groups or constellations, which were named by the earliest astronomers,
and compose a list of the most ridiculous imaginary figures, as useless
to the casual observer of the stars as to the astronomer, and have
not the most distant resemblance to the figures after which they are
named, as for instance the "Great Bear" (Ursa Major) fig. 24. The
grouping of stars into constellations serves, however, to find any
one required (provided the groups on the chart can be identified with
those in the heavens), as for instance the three conspicuous stars
forming "Orion's belt" (fig. 25), from which a line produced eastward
will point to Sirius, the brightest star in the heavens, and another
line produced westward will serve to point out "Aldebaran," also a
star of the first magnitude, &c.; also a line drawn from _a_ to _b_ in
the Great Bear will nearly point to the pole-star--the star situated
nearly (although not exactly) at the pole, or that part of the heavens
which would be indicated by a line drawn through the earth at its axis
of rotation. Amongst the stars scattered over the vault of heaven
there are many which to the naked eye appear single, but which when
seen through the telescope prove to be two stars closely approaching
each other. These "double stars" are of two kinds, "optical" and
"physical;" optical double stars are those which appear to be near
each other merely from the accident of one being placed behind the
other, nearly in a straight line, although in reality at an immense
distance and in no way connected. Physical double stars (usually called
"binary systems") consist of two suns comparatively near each other and
revolving about their common centre of gravity. Sir William Herschell
first discovered their physical connection, and thereby proved that the
great law of gravitation was not confined to our system, but was the
ruling power which controlled and regulated other systems. Astronomers
have calculated the orbits of only fourteen such "binary systems" at
present with any degree of certainty; about a thousand double stars are
known, but only about one hundred "binary systems." In some "binary
systems" the two suns are of different colours; white and purple, red
and green, or yellow and blue. If any planets revolve about such suns,
when situated between the two, what strange phenomena must occur!
Imagine a day during which a red sun had tinted everything crimson,
being succeeded at sunset (not, as with us, by darkness) by the rising
of a green sun, changing the colour of every object from red to green,
and how curious and beautiful must be the shades of light and colour
during the transition from one day to the other. Can anything be
conceived more gorgeous?--no shadow, but every object tinted with a
combination of colours of the most brilliant hues! Triple, quadruple,
and multiple stars are known in abundance, but all probably optically
so, and not in any way connected with each other by attraction.
Others of the stars are called "variable," they are those whose light
gradually diminishes and after a time regains its former brightness;
some of them not only vary in brilliancy but in colour; the cause
of these phenomena is totally unknown, and fortunate will he be who
discovers it; it will be a great stride in the science of astronomy,
for at present everything proposed has been quite inadequate to account
for such phenomena. About 60 of these variable stars are at present
known, but others are constantly being added to the list. Hind has
discovered twenty-one, Pogson seven, and other astronomers have taken
their share in discerning them.
But, of all the heavenly bodies, none have excited so much wonder, and
in former times so much fear, as the Comets. The orbits of these are
for the most part in very eccentric ellipses, some of them comprised
within the limits of our solar system, while those of others extend
millions of miles beyond it, so that they only reappear to us after
hundreds of years. The matter of which these bodies is composed is
certainly not solid, as the smallest telescopic stars may be seen
through the very middle of it, although many thousand miles in
thickness. When comets approach the sun, their substance appears to
become more condensed, and therefore to possess a greater power of
reflecting light. The cause of the "tails" which appear appended to
comets when they approach the sun is not known, but this tail extends
always in a direction from the sun, and passes over many degrees of
the heavens' space. As comets obey strictly the law of gravitation,
they must possess some amount of weight or mass, however small it may
be, and that it is inconceivably small is proved by the fact of comets
having passed close to Jupiter, right amongst his four moons, without
in any way affecting or disturbing their well-known motions; the
comets have, however, been terribly tossed about on such occasions, one
having been totally lost to us by such an occurrence.
