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showers into the lower regions, and if not evaporated
in their fall, reach the surface of the earth.

The varying occurrences thus produced, tend to
multiply and extend their own variety. The ascend-
ing streams of vapour carry with them that latent heat
belonging to their gaseous state, which, when they are
condensed, they give out as sensible heat. They thus



LAWS or HEAT. AIR. 89

raise the temperature of the upper regions of air, and
occasion changes in the pressure and motion of its
currents. The clouds, again, by shading the surface of
the earth from the sun, diminish the evaporation by
which their own substance is supplied, and the heating
effects by which currents are caused. Even the mere
mechanical effects of the currents of fluid on the dis-
tribution of its own pressure, and the dynamical con-
ditions of its motion, are in a high degree abstruse in
their principles and complex in their results. It need
not be wondered, therefore, if the study of this subject
is very difficult and entangled, and our knowledge, after
all, very imperfect.

In the midst of all this apparent confusion, however,
we can see much that we can understand. And, among
other things, we may notice some of the consequences
of the difference of the laws of temperature followed by
steam and by air in going upwards. One important
result is that the atmosphere is much drier, near the
surface, than it would have been if the laws of density
and temperature had been the same for both gases.
If this had been so, the air would always have been
saturated with vapour. It would have contained as
much as the existing temperature could support, and
the slightest cooling of any object would have covered
it with a watery film like dew. As it is, the air contains
much less than its full quantity of vapour : we may
often cool an object 10, 20, or 30 degrees without
obtaining a deposition of water upon it, or reaching the
deio-point, as it is called. To have had such a dripping
state of the atmosphere as the former arrangement



90 TEEEESTRIAL ADAPTATIONS.

would have produced, would have been inconvenient,
and, so far as we can judge, unsuited to vegetables as
well as animals. No evaporation from the surface of
either could have taken place under such conditions.

The sizes and forms of clouds appear to depend on
the same circumstance, of the air not being saturated
with moisture. And it is seemingly much better that
clouds should be comparatively small and well defined,
as they are, than that they should fill vast depths of
the atmosphere with a thin mist, which would have
been the consequence of the imaginary condition of
things just mentioned.

Here then we have another remarkable exliibition of
two laws, in two nearly similar gaseous fluids, producing
effects alike in kind, but different in degree, and by the
'play of their difference giving rise to a new set of
results, peculiar in their nature and beneficial in their
tendency. The form of the laws of air and of steam
with regard to heat might, so far as we can see, have
been more similar, or more dissimilar, than it now is :
the rate of each law might have had a different amount
from its present one, so as quite to alter the relation
of the two. By the laws having such forms and
such rates as they have, effects are produced,
some of which we can distinctly perceive to be bene-
ficial. Perhaps most ]Dersons will feel a strong per-
suasion, that if we understood the operation of these
laws more distinctly, we should see still more clearly
the beneficial tendency of these effects, and should
probably discover others, at present concealed in the
apparent perplexity of the subject.



LAWS OF HEAT, AIR. 91

III. From what has been said, we may see, in a
general way, both the causes and the effects of winds.
They arise from any disturbance by temperature,
motion, pressure, &c., of the equilibrium of the atmo-
sphere, and are the efforts of nature to restore the
balance. Their office in the economy of nature is to
carry heat and moisture from one tract to another, and
they are the great agents in the distribution of tempe-
rature and the changes of weather. Other purposes
might easily be ascribed to them in the business of the
vegetable and animal kingdoms, and in the arts of
human life, of wliich we shall not here treat. That
character in which we now consider them, that of the
machinery of atmospheric changes, and thus, imme-
diately or remotely, the instruments of atmospheric
influences, cannot well be refused them by any
person.

