light, and scattering it in various directions, no objects would be
visible to us out of direct sunshine; every shadow of a passing cloud
would involve us in midnight darkness; the stars would be visible all
day; and every apartment into which the sun had not direct admission
would be involved in the obscurity of night. This scattering action of
the atmosphere on the solar light is greatly increased by the
irregularity of temperature caused by the sun, which throws the
atmosphere into a constant state of undulation; and by thus bringing
together masses of air of different temperatures, produces partial
reflections and refractions at their common boundaries, by which means
much light is turned aside from a direct course, and diverted to the
purposes of general illumination.[6] In the upper regions of the
atmosphere, as on the tops of very high mountains, where the air is too
much rarefied to reflect much light, the sky assumes a black appearance,
and stars become visible in the day time.
Although the atmosphere is usually so transparent, that it is invisible
to us, yet we as truly move and live in a fluid as fishes that swim in
the sea. Considered in comparison with the whole earth, the atmosphere
is to be regarded as a thin layer investing the surface, like a film of
water covering the surface of an orange. Its actual height, however, is
over a hundred miles, though we cannot assign its precise boundaries.
Being perfectly elastic, the lower portions, bearing as they do, the
weight of all the mass above them, are greatly compressed, while the
upper portions having little to oppose the natural tendency of air to
expand, diffuse themselves widely. The consequence is, that the
atmosphere undergoes a rapid diminution of density, as we ascend from
the earth, and soon becomes exceedingly rare. At so moderate a height as
seven miles, it is four times rarer than at the surface, and continues
to grow rare in the same proportion, namely, being four times less for
every seven miles of ascent. It is only, therefore, within a few miles
of the earth, that the atmosphere is sufficiently dense to sustain
clouds and vapors, which seldom rise so high as eight miles, and are
usually much nearer to the earth than this. So rare does the air become
on the top of Mount Chimborazo, in South America, that it is incompetent
to support most of the birds that fly near the level of the sea. The
condor, a bird which has remarkably long wings, and a light body, is the
only bird seen towering above this lofty summit. The transparency of the
atmosphere,--a quality so essential to fine views of the starry
heavens,--is much increased by containing a large proportion of water,
provided it is perfectly dissolved, or in a state of invisible vapor. A
country at once hot and humid, like some portions of the torrid zone,
presents a much brighter and more beautiful view of the moon and stars,
than is seen in cold climates. Before a copious rain, especially in hot
weather, when the atmosphere is unusually humid, we sometimes observe
the sky to be remarkably resplendent, even in our own latitude.
Accordingly, this unusual clearness of the sky, when the stars shine
with unwonted brilliancy, is regarded as a sign of approaching rain; and
when, after the rain is apparently over, the air is remarkably
transparent, and distant objects on the earth are seen with uncommon
distinctness, while the sky exhibits an unusually deep azure, we may
conclude that the serenity is only temporary, and that the rain will
probably soon return.
FOOTNOTE:
[6] Sir J. Herschel.
LETTER X.
THE SUN.
"Great source of day! best image here below
Of thy Creator, ever pouring wide,
From world to world, the vital ocean round,
On Nature write, with every beam, His praise."--_Thomson._
THE subjects which have occupied the preceding Letters are by no means
the most interesting parts of our science. They constitute, indeed,
little more than an introduction to our main subject, but comprise such
matters as are very necessary to be clearly understood, before one is
prepared to enter with profit and delight upon the more sublime and
interesting field which now opens before us.
We will begin our survey of the heavenly bodies with the SUN, which
first claims our homage, as the natural monarch of the skies. The moon
will next occupy our attention; then the other bodies which compose the
solar system, namely, the planets and comets; and, finally, we shall
leave behind this little province in the great empire of Nature, and
wing a bolder flight to the fixed stars.
