant regions. The belief in the uncommon and the wonderful lends a
definite outline to every manifestation of ideal creation; and the realm of
fancy -- a fairy-land of cosmological, geognostical, and magnetic visions --
becomes thus involuntarily blended with the domain of reality.
Nature, in the manifold signification of the word -- whether considered as
the universality of all that is and ever will be -- as the inner moving
force of all phenomena, or as their mysterious prototype -- reveals itself
to the simple mind and feelings of man as something earthly, and closely
allied to himself. It is only within the animated circles of organic
structure that we feel ourselves peculiarly at home. Thus, wherever the
earth unfolds her fruits and flowers, and gives food to countless tribes of
animals, there the image of nature impresses itself most vividly upon our
senses. The impression thus produced upon our minds limits itself almost
exclusively to the reflection of the earthly. The starry vault and the wide
expanse of the heavens belong to a picture of the universe, in which the
magnitude of masses, the number of congregated suns and faintly glimmering
nebulae, although they excite our wonder and astonishment, manifest
themselves to us in apparent isolation, and as utterly devoid of all
evidence of their being the scenes of organic life. Thus, even in the
earliest physical views of mankind, heaven and earth have been separated and
opposed to one another as an upper and lower portion of space. If, then, a
picture of nature were to correspond to the requirements of contemplation by
the senses, it ought to begin with a delineation of our native earth. It
should depict, first, the terrestrial planet as to its size and form; its
increasing density and heat at increasing depths in its superimposed solid
and liquid strate; the separation of sea and land, and the vital forms
animating both, developed in the cellular tissues of plants and animals; the
atmospheric ocean, with its waves and currents, through which pierce the
forest-crowned summits of our mountain chains. After this delineation of
purely telluric relations, the eye would rise to the celestial regions, and
the Earth would then, as the well-known seat of organic development, be
considered as a planet, occupying a place in the series of those heavenly
bodies which circle round one of the innumerable host of self-luminous
stars. This succession of ideas indicates the course pursued in the
earliest stages of perceptive contemplation, and reminds us of the ancient
conception of the "sea-girt disk of earth," supporting the vault of heaven.
It begins to exercise in action
p 83
at the spot where it originated, and passes from the consideration of the
known to the unknown, of the near to the distant. It corresponds with the
method pursued in our elementary works on astronomy (and which is so
admirable in a mathematical point of view), of proceeding from the apparent
to the real movements of the heavenly bodies.
Another course of ideas must, however, be pursued in a work which proposes
merely to give an exposition of what is known -- of what may in the present
state of our knowledge be regarded as certain, or as merely probable in a
greater or lesser degree -- and does not enter into a consideration of the
proofs on which such results have been based. Here, therefore, we do not
proceed from the subjective point of view of human interests. The
terrestrial must be treated only as grand and free, uninfluenced by motives
of proximity, social sympathy, or relative utility. A physical cosmography
-- a picture of the universe -- does not begin, therefore, with the picture
of the universe -- does not begin, therefore, with the terrestrial, but with
that which fills the regions of space. But as the sphere of contemplation
contracts in dimension our perception of the richness of individual parts,
the fullness of physical phenomena, and of the heterogeneous properties of
matter becomes enlarged. From the regions in which we recognize ony the
dominion of the laws of attraction, we descend to our own planet, and to the
intricate play of terrestrial forces. The method here described for the
delineation of nature is opposed to that which mst be pursued in
establishing conclusive results. The one enumerates what the other
demonstrates.
Man learns to know the external world through the organs of the senses.
Phenomena of light proclaim the existence of matter in remotest space, and
the eye is thus made the medium through which we may contemplate the
universe. The discovery of telescopic vision more than two centuries ago,
has transmitted to latest generations a power whose limits are as yet
unattained.
The first and most general consideration of the Cosmos is that of the
'contents of space' -- the distribution of matter, or of creation, as we are
wont to designate the assemblage of all that is and ever will be developed.
