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CHAPTER VIII. MODIFICATION OF TEMPERATURE.
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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-being1 of the human race. The different zones give the world a richer and more varied2 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 ripens3 the fruit and grain, and the cold of winter tones up the physical strength; nay4, the winter’s frost is a natural subsoiler, loosening up the hard earth and promoting vegetable growth. As for man’s[Pg 177] 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 virtues5, and religion are greatly indebted to our winters. But were these agencies which tend to produce inequality of temperature suffered to operate without counteracting6 influences, the extremes of heat and cold would cease to be genial7 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 fluctuation8 of temperature caused by the revolution of the earth upon its axis9. The rotation10 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 diurnal11 change of temperature alleviated12?”

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[Pg 178] replied—“first, the absorption of heat during the day and the radiation of that heat during the night; and, secondly13, the formation of watery14 vapor15 during the day and the deposition16 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 conspicuous17 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[Pg 179] 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 fluctuations18 of temperature by the process of evaporation19 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 Alpine20 glaciers21, or rivers of ice, sink away[Pg 180] 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 elastic22 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.[Pg 181] 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 elasticity23; 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 ascending24 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 [Pg 182]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 condensation25 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 kindled26 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[Pg 183] 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 diminution27 of the temperature must, of course,[Pg 184] cause dew to be deposited. Do you know, Ansel, how to ascertain28 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 pitcher29 or goblet30 whose outer surface has been wiped perfectly31 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.”

[Pg 185]“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[Pg 186] 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 exhausted32 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[Pg 187] 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 pointed33 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[Pg 188] 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 commingled34, thus securing a more even temperature upon the land. This softens35 the [Pg 189]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 refreshing36 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 salvation37, and the ways of divine providence38 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.”