The _distance_ of the sun from the earth is about ninety-five millions
of miles. It may perhaps seem incredible to you, that astronomers should
be able to determine this fact with any degree of certainty. Some,
indeed, not so well informed as yourself, have looked upon the
marvellous things that are told respecting the distances, magnitudes,
and velocities, of the heavenly bodies, as attempts of astronomers to
impose on the credulity of the world at large; but the certainty and
exactness with which the predictions of astronomers are fulfilled, as an
eclipse, for example, ought to inspire full confidence in their
statements. I can assure you, my dear friend, that the evidence on which
these statements are founded is perfectly satisfactory to those whose
attainments in the sciences qualify them to understand them; and, so far
are astronomers from wishing to impose on the unlearned, by announcing
such wonderful discoveries as they have made among the heavenly bodies,
no class of men have ever shown a stricter regard and zeal than they for
the exact truth, wherever it is attainable.
Ninety-five millions of miles is indeed a vast distance. No human mind
is adequate to comprehend it fully; but the nearest approaches we can
make towards it are gained by successive efforts of the mind to conceive
of great distances, beginning with such as are clearly within our grasp.
Let us, then, first take so small a distance as that of the breadth of
the Atlantic ocean, and follow, in mind, a ship, as she leaves the port
of New York, and, after twenty days' steady sail, reaches Liverpool.
Having formed the best idea we are able of this distance, we may then
reflect, that it would take a ship, moving constantly at the rate of ten
miles per hour, more than a thousand years to reach the sun.
The diameter of the sun is towards a million of miles; or, more exactly,
it is eight hundred and eighty-five thousand miles. One hundred and
twelve bodies as large as the earth, lying side by side, would be
required to reach across the solar disk; and our ship, sailing at the
same rate as before, would be ten years in passing over the same space.
Immense as is the sun, we can readily understand why it appears no
larger than it does, when we reflect, that its distance is still more
vast. Even large objects on the earth, when seen on a distant eminence,
or over a wide expanse of water, dwindle almost to a point. Could we
approach nearer and nearer to the sun, it would constantly expand its
volume, until finally it would fill the whole vault of heaven. We could,
however, approach but little nearer to the sun without being consumed by
the intensity of his heat. Whenever we come nearer to any fire, the heat
rapidly increases, being four times as great at half the distance, and
one hundred times as great at one tenth the distance. This fact is
expressed by saying, that the heat increases as the square of the
distance decreases. Our globe is situated at such a distance from the
sun, as exactly suits the animal and vegetable kingdoms. Were it either
much nearer or much more remote, they could not exist, constituted as
they are. The intensity of the solar light also follows the same law.
Consequently, were we nearer to the sun than we are, its blaze would be
insufferable; or, were we much further off, the light would be too dim
to serve all the purposes of vision.
The sun is one million four hundred thousand times as large as the
earth; but its matter is not more than about one fourth as dense as that
of the earth, being only a little heavier than water, while the average
density of the earth is more than five times that of water. Still, on
account of the immense magnitude of the sun, its entire quantity of
matter is three hundred and fifty thousand times as great as that of the
earth. Now, the force of gravity in a body is greater, in proportion as
its quantity of matter is greater. Consequently, we might suppose, that
the weight of a body (weight being nothing else than the measure of the
force of gravity) would be increased in the same proportion; or, that a
body, which weighs only one pound at the surface of the earth, would
weigh three hundred and fifty thousand pounds at the sun. But we must
consider, that the attraction exerted by any body is the same as though
all the matter were concentrated in the centre. Thus, the attraction
exerted by the earth and by the sun is the same as though the entire
matter of each body were in its centre. Hence, on account of the vast
dimensions of the sun, its surface is one hundred and twelve times
further from its centre than the surface of the earth is from its
centre; and, since the force of gravity diminishes as the square of the
distance increases, that of the sun, exerted on bodies at its surface,
is (so far as this cause operates) the square of one hundred and twelve,
or twelve thousand five hundred and forty-four times less than that of
the earth. If, therefore, we increase the weight of a body three hundred
and fifty-four thousand times, in consequence of the greater amount of
matter in the sun, and diminish it twelve thousand five hundred and
forty-four times, in consequence of the force acting at a greater
distance from the body, we shall find that the body would weigh about
twenty-eight times more on the sun than on the earth. Hence, a man
weighing three hundred pounds would, if conveyed to the surface of the
sun, weigh eight thousand four hundred pounds, or nearly three tons and
three quarters. A limb of our bodies, weighing forty pounds, would
require to lift it a force of one thousand one hundred and twenty
pounds, which would be beyond the ordinary power of the muscles. At the
surface of the earth, a body falls from rest by the force of gravity, in
one second, sixteen and one twelfth feet; but at the surface of the sun,
a body would, in the same time, fall through four hundred and
forty-eight and seven tenths feet.
