Book HI. 



DRAINING RETENTIVE SOILS. 



703 



637 



taining water ; for in this case it is gene- 

 rally desirable to raise the stones above the 

 level of the bottom of the upper seam, so 

 as to convey away any water which may 

 remain in it, to the conduit beneath 

 {Jig. 637. a, sand or gravel ; b, clay) ; 

 and it was a circumstance very generally 

 observed in the course of operations, that 

 where the upper stratum containing water 

 was only a few feet in depth or thickness, 

 another open stratum was generally found 

 a few feet deeper. 



4265. Stones. When the stones to be used are only brought forward at the time of 

 cutting the drain, the carts are often run back to the edge of it, and the stones, after the 

 conduit is built, tumbled straight out of the cart into the drain ; but, in this case, it is 

 necessary to take care that the sides of the drain be not injured by the cart-wheels or 

 otherwise, lest the earth should fall into the drains, and so through the intervals of the 

 stones. A part of the stones for filling were recommended by Mr. Stephens to be broken 

 like large road metal. This, however, is very expensive, and was found by experience 

 not to be necessary, though usually large stones should be broken. When the stones 

 are small, that is, ten or twelve ounces, it is as well ; but no inconvenience has been 

 found from the constant use of stones of a much larger and very unequal size. When 

 a sufficient quantity of stones has been thrown in, the mason levels them at the top, 

 filling up the intervals of larger stones with smaller ones, so as to make the top of them 

 level. If the sod which has been cut off the surface of the drain is sufficiently solid, it 

 should be laid carefully by itself on the upper edge of the drain at the side of the stones. 

 It should again be laid with its grassy side undermost, on the top of the stones, as a 

 covering, to prevent the earth from getting down amongst them. If the sods are not 

 sufficiently coherent or plentiful to cover the whole completely, old coarse hay, or straw, 

 or heath, may be used as a substitute. When all this is completely dpne, the earth is 

 shovelled in upon the top, until the drain is full. It is then heaped up, somewhat after 

 the manner of a grave, to allow for the earth's subsiding to the level of the surface. It 

 is a circumstance deserving of notice, that, in digging the trial-pits, the earth taken out 

 is in most cases insufficient to fill them again, if allowed to lie open for any time ; so 

 that, in fact, contrary to what would be naturally inferred, the earth must become more 

 compact by being removed. 



4266. Repairs. When the drain is thus completed, it is still necessary, and parti- 

 cularly when the land is under tillage, carefully to inspect it from time to time, and to 

 see that no surface-water finds its way into it. If any hole is found, it ought to be im- 

 mediately stopped up, as a channel of this kind will sometimes very speedily carry 

 enough of mud into a conduit to choke it entirely, and spoil the drain. Under- 

 draining, it will be kept in mind, will not supersede the necessity of surface-drains, 

 where these are necessary to carry off water stagnant upon the ground. Besides the 

 danger to drains by the flowing in of surface-water, there are other sources of injury 

 which must be guarded against by a vigilant care. Animals, by burrowing in the 

 earth, or finding their way from any course in the conduit, are sometimes apt to injure 

 it, and cause the earth to crumble in ; but a more frequent source of injury is from 

 vegetable substances, as roots of trees, and particularly of the ash. As an instance of 

 this, there happened, on this property, to be an ash tree growing near a drain, the fibres 

 of which took possession of the conduit, and so obstructed the passage of the water, as 

 to produce a new swamp, in consequence of which it became necessary to lift the ma- 

 terials of the drain, and form it anew. It is often very difficult to eradicate certain 

 plants, whose long and creeping roots get intervened in the interstices of the conduit. 

 The advance of those larger animals which enter the conduits for safety, or in pursuit 

 of prey, may be prevented by an iron grating at the outlet, {^Trans. Highl. Soc 

 vol. vii.) 



Sect. V. Methods of draining Retentive Soils. 



4267. The mode of draining retentive soils is materially different from that which 

 has been described above. Many tracts of level land are injured by the stagnation 

 of a superabundant quantity of water in the upper parts of the surface materials, which 

 does not rise up into them from any reservoirs or springs below. The removal of the 

 wetness in these cases may, for the most part, be effected without any very heavy expense. 

 From the upper or surface soil, in such cases, being constituted of a loose porous stratum 

 of materials, to the depth of from two to four or five feet, and having a stiff retentive 

 body of clay underneath it, any water that may come upon the surface, from heavy rains, 

 or other causes, readily filtrates and sinks down through it, until it reaches the obstruct- 



