of considerable density. The attraction of the earth has probably
deprived the moon of hers, for the refraction of the air at the surface
of the earth is at least a thousand times as great as at the moon. The
lunar atmosphere, therefore, must be of a greater degree of rarity than
can be produced by our best air-pumps; consequently no terrestrial
animal could exist in it.
Many philosophers of the highest authority concur in the belief that
light consists in the undulations of a highly elastic ethereal medium
pervading space, which, communicated to the optic nerves produce the
phenomena of vision. The experiments of our illustrious countryman, Dr.
Thomas Young, and those of the celebrated Fresnel, show that this theory
accords better with all the observed phenomena than that of the emission
of particles from the luminous body. As sound is propagated by the
undulations of the air, its theory is in a great many respects similar
to that of light. The grave or low tones are produced by very slow
vibrations, which increase in frequency progressively as the note
becomes more acute. When the vibrations of a musical chord, for example,
are less than sixteen in a second, it will not communicate a continued
sound to the ear; the vibrations or pulses increase in number with the
acuteness of the note, till at last all sense of pitch is lost. The
whole extent of human hearing, from the lowest notes of the organ to the
highest known cry of insects, as of the cricket, includes about nine
octaves.
The undulations of light are much more rapid than those of sound, but
they are analogous in this respect, that as the frequency of the
pulsations in sound increases from the low tones to the higher, so those
of light augment in frequency, from the red rays of the solar spectrum
to the extreme violet. By the experiments of Sir William Herschel, it
appears that the heat communicated by the spectrum increases from the
violet to the red rays; but that the maximum of the hot invisible rays
is beyond the extreme red. Heat in all probability consists, like light
and sound, in the undulations of an elastic medium. All the principal
phenomena of heat may actually be illustrated by a comparison with those
of sound. The excitation of heat and sound are not only similar, but
often identical, as in friction and percussion; they are both
communicated by contact and by radiation; and Dr. Young observes, that
the effect of radiant heat in raising the temperature of a body upon
which it falls, resembles the sympathetic agitation of a string, when
the sound of another string, which is in unison with it, is transmitted
to it through the air. Light, heat, sound, and the waves of fluids are
all subject to the same laws of reflection, and, indeed, their
undulating theories are perfectly similar. If, therefore, we may judge
from analogy, the undulations of the heat producing rays must be less
frequent than those of the extreme red of the solar spectrum; but if the
analogy were perfect, the interference of two hot rays ought to produce
cold, since darkness results from the interference of two undulations of
light, silence ensues from the interference of two undulations of sound;
and still water, or no tide, is the consequence of the interference of
two tides.
The propagation of sound requires a much denser medium than that of
either light or heat; its intensity diminishes as the rarity of the air
increases; so that, at a very small height above the surface of the
earth, the noise of the tempest ceases, and the thunder is heard no more
in those boundless regions where the heavenly bodies accomplish their
periods in eternal and sublime silence.
What the body of the sun may be, it is impossible to conjecture; but he
seems to be surrounded by an ocean of flame through which his dark
nucleus appears like black spots, often of enormous size. The solar
rays, which probably arise from the chemical processes that continually
take place at his surface, are transmitted through space in all
directions; but, notwithstanding the sun's magnitude, and the
inconceivable heat that must exist where such combustion is going on, as
the intensity both of his light and heat diminishes with the square of
the distance, his kindly influence can hardly be felt at the boundaries
of our system. Much depends on the manner in which the rays fall, as we
readily perceive from the different climates on our globe. In winter the
earth is nearer the sun by 1/30th than in summer, but the rays strike
the northern hemisphere more obliquely in winter than in the other half
of the year. In Uranus the sun must be seen like a small but brilliant
star, not above the hundred and fiftieth part so bright as he appears
to us; that is however 2000 times brighter than our moon to us, so
that he really is a sun to Uranus, and probably imparts some degree
of warmth. But if we consider that water would not remain fluid in any
part of Mars, even at his equator, and that in the temperate zones of
the same planet even alcohol and quicksilver would freeze, we may form
some idea of the cold that must reign in Uranus, unless indeed the
ether has a temperature. The climate of Venus more nearly resembles
that of the earth, though, excepting perhaps at her poles, much too
hot for animal and vegetable life as they exist here; but in Mercury
the mean heat, arising only from the intensity of the sun's rays,
must be above that of boiling quick-silver, and water would boil even
at his poles. Thus the planets, though kindred with the earth in
motion and structure, are totally unfit for the habitation of such
a being as man.