The sun turns on his own axis once in a little more than twenty-five
days. This fact is known by observing certain dark places seen by the
telescope on the sun's disk, called _solar spots_. These are very
variable in size and number. Sometimes, the sun's disk, when viewed with
a telescope, is quite free from spots, while at other times we may see a
dozen or more distinct clusters, each containing a great number of
spots, some large and some very minute. Occasionally, a single spot is
so large as to be visible to the naked eye, especially when the sun is
near the horizon, and the glare of his light is taken off. One measured
by Dr. Herschel was no less than fifty thousand miles in diameter. A
solar spot usually consists of two parts, the _nucleus_ and the _umbra_.
The nucleus is black, of a very irregular shape, and is subject to great
and sudden changes, both in form and size. Spots have sometimes seemed
to burst asunder, and to project fragments in different directions. The
umbra is a wide margin, of lighter shade, and is often of greater extent
than the nucleus. The spots are usually confined to a zone extending
across the central regions of the sun, not exceeding sixty degrees in
breadth. Fig. 23 exhibits the appearance of the solar spots. As these
spots have all a common motion from day to day, across the sun's disk;
as they go off at one limb, and, after a certain interval, sometimes
come on again on the opposite limb, it is inferred that this apparent
motion is imparted to them by an actual revolution of the sun on his own
axis. We know that the spots must be in contact, or very nearly so, at
least, with the body of the sun, and cannot be bodies revolving around
it, which are projected on the solar disk when they are between us and
the sun; for, in that case, they would not be so long in view as out of
view, as will be evident from inspecting the following diagram. Let S,
Fig. 24, page 106, represent the sun, and _b_ a body revolving round it
in the orbit _a b c_; and let E represent the earth, where, of course,
the spectator is situated. The body would be seen projected on the sun
only while passing from _b_ to _c_, while, throughout the remainder of
its orbit, it would be out of view, whereas no such inequality exists in
respect to the two periods.
[Illustration Fig. 23.]
[Illustration Fig. 24.]
If you ask, what is the _cause_ of the solar spots, I can only tell you
what different astronomers have supposed respecting them. They attracted
the notice of Galileo soon after the invention of the telescope, and he
formed an hypothesis respecting their nature. Supposing the sun to
consist of a solid body embosomed in a sea of liquid fire, he believed
that the spots are composed of black cinders, formed in the interior of
the sun by volcanic action, which rise and float on the surface of the
fiery sea. The chief objections to this hypothesis are, first, the _vast
extent_ of some of the spots, covering, as they do, two thousand
millions of square miles, or more,--a space which it is incredible
should be filled by lava in so short a time as that in which the spots
are sometimes formed; and, secondly, the _sudden disappearance_ which
the spots sometimes undergo, a fact which can hardly be accounted for by
supposing, as Galileo did, that such a vast accumulation of matter all
at once sunk beneath the fiery flood. Moreover, we have many reasons for
believing that the spots are _depressions_ below the general surface.
La Lande, an eminent French astronomer of the last century, held that
the sun is a solid, opaque body, having its exterior diversified with
high mountains and deep valleys, and covered all over with a burning sea
of liquid matter. The spots he supposed to be produced by the flux and
reflux of this fiery sea, retreating occasionally from the mountains,
and exposing to view a portion of the dark body of the sun. But it is
inconsistent with the nature of fluids, that a liquid, like the sea
supposed, should depart so far from its equilibrium and remain so long
fixed, as to lay bare the immense spaces occupied by some of the solar
spots.
