TRANSPORTATION OF HEAT.
"To-day we come to that subject which we should have looked at a week ago,
if that I hope not unprofitable discussion of the uses of trials and the
ministry of pain had not prevented. We must now examine the arrangement
for softening the rigors of winter and toning down the heat of summer. The
general principle is that in summer the earth receives an excess of heat,
while in winter the opposite is true. These extremes are mitigated by
transferring heat from summer to winter. How is this accomplished? Any one
who has thoughts upon this subject may answer."
"I have some thoughts," said Ansel, "but whether right or wrong, I cannot
tell. I should think heat might be carried from summer to winter in the
same way as from day to night."
"What are some of those means for transferring heat which seem to you to
operate the same in the annual as in the daily changes of temperature?"
"One is the absorption and radiation of heat, and another is the
evaporation of water and the condensation of vapor."
"You are right," said Mr. Wilton. "The effect of these operations in the
equalization of the annual extremes of heat is in no wise different from
their effect upon the temperature of day and night, but from summer to
winter their effect is vaster and more impressive. During the summer, sea
and land, and 'all that in them is,' are receiving heat and rising in
temperature. The heat of summer penetrates and warms the earth nearly a
hundred feet in depth. Into the sea heat penetrates still deeper. How vast
the amount of heat required to warm the whole surface of the earth and sea
to such depths! By withdrawing so much heat from active use the intensity
of the summer temperature is softened. During the colder months the land
and sea slowly radiate their heat. We can hardly over-estimate the effect
of this alternate absorption and radiation of heat. So great is the
effect of this stored up heat that the sea and the great lakes never
freeze even in the coldest winter weather, except in the polar regions,
and the temperature must fall far below freezing and continue for a long
time below the freezing point before the earth begins to freeze. The great
bodies of water, remaining always at a temperature above thirty-two
degrees, are especially important in warming the wintry air. In the
coldest weather they seem like steaming caldrons throwing up their warm
vapor. It is the absorption and radiation of heat alone which prevent the
temperature of the atmosphere from rising or falling suddenly to the
highest or lowest point possible. The sun breaks forth in all its splendor
at noonday in summer: what if the sun were to remain stationary, shining
thus in his strength for days and months? Everything would be consumed
with heat. But why do not the glowing rays of the sun raise the
temperature at once to the highest possible point? Because the earth and
sea and every object upon the earth absorb the heat, storing it up and
holding it in reserve. On the other hand, when the sun sets and his heat
is withdrawn, why does not the temperature fall suddenly to the lowest
possible point? Because the heat held in store is slowly radiated and the
change of temperature rendered gradual.
"In this work of absorbing and radiating heat every object, earth, air,
and sea, does its appropriate share. But water stands chief, and performs
the largest service. Its high specific heat enables it to hold in store
the largest calorific treasure, and causes it to change its temperature
more slowly.
"The formation and condensation of vapor also operate in the same manner
as in the transitions of day and night. During the summer the higher
average temperature makes it possible for a much larger amount of vapor to
be formed than in winter. You remember that at eighty degrees vapor equal
to thirteen inches of water can sustain itself, while at thirty-six
degrees the elastic force of vapor is equal to the pressure of only two
inches and two-fifths of water, and at four degrees to three-fifths of an
inch. If the mean summer temperature at any place were eighty degrees, it
would be possible for more than one foot of water to be held in the form
of vapor. In the formation of this vapor heat would be consumed
sufficient to boil more than five and a half feet of ice water. If the
mean winter temperature at the same place be thirty-six degrees, more than
three-fourths of this vapor must be condensed and give out its latent heat
to warm the air. It is not to be supposed that the full amount of vapor
which can support itself does commonly exist, but the difference between
the average amount of vapor in summer and in winter must be very great. I
suppose this difference often amounts to four or six inches of water. If
we suppose it to be four inches, an amount of heat is transferred from
summer to winter sufficient to boil twenty-two inches of ice water. In
estimating the effect of this we must consider that this heat is not given
out gradually and regularly for three months, but whenever there is a
sudden fall of temperature vapor is condensed, latent heat becomes
sensible, and the suddenness and intensity of the fall are diminished. We
need also to bear in mind that every open body of water is sending up its
clouds of vapor constantly. The open lakes, and especially the sea, are
like a seething caldron; and thus immensely more vapor is condensed during
the winter months than is brought over from summer to winter. Much of the
vapor formed in winter is to be set to the account of summer, for it is
the summer's heat absorbed by the water, which maintains its temperature
and enables it to throw up such clouds of vapor, even in midwinter. But
this comes in more properly at another place, and we will leave it for the
present.
