Vegetable Need of Protection Illustrated. — Observed Fallacies and Reasonable Contradictions.—Laws of Heat Radiation Demonstrated. —Nightly Atmospheric Heating.—Condition and Elevation of Air Favorable to Vegetable Life.—Atmospheric Vapor, How Supplied. —"The Benefits of Transpiration of Forests. — Observations in Europe, and What they Prove.—A Conclusion Established.—Adduced Facts.—Motion of the Atmosphere.—Liquid and Aerial Motion Contrasted.—Aerial Motion Illustrated.—Protective Systems and their Controlling Influences.—Experienced Facts versus Theory.—A Study for the Orchardist and Farmer.—Experienced Testimony on the Influence of Shelter-Belts.
The following article from the pen of Professor Gale speaks for itself, and I need make no apology for inserting it here.
Both animal and vegetable life need protection. Nor do we all see eye to eye in regard to the theory of protection. This is well illustrated in the following statement from a late number of the Scientific American: 'A well-grown evergreen - tree gives off continually an exodium of warmth and moisture that reaches a distance of its area in height; when the tree-planters advocate shelter - belts surrounding a tract of fifty or more acres, when the influence of such belt can only reach the height of the trees of such belt, they do that which will prove of little value.' There are two fallacies here. First, that the climatic influence of a tree arises from its power to send off an' exodium of warmth' into the surrounding atmosphere. In relation to this we will only ask how many Christmas-trees will be required to keep our parlor warm next winter ? The second fallacy is that shelter-belts can effect climatic changes only through their power to send off an ' exodium of warmth.' While the writer of this article may have aimed at a very good thing, he has certainly missed the point as far as shelter-belts are concerned.
Holding that forest-culture in Kansas can be made a success, and that it is necessary to the prosperous settlement of the state, we desire to prove that forest-culture in the form of extended and carefully arranged shelter-belts must have efficient climatic influence. In proof of this let us state some of the simple laws which govern the radiation of heat and the motion of the atmosphere.
Laws of heat.
1. Heat is radiated from all bodies and in all directions, the angle of incidence and of reflection being equal.
2. Heat of high intensity passes almost unobstructed through some bodies, while the* same bodies are opaque to heat of a lower intensity; thus the sun sends its intense heat through the glass into the green-house, while the plants cannot radiate that heat back again through the glass into the open air. This fact can be illustrated by a heated ball and a plate of glass, showing the heat of low intensity is almost entirely retained by the glass. The vapor of water operates almost like the plate of glass, permitting the free passage of the heat from the sun, but checking very largely the radiation from the earth. Thus an atmosphere saturated with vapor will check radiation with seventy times the power of a dry atmosphere.
3. The point of saturation varies with the temperature of the atmosphere. Then the cooler the atmosphere the drier it will be, and hence the more rapid the radiation of heat; or, the drier the atmosphere under any circumstances, the more rapid the radiation of heat. It is calculated by Professor Tyndall that one tenth of the heat radiated from the earth is retained within ten feet of the earth's surface by the vapor held in the atmosphere.
4. It is found that during the night-time the atmosphere becomes sensibly warmer to the height of one hundred and fifty feet, as shown in the following table:
Let the thermometer upon the grass represent zero, and at one inch above the grass it will read three degrees higher; and
At 6 inches above the grass it will read
You will notice that two thirds of the entire rise of temperature occurs below twelve feet, and five sixths of the increase in temperature below fifty feet. That is, the vapor within fifty feet of the earth is five times more important to vegetable life than that contained within one hundred feet above that point, and the vapor within twelve feet of the earth's surface has twice as much influence upon climatic conditions as one hundred and thirty-eight feet of atmosphere above that point.
These facts lead us at once to the conclusion that, as far as vegetable life is concerned, we are most interested in the condition of the air within twelve or fifteen feet of the earth's surface, and that a vapor-laden atmosphere near the surface of the earth, not subject to violent commotion, must be a matter of the gravest moment.
Now it is well known that vegetable life, as well as the earth itself, is sending off continually a vast amount of vapor into the atmosphere. Every spear of grass and every leaf is pumping up the moisture from the earth, and sending it forth into the air in the form of vapor, thus giving the earth a glassy covering, opaque to radiated heat of a low intensity. The amount of water drawn from the soil by growing trees and given off in the form of vapor from the leaves is simply immense. Thus it is stated that the eucalyptus of Australia will absorb ten times its weight in a single day (Kept. No. 159, H. E. U. S., on Timber Culture, page 94). A small pear-tree has been found to absorb and give off more than its own weight of water in forty hours. The effect of this transpiration is seen in the prevailing moisture of the forest. We have only to surround a house with a dense growth of timber, and we learn the immediate result in the dampness and mildew which pervade the dwelling. Hence the amount of moisture pumped up by the growing trees, often from great depths, can hardly be measured. This process will be constantly varying in its activity with the conditions of vegetable life.
Extended observations in Europe have proved that there is a marked excess in the rainfall of an extensive forest over that of the open country. This should be expected, since the falling rain, as it reaches the prevailing moisture of the forest, must condense and carry much of its vapor to the ground.
If the positions above taken be correct, we should expect that wooded lands should be cooler than the open fields in the daytime, arid warmer in the night; and such a conclusion has been clearly established by extended observations, made under the direction of the Bavarian government during the last six years.
The facts adduced prove that all vegetable life will cover itself with a glassy mantle, in density proportioned to the luxuriance of growth, and nearly opaque to the heat radiated from the earth.
How can this glassy mantle be retained as a nightly and constant protection to vegetable life, or must it be swept away by the prevailing winds ? To answer this question intelligently, we must consider briefly some of the simple laws which govern atmospheric motion.