William Whewell.

Astronomy and general physics considered with reference to natural theology online

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with a beneficial design.

III. As water becomes ice by cold, it becomes steam
by heat. In common language, steam is the name
given to the vapour of hot water ; but in fact a vapour
or steam rises from water at all temperatures, however
low, and even from ice. The expansive force of this
vapour increases rapidly as the heat increases ; so that
when we reach the heat of boiling w^ater, it operates in
a far more striking manner than when it is colder ; but


in all cases the surface of water is covered with an
atmosphere of aqueous vapour, the pressure or tension
of which is limited by the temperature of the water.
To each degree of pressure in steam there is a con-
stituent temperature corresponding. If the surface of
water is not pressed by vapour with the force thus
corresponding to its temperature, an immediate evapo-
ration will supply the deficiency. We can compare
the tension of such vapour with that of our common
atmosphere; the pressure of the latter is measured
by the barometrical column, about thirty inches of
mercury ; that of watery vapour is equal to one inch of
mercury at the constituent temperature of 80 degrees,
and to one-fifth of an inch at the temperature of 82

Hence, if that part of the atmosphere which consists
of common air were annihilated, there would still
remain an atmosphere of aqueous vapour, arising from
the waters and moist parts of the earth; and in the
existing state of things this vapour rises in the atmo-
sphere of dry air. Its distribution and effects are
materially influenced by the vehicle in which it is thus
carried, as we shall hereafter notice ; but at present
we have to observe the exceeding utility of water in this
shape. We remark how suitable and indispensable to
the well-being of the creation it is, that the fluid should
possess the property of assuming such a form under
such circumstances.

The moisture which floats in the atmosphere is of
most essential use to vegetable life.* *' The leaves

* Loudon, 1219.


of living plants appear to act upon this vapour in its
elastic form, and to absorb it. Some vegetables increase
in weight from this cause when suspended in the
atmosphere and unconnected with the soil, as the
house-leek and the aloe. In very intense heats, and
when the soil is dry, the life of plants seems to be
preserved by the absorbent power of their leaves." It
follows from what has already been said, that, with
an increasing heat of the atmosphere, an increasing
quantity of vapour will rise into it, if supplied from
any quarter. Hence it appears that aqueous vapour
is most abundant in the atmosphere when it is most
needed for the purposes of life ; and that when other
sources of moisture are cut off, this is most copious.

IV. Clouds are produced by aqueous vapour when it
returns to the state of water. This process is con-
densation, the reverse of evaporation. When vapour
exists in the atmosphere, if in any manner the tempe-
rature becomes lower than the constituent temperature,
requisite for the maintenance of the vapoury state,
some of the steam will be condensed and will become
water. It is in this manner that the curl of steam
from the spout of a boiling tea-kettle becomes visible,
being cooled down as it rushes to the air. The steam
condenses into a fine watery powder, w^hich is carried
about by the little aerial currents. Clouds are of the
same nature with such curls, the condensation being
generally produced when air, charged with aqueous
vapour, is mixed with a colder current, or has its
temperature diminished in any other manner.

Clouds, while they retain that shape, are of the


most essential use to vegetable and animal life. They
moderate the fervour of the sun, in a manner agreeable,
to a greater or less degree, in all climates, and grateful
no less to vegetables than to animals. Duhamel says
that plants grow more during a week of cloudy weather
than a month of dry and hot. It has been observed
that vegetables are far more refreshed by being watered
in cloudy than in clear weather. In the latter case,
probably the supply of fluid is too rapidly carried off by
evaporation. Clouds also moderate the alternations of
temperature, by checking the radiation from the earth.
The coldest nights are those which occur under a
cloudless winter sky.

The uses of clouds, therefore, in this stage of their
history, are by no means inconsiderable, and seem to
indicate to us that the laws of their formation were
constructed with a view to the purposes of organised

V. Clouds produce rain. In the formation of a cloud
the precipitation of moisture probably forms a fine
watery powder, which remains suspended in the air in
consequence of the minuteness of its particles : but if
from any cause the precipitation is collected in larger
portions, and becomes drops, these descend by their
weight and produce a shower.

Thus rain is another of the consequences of the
properties of water with respect to heat ; its uses are
the results of the laws of evaporation and condensation.
These uses, with reference to plants, are too obvious
and too numerous to be described. It is evident that
on its quantity and distribution depend in a great


measure the prosperity of the vegetable kingdom : and
different climates are fitted for different productions,
no less by the relations of dry weather and showers,
than by those of hot and cold.