点击收听单词发音收听单词发音  

1 well-being Fe3zbn     
n.安康,安乐,幸福
参考例句:
  • He always has the well-being of the masses at heart.他总是把群众的疾苦挂在心上。
  • My concern for their well-being was misunderstood as interference.我关心他们的幸福,却被误解为多管闲事。
2 varied giIw9     
adj.多样的,多变化的
参考例句:
  • The forms of art are many and varied.艺术的形式是多种多样的。
  • The hotel has a varied programme of nightly entertainment.宾馆有各种晚间娱乐活动。
3 ripens 51963c68379ce47fb3f18e4b6ed340d0     
v.成熟,使熟( ripen的第三人称单数 )
参考例句:
  • The sun ripens the crops. 太阳使庄稼成熟。 来自《简明英汉词典》
  • Then their seed ripens, and soon they turn brown and shrivel up. 随后,它们的种子熟了,不久就变枯萎。 来自辞典例句
4 nay unjzAQ     
adv.不;n.反对票,投反对票者
参考例句:
  • He was grateful for and proud of his son's remarkable,nay,unique performance.他为儿子出色的,不,应该是独一无二的表演心怀感激和骄傲。
  • Long essays,nay,whole books have been written on this.许多长篇大论的文章,不,应该说是整部整部的书都是关于这件事的。
5 virtues cd5228c842b227ac02d36dd986c5cd53     
美德( virtue的名词复数 ); 德行; 优点; 长处
参考例句:
  • Doctors often extol the virtues of eating less fat. 医生常常宣扬少吃脂肪的好处。
  • She delivered a homily on the virtues of family life. 她进行了一场家庭生活美德方面的说教。
6 counteracting 5c99b70b8018c41ba8de9c512f4d61e1     
对抗,抵消( counteract的现在分词 )
参考例句:
  • The turmoil, he said, was "counteracting the course of global civilization. " 这次骚乱,他指出,“阻碍了世界文明的进程”。
  • But he notes that there are counteracting forces as well. 但是他也指出还有一些抵消因素。
7 genial egaxm     
adj.亲切的,和蔼的,愉快的,脾气好的
参考例句:
  • Orlando is a genial man.奥兰多是一位和蔼可亲的人。
  • He was a warm-hearted friend and genial host.他是个热心的朋友,也是友善待客的主人。
8 fluctuation OjaxE     
n.(物价的)波动,涨落;周期性变动;脉动
参考例句:
  • The erratic fluctuation of market prices are in consequence of unstable economy.经济波动致使市场物价忽起忽落。
  • Early and adequate drainage is essential if fluctuation occurs.有波动感时,应及早地充分引流。
9 axis sdXyz     
n.轴,轴线,中心线;坐标轴,基准线
参考例句:
  • The earth's axis is the line between the North and South Poles.地轴是南北极之间的线。
  • The axis of a circle is its diameter.圆的轴线是其直径。
10 rotation LXmxE     
n.旋转;循环,轮流
参考例句:
  • Crop rotation helps prevent soil erosion.农作物轮作有助于防止水土流失。
  • The workers in this workshop do day and night shifts in weekly rotation.这个车间的工人上白班和上夜班每周轮换一次。
11 diurnal ws5xi     
adj.白天的,每日的
参考例句:
  • Kangaroos are diurnal animals.袋鼠是日间活动的动物。
  • Over water the diurnal change in refraction is likely to be small. 在水面上,折光的周日变化可能是很小的。
12 alleviated a4745257ebd55707de96128297f486e1     
减轻,缓解,缓和( alleviate的过去式和过去分词 )
参考例句:
  • It is always completely alleviated by total gastrectomy. 全胃切除永远完全缓解症状。
  • Toxicity problem in manufacturing and storage might be alleviated by coating beryllium with aluminum. 但如果用铝包覆铍,则可避免加工过程中及储存期间的中毒问题。
13 secondly cjazXx     
adv.第二,其次
参考例句:
  • Secondly,use your own head and present your point of view.第二,动脑筋提出自己的见解。
  • Secondly it is necessary to define the applied load.其次,需要确定所作用的载荷。
14 watery bU5zW     
adj.有水的,水汪汪的;湿的,湿润的
参考例句:
  • In his watery eyes there is an expression of distrust.他那含泪的眼睛流露出惊惶失措的神情。
  • Her eyes became watery because of the smoke.因为烟熏,她的双眼变得泪汪汪的。
15 vapor DHJy2     
n.蒸汽,雾气
参考例句:
  • The cold wind condenses vapor into rain.冷风使水蒸气凝结成雨。
  • This new machine sometimes transpires a lot of hot vapor.这部机器有时排出大量的热气。
16 deposition MwOx4     
n.免职,罢官;作证;沉淀;沉淀物
参考例句:
  • It was this issue which led to the deposition of the king.正是这件事导致了国王被废黜。
  • This leads to calcium deposition in the blood-vessels.这导致钙在血管中沉积。
17 conspicuous spszE     
adj.明眼的,惹人注目的;炫耀的,摆阔气的
参考例句:
  • It is conspicuous that smoking is harmful to health.很明显,抽烟对健康有害。
  • Its colouring makes it highly conspicuous.它的色彩使它非常惹人注目。
18 fluctuations 5ffd9bfff797526ec241b97cfb872d61     
波动,涨落,起伏( fluctuation的名词复数 )
参考例句:
  • He showed the price fluctuations in a statistical table. 他用统计表显示价格的波动。
  • There were so many unpredictable fluctuations on the Stock Exchange. 股票市场瞬息万变。
19 evaporation Pnoxc     
n.蒸发,消失
参考例句:
  • Be careful not to lose too much liquid by evaporation.小心不要因蒸发失去太多水分。