We see matter either agglomerated into rotating, revolving spheres of
different density and size, or scattered through space in the form of
self-luminous vapor. If we consider first the cosmical vapor dispersed in
definite nebulous spots, its state of aggregation will
p 84
appear constantly to vary, sometimes appearing separated into round or
elliptical disks, single or in pairs, occasionally connected by a thread of
light; while, at another time, these nebulae occur in forms of larger
dimensions, and are either elongated, or variously branched or fan-shaped or
appear like well-defined rings, including a dark interior. It is
conjectured that these bodies are undergoing variously developed formative
processes, as the cosmical vapor becomes condensed in conformity with the
laws of attraction, either round one or more of the nuclei. Between two and
three thousand of such unresolvable nebulae, in which the most powerful
telescopes have hitherto been unable to distinguish the presence of stars,
have been counted, and their positions determined.
The genetic evolution -- that perpetual state of development which seems to
affect this portion of the regions of space -- has led philosophical
observers to the discovery of the analogy existing among organic phenomena.
As in our forests we see the same kind of tree in all the various stages of
its growth, and are thus enabled to form an idea of progressive, vital
development, so do we also in the great garden of the universe, recognise
the most different phases of sidereal formation. The process of
condensation, which formed a part of the doctrines of Anaximenes and of the
Ionian School, appears to be going on before our eyes. This subject of
investigation and conjecture is especially attractive to the imagination,
for in the study of the animated circles of nature, and of the action of all
the moving forces of the universe, the charm that exercises the most
powerful influence on the mind is derived less from a knowledge of that
which 'is' than from a perception of that which 'will be', even though the
latter be nothing more than a new condition of a known material existence;
for of actual creation, of origin, the beginning of existence from
non-existence, we have no experience, and can therefore form no conception.
A comparison of the various causes influencing the development manifested by
the greater or less degree of condensation in the interior of nebulae, no
less than a successive course of direct observations, have led to the belief
that changes of form have been recognized first in Andromeda, next in the
constallation Argo, and in the isolated filamentous portion of the nebula in
Orion. But want of uniformity in the power of the instruments employed,
different conditions of our atmosphere, and other optical relations, render
a part of the results invalid as historical evidence.
p 85
'Nebulous stars' must not be confounded either with irregularly-shaped
nebulous spots, properly so called, whose separate parts have an unequal
degree of brightness (and which may, perhaps, become concentrated into stars
as their circumference contracts), nor with the so-called planetary nebulae,
whose circular or slightly oval disks manifest in all their parts a
perfectly uniform degree of faint light. 'Nebulous stars' are not merely
accidental bodies projected upon a nebulous ground, but are a part of the
nebulous matter constituting one mass with the body which it surrounds. The
not unfrequently considerable magnitude of their apparent diameter, and the
remote distance from which they are revealed to us, show that both the
planetary nebulae and the nebulous stars must be of enormous dimensions.
New and ingenious considerations of the different influence exercised by
distance* on the intensity of light of a disk of appreciable diameter, and
of a single self-luminous point, render it not improbable that the planetary
nebulae are very remote nebulous stars, in which the difference between the
central body and the surrounding nebulous covering can no longer be detected
by our telescopic instruments.
[footnote] * The optical considerations relative to the difference
presented by a single luminous point, and by a disk subtending an
appreciable angle, in which the intensity of light is constant at every
distance, are explained in Arago's 'Analyse des Travaux de Sir William
Herschel' ('Annuaire du Bureau des Long.', 1842, p. 410-412, and 441).
The magnificent zones of the southern heavens, between 50 degrees and 80
degrees, are especially rich in nebulous stars, and in compressed
unresolvable nebua e. The larger of the two Magellanic clouds, which circle
round the starless, desert pole of the south, appears, according to the most
recent researches,* as "a collection of clusters of stars, composed of
globular clusters and nebulae of different magnitude, and of large nebulous
spots
p 86
not resolvable, which, producing a general brightness in the field of view,
form, as it were, the back-ground of the picture."
