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drainage necessary. It is a fixed rule that water, descending in the soil, will find the lowest outlet to which there exists a channel through which it can flow, and that if, after heavy rains, it rise too near the surface of the ground, the proper remedy is to tap it at a lower level, and thus remove the water table to the proper distance from the surface. This subject will be more fully treated in a future chapter, in considering the question of the depth, and the intervals, at which drains should be placed.
Evaporation.—By evaporation is meant the process by which a liquid assumes the form of a gas or vapor, or "dries up." Water, exposed to the air, is constantly undergoing this change. It is changed from the liquid form, and becomes a vapor in the air. Water in the form of vapor occupies nearly 2000 times the space that it filled as a liquid. As the vapor at the time of its formation is of the same temperature with the water, and, from its highly expanded condition, requires a great amount of heat to maintain it as vapor, it follows that a given quantity of water contains, in the vapory form, many times as much heat as m the liquid form. This heat is taken from surrounding substances,—from the ground and from the air,—which are thereby made much cooler. For instance, if a showei moisten the ground, on a hot summer day, the drying up ot the water will cool both the ground and the air. If we place a wet cloth on the head, and hasten the evaporation of the water by fanning, we cool the head; if we wrap a wet napkin around a pitcher of water, and place it in a current of air, the water in the pitcher is made cooler, by giving up its heat to the evaporating water of the napkin; when we sprinkle water on the floor of a room, its evaporation cools the air of the room.
So great is the effect of evaporation, on the temperature of the soil, that Dr. Madden found that the soil of ■ drained field, in which most of the water was removed from below, was 6J-° Far. warmer than a similar soil un> drained, from which the water had to be removed by evaporation. This difference of 6£° is equal to a differeno* of elevation of 1,950 feet.
It has been found, by experiments made in England, that the average evaporation of water from wet soils is equal to a depth of two inches per month, from May to August, inclusive; in America it must be very much greater than this in the summer months, but this is surely enough for the purposes of illustration, as two inches of water, over an acre of land, would weigh about two hundred tons. The amount of heat required to evaporate this is immense, and a very large part of it is taken from the soil, which, thereby, becomes cooler, and less favorable for a rapid growth It is usual to speak of heavy, wet lands as being " cold," and it is now seen why they are so.
If none of the water which falls on a field is removed by drainage, (natural or artificial,) and if none runs off from the surf-ice, the whole rain-fall of a year must be removed by evaporation, and the cooling of the soil will be proportionately great. The more completely we withdraw this water from the surface, and carry it off in under-ground drains, the more do we reduce the amount to be removed by evaporation. In land which is well drained, the amount evaporated, even in summer, will not be sufficient to so lower the temperature of the soil as to retard the growth of plants; the small amount dried out of the particles of the soil, (water of absorption,) will only keep it from being raised to too great a heat by the mid-summer sun.
An idea of the amount of heat lost to the soil, by the evaporation of water, may be formed from the fact that to evaporate, by artificial heat, the amount of water contained in a rain-fall of two inches on an acre, (200 tons,) would require over 20 tons of coal. Of course a considerable— probably by far the larger,—part of the heat taken up in the process of evaporation is furnished by the air; but the amount abstracted from the soil is great, and is in direct proportion to the amount of water removed by this process; hence, the more we remove by draining, the more heat we retain in the ground.
The season of growth is lengthened by draining, be cause, by avoiding the cooling effects of evaporation, germination is more rapid, and the young plant grows steadily from the start, instead of struggling against the retarding influence of a cold soil.
Temperature.—The temperature of the soil has great effect on the germination of seeds, the growth of plants, and the ripening of crops.
