« AnteriorContinuar »
The general use of all these materials for making drains was confined to a system of partial drainage, until the publication of a pamphlet, in 1833, by Mr. Smith, of Deanston, who advocated the drainage of the whole field, without reference to springs. From this plan, but with impor tant modifications in matters of detail, the modern sys tern of tile draining has grown. Many able men havf aided its progress, and have helped to disseminate a knowledge of its processes and its effects, yet there are few books on draining, even the most modern ones, which do not devote much attention to Elkington's discovery; to the various sorts of stone and brush drains; and to the manufacture and use of horse-shoe tile;—not treating them as matters of antiquarian interest, but repeating the instructions for their application, and allowing the reasoning on which their early use was based,to influence, often to a damaging extent, their general consideration of the modern practice of tile draining.
These processes are all of occasional use, even at this day, but they are based on no fixed rules, and are so much a matter of traditional knowledge, with all farmers, that instruction concerning them is not needed. The kind of draining which is now under consideration, has for its object the complete removal of all of the surplus water that reaches the soil, from whatever source, and the assimilation of all wet soils to a somewhat uniform condition, as to the ease with which water passes through them
There are instances, as has been shown, where a large spring, overflowing a considerable area, or supplying the water of an annoying brook, ought to be directly connected with the under-ground drainage, and its flow neatly carried away; and, in other cases, the surface flow over large masses of rock should be given easy entrance into the tile; but, in all ordinary lands, whether swamps, springy hill sides, heavy clays, or light soils lying on retentive subsoil, all ground, in fact, which needs under
draining at all, should be laid dry above the level to which it is deemed best to place the drains;—not only secured against the wetting of springs and soakage water, but rapidly relieved of the water of heavy rains. The water table, in short, should be lowered to the proper depth, and, by permanent outlets at that depth, be prevented from ever rising, for any considerable time, to a higher level. This being accomplished, it is of no consequence to know whence the water comes, and Elkington's system need have no place in our calculations. As round pipes, with collars, are far superior to the "horse-shoe" tiles, and are equally easy to obtain, it is not necessary to consider the manner in which these latter should be used,—only to say that they ought not to be used at all.
The water which falls upon the surface is at once absorbed, settles through the ground, until it reaches a point where the soil is completely saturated, and raises the general water level. When this level reaches the floor of the drains, the water enters at the joints and is carried off. That which passes down through the land lying between the drains, bears down upon that which has already accumulated in the soil, and forces it to seek an outlet by rising into the drains.* For example, if a barrel, standing on end, be filled with earth which is saturated with water, and its bung be removed, the water of saturation, (that is, all which is not held by attraction in the par tides of earth,) will be removed from so much of the mass as lies above the bottom of the bung-hole. If a bucket of water be now poured upon the top, it will not all run diagonally toward the opening; it will trickle down to the level of the water remaining in the barrel, and this level will rise and water will run off at the bottom of the orifice. In this manner, the water, even below the drainage level,
* Except from quite near to the drain, It Is not probable that the water In the soU runs lateral); towards It
is changed with each addition at the surface. In a barrel filled with coarse pebbles, the water of saturation would maintain a nearly level surface; if the material were more compact and retentive, a true level would be attained only after a considerable time. Toward the end of the flow, the water would stand highest at the points furthest distant from the outlet. So, in the land, after a drenching rain, the water is first removed to the full depth, near the line of the drain, and that midway between two drains settles much more slowly, meeting more resistance from below, and, for a long time, will remain some inches higher than the floor of the drain. The usual condition of the soil, (except in very dry weather,) would be somewhat as represented in the accompanying cut, (Fig. 12.)
Fig. 12.—LINE OF SATURATION BETWEEN DRAINS. T Tare the drains. The curved line b is the line of saturation, which has descended from a. and is approaching c.
To provide for this deviation of the line of saturation, in practice, drains are placed deeper than would be neces* sary if the water sank at once to the level of the drain floor, the depth of the drains being increased with the increasing distance between them.
Theoretically, every drop of water which falls on a field should sink straight down to the level of the drains, and force a drop of water below that level to rise into the drain and flow ofi". How exactly this is true in nature cannot be known, and is not material. Drains made in pursuance of this theory will be effective for any actual condition.
The depth to which the water table should be withdrawn depends, not at all on the character of the soil, but on the requirements of the crops which are to be grown upca it, and these requirements are the same in all soils,—consequently the depth should be the same in alL What, then, shall that depth be? The usual practice of the most experienced drainers seems to have fixed four feet as about the proper depth, and the arguments against anything less than this, as well as some reasons for sup posing that to he sufficient, are so clearly stated by Mr. Gisborne that it has been deemed best to quote his own words on the subject:
"Take a flower-pot a foot deep, filled with dry soil.
'Place it in a saucer containing three inches of water.
'The first effect will be, that the water will rise through "the hole in the bottom of the pot till the water which "fills the interstices between the soil is on a level with the "water in the saucer. This effect is by gravity. The "upper surface of this water is our water-table. From it water will ascend by attraction through the whole "body of soil till moisture is apparent at the surface. Put "in your soil at 60°, a reasonable summer heat for nine "inches in depth, your water at 47°, the seven inches' "temperature of Mr. Parke's undrained bog; the attracted "water will ascend at 47°, and will diligently occupy "itself in attempting to reduce the 60° soil to its own "temperature. Moreover, no sooner will the soil hold "water of attraction, than evaporation will begin to carry "it off, and will produce the cold consequent thereon. "This evaporated water will be replaced by water of at "traction at 47°, and this double cooling process will go "on till all the water in the water-table is exhausted. "Supply water to the saucer as fast as it disappears, and "then the process will be perpetual. The system of saucer"watering is reprobated by every intelligent gardener; it "is found by experience to chill vegetation; besides which, "scarcely any cultivated plant can dij. its roots into stag"nant water with impunity. Exactly the process which "we have described in the flower-pot is constantly in "operation on an undrained retentive soil; the water"table may not be within nine inches of the surface, but "in very many instances it is within a foot or eighteen "inches, at which level the cold surplus oozes into some "ditch or other superficial outlet. At eighteen inches, "attraction will, on the average of soils, act with consid"erable power. Here, then, you have two obnoxious "principles at work, both producing cold, and the one u administering to the other. The obvious remedy is, to "destroy their united action; to break through their line
* of communication. Remove your water of attraction "to such a depth that evaporation cannot act upon it, or "but feebly. What is that depth? In ascertaining this "point we are not altogether without data. No doubt "depth diminishes the power of evaporation rapidly. Still, "as water taken from a 30-inch drain is almost invariably "two or three degrees colder than water taken from four "feet, and as this latter is generally one or two degrees "colder than water from a contiguous well several feet "below, we can hardly avoid drawing the conclusion that M the cold of evaporation has considerable influence at 30 "inches, a much-diminished influence at four feet, and little '' or none below that depth. If the water-table is removed "to the depth of four feet, when we have allowed 18 "inches of attraction, we shall still have 30 inches of de"fence against evaporation; and we are inclined to be"lieve that any prejudicial combined action of attraction "and evaporation is thereby well guarded against. The
* facts stated sesm to prove that less will not suffice.
"So much on the score of temperature; but this is not "all. Do the roots of esculents wish to penetrate into "the earth—at least, to the depth of some feet? We be"lieve that they do. We are sure of the brassies tribe,