It is a common impression that clay is impervious to water, and that, therefore, a clay soil cannot be drained, especially by deep under-drains. A moment's reflection will satisfy any one that such land is not absolutely impervious. We find such land is wet in Spring, at any depth; and, in the latter part of Summer, we find it comparatively dry. How comes it wet, at any time, if water does not go into it? And how comes it dry, at any time, if water does not come out of it? In treating of the power of the soil to absorb moisture, we have shown that a clay soil will absorb more than half its weight and bulk of water, and that it holds more water than any other soil, with, perhaps, the single exception of peat. The facts, however, that clay may be wet, and may be dried, and that it readily absorbs large quantities of water, though they prove conclusively that it is not impervious to water, yet do not prove that water will pass through it with sufficient rapidity to answer the practical purposes of drainage for agriculture. This point can only be satisfactorily determined by experiment. It is not necessary, however, that each farmer should try the experiment for himself; because, although we are very apt to think our own case an exception to all general rules, it is not really probable that any new kind of clay will be discovered hereafter, that is so different from all other clay that is known, that established principles will not apply to it. So far as our own observation extends, owners of clay farms always over-estimate the difficulty of draining their land. There are certain notorious facts with regard to clay, which mislead the judgment of men on this point. One of these facts is, that clay is used for stopping water, by the process called _puddling_. Puddled clay is used for the bottom of ponds, and of canals, and of reservoirs, and, for such purposes, is regarded as nearly, or quite impervious. We see that, on our clay fields, water stands upon the surface, especially in the ruts of wheels, and on headlands much trodden, late in the season, and when, in other places, it has disappeared. This is due, also, to puddling.