Dr. Herschel's views respecting the nature and constitution of the sun,
embracing an explanation of the solar spots, have, of late years, been
very generally received by the astronomical world. This great
astronomer, after attentively viewing the surface of the sun, for a long
time, with his large telescopes, came to the following conclusions
respecting the nature of this luminary. He supposes the sun to be a
planetary body like our earth, diversified with mountains and valleys,
to which, on account of the magnitude of the sun, he assigns a
prodigious extent, some of the mountains being six hundred miles high,
and the valleys proportionally deep. He employs in his explanation no
volcanic fires, but supposes two separate regions of dense clouds
floating in the solar atmosphere, at different distances from the sun.
The exterior stratum of clouds he considers as the depository of the
sun's light and heat, while the inferior stratum serves as an awning or
screen to the body of the sun itself, which thus becomes fitted to be
the residence of animals. The proofs offered in support of this
hypothesis are chiefly the following: first, that the appearances, as
presented to the telescope, are such as accord better with the idea that
the fluctuations arise from the motions of clouds, than that they are
owing to the agitations of a liquid, which could not depart far enough
from its general level to enable us to see its waves at so great a
distance, where a line forty miles in length would subtend an angle at
the eye of only the tenth part of a second; secondly, that, since clouds
are easily dispersed to any extent, the great dimensions which the solar
spots occasionally exhibit are more consistent with this than with any
other hypothesis; and, finally, that a lower stratum of clouds, similar
to those of our atmosphere, was frequently seen by the Doctor, far below
the luminous clouds which are the fountains of light and heat.
Such are the views of one who had, it must be acknowledged, great
powers of observation, and means of observation in higher perfection
than have ever been enjoyed by any other individual; but, with all
deference to such authority, I am compelled to think that the hypothesis
is encumbered with very serious objections. Clouds analogous to those of
our atmosphere (and the Doctor expressly asserts that his lower stratum
of clouds are analogous to ours, and reasons respecting the upper
stratum according to the same analogy) cannot exist in hot air; they are
tenants only of cold regions. How can they be supposed to exist in the
immediate vicinity of a fire so intense, that they are even dissipated
by it at the distance of ninety-five millions of miles? Much less can
they be supposed to be the depositories of such devouring fire, when any
thing in the form of clouds, floating in our atmosphere, is at once
scattered and dissolved by the accession of only a few degrees of heat.
Nothing, moreover, can be imagined more unfavorable for radiating heat
to such a distance, than the light, inconstant matter of which clouds
are composed, floating loosely in the solar atmosphere. There is a
logical difficulty in the case: it is ascribing to things properties
which they are not known to possess; nay, more, which they are known not
to possess. From variations of light and shade in objects seen at
moderate distances on the earth, we are often deceived in regard to
their appearances; and we must distrust the power of an astronomer to
decide upon the nature of matter seen at the distance of ninety-five
millions of miles.
I think, therefore, we must confess our ignorance of the nature and
constitution of the sun; nor can we, as astronomers, obtain much more
satisfactory knowledge respecting it than the common apprehension,
namely, that it is an immense globe of fire. We have not yet learned
what causes are in operation to maintain its undecaying fires; but our
confidence in the Divine wisdom and goodness leads us to believe, that
those causes are such as will preserve those fires from extinction, and
at a nearly uniform degree of intensity. Any material change in this
respect would jeopardize the safety of the animal and vegetable
kingdoms, which could not exist without the enlivening influence of the
solar heat, nor, indeed, were that heat any more or less intense than it
is at present.