Thus it is seen how the same laws of gravity exist throughout all the
infinity of space. The comets traverse our system of suns, and passing
onwards with incredible rapidity, for years upon years, through that
great space beyond it, at last visit some other system--perhaps to be
as much an object of regard and wonderment there as they were here. The
same ray of light which emanated from one of the distant stars of some
one of the clusters situated far out in space, and which has travelled
for thousands of years at the rate of 200,000 miles for every second
of time, serves us here on this globe to distinguish it--it enters the
tube of our telescope and affects our optic nerve; and the Creator who
caused it _there_ to be given off, has _here_ made our senses capable
of perceiving it. Is not this a clear demonstration of the fact, that
one hand has designed the whole; and one Creator provided for all?
THE AIR.
[Illustration: FIG. 2. STORM, WITH HEAVY RAIN.]
[Illustration: FIG. 1.]
The earth is surrounded in all directions by a covering of air about
forty miles in thickness, which bears about the proportion to the
earth itself that a coating one inch thick would to a ball seventeen
feet through, being about 1/200th of its diameter. This air, though
invisible, may at all times be felt while in motion (such is the
wind), and by rapidly passing the hand backwards and forwards it may
be perceived to press against and slightly obstruct it; this air is
composed of about twenty parts oxygen and eighty parts nitrogen,
together with about one part in every thousand of carbonic acid
(consisting of carbon and oxygen in union) and a small proportion of
vapour of carbonate of ammonia (ammonia in union with carbonic acid).
It is the oxygen which enables animals to live in the air, as it is
taken into their systems at every breath, it is thus constantly being
consumed and would ultimately be so far diminished, that they could
live no longer for want of it; but Providence has so arranged, that the
necessary quantity is always being supplied by vegetables (which give
out oxygen) to meet the deficiency. The air, although a gas, is yet
capable of being weighed, of great compressibility, and of expansion
to an unlimited extent, this causes the lower part (near the surface
of the earth) to be much more dense than the upper regions, and those
who have ascended to the tops of high mountains, have described the
difficulty of respiration as being very great, owing to the rarefaction
or lightness of the air. At the surface of the earth it presses with
a weight of about fifteen pounds upon every square inch, but as this
pressure is equal in every direction, it is not felt by us, nor does
it crush the most fragile flower or insect; but, remove the pressure
from one side of anything, and it will be found to press with violence
upon the other. If the air were removed from the inside of a drum, the
weight of the surrounding air would burst in the parchment and fill it.
By way of experiment, fill a large basin with water, take a tumbler
in the left hand and a piece of lighted paper in the right, hold the
lighted paper for a moment under the inverted tumbler and immediately
apply its open mouth to the surface of the water, letting it dip in
about half an inch, the heat expands and consequently gets rid of some
of the air in the tumbler, and as this cools again it resumes its
original bulk; the pressure of the air on the surface of the water in
the basin will force it up into the tumbler and nearly fill it (fig.
1). This is the principle of the barometer, which is a tube entirely
exhausted of air and the weight of the atmosphere forces a column
of mercury (which is many times heavier than water) up to the height
of about twenty-nine inches, according to the weight of the air at
any particular time; this weight varies according to the state of the
weather, whether wet or dry; but the average weight of air is found
to be perfectly stationary, and during the twenty years from 1816 to
1836, it was found at Paris not to have varied 1/1200th of an inch.
Winds or currents of air proceed from several causes; when the sun
shines on a large surface of the earth, it becomes heated and a column
of hot air is pressed upwards, for hot air is lighter than cold, and
the cold air all around, by its weight, forces it up and rushes in to
fill the space; this space may perhaps be many hundred square miles
in extent, so that a current of wind is caused to blow towards this
spot from all the regions round. This is the general state of things
at the equatorial parts of the earth and causes the "trade winds,"
which uniformly blow (the greater part of the year) from both north
and south towards the equator. Another cause of local winds, is the
condensation of vapours into water; when the air over a large region
is saturated with moisture it is greatly expanded by it, and when the
vapour is condensed and falls as rain, the air from the surrounding
parts forces itself in to fill the space occupied by the rain while in
a state of vapour, which is nearly one hundred thousand times greater
than it occupies when in the form of water. There are many causes to
determine the condensation of the vapour, which the atmosphere always
holds in larger or smaller quantities, but cold is the chief agent, for
when a current of air passes over seas or rivers, or the damp surface
of the earth, it becomes loaded with moisture, and being capable of
holding only a certain quantity (less when cold than hot), it follows
that warm air when saturated with moisture must let some of it fall
when it becomes cooled. This may occur from entering a colder region,
or uniting with another current of air colder than itself. In tropical
regions the air is so warm that it takes up a very large quantity
of watery vapour and upon the coming of colder weather, the rainy
season begins and the air empties itself of its superfluous moisture,
causing the most tremendous falls of rain in consequence (fig. 2).
