"Now, if your correspondent's soil and subsoil is similar to that soil I would advise him to feel his way cautiously in draining. Certainly, no man would be fool enough to dig ditches and lay tile, if there is no water to carry off."
In the Country Gentleman of Nov. 18th, 1858, we find an interesting statement, by John S. Pettibone, of Manchester, Vermont, partly in reply to the statement of Mr. Johnston.
The experiment by Mr. Pettibone, showing the increased permeability of clay, merely by the passage of water through it, is very interesting. He says, in his letter to the editor:
"When so experienced a drainer as Mr. Johnston expresses an opinion that some soils cannot be drained, it is important we should know what the soil is which cannot be drained. He uses the word stiff blue clay, as descriptive of the soil which cannot be drained. * * *
"I had taken a specimen of what I thought to be stiff blue clay. That clay, when wet, as taken out, would hold water about as well as iron: yet, from experiments I have made, I am confident that such clay soil can be drained, and at much less expense than a hard-pan soil. Water will pass through such clay, and the clay become dry; and after it becomes once dry, water will, I am convinced, readily pass down through such stiff blue clay. The specimen was taken about three feet below the surface, and on a level with a brook which runs through this clay soil. I filled a one hundred-pound nail-keg with clay taken from the same place. It was so wet, that by shaking, it came to a level, and water rose to the top of the clay. I had made holes in the bottom of the keg, and set it up on blocks. After twenty-four hours I came almost to the conclusion Mr. Johnston did, that water would not pass through this clay. This trial was during the hot, dry weather last Summer. After some ten or twelve days the clay appeared to be dry. I then made a basin-like excavation in the top of the clay, and put water in, and the water disappeared rather slowly. I filled the basin with water frequently, and the oftener I filled it, the more readily it passed off. I left it for more than a week, when we had a heavy shower. After the shower I examined the keg, and not a drop of water was to be seen. I then took a chisel and cut a hole six inches down. I took out a piece like the one I dried in the house, and laid that up till it was perfectly dry. There was a plain difference between the appearance of the two pieces. The texture, I should say, was quite different. That through which the water had passed, after it had been dried, was more open and porous. It did not possess so much of the blue cast. In less than one hour after the rain fell, the clay taken six inches from the top of the keg would crumble by rubbing in the hand."
When we observe the effect of heat in opening clays to water by cracking, and the effect of the water itself, aided, as it doubtless is, by the action of the air, in rendering the soil permeable, we hardly need feel discouraged if the question rested entirely on this evidence; but when we consider that thousands upon thousands of acres of the stiffest clays have been, in England and Scotland, rescued from utter barrenness by drainage, and made to yield the largest crops, we should regard the question of practicability as settled. The only question left for decision is whether, under all the circumstances of each particular case, the operation of draining our clay lands will be expedient—whether their increased value will pay the expense. It is often objected to deep drains in clays, that it is so far down to the drains that the water cannot readily pass through so large a mass. If we think merely of a drop of rain falling on the surface, and obliged to find its devious way through the mazes of cracks and particles till it gains an outlet at the bottom of four feet of clay, it does seem a discouraging journey for the poor little solitary thing; but there is a more correct view of the matter, which somewhat relieves the difficulty.
All the water that will run out of the soil has departed; but the soil holds a vast amount still, by attraction. The rain begins to fall; and when the soil is saturated, a portion passes into the drain; but it is, by no means, the water which last fell upon the surface, but that which was next the drain before the rain fell. If you pour water into a tube that is nearly full, the water which will first run from the other end is manifestly not that which you pour in. So the ground is full of little tubes, open at both ends, in which the water is held by attraction. A drop upon the surface drives out a drop at the lower end, into to the drain, and so the process goes on—the drains beginning to run as soon as the rain commences, and ceasing to flow only when the principle of attraction balances the power of gravitation.
PRESSURE OF WATER IN THE SOIL.
In connection with the passage of water through clay soil, it may be appropriate to advert to the question sometimes mooted, whether in a soil filled with water, at four feet depth, there is the same pressure as there would be, at the same depth, in a river or pond. The pressure of fluids on a given area, is, ordinarily, in proportion to their vertical height; and the pressure of a column of water, four feet high, would be sufficient to drive the lower particles into an opening like a drain, with considerable force, and the upper part of such a column would essentially aid the lower part in its downward passage. Does this pressure exist? Mr. Gisborne speaks undoubtingly on this point, thus:
"We will assume the drain to be four feet deep, and the water-table to be at one foot below the surface of the earth. Every particle of water which lies at three feet below the water-table, has on it the pressure of a column of water three feet high. This pressure will drive the particle in any direction in which it finds no resistance, with a rapidity varying inversely to the friction of the medium through which the column acts. The bottom of our drains will offer no resistance, and into it particles of water will be pushed, in conformity with the rule we have stated; rapidly, if the medium opposes little friction; slowly, if it opposes much. The water so pushed in runs off by the drain, the column of pressure being diminished in proportion to the water which runs off."