The direct light of the sun has been estimated to be equal to that of
5563 wax candles of a moderate size, supposed to be placed at the
distance of one foot from the object: that of the moon is probably only
equal to the light of one candle at the distance of twelve feet;
consequently the light of the sun is more than three hundred thousand
times greater than that of the moon; for which reason the light of the
moon imparts no heat, even when brought to a focus by a mirror.
In adverting to the peculiarities in the form and nature of the earth
and planets, it is impossible to pass in silence the magnetism of the
earth, the director of the mariner's compass, and his guide through the
ocean. This property probably arises from metallic iron in the interior
of the earth, or from the circulation of currents of electricity round
it: its influence extends over every part of its surface, but its
accumulation and deficiency determine the two poles of this great
magnet, which are by no means the same as the poles of the earth's
rotation. In consequence of their attraction and repulsion, a needle
freely suspended, whether it be magnetic or not, only remains in
equilibrio when in the magnetic meridian, that is, in the plane which
passes through the north and south magnetic poles. There are places
where the magnetic meridian coincides with the terrestrial meridian; in
these a magnetic needle freely suspended, points to the true north, but
if it be carried successively to different places on the earth's
surface, its direction will deviate sometimes to the east and sometimes
to the west of north. Lines drawn on the globe through all the places
where the needle points due north and south, are called lines of no
variation, and are extremely complicated. The direction of the needle is
not even constant in the same place, but changes in a few years,
according to a law not yet determined. In 1657, the line of no variation
passed through London. In the year 1819, Captain Parry, in his voyage to
discover the north-west passage round America, sailed directly over the
magnetic pole; and in 1824, Captain Lyon, when on en expedition for the
same purpose, found that the variation of the compass was 37° 30'
west, and that the magnetic pole was then situate in 63° 26' 51"
north latitude, and in 80° 51' 25" west longitude. It appears
however from later researches that the law of terrestrial magnetism is
of considerable complication, and the existence of more than one
magnetic pole in either hemisphere has been rendered highly probable.
The needle is also subject to diurnal variations; in our latitudes it
moves slowly westward from about three in the morning till two, and
returns to its former position in the evening.
A needle suspended so as only to be moveable in the vertical plane, dips
or become more and more inclined to the horizon the nearer it is brought
to the magnetic pole. Captain Lyon found that the dip in the latitude
and longitude mentioned was 86° 32'. What properties the planets may
have in this respect, it is impossible to know, but it is probable that
the moon has become highly magnetic, in consequence of her proximity to
the earth, and because her greatest diameter always points towards it.
The passage of comets has never sensibly disturbed the stability of the
solar system; their nucleus is rare, and their transit so rapid, that
the time has not been long enough to admit of a sufficient accumulation
of impetus to produce a perceptible effect. The comet of 1770 passed
within 80000 miles of the earth without even affecting our tides, and
swept through the midst of Jupiter's satellites without deranging the
motions of those little moons. Had the mass of that comet been equal to
the mass of the earth, its disturbing action would have shortened the
year by the ninth of a day; but, as Delambre's computations from the
Greenwich observations of the sun, show that the length of the year has
not been sensibly affected by the approach of the comet. La Place proved
that its mass could not be so much as the 5000th part of that of the
earth. The paths of comets have every possible inclination to the plane
of the ecliptic, and unlike the planets, their motion is frequently
retrograde. Comets are only visible when near their perihelia. Then
their velocity is such that its square is twice as great as that of a
body moving in a circle at the same distance; they consequently remain a
very short time within the planetary orbits; and as all the conic
sections of the same focal distance sensibly coincide through a small
arc on each side of the extremity of their axis, it is difficult to
ascertain in which of these curves the comets move, from observations
made, as they necessarily must be, at their perihelia: but probably they
all move in extremely eccentric ellipses, although, in most cases, the
parabolic curve coincides most nearly with their observed motions. Even
if the orbit be determined with all the accuracy that the case admits
of, it may be difficult, or even impossible, to recognise a comet on its
return, because its orbit would be very much changed if it passed near
any of the large planets of this or of any other system, in consequence
of their disturbing energy, which would be very great on bodies of so
rare a nature. Halley and Clairaut predicted that, in consequence of the
attraction of Jupiter and Saturn, the return of the comet of 1759 would
be retarded 618 days, which was verified by the event as nearly as could
be expected.
The nebulous appearance of comets is perhaps occasioned by the vapours
which the solar heat raises at their surfaces in their passage at the
perihelia, and which are again condensed as they recede from the sun.