When the clouds are overcharged with moisture in cold climates or in
cold weather, the vapour freezes as it condenses and forms snow, which
under the microscope presents a series of the most beautiful star-like
crystals (fig. 3); the same result is produced by the freezing of the
vapour or fog near to the earth's surface, this is called "hoar-frost,"
which is (like dew) deposited on those parts of the surface most
cooled by radiation. Hail is caused in all probability by the drops
of rain passing through a cold stratum of air and becoming frozen as
they fall. There are many wonderful accounts on record of great masses
of ice falling from the clouds, some of them several feet thick; the
cause of these phenomena (if they ever did occur) is not known. The
curious occurrence of red or green snow, which has sometimes been
known to fall, is owing to the snow being mixed with myriads of minute
cellular plants called the _Protococcus pluvialis_, which in one state
of its existence is green and in another red, and they colour the
snow accordingly. The cause of their sudden accumulation in such vast
quantities is not known, but may be sought for in some peculiar state
of the atmosphere favourable to their growth, which in many places is
prodigiously rapid.
[Illustration: FIG. 3.]
The temperature of the air differs very greatly in different situations
at the surface of the earth, the extreme difference being about 180
degrees; thus in winter, at the poles, the thermometer frequently
stands at 60 degrees below zero, and at the equator, in summer, 120
degrees above, but, in the higher regions of the air, it is even colder
than at the poles, for the sun shining on the earth heats it, and the
air is heated by contact with it; this is its only source of heat, of
which it receives less and less the further it is from the earth, so
that at a distance (depending upon the situation and climate) ranging
from 3,500 to 17,000 feet from the surface, the air is so cold that it
is called "the region of perpetual snow," and all mountains whose tops
reach above this altitude are covered with snow (formed by the freezing
of the rain and vapour) from this point to their summits (fig. 4), and
the sun (although it nearly always shines there, these regions being
above most of the clouds) is not able to melt it, for the radiation of
heat from its surface is greater than its absorption from the sun's
rays, white substances absorbing but very slowly, and rough surfaces
(as the snow) radiating very rapidly.
[Illustration: FIG. 4.]
Dew is caused by condensation of the watery vapour held in the air;
when the sun goes down the radiation continues from the surface of the
earth, and those surfaces which radiate most rapidly, such as fields
of grass, get cooled down below the temperature of the surrounding
air, and therefore cause a condensation of its watery vapour. This
may be imitated by filling a large glass goblet with very cold water,
and bringing it into a very warm room, the outside of the glass vessel
will become covered with dew, although previously quite dry; the same
may be shown by filling a similar glass with water at the ordinary
temperature, taking care that the outside is dry, then stirring in an
ounce or so of nitre or Rochelle salt, which will cause the water to
become cold, and the outside of the glass to be covered, as before,
with dew. Dew falls fastest when there are no clouds in the sky, as
radiation then takes place from the earth into space, and is not
compensated for by a return of radiation from the clouds to the earth.
[Illustration: FIG. 5.]
[Illustration: FIG. 6.]
