_The Laws of Heat with respect to Air._
We have seen in the preceding chapter how many and how important
are the offices discharged by the aqueous part of the atmosphere.
The aqueous part is, however, a very small part only; it may
vary, perhaps, from less than 1-100th to nearly as much as 1-20th
in weight, of the whole aerial ocean. We have to offer some
considerations with regard to the remainder of the mass.
1. In the first place we may observe that the aerial atmosphere is
necessary as a vehicle for the aqueous vapour. Salutary as is the
operation of this last element to the whole organized creation, it
is a substance which would not have answered its purposes if it had
been administered pure. It requires to be diluted and associated
with dry air, to make it serviceable. A little consideration will
show this.
We can suppose the earth with no atmosphere except the vapour
which arises from its watery parts: and if we suppose also the
equatorial parts of the globe to be hot, and the polar parts cold,
we may easily see what would be the consequence. The waters at
the equator, and near the equator, would produce steam of greater
elasticity, rarity, and temperature, than that which occupies the
regions further _polewards_; and such steam, as it came in contact
with the colder vapour of a higher latitude, would be precipitated
into the form of water. Hence there would be a perpetual current
of steam from the equatorial parts towards each pole, which would
be condensed, would fall to the surface, and flow back to the
equator in the form of fluid. We should have a circulation which
might be regarded as a species of regulated distillation.[11] On
a globe so constituted, the sky of the equatorial zone would be
perpetually cloudless; but in all other latitudes we should have an
uninterrupted shroud of clouds, fogs, rains, and, near the poles, a
continual fall of snow. This would be balanced by a constant flow
of the currents of the ocean from each pole towards the equator. We
should have an excessive circulation of moisture, but no sunshine,
and probably only minute changes in the intensity and appearances
of one eternal drizzle or shower.
It is plain that this state of things would but ill answer the ends
of vegetable and animal life: so that even if the lungs of animals
and the leaves of plants were so constructed as to breathe steam
instead of air, an atmosphere of unmixed steam would deprive those
creatures of most of the other external conditions of their well
being.
The real state of things which we enjoy, the steam being mixed
in our breath and in our sky in a moderate quantity, gives rise
to results very different from those which have been described.
The machinery by which these results are produced is not a little
curious. It is in fact the machinery of the _weather_, and
therefore the reader will not be surprised to find it both complex
and apparently uncertain in its working. At the same time some
of the general principles which govern it seem now to be pretty
well made out, and they offer no small evidence of beneficent
arrangement.
Besides our atmosphere of aqueous vapour, we have another and far
larger atmosphere of common air; a _permanently elastic_ fluid,
that is, one which is not condensed into a liquid form by pressure
or cold, such as it is exposed to in the order of natural events.
The pressure of the dry air is about twenty-nine and a half inches
of mercury; that of the watery vapour, perhaps, half an inch. Now
if we had the earth quite dry, and covered with an atmosphere of
dry air, we can trace in a great measure what would be the results,
supposing still the equatorial zone to be hot, and the temperature
of the surface to decrease perpetually as we advance into higher
latitudes. The air at the equator would be rarefied by the heat,
and would be perpetually displaced below by the denser portions
which belonged to cooler latitudes. We should have a current of
air from the equator to the poles in the higher regions of the
atmosphere, and at the surface a returning current setting towards
the equator to fill up the void so created. Such aerial currents,
combined with the rotatory motion of the earth, would produce
oblique winds; and we have in fact instances of winds so produced,
in the trade winds, which between the tropics blow constantly from
the quarters between east and north, and are, we know, balanced
by opposite currents in higher regions. The effect of a heated
surface of land would be the same as that of the heated zone of the
equator, and would attract to it a sea breeze during the day time,
a phenomenon, as we also know, of perpetual occurrence.
Now a mass of dry air of such a character as this, is by far the
dominant part of our atmosphere; and hence carries with it in its
motions the thinner and smaller eddies of aqueous vapour. The
latter fluid may be considered as permeating and moving in the
interstices of the former, as a spring of water flows through a
sand rock.[12] The lower current of air is, as has been said,
directed towards the equator, and hence it resists the motion of
the steam, the tendency of which is in the opposite direction; and
prevents or much retards that continual flow of hot vapour into
colder regions, by which a constant precipitation would take place
in the latter situations.
If, in this state of things, the flow of the current of air, which
blows from any colder place into a warmer region, be retarded or
stopped, the aqueous vapours will now be able to make their way
to the colder point, where they will be precipitated in clouds or
showers.
Thus, in the lower part of the atmosphere, there are tendencies
to a current of air in one direction, and a current of vapour in
the opposite; and these tendencies exist in the average weather
of places situated at a moderate distance from the equator. The
air tends from the colder to the warmer parts, the vapour from the
warmer to the colder.
