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[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 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[pg 178] 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[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 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[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 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.[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 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 [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 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[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 diminution of the temperature must, of course,[pg 184] 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.”

[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 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[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 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[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 commingled, thus securing a more even temperature upon the land. this softens 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 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.”