If we inquire whether the surface of the sun is in a state of actual
combustion, like burning fuel, or merely in a state of intense ignition,
like a stone heated to redness in a furnace, we shall find it most
reasonable to conclude that it is in a state of ignition. If the body of
the sun were composed of combustible matter and were actually on fire,
the material of the sun would be continually wasting away, while the
products of combustion would fill all the vast surrounding regions, and
obscure the solar light. But solid bodies may attain a very intense
state of ignition, and glow with the most fervent heat, while none of
their material is consumed, and no clouds or fumes rise to obscure their
brightness, or to impede their further emission of heat. An ignited
surface, moreover, is far better adapted than flame to the radiation of
heat. Flame, when made to act in contact with the surfaces of solid
bodies, heats them rapidly and powerfully; but it sends forth, or
_radiates_, very little heat, compared with solid matter in a high state
of ignition. These various considerations render it highly probable to
my mind, that the body of the sun is not in a state of actual
combustion, but merely in a state of high ignition.
The solar beam consists of a mixture of several different sorts of rays.
First, there are the _calorific_ rays, which afford heat, and are
entirely distinct from those which afford light, and may be separated
from them. Secondly, there are the _colorific_ rays, which give light,
consisting of rays of seven distinct colors, namely, violet, indigo,
blue, green, yellow, orange, red. These, when separated, as they may be
by a glass prism, compose the _prismatic spectrum_. They appear also in
the rainbow. When united again, in due proportions, they constitute
white light, as seen in the light of the sun. Thirdly, there are found
in the solar beam a class of rays which afford neither heat nor light,
but which produce chemical changes in certain bodies exposed to their
influence, and hence are called _chemical_ rays. Fourthly, there is
still another class, called _magnetizing_ rays, because they are capable
of imparting magnetic properties to steel. These different sorts of rays
are sent forth from the sun, to the remotest regions of the planetary
worlds, invigorating all things by their life-giving influence, and
dispelling the darkness that naturally fills all space.
But it was not alone to give heat and light, that the sun was placed in
the firmament. By his power of attraction, also, he serves as the great
regulator of the planetary motions, bending them continually from the
straight line in which they tend to move, and compelling them to
circulate around him, each at nearly a uniform distance, and all in
perfect harmony. I will hereafter explain to you the manner in which the
gravity of the sun thus acts, to control the planetary motions. For the
present, let us content ourselves with reflecting upon the wonderful
force which the sun must put forth, in order to bend out of their
courses, into circular orbits, such a number of planets, some of which
are more than a thousand times as large as the earth. Were a ship of war
under full sail, and it should be required to turn her aside from her
course by a rope attached to her bow, we can easily imagine that it
would take a great force to do it, especially were it required that the
force should remain stationary and the ship be so constantly diverted
from her course, as to be made to go round the force as round a centre.
Somewhat similar to this is the action which the sun exerts on each of
the planets by some invisible influence, called gravitation. The bodies
which he thus turns out of their course, and bends into a circular orbit
around himself, are, however, many millions of times as ponderous as the
ship, and are moving many thousand times as swiftly.
LETTER XI.
ANNUAL REVOLUTION.--SEASONS
"These, as they change, Almighty Father, these
Are but the varied God. The rolling year
Is full of Thee."--_Thomson._
WE have seen that the apparent revolution of the heavenly bodies, from
east to west, every twenty-four hours, is owing to a real revolution of
the earth on its own axis, in the opposite direction. This motion is
very easily understood, resembling, as it does, the spinning of a top.
We must, however, conceive of the top as turning without any visible
support, and not as resting in the usual manner on a plane. The annual
motion of the earth around the sun, which gives rise to an apparent
motion of the sun around the earth once a year, and occasions the change
of seasons, is somewhat more difficult to understand; and it may cost
you some reflection, before you will settle all the points respecting
the changes of the seasons clearly in your mind. We sometimes see these
two motions exemplified in a top. When, as the string is pulled, the top
is thrown forwards on the floor, we may see it move forward (sometimes
in a circle) at the same time that it spins on its axis. Let a candle be
placed on a table, to represent the sun, and let these two motions be
imagined to be given to a top around it, and we shall have a case
somewhat resembling the actual motions of the earth around the sun.