On this subject I shall add one important practical remark, which will readily suggest itself to the geologist who has studied the action of air and water on the various clay beds that occur here and there as members of the series of stratified rocks. _There are no clays which do not gradually soften under the united influence of air and of running water. It is false economy, therefore, to lay down tiles without soles_—however hard and stiff the clay subsoil may appear to be. In the course of ten or fifteen years the stiffest clays will soften, so as to allow the tile to sink; and many very much sooner. The passage for the water is thus gradually narrowed; and when the tile has sunk a couple of inches, the whole must be taken up. Thousands of miles of drains have been thus laid down, both in the low country of Scotland and in the southern counties of England, which have now become nearly useless; and yet the system still goes on. It would appear even as if the farmers and proprietors of each district—unwilling to believe in or to be benefitted by the experience of others—were determined to prove the matter in their own case also, before they will consent to adopt that surer system which, though demanding a slightly greater outlay at first, will return upon the drainer with no after-calls for either time or capital. If my reader live in a district where this practice is now exploded, and if he be inclined to doubt if other counties be farther behind the advance of knowledge than his own, I would invite him to spend a week in crossing the county of Durham, where he may find opportunities not only of satisfying his own doubts, but of scattering here and there a few words of useful advice among the more intelligent of our practical farmers. 2. _Subsoiling._—The subsoil plough is an auxiliary to the drain. Though there are few subsoils through which the water will not at length make its way, yet there are some so stiff either naturally or from long consolidation, that the good effect of a well-arranged line of drains is lessened by the slowness with which they allow the superfluous rains to pass through them. In such cases, the use of the subsoil plough is most advantageous in loosening the under layers of clay, and allowing the water to find a ready escape downwards and to either side until it reach the drains. It is well known that if a piece of stiff clay be cut into the shape of a brick, and then allowed to dry, it will contract and harden—it will form an air-dried brick, almost impervious to any kind of gas—wet it again, it will swell and become still more impervious. Cut up _while wet_, it will only be divided into so many pieces, each of which will harden when dry, or the whole of which will again attach themselves and stick together if exposed to pressure. But tear it asunder _when dry_, and it will fall into many pieces, will more or less crumble, and will readily admit the air into its inner parts. So it is with a clay subsoil.