one knows, or may know, that if into two flower pots, with holes in the bottom, are put respectively equal portions of gravel and clay, equally heated to any point short of torrefaction, and if equal quantities of water are administered to the surface of each, water (water of drainage) will run from the gravel long before it begins to run from the clay. Gravel can hold by attraction much less water than clay can. At the time when each is saturated by water of attraction, and neither holds any water of drainage, evaporation will begin to act upon the water in each, and will act most strongly in the vegetative period of the year. The cold produced will be in proportion to the quantities of water evaporated respectively, and will, of course, be greatest in the retentive soil. We will reserve a farther cause of coolness in retentive soils, which is also connected with evaporation, till we have spoken of the depths of drains."

"The temperature of retentive soils is very much raised during that period of the year in which vegetation is active, by the removal of water by drainage."

Many experiments have shown that in retentive soils the temperature at 2 or 3 feet below the surface of the water-table is at no period of the year higher than from 46° to 48°, that is, in agricultural Britain. This temperature is little affected by summer heats, for the following short reasons. Water, in a quiescent state, is one of the worst conductors of heat with which we are acquainted. Water warmed at the surface transmits little or no heat downwards. The small portion warmed expands, becomes lighter than that below, consequently retains its position on the surface, and carries no heat downwards. To ascertain the mean heat of the air at the surface of the earth, over any extended space, and for a period of eight or nine months, is no simple operation. More elements enter into such a calculation than we have space or ability to enumerate; but we know certainly that for seven months in the year, air, at the surface of the ground, is seldom lower than 48°, never much lower, and only for short periods; whereas at 4 feet from the surface, in the shade, from 70° to 80° is not an unusual temperature, and in a southern exposure, in hot sunshine, double that temperature is not unfrequently obtained on the surface."

"Now let us consider the effect of drains placed from 2 to 3 feet below the water-table, and acting during the seven months of which we have spoken. They draw out water of the temperature of 48°. Every particle of water which they withdraw at this temperature is replaced by an equal bulk of air, at a higher, and frequently at a much higher temperature. The warmth of the air is carried down into the earth. The temperature of the soil, to the depth to which the water is removed, is in a course of constant assimilation to the temperature of the air at the surface."

"From this it follows, necessarily, that during that period of the year when the temperature of air at the surface of the earth is generally below 48°, retentive soils, which have been drained, are colder than those which have not.”

“There are no satisfactory British experiments with reference to the surface-heat of the earth. Professor Leslie's only commence at 1 foot below the surface. Schubler's experiments, made near Genoa, in the year 1796, are strictly superficial. His thermometers were sunk in the soil only to the depth of 1-12th of an inch. In that sunny clime he found the mean heat of soil, at that depth, to be at noon for six successive months, 131°. If that were his mean heat for six months, we cannot doubt that it is frequently obtained as an extreme heat in the hottest portion of our year, in England."

"Mr. Parkes gives temperatures on a Lancashire Peat Moss, but they only commence at 7 inches below the surface, and do not extend to midsummer. 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 air in the shade, at 4 feet above the earth. At the depth of 13 inches, the soil was generally from 5° to 8° cooler than at 7 inches."

"Mr. Parkes' experiments were made simultaneously on a drained and on an undrained portion 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 thunderstorm the drained reached 66°, at 7 inches, and 48° at 31 inches.

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Such were the effects at an early period of the year on a black bog. They suggest some idea of what they are, 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 soils only acquire them by drainage."

How then shall we best get these advantages by the aid of drains? how deep shall the drain be in the soil? and how wide apart on the surface? That some absolute depth is right for average soils is self-evident, whilst different qualities of soil will vary this depth and distance.

We should not forget, in considering that the reasoning in the foregoing and following quotations is applied to England and Scotland, that the facts and deductions from them are equally applicable to America. The depth of the drains below the surface in all ordinary cases depends upon these two considerations: from what depth does water prefer to arise to the surface by evaporation, or to descend into subsoil drainage, and how much does the escape of water by natural evaporation affect the temperature of the soil.

Careful experiments have shown that the heat necessary to convert 1 pound of water into vapor by sun evaporation is just as great as to convert it into steam over a fire, and that the fire heat contained in 2 or 3 ounces of coal is that amount.