IV. There is still one reflection which ought not to
be omitted. All the changes of the weather, even the
most violent tempests and torrents of rain, may be
considered as oscillations about the mean or average
condition belonging to each place. All these oscilla-
tions are limited and transient ; the storm spends its
fury, the inundation passes off, the sky clears, the
calmer course of nature succeeds. In the forces which
produce this derangement, there is a provision for
making it short and moderate. The oscillation stops
of itself, like the rolling of a ship, when no longer
impelled by the wind. Now, why should this be so ?
Why should the oscillations, produced by the conflict
of so many laws, seemingly quite unconnected with



92 TERRESTEIAL ADAPTATIONS.

each other, be of this converging and subsiding cha-
racter? Wonld it be so under all arrangements ?
Is it a matter of mechanical necessity that disturbance
must end in the restoration of the medium condition ?
By no means. There may be an utter subversion of
the equihbrium. The ship may roll too far, and may
capsise. The oscillations may go on, becoming larger
and larger, till all trace of the original condition is
lost ; till new forces of inequality and disturbance are
brought into play ; and disorder and irregularity may
succeed, without apparent limit or check in its own
nature, like the spread of a conflagration in a city.
This is a possibility in any combination of mechanical
forces ; why does it not happen in the one now before
us ? By what good fortune are the powers of heat, of
water, of steam, of air, the effects of the earth's annual
and diurnal motions, and x^robably other causes, so
adjusted, that through all their struggles the elemental
world goes on, upon the whole, so quietly and steadily ?
Why is the whole fabric of the weather never utterly
deranged, its balance lost irrecoverably ? Why is
there not an eternal conflict, such as the poets imagine
to take place in their chaos ?

" For Hot, Cold, Moist, and Dry, four champions fierce.
Strive here for mastery, and to battle bring
Their embryon atoms : —

to whom these most adhere
He rules a moment : Chaos umpire sits,
And by decision more embroils the fray." *

A state of things something like that which Milton

* Par. Lost, b. ii.



LAWS OF HEAT. AlE. 93

here seems to have imagined is, so far as we know, not
mechanically impossible. It might have continued to
obtain, if Hot and Cold, and Moist and Dry had not
been compelled to " run into their places." It will be
hereafter seen, that in the comparatively simple problem
of the solar system, a number of very peculiar adjust-
ments were requisite, in order that the system might
retain a permanent form, in order that its motions
might have their cycles, its perturbations their limits
and period. The problem of the combination of such
laws and materials as enter into the constitution of the
atmosphere, is one manifestly of much greater com-
plexity, and indeed to us probably of insurmountable
difficulty as a mechanical problem. But all that inves-
tigation and analogy teach us, tends to show that it will
resemble the other problem in the nature of its result ;
and that certain relations of its data, and of the laws
of its elements, are necessary requisites, for securing
the stability of its mean condition, and for giving a
small and periodical character to its deviations from
such a condition.

It would then be probable, from this reflection alone,
that in determining the quantity and the law and
intensity of the forces, of earth, water, air, and heat,
the same regard has been shown to the permanency
and stability of the terrestrial system, which may be
traced in the adjustment of the masses, distances,
positions, and motions of the bodies of the celestial
machine.

This permanency appears to be, of itself, a suitable
object of contrivance. The purpose for which the



94 TERRESTRIAL ADAPTATIONS.

world was made could be answered only by its being
preserved. But it has appeared, from the preceding
part of this and the former chapter, that this per-
manence is a permanence of a state of things adapted
by the most remarkable and multiplied combinations
to the well-being of man, of animals, of vegetables. I
The adjustments and conditions therefore, beyond the
reach of our investigation as they are, by which its ,
permanence is secured, must be conceived as fitted
to add, in each of the instances above adduced, to
the admiration which the several manifestations of
IntelKgent Beneficence are calculated to excite.



Chap. XI. — The Laws of Electricity.



Electricity undoubtedly exists in the atmospher*
in most states of the air ; but we know very imper
fectly the laws of this agent, and are still more ignorant
of its atmospheric operation. The present state of
science does not therefore enable us to perceive those
adaptations of its laws to its uses, which we can discover
in those cases where the laws and the uses are both of
them more apparent.