[footnote] *The two Magellanic clouds, Nubecula major and Nubecula minor,
are very remarkable objects. The larger of the two is an accumulated mass
of stars, and consists of clusters of stars of irregular form, either
conical masses or nebulae of different magnitudes and degrees of
condensation. This is interspersed with nebulous spots, not resolvable into
stars, but which are probably 'star dust', appearing only as a general
radiance upon the telescopic field of a twenty-feet reflector, and forming a
luminous ground on which other objects of striking and indescribable form
are scattered. In no other portion of the heavens are so many nebulous and
stellar masses thronged together in an equally small space. Nubecula minor
is much less beautiful, has more unresolvable nebulous light, while the
stellar masses are fewer and fainter in intensity. -- (From a letter of Sir
John Herschel, Feldhuysen, Cape of Good Hope, 13th June, 1836.)
The appearance of these clouds, of the brightly-beaming constellation Argo,
of the Milky Way between Scorpio, the Centaur, and the Southern Cross, the
picturesque beauty, if one may so speak, of the whole expanse of the
southern celestial hemisphere, has left upon my mind an ineffaceable
impression. The zodiacal light, which rises in a pyramidal form, and
constantly contributes, by its mild radiance, to the external beauty of the
tropical nights, is either a vast nebulous ring, rotating between the Earth
and Mars, or, less probably, the exterior stratum of the solar atmosphere.
Besides these luminous clouds and nebulae of definite form, exact and
corresponding observations indicate the existence and the general
distribution of an apparently non-luminous, infinitely-divided matter, which
posssesses a force of resistance and manifests its presence in Encke's, and
perhaps also in Biela's comet, by diminishing their eccentricity and
shortening their period of revolution. Of this impending, ethereal, and
cosmical matter, it may be supposed that it is in motion; that it
gravitates, notwithstanding its original tenuity; that it is condensed in
the vicinity of the great mass of the Sun; and, finally, that it may, for
myriads of ages, have been augmented by the vapor emanating from the tails
of comets.
If we now pass from the consideration of the vaporous matter of the
immeasurable regions of space [(Greek)*] -- whether scattered without
definite form and limits, it exists as a cosmical other, or is condensed
into nebulous spots, and becomes comprised among the solid agglomerated
bodies of the universe -- we approach a class of phenomena exclusively
designated by the form of stars, or as the sidereal world.
[footnote] *I should have made use, in the place of garden of the universe,
of the beautiful expression [Greek], borrowed by Hesychius from an unknown
poet, if [Greek] had not rather signified in general an inclosed space. The
connection with the German 'garten' and the English 'garden', 'gards' in
Gothic (derived according to Jacob Grimm, from 'gairdan', 'to gird'), is,
however, evident, as is likewise the affinity with the Slavonic 'grad',
'gorod', and as Pott remarks, in his 'Etymol. Forschungen', th. i., s. 144
(Etymol. Researches), with the Latin 'chors', whence we have the Spanish
'corte', the French 'cour', and the English word 'court', together with the
Ossetic 'khart'. To these may be further added the Scandinavian 'gard',**
'gard', a place inclosed, as a court, or a country seat, and the Persian
'gerd', 'gird', a district, a circle, a princely country seat, a castle or
city, as we find the term applied to the names of places in Firdusi's
Schahnameh, as 'Siyawakschgird', 'Darabgird', etc.
