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 extreme............