Clouds are formed by the partial condensation of vapour, and are borne
along by the wind, instead of falling as rain; the reason of their
not falling is this, when the air below the clouds is saturated with
moisture it will absorb no more and the rain falls, but when it is
warm and dry, and passed in a constantly renewed current, then the
vapour is absorbed before it reaches the earth, and is carried off by
the wind. Clouds, therefore, although they often appear stationary,
are constantly altering their form and size, portions being absorbed
while others are being formed. Fogs are the same as clouds, the vapour
arising from wet grass, rivers, &c., being condensed as it ascends,
by a current of cold air passing over them. The fogs in London have a
brown colour, from admixture of smoke, dust, &c., with which the air
is contaminated (fig. 5). Clouds are of various forms and sizes, and
indeed of almost every variety, but certain kinds which are frequently
seen, have received different names. The "cirrhus" comprises all the
feathery white clouds which float high up in the air in fine weather;
the "cumulus" consists of the large mountain-like clouds which are
generally seen in summer; the "stratus," those horizontal layers
of clouds low down in the horizon, so often seen at sunset; and the
"nimbus" is the rain-cloud, of a dark grey or leaden hue, with sharp
well-marked edges (fig. 6).
[Illustration: FIG. 7.]
Clouds are amongst the most beautiful as well as useful things in
nature, and it is one of the greatest proofs of the active benevolence
of God, that all those things which serve man the most, are the most
beautiful to contemplate. Without clouds there would be no rain, and
without this no vegetation. In many parts of the tropical regions there
is little or no rain, and in such parts desert places abound. Clouds
are often of different states of electricity, and when they come near
enough to each other, a transfer of the fluid takes place, accompanied
with a flash of lightning (fig. 7) and a report, although this is not
always heard at the same time that the lightning is seen, as sound does
not travel nearly so fast as light; there is no danger from this kind
of lightning. But it sometimes happens that a cloud in an opposite
state of electricity to the surface of the earth is near enough
to produce a flash of lightning between them, in this case it is
extremely dangerous to be near; but when the thunder is not heard till
some time after the flash is seen, there can be no danger, as it is
then far away. Sound travels at about eleven hundred feet per second,
therefore (in round numbers) it may be known that the lightning is one
mile distant for every five seconds that elapse between the flash and
the thunder. The notion which prevails that iron and steel attract
lightning is entirely erroneous, they do not even conduct it so well as
copper and many other metals.
[Illustration: FIG. 8.]
Lightning conductors (fig. 8) are wires of iron or copper, made to
project above the highest parts of buildings and carried down to the
earth; they have the power of conducting the lightning down without
injury to the building, for electricity (which lightning is) travels
through metallic wires for any distance without disturbance or noise
(as in the electric telegraph), but bad conductors are apt to be burst
and rent by it if it be strong enough, creating flashes of light and
reports. Lightning strikes most readily any projecting substance or
point, the highest point is therefore selected for the protrusion of
the upper part of conductors. In a plain or open space, trees and
animals are often struck, as they form conductors (although very bad
ones) to the lightning; being bad conductors, instead of allowing it to
pass quietly to the earth as do the wires, they are generally destroyed
by it. It has been often stated that we should select an open space
in a thunder-storm, but this would render one liable to be struck,
especially if there were no object near of larger dimensions, the best
plan is to stand near to any trees or houses without touching them, the
danger would not then be so great, the trees or buildings being taller,
would be most liable to conduct the lightning to the earth. The danger
of touching or leaning against a tree in a thunder-storm, arises from
the person thus making himself (as it were) part of it and incurring
the same risk.
Thunder-storms, which would at first thought appear to be of no utility
to man, are indeed of great service; it is a common saying that thunder
clears the air, this is the result of feeling and experience, but
chemists have demonstrated the fact that a substance called "ozone" (a
peculiar state of the element oxygen) is produced by thunder-storms;
this ozone possesses the wonderful power of correcting (decomposing)
putrid and unwholesome gases and exhalations which might otherwise
produce fever, cholera, &c., it moreover destroys the ova or germs
of many animals and vegetables which might otherwise be injurious to
vegetation of more importance, the slight injury which these storms
inflict here and there should weigh as nothing in the balance of
utility with such universal good. Thus it is that the works of God
have all the stamps of goodness, and this ought to inspire us with
so much thankfulness as to overcome every fear for personal safety.