We can, however, easily make out that electrical
agency plays a very considerable part among the clouds,
in their usual conditions and changes. This may be
easily shown by Franklin's experiment of the electrical:
kite. The clouds are sometimes positively, sometimes
negatively, charged, and the rain which descends from'
them offers also indications of one or other kind of
electricity. The changes of wind and alterations of the



i



ELECTRICITY. 95

form of the clouds are generally accompanied with
changes in these electrical indications. Every one
knows that a thnnder-cloud is strongly charged with
the electric fluid, (if it he a fluid,) and that the stroke
of the lightning is an electrical discharge. "We may
add that it appears, hy recent experiments, that a
transfer of electricity between plants and the atmo-
sphere is perpetually going on during the process of
vegetation.

We cannot trace very exactly the precise circum-
stances, in the occurrences of the atmospheric regions,
which depend on the influence of the laws of electricity :
but we are tolerably certain, from what has been
already noticed, that if these laws did not exist, or were
very different from what they now are, the action of the
clouds and winds, and the course of vegetation, would
also be other than it now is.

It is therefore at any rate very probable that elec-
tricity has its appointed and important purposes in
the economy of the atmosphere. And this being so,
we may see a use in the thunder-storm and the stroke
of the lightning. These violent events are, with regard
to the electricity of the atmosphere, what winds are
with regard to heat and moisture. They restore the
equilibrium where it has been disturbed, and carry the
fluid from places where it is superfluous, to others
where it is deficient.

We are so constituted, however, that these crises
impress almost every one with a feeling of awe. The
deep lowering gloom of the thunder- cloud, the over-
whelmmg burst of the explosion, the flash from which



96 TEEEESTEIAL ADAPTATIONS.

the steadiest eye shrinks, and the irresistible arrow of
the lightning which no earthly substance can withstand,
speak of something fearful, even independently of the
personal danger which they may whisper. They
convey, far more than any other appearance does, the
idea of a superior and mighty power, manifesting dis-
pleasure and threatening punishment. Yet we find
that this is not the language which they speak to the
physical inquirer : he sees these formidable symptoms
only as the means or the consequences of good. "What
ofiice the thunderbolt and the whirlwind may have in
the moral world, we cannot here discuss : but certainly
he must speculate as far beyond the limits of philosophy
as of piety, who pretends to have learnt that there
their work has more of evil than of good. In the
natural world, these apparently destructive agents are,
like all the other movements and appearances of the
atmos^Dliere, parts of a great scheme, of which every
discoverable purpose is marked with beneficence as well
as wisdom.

Chap. XII. — The Laios of Magnetism,

Magnetism has no very obvious or apparently
extensive office in the mechanism of the atmosphere
and the earth : but the mention of it may be intro-
duced, because its ascertained relations to the other
powers which exist in the system are well suited to
show us the comiection subsisting throughout the
universe, and to check the suspicion, if any such should
arise, that any law of nature is without its use. The



♦ MAGNETISM. ~ 97

parts of creation when these uses are most obscure,
are precisely those parts when the laws themselves are
least known.

When indeed we consider the vast service of which
magnetism is to man, by supplying him with that
invaluable instrument the mariner's compass, many
persons will require no further evidence of this pro-
perty being introduced into the frame of things with a
worthy purpose. As however, we have hitherto excluded
ttse in the arts from our line of argument, we shall not
here make any exception in favour of navigation, and
what we shall observe belongs to another view of the
subject.

Magnetism has been discovered in modern times to
have so close a connexion with galvanism, that they
may be said to be almost different aspects of the same
agent. All the phenomena which we can produce with
magnets, we can imitate with coils of galvanic wire.
That galvanism exists in the earth, we need no proof.
Electricit}^, which appears to differ from galvanic
currents, much in the same manner in which a fluid at
rest differs from a fluid in motion, appears to be only
galvanism in equilibrium, is there in abundance ; and
recently, Mr. Fox* has show^n by experiment that
metalliferous veins, as they lie in the earth, exercise a
galvanic influence on each other. Something of this
kind might have been anticipated ; for masses of metal
in contact, if they differ in temperature or other
circumstances, are known to produce a galvanic
current. Hence we have undoubtedly streams of galvanic

* Phil. Trans., 1831.



