MODIFICATION OF TEMPERATURE.
Resuming the subject where it was left the previous Lord's Day, Mr. Wilton
said:
"We saw at our last session that the most prominent and permanent features
of the earth tend to produce differences and great extremes of
temperature. These variations of temperature within due limits must be
regarded as beneficial, if not absolutely essential, to the well-being of
the human race. The different zones give the world a richer and more
varied supply of food, and finer and more varied plants and animals. The
change of seasons gives variety in the experience of life; the warmth of
summer ripens the fruit and grain, and the cold of winter tones up the
physical strength; nay, the winter's frost is a natural subsoiler,
loosening up the hard earth and promoting vegetable growth. As for man's
higher interests, no one can tell how much the world is indebted to winter
evenings, to a period of darkness longer than is needed for sleep, and a
period of cold during which the work of husbandry may largely cease.
Learning, the domestic virtues, and religion are greatly indebted to our
winters. But were these agencies which tend to produce inequality of
temperature suffered to operate without counteracting influences, the
extremes of heat and cold would cease to be genial and healthful, and
become destructive. We are now to begin the consideration of those
counteracting agencies by which the extremes of temperature are moderated.
"Let us look first at the daily fluctuation of temperature caused by the
revolution of the earth upon its axis. The rotation of the earth brings
every place by turns under the influence of the sun's rays, and in turn
withdraws it from the heat of the sun, thus producing a daily change of
temperature. How is this diurnal change of temperature alleviated?"
This was addressed to all, but no one answered. "Mr. Hume, I should be
glad to have you suggest the answer."
"There are two chief agencies," Mr. Hume replied--"first, the absorption
of heat during the day and the radiation of that heat during the night;
and, secondly, the formation of watery vapor during the day and the
deposition of dew by night."
"The first of these agencies," said Mr. Wilton, "is so plain that very
little explanation need be made. During the day, while the sun is shining
and the temperature is rising, the surface of the earth, the rocks, the
trees, and all things are absorbing heat. This heat is, so to speak, laid
up in store, ready for use in time of need. In due time the sun sinks low
and sets behind the horizon; the supply of heat is cut off and the
temperature begins to fall. Then all those objects which during the day
were laying up heat in store begin to radiate heat into the air, and by
their contact with it keep up its warmth. Commonly, the temperature falls
so low that bodies radiate more heat than they absorb before the setting
of the sun. In this process water plays a very conspicuous part. You will
call to mind what was said before about the large specific heat of water.
By means of this, water is able to store up heat in large amounts--larger
in proportion to its weight than any other substance except hydrogen gas.
The heat that is stored up during the day is given off by contact with the
air and by radiation during the night.
"But water plays a still more important part in moderating the daily
fluctuations of temperature by the process of evaporation and the
formation of dew. Call to mind what was said of the formation of vapor
when we were speaking of latent heat. Heat water to two hundred and twelve
degrees--the boiling point: it must still be heated a long time before it
evaporates. Boiling water must receive five and a half times more heat to
give it the form of vapor than to raise it from the freezing to the
boiling point; that is, about one thousand degrees of heat are required to
turn boiling hot water to vapor. The same amount of heat is required for
the formation of vapor whatever the temperature of the water from which
the vapor rises. There is only this difference--vapor from cold water is
cold, while vapor from hot water is hot. Evaporation goes on more rapidly
in proportion as the temperature rises, but vapor is formed at all
temperatures. Evaporation goes on from ice. The Alpine glaciers, or rivers
of ice, sink away several feet by evaporation from their surface during
their slow course of many years down the mountain ravines. This process of
evaporation goes on, I say, during the day, and in the formation of vapor
an amount of heat which would raise an equal weight of water through one
thousand degrees of temperature is used up.