Were these grand phenomena of nature, (as materialists would make
us believe) the result of laws depending solely upon the physical
co-operation of mere matter, good would be the exception instead of
the rule, and most of them would produce effects, if not injurious at
least not beneficial, but such is not the case, and throughout all the
wonderful operations of nature there is not one but tends to good,
for God often inflicts a small injury that a great good may result;
we say a small injury, but it is questionable if the injury is not
often a benefit, which appears to us injurious only because we do not
understand it fully, every disturbing cause tends to produce some
apparent disorder, such as storms, hurricanes, &c., some indeed so
terrific as to destroy ships and houses, but what would be the result
if the atmosphere were never disturbed from any cause? Why the lower
stratum would become so loaded with impurities that it would be unfit
to breathe, miasms and noxious gases would for ever remain a curse
to the races of men and animals who might be doomed to inhabit such
regions, but the very regions where these miasms are most likely to
form are those about the tropics, and here it is that the greatest
storms occur to remove them. Even the great deserts, which appear so
useless to man, and which are uninhabitable to a great extent, have
their office, and an important one too; they are to the earth what
ventilators are to buildings, drawing the cold air from the poles to
cool the regions that are too hot, and sending a current of heated air
through the upper regions of the atmosphere (where it can do no injury
to anything) to warm the colder parts of the earth, another instance
of the wonderful care and goodness on the part of the Creator. The
various and beautiful colours of the clouds, particularly at the rising
and setting of the sun, are caused by refraction separating the white
light into its primitive constituents, blue, red, and yellow light and
their combinations, purple, orange, green, &c., and the more obliquely
the rays impinge upon the earth the greater will be this refraction,
this accounts for colours seldom appearing in the clouds at midday. But
of all the beautiful effects of the refraction of light, the Rainbow
(fig. 9) is the most glorious, it has been celebrated in all ages for
its transient beauty. It is only seen when rain is falling in front of
a brightly illuminated cloud, the sun being behind the spectator; it
is a reflection of the sun by the cloud transmitted through millions
of drops of rain, each of which acts as a prism, and produces rings
of colour; for each of the rays of light (red, blue, and yellow) are
refracted in unequal degrees, and therefore take separate places,
forming the rings of colour seen in the rainbow.
[Illustration: FIG. 9.]
The air is the great source from which all the nourishment of the
organic creation, whether vegetable or animal, is derived; its carbonic
acid is decomposed by the vegetables, which appropriate the carbon,
turning over the oxygen to their companions, the animals; the ammonia
of the air furnishes all the nitrogen of seeds and other nutritious
parts of vegetables, which are eaten by animals; and water, the chief
source of all nutriment, passes first from the air before it enters
the soil, bringing both carbonic acid and ammonia to fertilise it.
That all this nourishment is derived from the air is evidently shown
by the formation and increase of mould in forests which have grown for
centuries; this black mould is nearly all decayed vegetable matter,
formed by the continuous fall and decay of leaves and trees, but which,
instead of diminishing, increases. Now, where does all this come from?
certainly not out of the earth, for it does not contain the necessary
elements. Expose the surface of the bare earth for several centuries,
and first small plants, then larger ones, will grow upon it, until the
state of things described above takes place, the earth being just as
rich in organic matter now as at the beginning, and much richer on the
surface, where a thick stratum of black mould forms from the repeated
fall and decay of leaves and wood; all the substance which the forest
shall have drawn from the earth (with the exception of certain salts
and earthy matters) must therefore have been derived from the air,
which contains every ingredient necessary for its formation, while in
the earth itself no kind of organic matter is ever found.
There was formerly a time when the carbonic acid of the air was in much
greater abundance, and favoured the growth of those plants which thrive
where there is plenty of water, as in swamps and marshes. They grew and
decayed for a vast period of time, till a thick stratum of carbonaceous
matter was deposited, which, after being buried (by some convulsion of
nature) at a great depth, and pressed by the enormous weight of the
superincumbent earth into a hard solid substance, is now being dug up
by man, and forms that most valuable of all products of the mine, coal
(fig. 10).
[Illustration: FIG. 10.]