412 
On Drahiinr/, 
the clialk ; the subsoil consists of a very retentive stiff cla}-, of a 
li<i^ht colour, from 2 to 15 feet thick, enclosing small stones, shells, 
and nodules of chalk, rising generally to within 6 or 7 inches of 
the surface, and resting upon the Oxford clay, which is computed 
to be from 100 to 1500 feet thick, mostly devoid of sand or cal- 
careous matter, containing surface-water only. The number of 
springs is so small as not to be worthy of notice. 
2)1(1. Dcptli (uul frequency of drains, and tlieir operation, in con- 
junction icith the atmospliere, in contracting the subsoil, enlarcjing 
tlie fissures for the ivater to percolcdc freely to the drains, and pre- 
venting, in wet seasons, such an expansion of the subsoil and closing 
of the fissui'cs as would hinder the free infiltration of the water ; 
and that the drainage of land cannot be perfect where the water runs 
over the land and Jitters from the surface through the filling-in over 
the drain. 
There is no part of cold clay land draining upon which greater 
difference of opinion exists than upon the depth at which the drains 
should be laid ; the plan generally followed by persons without 
experience, or reflection upon the subject, is to lay the drains near 
the surface, on the supposition that they cannot be too shallow, if 
laid so as not to be injured by the horses, &c., in the necessary 
acts of cultivation ; and a very general but erroneous opinion is 
entertained, that if under-drains are laid deep in tenacious clay soils 
the water will not enter so freely.* From experience I have found 
that the more tenacious the clay land to be drained, the greater 
should be the depth of the drains ; for although, on some lands of 
a mixed soil, drains 20 inches deep may be effectual, they are 
very inferior to drains of 30 inches, or 3 feet for stiff clays, for the 
more pure and tenacious the clay, the greater is the contraction 
of the subsoil, when the water is displaced by the operation of 
under-drains and evaporation by the atmosphere, and the larger 
will be the fissures in the same to convey the water to the drains. 
On well-drained land the water does not enter the drain by the 
furrow, or from the surface immediately over the drain (or where 
the drain is not in the furrow, through the filling-in'), but as it falls 
enters first the tilth or surface, and afterwards the fissures and the 
borings of the common earth-worm. The utility of the earth-worm 
in the drainage of land is unquestionable, for it loosens the soil by 
its boring operations ; the bores of the worms alone on some 
grass-lands would be sufficient for the infiltration of the water, if 
the drains were laid a proper depth to carry it off. I have found 
* An observation very general among cold land farmers, and which is 
considered conclusive against deep draining, is, that if a horse only sets his 
loot upon heavy land near the edge of the open fnrrow, the cavity will 
hold water like a basin. The cause of this is very evident: the horse on 
wet clay land will smooth the bottom of the cavity with his foot. , 