"There is another transition experienced by water by which heat treasured
up in summer is made available for softening the rigors of winter. Who
will suggest it?"
"It is the freezing of water," said Mr. Hume. "In the process of
crystallization one hundred and forty degrees of latent heat become
sensible."
"And this," continued Mr. Wilton, "is no inconsiderable matter. Every
pound of water frozen upon the surface of our lakes and rivers, every
pound of water frozen in the wet earth, every pound of water frozen as
snow or sleet in the air, gives out as much heat as would boil an equal
amount of water at seventy-two degrees. Have you never heard of setting
tubs of water in cellars to keep vegetables from freezing?"
"I have," replied Peter. "I visited my grandfather two years ago, and his
cellar sometimes froze. I asked him why he put tubs of water in his
cellar, but he could not tell me, only he said that he knew that tubs of
water in his cellar did keep his vegetables from being nipped with the
frost."
"Can you tell us, Peter, why tubs of water set in a cellar should have
this effect?"
"I suppose that when the water begins to freeze it begins to give out its
latent heat."
"That is one part of the reason. The water is drawn from the well at
perhaps fifty degrees; it must lose eighteen degrees of heat before it
begins to freeze, and all the heat which the water loses the air of the
cellar gains. And then, as you said, as soon as the water begins to freeze
latent heat begins to become sensible. Every pound of water frozen sets
free heat enough to raise a pound of water through one hundred and forty
degrees. But why do not the vegetables begin to freeze as soon as the
water?"
"I don't know."
"Water holding salt or other minerals in solution freezes at a lower
temperature than pure water. For this reason the juices of vegetables and
fruits and the sap of trees may be cooled below thirty-two degrees
without freezing. On this account the water set in cellars tends to
prevent vegetables from freezing; the water begins to freeze at thirty-two
degrees, while potatoes and turnips may be cooled a little lower than
thirty-two degrees without harm. In this manner the buds of trees are
sometimes warmed and protected by the coating of ice which forms around
them. The drops of water, falling through the sleety air, touch upon the
twigs of trees and freeze upon them, an icy coat embracing them all
around. In freezing, the water gives out one hundred and forty degrees of
heat, a part of which goes to the air and a part to the twig."
"This reminds me," said Ansel, "of what the Irishman said on being told
that snow contains heat, that 'it would be a blessed thing for the poor if
one could tell how many snowballs it would take to boil a tea-kettle.'"
"It might be difficult to use snowballs to boil the tea-kettle, but the
heat given out in the formation of the snowflakes is doubtless employed
quite as usefully for the poor as if used in preparing their tea. You have
all noticed that before a snow-storm, or perhaps during the early part of
the storm, the temperature generally becomes milder, and you have often
heard the remark, 'It is too cold to snow.' Men have learned that the
coming of a snow-storm is attended by a warming of the air. This popular
impression is philosophical, yet few understand its philosophy. A foot of
snow falls, equal to two or three inches of water. In the condensation of
the vapor which formed this snow one thousand degrees of latent heat
become sensible, and then in the congelation of the clouds into snowflakes
one hundred and forty degrees of heat are evolved. This softening of the
rigors of winter is, I think, as great a blessing to the poor as the
heating of the tea-kettle. Let us make an estimate of the amount of heat
set free in the production of one great snow-storm. Two feet of snow
falls, equal, we will suppose, to five inches of water. In the
condensation of the watery vapor one thousand degrees of heat are evolved,
and in the congelation one hundred and forty degrees--an amount of heat
which would boil three feet of cold spring water. In every square mile
there are 27,878,400 square feet, and a square mile of water three feet in
depth would contain 83,625,200 cubic feet. The production of such a
snow-storm sets free for every square mile of surface heat which would
boil more than 80,000,000 of cubic feet of spring water. Such a storm
sometimes extends over a region of country a thousand miles square, that
is, over a million of square miles. In the production of one such storm--a
very heavy and extensive storm, I have supposed--heat is generated which
would boil eighty millions of millions (80,000,000,000,000) of cubic feet
of spring water--an amount altogether too vast for our comprehension. To
accomplish this result by combustion would require more than 500,000,000
of tons of anthracite coal--an amount at least three times as great as the
yearly product of all the coal-mines of the world. And this is but one
heavy storm. The amount of rainfall in the United States may be thirty-six
inches or forty or forty-five inches. Supposing the average rainfall of
the whole earth to be twenty-four inches--an estimate very far below the
truth--we have this result: There are, in round numbers, two hundred
millions of square miles of surface, more than five and a half
quadrillions (5,575,680,000,000,000) of square feet and more than eleven
quadrillions of cubic feet of water. The condensation of this amount of
vapor would boil more than sixty quadrillions of cubic feet of ice water.