"This vapor which is formed is not supported _by the_ air, as men commonly
suppose. It is true that clouds are held up by the atmosphere, but clouds
are condensed vapor--minute globules of water floating in the air. Vapor
is invisible. You must have noticed that steam is invisible till it is
condensed by contact with the colder air. Vapor rests upon the earth and
supports itself by its own elastic force, just as the atmosphere supports
itself. The presence of air makes no difference with the formation of
vapor, except that in a vacuum vapor forms very much more rapidly, because
no air stands in its way. But at any given temperature, in the air or in a
vacuum, the same amount of vapor rises in due time, and the same amount
can support itself. Vapor seems to circulate between the atoms of air, as
sand fills the spaces between marbles. At the temperature of four degrees
below zero vapor equal to two-thirds of an inch of water can be formed and
support itself by its elasticity; that is, the elastic force of vapor at
four degrees below zero is equal to two-fifths of an ounce per square
inch; at thirty-six degrees vapor equal to two and two-thirds inches of
water can support itself; at eighty degrees vapor equal to thirteen inches
of water can exist; at one hundred and seventy-nine degrees, seventeen
feet; and at two hundred and twelve degrees nearly thirty-four feet; that
is, vapor at two hundred and twelve degrees has an elastic force of
fifteen pounds to the square inch. Let us suppose that at sunrise the air
has a temperature of thirty-six degrees, and that as much vapor is already
formed as can sustain itself at that temperature. As the sun sheds down
his rays the temperature rises and more vapor is formed. We will suppose
that half an inch of water is evaporated. Some of this vapor will be
carried by ascending currents of air into the higher regions and condensed
into clouds, some will be carried by winds into drier and warmer regions,
yet the amount of vapor will increase during the day. We will suppose that
during the night the temperature falls again to thirty-six degrees; all
the excess of vapor above two inches and two-thirds of water will be
condensed and become dew or fog, and in this condensation the thousand
degrees of heat absorbed in the formation of the vapor will be given out
again. If vapor equal to one inch of water be condensed, heat is set free
sufficient to boil a sheet of ice water, five and a half inches in
thickness, extending over the whole region; that is, it would be all the
same as if a fire were kindled on every square rod of land hot enough to
boil during the night more than twenty barrels of ice water. In this
illustration I have supposed a larger condensation than commonly takes
place, but very much less than is conceivable. Suppose that the
temperature is eighty degrees, and that, as is possible, more than one
foot of water exists in the state of vapor. Let the temperature fall to
thirty-six degrees, and full ten inches of water must be condensed,
setting free heat which would boil four and a half feet of ice water. So
large a condensation as this never takes place in twelve hours, partly
because the full amount of vapor which might be formed is never actually
produced, and partly because the condensation of but a small part of this
vapor would check the fall of temperature and prevent farther
condensation. The supposition that I have made shows the possibilities of
this method of moderating extremes of heat and cold. Were it not for these
processes, our days would be much warmer and our nights much cooler than
they now are. By the formation of vapor the excess of heat during the day
is stored up in a latent form; that is, it is used, not as heat, but as
force, and is employed in bringing the atoms of water into new
relationship; during the night the vapor returns to its former state as
water, and the heat-force again becomes sensible heat. Thus the day is
cooled and the night made warmer.
"Ansel, have you ever heard the 'dew point' spoken of?"
"Yes, sir, I have."
"Do you know what is meant by it?"
"That point or degree of temperature at which dew begins to be formed."
"Upon what does the dew point depend?"
"Upon the amount of vapor in the air."
"That is right, Ansel. If at any time the full possible amount of vapor
should exist, any diminution of the temperature must, of course, cause
dew to be deposited. Do you know, Ansel, how to ascertain the dew point at
any time?"
"No, sir, I do not."
"There is a beautiful instrument known as Daniell's Hygrometer which shows
the dew point as a thermometer shows the temperature. But any one can
easily determine the dew point without a special instrument for that
purpose. Pour warm water into a glass pitcher or goblet whose outer
surface has been wiped perfectly dry, and polished. Into this set a common
thermometer. Cool down this warm water by dropping into it small pieces of
ice, and notice carefully when the polished glass begins to be dimmed as
if it had been breathed upon. When that begins to take place the
thermometer will show the dew point. In this manner we can determine the
amount of vapor in the air, and by estimating the probable temperature of
the night judge of the probability that dew will fall."
"I have noticed some things," said Peter, "about the formation of dew
which I do not understand, and I wish very much to ask about them."
"I should be glad to hear your questions, and will answer them if I can."
"I have noticed that dew falls on clear nights, but not very often on
cloudy nights. I don't see why that is so."
"Have you ever noticed whether cloudy nights or clear nights are the
warmer?"