98 TERRESTEIAL ADAPTATIONS,

influence moving along in the earth. Whether or not
such causes as these produce the directive power of
the magnetic needle, we cannot here pretend to decide ;
they can hardly fail to affect it. The Aurora Borealis
too, probably an electrical phenomenon, is said, under
particular circumstances, to agitate the magnetic needle.
It is not surprising, therefore, that, if electricity have
an important office in the atmosphere, magnetism
should exist in the earth. It seems, likely, that the
magnetic properties of the earth may be collateral
results of the existence of the same cause by which
electrical agency operates ; an agency which, as we
have already seen, has important offices in the processes
of vegetable life. And thus magnetism belongs to the
same system of beneficial contrivance to which elec-
tricity has been abeady traced.

We see, however, on this subject very dimly and a
very small way. It can hardly be doubted that
magnetism has other functions than those we have
noticed.

Chap. XIII. — The Properties of Light with regard to Vegetation,

The illuminating power of light will come under our
consideration hereafter. Its agency, with regard to
organic life, is too important not to be noticed, though
this must be done briefly. Light appears to be as neces-
sary to the health of plants as air or moisture. A plant
may, indeed, gTow without it, but it does not appear
that a species could be so continued. Under such a
privation, the parts which are usually green, assume a



LIGHT AND PLANTS. 99

white colour, as is the case with vegetables growii in a
cellar, or protected by a covering for the sake of
producing this very effect ; thus, celery is in this
manner blanched, or etiolated.

The part of the process of vegetable life for which
light is especially essential, appears to be the functions
of the leaves ; these are affected by this agent in a
very remarkable manner. The moisture which plants
imbibe is, by their vital energies, carried to their leaves ;
and is there brought in contact with the atmosphere,
which, besides other ingredients, contains, in general,
a portion of carbonic acid. So long as light is 'present,
the leaf decomposes the carbonic acid, appropriates
the carbon to the formation of its own proper juices,
and returns the disengaged oxygen into the atmos-
phere ; thus restoring the atmospheric air to a con-
dition in which it is more fitted than it was before for
the support of animal life. The plant thus prepares the
support of life for other creatures at the same time that
it absorbs its own. The greenness of those members
which affect that colour, and the disengagement of
oxygen, are the indications that its vital powers are in
healthful action : as soon as we remove light from the
plant, these indications cease : it has no longer power to
imbibe carbon and disengage oxygen, but, on the con-
trary, it gives back some of the carbon already obtained,
and robs the atmosphere of oxygen for the purpose of
re-converting this into carbonic acid-.

It cannot well be conceived that such effects of light
on vegetables, as we have described, should occur, if
that agent, of whatever nature it is, and those organs,

h2



100 TERRESTRIAL ADAPTATIONS.

had not been adapted to each other. But the subject
is here introduced that the reader may the more readily
receive the conviction of combining purpose which
must arise, on finding that an agent, possessing these
very peculiar chemical properties, is employed to
produce also those effects of illumination, vision, &c.,
which form the most obvious portion of the properties
of light.

Chap. XIY.—Sowid.

Besides the function which air discharges as the
great agent in the changes of meteorology and vege-
tation, it has another office, also of great and extensive
importance, as the vehicle of sound.

I. The communication of sound through the air takes
place by means of a process altogether different from
anything of which we have yet spoken : namely, by the
propagation of minute vibrations of the particles from
one part of the fluid mass to another, without any local
motion of the fluid itself.

Perhaps we may most distinctly conceive the kind of
effect here spoken of, by comparing it to the motion
produced by the wind in a field of standing corn ;
grassy waves travel visibly over the field, in the direction
in which the wind blows, but this appearance of an
object moving is delusive. The only real motion is
that of the ears of grain, of which each goes and
returns, as the stalk stoops and recovers itself. This
motion affects successively a line of ears in the direction
of the wind, and affects simultaneously all those ears of



SOUND. 101

which the elevation or depression forms one visible
wave. The elevations and depressions are propagated
in a constant direction, while the parts with which the
space is filled only vibrate to and fro. Of exactly such
a nature is the propagation of sound through the air.
The particles of air go and return through very minute
spaces, and this vibratory motion runs through the
atmosphere from the sounding body to the ear. Waves,
not of elevation and depression, but of condensation
and rarefaction, are transmitted ; and the sound thus
becomes an object of sense to the organ.