Gisborne says: "The evaporation of 1 lb. of water "lowers the temperature of 100 lbs. of soil 10°,—that is "to say, that, if to 100 lbs. of soil, holding all the water "it can by attraction, but containing no water of drain"age, is added 1 lb. of water which it has no means of "discharging, except by evaporation, it will, by the time "that it has so discharged it, be 10° colder than it would "have been, if it had the power of discharging this 1 lb. "by filtration; or, more practically, that, if rain, entering "in the proportion of 1 lb. to 100 lbs. into a retentive "soil, which is saturated with water of attraction, is dis"charged by evaporation, it lowers the temperature of ** that soil 10°. If the soil has the means of discharging "that 1 lb. of water by filtration, no effect is produced beu yond what is due to the relative temperatures of the "rain and of the soil."
It has been established by experiment that four timei as much heat is required to evaporate a certain quantity of water, as to raise the same quantity from the freezing to the boiling point.
It is, probably, in consequence of this cooling effect of evaporation, that wet lands are warmest when shaded, because, under this condition, evaporation is lest active Such lands, in cloudy weather, form an unnatural growth, such as results in the "lodging" of grain crops, from the; deficient strength of the straw which this growth produces.
In hot weather, the temperature of the lower soil is, of course, much lower than that of the air, and lower than that of the water of warm rains. If the soil is saturated with water, the water will, of course, be of an even temperature with the soil in which it lies, but if this be drained off, warm air will enter from above, and give its heat to the soil, while each rain, as it falls, will also carry its heat with it. Furthermore, the surface of the ground is sometimes excessively heated by the summer sun, and the heat thus contained is carried down to the lower soil by the descending water of rains, which thus cool the surface and warm the subsoil, both beneficial.
Mr. Josiah Parkes, one of the leading draining engineers of England, has made some experiments to test the extent to which draining affects the temperature of the soil. The results of his observations are thus stated by Gisborne: "Mr. Parkes gives the temperature on a "Lancashire flat moss, but they only commence 7 inches "below the surface, and do not extend to mid-summer. "At that period of the .year the temperature, at 7 inches, "never exceeded 66°, and was generally from 10° to 15° "below the temperature of the air in the shade, at 4 feet "above the earth. Mr. Parkes' experiments were made "simultaneously, on a drained, and on an undrained por"tion of the moss; and the result was, that, on a mean "of 35 observations, the drained soil at 7 inches in depth "was 10° warmer than the undrained, at the same depth. "The undrained soil never exceeded 47°, whereas, after a u thunder storm, the drained reached 66° at 7 inches, ana "48° at 31 inohes. Such were the effects, at an early u period of the year, on a black bog. They suggest some
"idea of what they were, when, in July or August, thunder "rain at 60° or 70° falls on a surface heated to 130°, and
carries down with it, into the greedy fissures of the earth, "its augmented temperature. These advantages, porous "soils possess by nature, and retentive ones only acquire "them by drainage."
Drained land, being more open to atmospheric circulation, and having lost the water which prevented the tern.perature of its lower portions from being so readily affected by the temperature of the air as it is when dry, . will freeze to a greater depth in winter and thaw out earlier in the spring. The deep freezing has the effect to greatly pulverize the lower soil, thus better fitting it for the support of vegetation; and the earlier thawing makes it earlier ready for spring work.
Drought.—At first thought, it is not unnatural to sup pose that draining will increase the ill effect of too dry seasons, by removing water which might keep the soil moist. Experience has proven, however, that the result is exactly the opposite of this. Lands which suffer most from drought are most benefited by draining,—more in their greater ability to withstand drought than in any other particular. This applies to heavy soils.
The reasons for this action of draining become obvious, when its effects on the character of the soil are examined. There is always the same amount of water in, and about, the surface of the earth. In winter there is more in the soil than in summer, while in summer, that which has been dried out of the soil exists in the atmosphere in the form of a vapor. It is held in the vapory form by heatx which may be regarded as braces to keep it distended. When vapor comes in contact with substances sufficiently colder than itself, it gives up its heat,—thus losing its braces,—contracts, becomes liquid water, and is deposited u dew.