By consequence, then, were all the water which falls upon an acre of land in a year, 40 inches or 4,040 tons, to be so removed, there would be for each day 11 tons, which would consume the heat of 32 cwt. of coal per hour. We know that all this water is not so removed, but all that is removed by evaporation consumes heat pro rata; and the amount we know, under all circumstances, is large, whilst in saturated land it is enormous.

Bring before your imagination an acre of close clay land, nearly impermeable by water; in its natural condition, this land, in early spring, is unapproachable for its saturation; by degrees it dries, and is meagrely tilled, the crops feebly budding into life are not only grievously choked and poisoned by the standing water, but

also the heat, so vitally necessary to their rapid growth, is taken away at the rate of 24 cwt. of burning coal per hour, whilst the plants are half frozen."

"It has been proved that the heat of a pound of water in a state of steam would raise the temperature of 1,000 pounds of water one degree, and consequently the heat abstracted to convert one pound of water in the soil into steam, reduces 1,000 pounds of earth one degree, or 500 lbs. two degrees, and so on."

Besides this abstraction of heat by evaporating water, another solid objection should be considered: that water is the best of nonconductors of heat. If a kettle of water is heated below, it soon boils by the rising of the lighter hot water to the top, and the falling of the heavy drops at the top. But if a fire were made on top of the kettle, it would never warm, as the hot and light particles are at the top, and cannot ascend and allow new cold particles to have access to the fire.

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Apply this to poor land lying exposed to the heat of the sun, the warmth of whose rays may penetrate into the ground only so fast as the water evaporates and reduces its level. But although, so poor a conductor, it is a very rapid radiator, as we have seen. Were the soil warmed, we will say to 60°, and the water at any depth, not over two feet, to 40°, the water would rapidly abstract the heat and rise into the air, carrying away with it as is above. seen the warmth of thousands of pounds of soil.”

But let the land be readily permeable by water and it becomes a carrier of heat, not a remover. Supposing the water to be that of a warm thunder-storm, say warmed to 70°, it will run down into the soil to the drains, carrying that warmth and imparting it on its way; or, supposing the upper stratum to be warmed to 70° or 120°, the water which falls upon it instead of standing there, awaiting to absorb heat enough to evaporate, and so cooling the soil to its dead loss, now runs through it, abstracting much of its heat, and carrying it down to the subsoil, and; of course, as it gradually cools, imparting its heat to the soil.

We are thus brought to the edge of the question of how deep drains are to be. Set a pot containing a plant into a saucer full of water; at first the water will rise as high in the pot, or owing to

capillary attraction a little higher than, the water in the saucer. The top of this water is the water table; from this point it rises by evaporation through the soil till it appears at the top, and is thence discharged into the air.

Observe how largely heat must be abstracted to make the water thus escape; and it is for this reason, though perhaps not knowing how to explain it, that gardeners object to plants standing in water.

So, too, whenever water stands in soils it assumes a level which we call the water table and from its top it will rise to the surface by evaporation. How low ought this water table to be, to reduce its injurious action to a minimum? Experiments have shown that the water in the soil, from its surface to a depth of not less than 30 inches, is colder by several degrees than below that depth; or in other words, the evaporation acts upon the water to that depth, and consequently cools the water and the earth holding it; below that depth it does not affect it materially; only water enough rises to supply the roots of plants, which will descend if the strata is open to the water level, if it be even 4 feet below the surface. Not less than 30 inches should be the depth to the top of the water table, and as much lower as circumstances will admit, until we arrive at 4 feet, where the temperature seems to be about the same with water at much greater depths. At first, we might suppose that deep drains would not remove the water with rapidity after a rain, but, on the contrary, in lands drained at different depths, the drains 4 feet deep begin to run the soonest and stop the earliest, showing that the flood about the roots, immediately after a shower, is the water of evaporation, condensed and checked, together with the water of saturation, not the rain. For instance, Mr. Parkes relates, p. 153, Royal Ag. Soc. for 1844, "that on the 7th and 8th November, rain fell by guage to the depth of 48 of an inch. On the 9th, he inspected some drains on Mr. Hammond's farm, and found that after a rain of 12 hours' duration on the 7th, in a nine-acre piece, the drains 3 feet deep were just dribbling, whilst in a four-acre hop ground adjoining, the four-feet drains were already exhausted."

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Mr. Hammond states that after the late rains Feb. 17, 1844, a drain 4 feet deep ran 8 pints of water, whilst another, 3 feet deep, ran 5 pints, although placed at equal distances.