** (This word is written 'gaard' in the Danish) -- Tr.
p 87
Here, too, we find differences existing in the solidity or density of the
spheroidally agglomerated matter. Our own solar system presents all stages
of 'mean' density (or of the relation of 'volume' to 'mass'.) On comparing
the planets from Mercury to Mars with the Sun and with Jupiter, and these
two last named with the yet inferior density of Saturn, we arrive, by a
descending scale -- to draw our illustration from the terrestrial substances
-- at the respective densities of antimony, honey, water, and pine wood. In
comets, which actually constitute the most considerable portion of our solar
system with respect to the number of individual forms, the concentrated
part, usually termed the 'head', or 'nucleus', transmits sidereal light
unimpaired. The mass of a comet probably in no case equals the five
thousandth part of that of the earth, so dissimilar are the formative
processes manifested in the original and perhaps still progressive
agglomerations of matter. In proceeding from general to special
considerations, it was particularly desirable to draw attention to this
diversity, not merely as a possible, but as an actually proved fact.
The purely speculative conclusions arrived at by Wright, Kant, and Lambert,
concerning the general structural arrangement of the universe, and of the
distribution of matter in space, have been confirmed by Sir William
Herschel, on the more certain path of observation and measurement. That
great and enthusiastic, although cautious observer, was the first to sound
the depths of heaven in order to determine the limits and form of the starry
stratum which we inhabit, and he, too, was the first who ventured to throw
the light of investigation upon the relations existing between the position
and distance of remote nebulae and our own portion of the sidereal universe.
William Herschel, as is well expressed in the elegant inscription on his
monument at Upton, broke through the inclosures of heaven ('caelorum
perrupit claustra'), and, like another Columbus, penetrated into an unknown
ocean, from which he beheld coasts and groups of islands, whose true
position it remains for future ages to determine.
Considerations regarding the different intensity of light in stars, and
their relative number, that is to say, their numerical frequency on
telescopic fields of equal magnitude, have led to the assumption of unequal
distances and distribution in space in the strata which they compose. Such
assumptions, in as far as they may lead us to draw the limits of the
individual portions of the universe, can not offer the same degree of
mathematical certainty as that which may be attained in all that
p 88
relates to our solar system, whether we consider the rotation of double
stars with unequal velocity round one common center of gravity, or the
apparent or true movements of all the heavenly bodies. If we take up the
physical description of the universe from the remotest nebulae, we may be
inclined to compare it with the mythical portions of history. The one
begins in the obscurity of antiquity, the other in that of inaccessible
space; and at the point where reality seems to flee before us, imagination
becomes doubly incited to draw from its own fullness, and give definite
outline and permanence to the changing forms of objects.
If we compare the regions of the universe with one of the island-studded
seas of our own planet, we may imagine matter to be distributed in groups,
either as unresolvable nebulae of different ages, condensed around one or
more nuclei, or as already agglomerated into clusters of stars, or isolated
spheroidal bodies. The cluster of stars, to which our cosmical island
belongs, forms a lens-shaped, flattened stratum, detached on every side,
whose major axis is estimated at seven or eight hundred, and its minor one
at a hundred and fifty times the distance of Sirius. It would appear, on
the supposition that the parallax of Sirius is not greater than that
accurately determined for the brightest star in the Centaur (0".9128), that
light traverses one distance of Sirius in three years, while it also
follows, from Bessel's earlier excellent Memoir* on the parallax of the
remarkable star 61 Cygni (0".3483), (whose considerable motion might lead to
the inference of great proximity), that a period of nine years and a quarter
is required for the transmission of light from this star to our planet.
[footnote] *See Maclear's "Results from 1839 to 1840," in the 'Trans. of
the Astronomical Soc.', vol. xii., p. 370, on 'a' Centauri, the probable
mean error being 0".0649. For 61 Cygni, see Bessel, in Schumacher's
'Jahrbuch', 1839, s. 47, and Schumacher's 'Astron. Nachr.', bd. xviii., s.
401, 402, probable mean error, 0".0141. With reference to the relative
distances of stars of different magnitudes, how those of the third magnitude
may probably be three times more remote, and the manner in which we
represent to ourselves the material arrangement of the starry strata, I have
found the following remarkable passage in Kepler's 'Epitome Astronomiae
Copernicanae', 1618, t. i., lib. 1, p. 34-39: "Sol hic noster nil aliud est
quam una ex fixis, nobis major et clarior visa, quia propior quam fixa.