One pound of anthracite coal burned under the most favorable circumstances
will boil sixty pounds of ice water. To boil sixty quadrillions of cubic
feet of ice water would require sixty quadrillions of pounds of
coal--thirty billions of tons--not less than twenty-five tons to every
inhabitant of the globe. At this rate a very few years would exhaust the
coal-fields of the world. Calculations like these are useful in showing
upon how stupendous a scale the Creator carries on his operations. But we
must remember that these works are carried on, not to amaze men, but to
benefit them. The works go on silently and unseen, challenging no
attention from fools, receiving no thought except from the patient student
of Nature, and eliciting no thankful recognition save from a few reverent
worshipers.
"But I have been led away from a point which I had in mind. While
considering the effect of heat in expanding bodies, I reminded you that
water presents a marked peculiarity, and promised to speak of it more
fully. This is the place for us to look at this singular and beautiful
peculiarity of water. What is the general principle touching the effect of
heat upon bodies?"
"Heat expands bodies and cold contracts them," answered Ansel.
"Water both illustrates this rule and presents some very interesting
apparent exceptions. It contracts by cold like other bodies till it
reaches the temperature of thirty-nine and a half degrees; it then begins
to expand, and expands regularly till it falls to thirty-two degrees; at
that point it freezes, and in freezing it expands at once about one-ninth
of its bulk. If the cooling process be continued, the ice produced
contracts like any other solid. This peculiarity of the interrupted and
unequal expansion of water is of the utmost importance in the affairs of
our world. Consider the result if the water were to contract by cold as do
other bodies down to the freezing point and below it. Water is cooled from
the top by contact with the cold air. As the upper film of water cooled it
would sink and a new stratum be brought to the surface; that in turn would
be cooled and sink, and thus the cooling process would go on with the
utmost rapidity till the whole body of water should be reduced to the
freezing temperature. Then congelation would begin, and the first
particles of ice formed would sink to the bottom, and as fast as the water
became frozen at the top the ice would sink. In this manner a solid body
of ice would be formed at the bottom of our lakes and rivers, while the
surface would remain unfrozen in contact with the cold air till the whole
body of water became a compact mass of ice. Great lakes turned to solid
ice would not be thawed during the whole of the summer, for the water
warmed from the top would not sink, but would form a warm stratum of water
upon the surface, while, below, the solid ice would lie hardly feeling the
summer heat. Nay, more; in the higher latitudes it would seem as if the
very ocean must be turned to solid ice, never to be melted till the end of
time. By the singular expansion of water below thirty-nine and half
degrees and its great expansion in congelation, these disastrous
consequences are prevented. Our lakes are cooled even in winter only to
thirty-nine and a half degrees; below this temperature the colder water is
lighter and remains upon the surface; ice floats upon the surface. The top
becomes ice, but the great mass of the water remains at thirty-nine
degrees, and the inhabitants of the waters live on unharmed. Spring comes,
and the ice, being upon the surface, is soon melted, and the unbound waves
begin again to ripple forth their unconscious joy."
"Do you look upon this irregular expansion and contraction of water,"
asked Mr. Hume, "as a real exception to the rule that heat expands
bodies?"
"Not at all. In freezing, a new force comes in and asserts itself--the
force of crystallization; or, more exactly, as the force of heat
diminishes the force of crystallization becomes predominant, and throws
the atoms into new positions and new relationships. To this new
arrangement of atoms is due the expansion in freezing. Ice contracts and
expands by cold and heat the same as any other solid. The attraction of
crystallization begins, doubtless, to throw the atoms into their new and
crystalline arrangement at the temperature of thirty-nine and a half
degrees.
"We must remember that the heat which is set free in the condensation of
vapor and in the freezing of water is absorbed in the formation of vapor
and the melting of water. As much heat is taken from summer as is
conferred upon winter. The summer is cooled as much as winter is warmed.
The formation of vapor is a cooling process. Water is prevented from
rising above the boiling point by the formation of vapor. Perspiration
cools us by the evaporation to which it gives rise from the whole surface
of our bodies. And the higher the temperature, the more rapid the
evaporation, and the more vigorous the cooling process.