"Cloudy nights are commonly warmer, I think, but I never could see the
reason for that, either."
"Can you tell why a newspaper spread over a tomato vine keeps the frost
from the vine?"
"Because the frost comes upon the paper instead of the vine, of course."
"But why do you say, of course? Why does not the dew--for frost is nothing
but dew frozen as it forms--come upon the under side of the paper?"
"How could the dew fall upon the under side?"
"That is just the point which we need first of all to understand. Men
commonly speak of dew as if it fell. I don't know but I have spoken of the
falling of dew in this lesson. But dew does not fall at all. The vapor
simply touches some cold object, and is condensed upon it. The vapor by
its elasticity presses against the cold body, and the process of
condensation continues until either the body is warmed by the heat set
free so that its temperature rises above the dew point, or till the vapor
is so far exhausted that the dew point falls below the existing
temperature. Dew is formed upon the upper surface and not upon the under,
because the upper surface is cool and the under surface is warmer. Beneath
the paper spread over the tomato vine, the earth is radiating heat and the
paper is radiating it back again. If the paper were not there, the heat
would be radiated into space and not returned again. The vine would soon
radiate away its little store of heat, its temperature would sink, below
the dew point, and dew or frost would be deposited upon it. The under
surfaces of objects are kept warm by the radiation from the earth. In the
same manner clouds are wrapped around the earth and keep it warm by
radiating back its radiant heat. Dew is not formed on cloudy nights,
because they are warmer: the clouds throw back the heat which otherwise
would be lost in open space."
"I never knew before," said Peter, "that clouds were of any great use
except to send down rain."
"We shall see in the course of our lessons that clouds are of very great
use in warming the earth in other ways, as well as by serving as blankets
and radiating back the heat which otherwise would escape."
"I wanted also to ask why dew falls--I mean, is formed--on grass and
leaves of plants while stones are dry."
"I will answer your question by asking another. Did you ever see barefoot
boys running in the cold dew stop and stand upon a stone or rock to get
their feet warm?"
"Oh yes, sir; I have done it myself."
"Why did you stand upon a rock?"
"Because I had learned that the rocks would be warm."
"I think that answers your question. The rocks and stones are warmer than
the grass and the leaves. The blades of grass and the leaves are thin and
pointed and rough, and have a very large radiating surface. They have but
little heat, and that little they part with rapidly. The rocks and stones,
on the other hand, are bulky, and contain a much larger store of heat,
their radiating surface is comparatively small; only one side is exposed,
the other being covered by the warm earth, from which they are drinking in
heat almost as rapidly as they lose it. They therefore do not lose heat
enough to sink their temperature to the dew point.
"So much, then, for the means employed to moderate the changes of
temperature from day to night and from night to day. But upon the
sea-coast and upon certain islands of the sea another agency is employed.
Will some one suggest what this agency is?"
No one else answered, and finally Mr. Hume said: "I suppose, of course,
that you refer to the land and sea breezes?"
"This is what I had in mind. During the day the land is warmer than the
sea, and the breeze from the sea blowing upon the land cools the air.
During the night the land radiates its heat more rapidly than the water,
and soon the sea becomes the warmer. Then a breeze springs up in the
opposite direction; the cooler air of the land flows out upon the sea. By
this means the air upon the land and the air upon the sea are daily
commingled, thus securing a more even temperature upon the land. This
softens the extremes of daily temperature. I make only this brief
reference to the land and sea breezes, because in another connection we
shall examine the general subject of winds and their influence in the
equalization of temperature.
"The result of all these influences is that the changes of temperature
from day to night and from night to day, while not inconsiderable, are by
no means destructive, and in many cases are no greater than is refreshing
and agreeable. These agencies remind us every day of the wise provision of
the Creator for the well-being of his creatures. 'Day unto day uttereth
speech and night unto night showeth knowledge. There is no speech nor
language where their voice is not heard.' This care for the earthly
well-being of men is but a type of his care for their spiritual happiness.
The plan of salvation, and the ways of divine providence working in
accordance therewith, are more wonderful both in their means and their end
than the greatest of the works of Nature. If while we study the natural we
forget the supernatural, we commit the greatest mistake: we pass by the
greater to examine the less. The natural is valuable only as it leads to
the spiritual."