Pone terram stare ad latus, una semi-diametro via e lactea e, tunc ha ec via
lactea apparebit circulus parvus, vel ellipsis parva, tota declinans ad
latus alterum; eritque simul uno intuitu conspicua, quae nunc no potest nisi
dimidia conspici quovis momento. Itaque fix arum spha era non tantum orbe
stellarum, sed etiam circulo lactis versus not deorsum est terminata."
Our starry stratum is a disk of inconsiderable thickness, divided a
p 89
third of its length into two branches; it is supposed that we are near this
division, and nearer to the region of Sirius than to the constellation
Aquila, almost in the middle of the stratum in the line of its thickness or
minor axis.
This position of our solar system, and the form of the whole discoidal
stratum, have been inferred from sidereal scales, that is to say, from that
method of counting the stars to which I have already alluded, and which is
based upon the equidistant subdivision of the telescopic field of view. The
relative depth of the stratum in all directions is measured by the greater
or smaller number of stars appearing in each division. These divisions give
the length of the ray of vision in the same manner as we measure the depth
to which the plummet has been thrown, before it reaches the bottom, although
in the case of a starry stratum there can not, correctly speaking, be any
idea of depth, but merely of outer limits. In the direction of the longer
axis, where the stars lie behind one another, the more remote ones appear
closely crowded together, united, as it were, by a milky-white radiance or
luminous vapor, and are perspectively grouped, encircling as in a zone, the
visible vault of heaven. This narrow and branched girdle, studded with a
radiant light, and here and there interrupted by dark spots, deviates only
by a few degrees from forming a perfect large circle round the concave
sphere of heaven, owing to our being near the center of the large starry
cluster, and almost on the plane of the Milky Way. If our planetary system
were far 'outside' this cluster, the Milky Way would appear to telescopic
vision as a ring, and at a still greater distance as a resolvable discoidal
nebula.
Among the many self-luminous moving suns, erroneously called 'fixed stars',
which constitute our cosmical island, our own sun is the only one known by
direct observation to be a 'central body' in its relations to spherical
agglomerations of matter directly depending upon and revolving round it,
either in the form of planets, comets, or aerolite asteroids. As far as we
have hitherto been able to investigate 'multiple' stars (double stars or
suns), these bodies are not subject, with respect to relative motion and
illumination, to the same planetary dependence that characterizes our own
solar system. Two or more self-luminous bodies, whose planets and moon, if
such exist, have hitherto escaped our telescopic powers of vision, certainly
revolve around one common center of gravity; but this is in a portion of
space which is probably occupied merely by unagglomerated matter or cosmical
vapor, while in our system
p 90
the center of gravity is often comprised within the innermost limits of a
'visible' central body. If, therefore, we regard the Sun and the Earth, or
the Earth and the Moon, as double-stars, and the whole of our planetary
solar system as a multiple cluster of stars, the analogy thus suggested must
be limited to the universality of the laws of attraction in different
systems, being alike applicable to the independent processes of light and to
the method of illumination.
For the generalization of cosmical views, corresponding with the plan we
have proposed to follow in giving a delineation of nature or of the
universe, the solar system to which the Earth belongs may be considered in a
two-fold relation: first, with respect to the different classes of
individually agglomerated matter, and the relative size, conformation,
density, and distance of the heavenly bodies of this system; and secondly,
with reference to other portions of our starry cluster, and of the changes
of position of its central body, the Sun.
The solar system, that is to say, the variously-formed matter circling round
the Sun, consists, according to the present state of our knowledge of
'eleven primary planets',* eighteen satellites
p 91
or secondary planets, and myriads of comets, three of which, known as the
"planetary comets," do not pass beyond the narrow limits of the orbits
described by the principal planets.