VI. Returning back still further in the changes which
cold can produce on water, we come to snow and ice :
snow bemg apparently frozen cloud or vapour, aggre-
gated by a confused action of crystalline laws ; and ice
being water in its fluid state, solidified by the same
crystalline forces. The impression of these agents on
the animal feelings is generally unpleasant, and we are
in the habit of considering them as symptoms of the
power of winter to interrupt that state of the elements
in which they are subservient to life. Yet, even in this
form, they are not without their uses.* " Snow and
ice are bad conductors of cold ; and when the ground
is covered with snow, or the surface of the soil or of
water is frozen, the roots or bulbs of plants beneath
are protected by the congealed water from the influence
of the atmosphere, the temperature of which, in northern
winters, is usually very much below the freezing point :
and this water becomes the first nourishment of the
plant in early spring. The expansion of water during
its congelation, at which time its volume increases
one -twelfth, and its contraction in bulk during a thaw",
tend to pulverise the soil, to separate its parts from
each other, and to make it more permeable to the
influence of the air." In consequence of the same
slowness in the conduction of heat which snow thus
possesses, the arctic traveller finds his bed of snow of

* Loudon, 1214.


no intolerable coldness; the Esquimaux is sheltered
from the inclemency of the season in his snow hut, and
travels rapidly and agreeably over the frozen surface
of the sea. The uses of those arrangements, which at
first appear productive only of pain and inconvenience,
are well suited to give confidence and hope to our
researches for such usefulness in every part of the
creation. They have thus a peculiar value in adding
connexion and universality to our perception of bene-
ficial design.

VII. There is a peculiar circumstance still to be
noticed in the changes from ice to water and from
water to steam. These changes take place at a parti-
cular and invariable degree of heat ; yet they do not
take place suddenly when we increase the heat to this
degree. This is a ycyj curious arrangement. The
temperature makes a stand, as it were, at the point
where thaw and where boiling take place. It is
necessary to a]3ply a considerable quantity of heat to
produce these effects; all which heat disappears, or
becomes latent, as it is called. We cannot raise the
temperatm^e of a thawing mass of ice till we have
thawed the whole. We cannot raise the temperature
of boiling water, or of steam rising from it, till we have
converted all the water into steam. Any heat that we
apply while these changes are going on is absorbed in
producing the changes.

The consequences of this property of latent heat are
very important. It is on this account that the changes
now spoken of necessarily occupy a considerable time.
Each part in succession must have a proper degree of


heat applied to it. If it were otherwise, thaw and
evaporation must be instantaneous ; at the first touch
of warmth, all the snow which lies on the roofs of our
houses would descend like a water-spout into the
streets : all that which rests on the ground would rush
like an inundation into the water courses. The hut of
the Esquimaux would vanish like a house in a panto-
mime : the icy floor of the river would be gone without
giving any warning to the skater or the traveller : and
when, in heating our water, we reached the boiling
point, the whole fluid would " flash into steam," (to use
the expression of engineers,) and dissipate itself in
the atmosphere, or settle in dew on the neighbouring

It is obviously necessary for the purposes of human
life, that these changes should be of a more gradual
and manageable kmd than such as we have now
described. Yet this gradual progress of freezing and
thawing, of evaporation and condensation, is produced,
so far as we can discover, by a particular contrivance.
Like the freezing of water from the top, or the floating
of ice, the moderation of the rate of these changes
seems to be the result of a violation of a law : that is,
the simple rule regarding the effects of change of tem-
perature, which at first sight appears to be the law,
and which, from its simplicity, would seem to us the
most obvious law for these as well as other cases, is
modified at certain critical points, so as to produce
these advantageous effects : — why may we not say in
order to produce such effects ?