The comet of 1680 when in its perihelion was only at the distance of
one-sixth of the sun's diameter, or about 148000 miles from its surface;
it consequently would be exposed to a heat 27500 times greater than that
received by the earth. As the sun's heat is supposed to be in proportion
to the intensity of his height, it is probable that a degree of heat so
very intense would be sufficient to convert into vapour every
terrestrial substance with which we are acquainted.
In those positions of comets where only half of their enlightened
hemisphere ought to be seen, they exhibit no phases even when viewed
with high magnifying powers. Some slight indications however were once
observed by Hevelius and La Hire in 1682; and in 1811 Sir William
Herschel discovered a small luminous point, which he concluded to be the
disc of the comet. In general their masses are so minute, that they have
no sensible diameters, the nucleus being principally formed of denser
strata of the nebulous matter, but so rare that stars have been seen
through them. The transit of a comet over the sun's disc would afford
the best information on this point. It was computed that such an event
was to take place in the year 1627; unfortunately the sun was hid by
clouds in this country, but it was observed at Viviers and at Marseilles
at the time the comet must have been on it, but no spot was seen. The
tails are often of very great length, and are generally situate in the
planes of their orbits; they follow them in their descent towards the
sun, but precede them in their return, with a small degree of curvature;
but their extent and form must vary in appearance, according to the
position of their orbits with regard to the ecliptic. The tail of the
comet of 1680 appeared, at Paris, to extend over sixty-two degrees. The
matter of which the tail is composed must be extremely buoyant to
precede a body moving with such velocity; indeed the rapidity of its
ascent cannot be accounted for. The nebulous part of comets diminishes
every time they return to their perihelia; after frequent returns they
ought to lose it altogether, and present the appearance of a fixed
nucleus; this ought to happen sooner in comets of short periods. La
Place supposes that the comet of 1682 must be approaching rapidly to
that state. Should the substances be altogether or even to a great
degree evaporated, the comet wilt disappear for ever. Possibly comets
may have vanished from our view sooner than they otherwise would have
done from this cause. Of about six hundred comets that have been seen at
different times, three are now perfectly ascertained to form part of our
system; that is to say, they return to the sun at intervals of 76, 6 1/3,
and 3 1/4 years nearly.
A hundred and forty comets have appeared within the earth's orbit during
the last century that have not again been seen; if a thousand years be
allowed as the average period of each, it may be computed by the theory
of probabilities, that the whole number that range within the earth's
orbit must be 1400; but Uranus being twenty times more distant, there
may be no less than 11,200,000 comets that come within the known extent
of our system. In such a multitude of wandering bodies it is just
possible that one of them may come in collision with the earth; but even
if it should, the mischief would be local, and the equilibrium soon
restored. It is however more probable that the earth would only be
deflected a little from its course by the near approach of the comet,
without being touched. Great as the number of comets appears to be, it
is absolutely nothing when compared to the number of the fixed stars.
About two thousand only are visible to the naked eye, but when we view
the heavens with a telescope, their number seems to be limited only by
the imperfection of the instrument. In one quarter of an hour Sir
William Herschel estimated that 116000 stars passed through the field of
his telescope, which subtended an angle of 15'. This however was
stated as a specimen of extraordinary crowding; but at an average the
whole expanse of the heavens must exhibit about a hundred millions of
fixed stars that come within the reach of telescopic vision.
Many of the stars have a very small progressive motion, especially _μ_
Cassiopeia and 61 Cygni, both small stars; and, as the sun is decidedly
a star, it is an additional reason for supposing the solar system to be
in motion. The distance of the fixed stars is too great to admit of
their exhibiting a sensible disc; but in all probability they are
spherical, and must certainly be so, if gravitation pervades all space.
With a fine telescope they appear like a point of light; their twinkling
arises from sudden changes in the refractive power of the air, which
would not be sensible if they had discs like the planets. Thus we can
learn nothing of the relative distances of the stars from us and from
one another, by their apparent diameters; but their annual parallax
being insensible, shows that we must be one hundred millions of millions
of miles from the nearest; many of them however must be vastly more
remote, for of two stars that appear close together, one may be far
beyond the other in the depth of space. The light of Sirius, according
to the observations of Mr. Herschel, is 324 times greater than that of a
star of the sixth magnitude; if we suppose the two to be really of the
same size, their distances from us must be in the ratio of 57.3 to 1,
because light diminishes as the square of the distance of the luminous
body increases.