Another familiar instance of the propagation of
vibrations we have in the circles on the surface of
smooth water, which diverge from the point where it is
touched by a small object, as a drop of rain. In the
begmning of a shower for instance, when the drops
come distinct, though frequent, we may see each drop
giving rise to a ring, formed of two or three close
concentric circles, which grow and spread, leaving the
interior of the circles smooth, and gradually reaching
' parts of the surface more and more distant from their
origin. In this instance, it is clearly not a portion of
the water which flows onwards ; but the disturbance,
the rise and fall of the surface which makes the ring-
formed waves, passes into wider and wider circles, and
thus the undulation is transmitted from its starting-
place, to points in all directions on the surface of the
fluid.

The diffusion of these ring-formed undulations from
their centre resembles the diffusion of a sound from
the place where it is produced to the points where it is



102 TERRESTRIAL ADAPTATIONS.

lieard. The disturbance, or vibration, by which it is
conveyed, travels at the same rate in all directions, and
the waves which are propagated are hence of a circular
form. They differ, however, from those on the surface
of water ; for sound is communicated upwards and
downwards, and in all intermediate directions, as well
as horizontally; hence the waves of sound are spherical,
the point where the sound is produced being the centre
of the sphere.

This diffusion of vibrations in spherical shells of
successive condensation and rarefaction, will easily be
seen to be different from any local motion of the
air, as wind, and to be independent of that. The
circles on the surface of water will spread on a river
which is flowing, provided it be smooth, as well as on
a standing canal.

Not only are such undulations propagated almost
undisturbed by any local motion of the fluid in which
they take place, but also, many may be propagated in
the same fluid at the same time, without disturbing
each other. We may see this effect on water. When
several drops fall near each other, the circles which
they produce cross each other, without either of them
being lost, and the separate courses of the rings ma}'"
still be traced.

All these consequences, both in water, in air, and in
any other fluid, can be very exactly investigated upon
mechanical principles, and the greater part of the
phenomena can thus be shown to result from the
properties of the fluids.

There are several remarkable circumstances in the



I



SOUND. 103

way in which air answers its purpose as the vehicle of
sound, of which we will now point out a few.

II. The loudness of sound is such as is convenient
for common purposes. The organs of speech can, in
the present constitution of the air, produce, without
fatigue, such a tone of voice as can be heard with
distinctness and with comfort. That any great alte-
ration in this element might be incommodious, we may
judge from the difficulties to which persons are subject
who are dull of hearing, and from the disagreeable
effects of a voice much louder than usual, or so low as
to be indistinct. Sounds produced by the human

, organs, with other kinds of air, are very different from
those in our common air. If a man inhale a quantity
of hydrogen gas, and then speak, his voice is scarcely
audible.

The loudness of sounds becomes smaller in propor-
tion as they come from a greater distance. This enables
us to judge of the distance of objects, in some degree
at least, by the sounds which proceed from them.
Moreover it is found that we can judge of the position
of objects by the ear : and this judgment seems to be
formed by comparing the loudness of the impression
of the same sound on the two ears and two sides of
the head.*

The loudness of sounds appears to depend on the
extent of vibration of the particles of air, and this is
determined by the vibrations of the sounding body.

III. The ^itch or the differences of acute and grave,
in sounds, form another important property, and one

■^'■' Mr. Gough in Manch. Mem. vol.



104 TERRESTEIAL ADAPTATIONS.

which fits them for a great part of their purposes.
By the association of different notes, we have all the
results of melody and harmony in musical somid ; and
of intonation and modulation of the voice, of accent,
cadence, emphasis, expression, passion, in speech. The
song of birds, which is one of theii' princi^Dal modes of
communication, depends chiefly for its distinctions and
its significance upon the combinations of acute and grave.


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Online LibraryWilliam WhewellAstronomy and general physics considered with reference to natural theology → online text (page 7 of 22)