VIII. Another office of water, which it discharges by


means of its relations to lieat, is that of supplying onr
springs. There can be no doubt that the old hypotheses,
which represent springs as drawing their supplies from
large subterranean reservoirs of water, or from the sea
by a process of subterraneous filtration, are erroneous
and untenable. The quantity of evaporation from water
and from tvet ground is found to be amply sufficient
to supply the requisite drain. Mr. Dalton calculated*
that the quantity of rain which falls in England
is thirty-six inches a year. Of this he reckoned
that thirteen inches flow off to the sea by the rivers,
and that the remaining twenty-three inches are raised
again from the ground by evaporation. The thirteen
inches of water are of course supplied by evaporation
from the sea, and are carried back to the land through
the atmosphere. Vapour is perpetually rising from the
ocean, and is condensed in the hills and high lands,
and through their pores and crevices descends, till it is
deflected, collected, and conducted out to the day, by
some stratum or channel which is watertight. The
condensation which takes place in the higher parts of
a country, may easily be recognised in the mists and
rains which are the frequent occupants of such regions.
The coldness of the atmosphere and other causes pre-
cipitate the moisture in clouds and show^ers, and in the
former as well as in the latter shape, it is condensed
and absorbed by the cool ground. Thus a j)erpetual
and compound circulation of the waters is kept up ; a
narrower circle between the evaporation and precipita-
tion of the land itself, the rivers and streams only

* Manchester Memoirs, v. 357 j


occasionally and partially forming a portion of the
circuit ; and a wider interchange between the sea and
the lands which feed the springs, the water ascending
perpetually by a thousand currents through the air, and
descending by the gradually converging branches of the
rivers, till it is again returned into the great reservoir
of the ocean.

In every country, these two portions of the aqueous
circulation have their regular, and nearly constant,
proportion. In this kingdom the relative quantities
are, as we have said, 23 and 13. A due distribution of
these circulating fluids in each country appears to be
necessary to its organic health ; to the habits of vege-
tables, and of man. We have every reason to believe
that it is kept up from year to year as steadily as the
circulation of the blood in the veins and arteries of
man. It is maintained by machinery very different,
indeed, from that of the human system, but apparently
as well, and therefore we may say as clearly, as that,
adapted to its purposes.

By this machinery we have a connection established
between the atmospheric changes of remote countries.
Eains in England are often introduced by a south-east
wind. '' Vapour brought to us by such a wind, must
have been generated in countries to the south and east
of our island. It is therefore, probably, in the extensive
valleys watered by the Mouse, the Moselle, and the
Bhine, if not from the more distant Elbe, with the
Oder and the Weser, that the water rises, in the midst
of sunshine, which is soon afterwards to form our
clouds, and pour down our thunder- showers." "Drought


and sunshine in one j)art of Europe may be as neces-
sary to the production of a wet season in another, as it
is on the great scale of the continents of Africa and
South America ; where the plains, during one half the
year, are burnt up, to feed the springs of the moun-
tains ; which in their turn contribute to inundate the
fertile valleys, and prepare them for a luxuriant vege-
tation."* The properties of water which regard heat
make one vast watering -engine of the atmosphere.

Chap. X.—T/ie 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-lOOdth to nearly as much as l-20th
in weight of the whole aerial ocean. "We have to offer
some considerations with regard to the remainder of
the mass.

I. 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 organised creation, it is a sub-
stance 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 service-
able. A little consideration will show this.

We can suppose the earth with no atmosphere except
the vapour which arises from its watery parts : gmd if

* Howard on tlie Climate of London, vol. ii., pp. 216, 217.


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 poleioards; 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.* 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 sun-
shine, 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.

* Daniell, Meteor. Ess., p. ^Q.

G 2


The real state of things which we enjoy, the steam
Jbeing 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 imcertain in its worldng. 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 con-
densed 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 29 J 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 lati-
tudes. The air at the equator would be rarefied by the
heat, and would be perpetually displaced below by the
denser portions which belong 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.* 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

* Daniell. p. 129.


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 succes-
sively developed and matured. Why should such laws
of heat and elastic fluids so obtain, and be so com-
bined? Is it not in order that they may be fit for
such oflices? 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 om^ sight. Such circumstances


do not make it the less a curious and beautiful con-
trivance : they need not prevent our recognising the
skill and benevolence wliich we can discover.

II. But we have not yet done with the macliinery 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 con-
firm these indications. And here again w^e find some-
thing to reinark.

In both the simple atmospheres of which we have
spoken, the one of air and the one of steam, the pro-
perty 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 tempera-
ture of 80 degrees at the surface, on ascending 5,000
feet the steam is still 76^ degrees, the air is only 64^
degrees ; at 10,000 feet, the steam is 73 degrees, the
air 48 J degrees; at 15,000 feet, steam is at 70 degrees;
air has fallen below the freezing point to 81|^ 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 behig of much the greater bullc. 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 atmo-
sphere, 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 acces-
sion of humidity, are formed into drops, and descend in

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Online LibraryWilliam WhewellAstronomy and general physics considered with reference to natural theology → online text (page 6 of 22)