Of the absolute magnitude of the stars, nothing is known, only that many
of them must be much larger than the sun, from the quantity of light
emitted by them. Dr. Wollaston determined the approximate ratio that the
light of a wax candle bears to that of the sun, moon, and stars, by
comparing their respective images reflected from small glass globes
filled, with mercury, whence a comparison was established between the
quantities of light emitted by the celestial bodies themselves. By this
method he found that the light of the sun is about twenty millions of
millions of times greater than that of Sirius, the brightest, and
supposed to be the nearest of the fixed stars. If Sirius had a parallax
of half a second, its distance from the earth would be 525481 times the
distance of the sun from the earth; and therefore Sirius, placed where
the sun is, would appear to us to be 3.7 times as large as the sun, and
would give 13.8 times more light; but many of the fixed stars must be
immensely greater than Sirius. Sometimes stars have all at once
appeared, shone with a brilliant light, and then vanished. In 1572 a
star was discovered in Cassiopeia, which rapidly increased in brightness
till it even surpassed that of Jupiter; it then gradually diminished in
splendour, and after exhibiting all the variety of tints that indicates
the changes of combustion, vanished sixteen months after its discovery,
without altering its position. It is impossible to imagine any thing
more tremendous than a conflagration that could be visible at such a
distance. Some stars are periodic, possibly from the intervention of
opaque bodies revolving about them, or from extensive spots on their
surfaces. Many thousands of stars that seem to be only brilliant points,
when carefully examined are found to be in reality systems of two or
more suns revolving about a common centre. These double and multiple
stars are extremely remote, requiring the most powerful telescopes to
show them separately.
The first catalogue of double stars in which their places and relative
positions are determined, was accomplished by the talents and industry
of Sir William Herschel, to whom astronomy is indebted for so many
brilliant discoveries, and with whom originated the idea of their
combination in binary and multiple systems, an idea which his own
observations had completely established, but which has since received
additional confirmation from those of his son and Sir James South, the
former of whom, as well as Professor Struve of Dorpat, have added many
thousands to their numbers. The motions of revolution round a common
centre of many have been clearly established, and their periods
determined with considerable accuracy. Some have already since their
first discovery accomplished nearly a whole revolution, and one, if the
latest observations can be depended on, is actually considerably
advanced in its second period. These interesting systems thus present a
species of sidereal chronometer, by which the chronology of the heavens
will be marked out to future ages by epochs of their own, liable to no
fluctuations from planetary disturbances such as obtain in our system.
Possibly among the multitudes of small stars, whether double or
insulated, some may be found near enough to exhibit distinct parallactic
motions, or perhaps something approaching to planetary motion, which may
prove that solar attraction is not confined to our system, or may lead
to the discovery of the proper motion of the sun. The double stars are
of various hues, but most frequently exhibit the contrasted colours. The
large star is generally yellow, orange, or red; and the small star blue,
purple, or green. Sometimes a white star is combined with a blue or
purple, and more rarely a red and white are united. In many cases, these
appearances are due to the influences of contrast on our judgment of
colours. For example, in observing a double star where the large one is
of a full ruby red, or almost blood colour, and the small one a fine
green, the latter lost its colour when the former was hid by the cross
wires of the telescope. But there are a vast number of instances where
the colours are too strongly marked to be merely imaginary. Mr. Herschel
observes in one of his papers in the _Philosophical Transactions_, as a
very remarkable fact, that although red single stars are common enough,
no example of an insulated blue, green, or purple one has as yet been
produced.
In some parts of the heavens, the stars are so near together as to form
clusters, which to the unassisted eye appear like thin white clouds;
such is the milky way, which has its brightness from the diffused light
of myriads of stars. Many of these clouds, however, are never resolved
into separate stars, even by the highest magnifying powers. This
nebulous matter exists in vast abundance in space. No fewer than 2500
nebulæ were observed by Sir William Herschel, whose places have been
computed from his observations, reduced to a common epoch, and arranged
into a catalogue in order of right ascension by his sister Miss Caroline
Herschel, a lady so justly celebrated for astronomical knowledge and
discovery. The nature and use of this matter scattered over the heavens
in such a variety of forms is involved in the greatest obscurity. That
it is a self-luminous, phosphorescent material substance, in a highly
dilated or gaseous state, but gradually subsiding by the mutual
gravitation of its particles into stars and sidereal systems, is the
hypothesis which seems to be most generally received; but the only way
that any real knowledge on this mysterious subject can be obtained, is
by the determination of the form, place, and present state of each
individual nebula, and a comparison of these with future observations
will show generations to come the changes that may now be going on in
these rudiments of future systems. With this view, Mr. Herschel is now
engaged in the difficult and laborious investigation, which is
understood to be nearly approaching its completion, and the results of
which we may therefore hope ere long to see made public. The most
conspicuous of these appearances are found in Orion, and in the girdle
of Andromeda. It is probable that light must be millions of years
travelling to the earth from some of the nebulæ.