The various distribution of land and sea, and many other causes
make these currents far from simple. But in general the air current
predominates, and keeps the skies clear and the moisture dissolved.
Occasional and irregular occurrences disturb this predominance;
the moisture is then precipitated, the skies are clouded, and the
clouds may descend in copious rains.
These alternations of fair weather and showers, appear to be much
more favourable to vegetable and animal life than any uniform
course of weather could have been. To produce this variety, we have
two antagonist forces, by the struggle of which such changes occur.
Steam and air, two transparent and elastic fluids, expansible by
heat, are in many respects and properties very like each other.
Yet, the same heat similarly applied to the globe, produces at the
surface currents of these fluids, tending in opposite directions.
And these currents mix and balance, conspire and interfere, so
that our trees and fields have alternately water and sunshine;
our fruits and grain are successively developed and matured. Why
should such laws of heat and elastic fluids so obtain, and be so
combined? Is it not in order that they may be fit for such offices?
There is here an arrangement, which no chance could have produced.
The details of this apparatus may be beyond our power of tracing;
its springs may be out of our sight. Such circumstances do not make
it the less a curious and beautiful contrivance: they need not
prevent our recognizing the skill and benevolence which we _can_
discover.
2. But we have not yet done with the machinery of the weather. In
ascending from the earth’s surface through the atmosphere, we find
a remarkable difference in the heat and in the pressure of the air.
It becomes much colder, and much lighter; men’s feelings tell them
this; and the thermometer and barometer confirm these indications.
And here again we find something to remark.
In both the simple atmospheres of which we have spoken, the one of
air and the one of steam, the property which we have mentioned must
exist. In each of them, both the temperature and the tension would
diminish in ascending. But they would diminish at very different
rates. The temperature, for instance, would decrease much more
rapidly for the same height in dry air than in steam. If we begin
with a temperature of 80 degrees at the surface, on ascending five
thousand feet the steam is still 76½ degrees, the air is only 64½
degrees; at ten thousand feet, the steam is 73 degrees, the air
48½ degrees; at fifteen thousand feet, steam is at 70 degrees,
air has fallen below the freezing point to 31½ degrees. Hence
these two atmospheres cannot exist together without modifying one
another: one must heat or cool the other, so that the coincident
parts may be of the same temperature. This accordingly does take
place, and this effect influences very greatly the constitution
of the atmosphere. For the most part, the steam is compelled to
accommodate itself to the temperature of the air, the latter being
of much the greater bulk. But if the upper parts of the aqueous
vapour be cooled down to the temperature of the air, they will not
by any means exert on the lower parts of the same vapour so great a
pressure as the gaseous form of these could bear. Hence, there will
be a deficiency of moisture in the lower part of the atmosphere,
and if water exist there, it will rise by evaporation, the surface
feeling an insufficient tension; and there will thus be a fresh
supply of vapour upwards. As, however, the upper regions already
contain as much as their temperature will support in the state of
gas, a precipitation will now take place, and the fluid thus formed
will descend till it arrives in a lower region, where the tension
and temperature are again adapted to its evaporation.
Thus, we can have no equilibrium in such an atmosphere, but a
perpetual circulation of vapour between its upper and lower parts.
The currents of air which move about in different directions, at
different altitudes, will be differently charged with moisture, and
as they touch and mingle, lines of cloud are formed, which grow and
join, and are spread out in floors, or rolled together in piles.
These, again, by an additional accession of humidity, are formed
into drops, and descend in 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 ascending 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
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 distribution of
its own pressure, and the dynamical conditions 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 middle 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 ten,
twenty, or thirty degrees without obtaining a deposition of water
upon it, or reaching the _dew-point_, as it is called. To have had
such a _dripping_ state of the atmosphere as the former arrangement
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 exhibition 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 beneficial. Perhaps
most persons will feel a strong persuasion, 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.
3. 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
atmosphere, 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 temperature 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
which we shall not here treat. That character in which we now
consider them, that of the machinery of atmospheric changes, and
thus, immediately or remotely, the instruments of atmospheric
influences, cannot well be refused them by any person.
4. There is still one reflexion 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
oscillations 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 each other, be of this converging
and subsiding character? Would 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 equilibrium. The ship may roll too
far, and may _capsize_. 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 probably
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.”--_Par. Lost._ b. ii.
A state of things something like that which Milton 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 adjustments 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 continuation of such laws and
materials as enter into the constitution of the atmosphere, is one
manifestly of much greater complexity, and indeed to us probably
of insurmountable difficulty as a mechanical problem. But all that
investigation 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 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
permanence 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. 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 Intelligent Beneficence are calculated to excite.