  • Our bodies can sweat,thereby losing heat by evaporation.我们的身体能出汗,由此可以蒸发散热。
20 alpine ozCz0j     
adj.高山的;n.高山植物
参考例句:
  • Alpine flowers are abundant there.那里有很多高山地带的花。
  • Its main attractions are alpine lakes and waterfalls .它以高山湖泊和瀑布群为主要特色。
21 glaciers e815ddf266946d55974cdc5579cbd89b     
冰河,冰川( glacier的名词复数 )
参考例句:
  • Glaciers gouged out valleys from the hills. 冰川把丘陵地带冲出一条条山谷。
  • It has ice and snow glaciers, rainforests and beautiful mountains. 既有冰川,又有雨林和秀丽的山峰。 来自英语晨读30分(高一)
22 elastic Tjbzq     
n.橡皮圈,松紧带;adj.有弹性的;灵活的
参考例句:
  • Rubber is an elastic material.橡胶是一种弹性材料。
  • These regulations are elastic.这些规定是有弹性的。
23 elasticity 8jlzp     
n.弹性,伸缩力
参考例句:
  • The skin eventually loses its elasticity.皮肤最终会失去弹性。
  • Every sort of spring has a definite elasticity.每一种弹簧都有一定的弹性。
24 ascending CyCzrc     
adj.上升的,向上的
参考例句:
  • Now draw or trace ten dinosaurs in ascending order of size.现在按照体型由小到大的顺序画出或是临摹出10只恐龙。
25 condensation YYyyr     
n.压缩,浓缩;凝结的水珠
参考例句:
  • A cloud is a condensation of water vapour in the atmosphere.云是由大气中的水蒸气凝结成的。
  • He used his sleeve to wipe the condensation off the glass.他用袖子擦掉玻璃上凝结的水珠。
26 kindled d35b7382b991feaaaa3e8ddbbcca9c46     
(使某物)燃烧,着火( kindle的过去式和过去分词 ); 激起(感情等); 发亮,放光
参考例句:
  • We watched as the fire slowly kindled. 我们看着火慢慢地燃烧起来。
  • The teacher's praise kindled a spark of hope inside her. 老师的赞扬激起了她内心的希望。
27 diminution 2l9zc     
n.减少;变小
参考例句:
  • They hope for a small diminution in taxes.他们希望捐税能稍有减少。
  • He experienced no diminution of his physical strength.他并未感觉体力衰落。
28 ascertain WNVyN     
vt.发现,确定,查明,弄清
参考例句:
  • It's difficult to ascertain the coal deposits.煤储量很难探明。
  • We must ascertain the responsibility in light of different situtations.我们必须根据不同情况判定责任。
29 pitcher S2Gz7     
n.(有嘴和柄的)大水罐;(棒球)投手
参考例句:
  • He poured the milk out of the pitcher.他从大罐中倒出牛奶。
  • Any pitcher is liable to crack during a tight game.任何投手在紧张的比赛中都可能会失常。
30 goblet S66yI     
n.高脚酒杯
参考例句:
  • He poured some wine into the goblet.他向高脚酒杯里倒了一些葡萄酒。
  • He swirled the brandy around in the huge goblet.他摇晃着高脚大玻璃杯使里面的白兰地酒旋动起来。
31 perfectly 8Mzxb     
adv.完美地,无可非议地,彻底地
参考例句:
  • The witnesses were each perfectly certain of what they said.证人们个个对自己所说的话十分肯定。
  • Everything that we're doing is all perfectly above board.我们做的每件事情都是光明正大的。
32 exhausted 7taz4r     
adj.极其疲惫的,精疲力尽的
参考例句:
  • It was a long haul home and we arrived exhausted.搬运回家的这段路程特别长,到家时我们已筋疲力尽。
  • Jenny was exhausted by the hustle of city life.珍妮被城市生活的忙乱弄得筋疲力尽。
33 pointed Il8zB4     
adj.尖的,直截了当的
参考例句:
  • He gave me a very sharp pointed pencil.他给我一支削得非常尖的铅笔。
  • She wished to show Mrs.John Dashwood by this pointed invitation to her brother.她想通过对达茨伍德夫人提出直截了当的邀请向她的哥哥表示出来。
34 commingled f7055852d95e8d338b4df7040663fa94     
v.混合,掺和,合并( commingle的过去式和过去分词 )
参考例句:
  • Tears commingled with the blood from the cut on his face. 眼泪和他脸上伤口流的血混在一起了。 来自《简明英汉词典》
  • Fact is inextricably commingled with fiction. 事实与虚构混杂难分。 来自《简明英汉词典》
35 softens 8f06d4fce5859f2737f5a09a715a2d27     
(使)变软( soften的第三人称单数 ); 缓解打击; 缓和; 安慰
参考例句:
  • Iron softens with heat. 铁受热就软化。
  • Moonlight softens our faults; all shabbiness dissolves into shadow. 月光淡化了我们的各种缺点,所有的卑微都化解为依稀朦胧的阴影。 来自名作英译部分
36 refreshing HkozPQ     
adj.使精神振作的,使人清爽的,使人喜欢的
参考例句:
  • I find it'so refreshing to work with young people in this department.我发现和这一部门的青年一起工作令人精神振奋。
  • The water was cold and wonderfully refreshing.水很涼,特别解乏提神。
37 salvation nC2zC     
n.(尤指基督)救世,超度,拯救,解困
参考例句:
  • Salvation lay in political reform.解救办法在于政治改革。
  • Christians hope and pray for salvation.基督教徒希望并祈祷灵魂得救。
38 providence 8tdyh     
n.深谋远虑,天道,天意;远见;节约;上帝
参考例句:
  • It is tempting Providence to go in that old boat.乘那艘旧船前往是冒大险。
  • To act as you have done is to fly in the face of Providence.照你的所作所为那样去行事,是违背上帝的意志的。


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