"We might look at other appliances for transferring heat from summer to
winter, but they belong in principle to another department. We have now
looked at some of the means for transferring heat in time. The heat is
treasured up at the heated noonday, to be brought out for use during the
cool hours of night; it is garnered from the excessive heats of summer to
supply the deficiencies of winter. It is laid up in store to-day to be
expended at any future time when needed. The transfer is a transfer not in
space, but in time. We must hereafter examine those arrangements by which
heat is transported through space. Some of these arrangements exert an
influence upon day and night and upon summer and winter, and thus throw
further light upon the subjects already discussed. Already more than once
topics have been suggested and their full consideration put off till some
more fitting time. In our next lesson we must begin the examination of
these new principles. We have before spoken of the vicissitudes of days
and seasons and years. We shall now have to do with the vicissitudes of
zones and lands and seas, of deserts and mountain ranges. The elements
become vaster, the stage is broader, and the movements more sublime.
"I am glad that you are so well interested in these great and beautiful
works of God's wisdom and power, but I hope that you do not forget that
the crucified Christ is pre-eminently the power of God and the wisdom of
God. These natural works are but the husk of which salvation from sin by
Christ is the kernel. These outward things are wonderful and beautiful for
the setting, but the gem, the royal precious stone, the Koh-i-noor, the
'mountain of light,' for which the setting was made, is the true knowledge
of the true God and of his Son Jesus Christ. During the past few weeks you
have heard others asking, 'What shall we do to be saved?' I should be
greatly guilty if I allowed you to think earth, air, and sea, with all
their silent and solemn movements, more important than our spiritual
attitude toward God the Father and Christ the Saviour. Are you, Samuel, in
your interest in studying Nature, forgetting Christ and the souls of men?"
"I hope not, and I think not. During the three years since my baptism I
have never felt so much my obligation to Christ as now. I never felt
before so deep a desire that my friends should repent and believe in
Jesus. I think the love of Christ constrains me. I have not felt before
that my work was very important; I have been expecting to work more
earnestly by and by; but lately I have felt that Christ gives me something
to do now for which he holds me responsible."
"What have you tried to do for Christ?"
"I have been praying for some of my young friends, and especially for
Ansel and Peter. And then I felt that I must talk with them as well as
pray for them."
"And can you, my young friends, be careless about your own salvation while
Samuel is so anxious for you? Are you contented to live 'having no hope
and without God in the world'? Is your happiness here and hereafter more
important to Samuel than to yourselves?"
"We are interested," said Ansel. "We have been talking together about
being Christians, but we don't know what to do."
"They said," broke in Samuel, "that they wished I would ask you to preach
a sermon and tell them what they must do to be saved. They wished to go on
with these lessons, but they thought that perhaps you would be willing to
preach a sermon just upon that subject."
"You know that I often speak of that subject, and when persons have come
to the inquiry-meeting I have told them what they must do. But I know that
there must needs be 'line upon line.' If Ansel and Peter wish it, I will
devote a sermon to the subject, and make it as plain as I can. Hardly
anything gives me more pleasure than to explain the way of salvation when
I know that my hearers are interested."
"We do wish to have you preach upon that subject, and I am sure that you
will have a great many interested hearers besides Ansel and myself."
"But, Samuel, did you not pray for Mr. Hume also, and talk with him?"
"I prayed for him, but I was afraid to speak with him. I have tried to
pray for him a double portion because I could not speak with him."
Tears gathered in Mr. Hume's eyes; the thought came to him that his
unbelief had raised a barrier between himself and both God and his people.
This pious young man was afraid to come to him lest he should meet the
scornful arguments and cold derision of a proud unbeliever. He felt
humbled--he, a subtle, well-read unbeliever, and Samuel a pious lad
yearning for the salvation of his soul, but daring only to pray in secret
for him.
"Have not you, Mr. Hume, been treating Christ and the Holy Spirit as
Samuel feared that you would treat him?"
"Perhaps so," he answered. "I am sorry that Samuel did not come to me
freely. I think he need not be afraid of me now. I also hope you will
preach the sermon which Ansel and Peter wish to hear."
Mr. Wilton assured them that he would do as they wished unless the Spirit
clearly drew him to some other subject. "I always look," he said, "to the
Holy Spirit for direction in my preaching. 'When he, the Spirit of truth,
is come,' said Jesus, 'he will lead you into all truth.' This was
fulfilled pre-eminently, I suppose, in the inspired men who laid the
foundation of the Church, but the Spirit still dwells in believers and
leads those who love and follow Christ. The preacher of the gospel can do
nothing without the power of the Spirit of God."
And I, kind reader, will give you the outline of the sermon if the Spirit
bids him preach it.