unless an earthquake or some similar catastrophe has filled it up: but regular and frequent outlays are necessary to keep it in repair. The cost of opening a mine needs not be incurred a second time; but unless some one goes to the expense of keeping the mine clear of water, it is soon rendered useless. The most permanent of all kinds of fixed capital is that employed in giving increased productiveness to a natural agent, such as land. The draining of marshy or inundated tracts like the Bedford Level, the reclaiming of land from the sea, or its protection by embankments, are improvements calculated for perpetuity; but drains and dykes require frequent repairs. The same character of perpetuity belongs to the improvement of land by subsoil draining, which adds so much to the productiveness of the clay soils; or by permanent manures, that is, by the addition to the soil, not of the substances which enter into the composition of vegetables, and which are therefore consumed by vegetation, but of those which merely alter the relation of the soil to air and water; as sand and lime on the heavy soils, clay and marl on the light. Even such works, however, require some, though it may be very little, occasional outlay to maintain their full effect.
These improvements, however, by the very fact of their deserving that title, produce an increase of return, which, after defraying all expenditure necessary for keeping them up, still leaves a surplus. This surplus forms the return to the capital sunk in the first instance, and that return does not, as in the case of machinery, terminate by the wearing out of the machine, but continues for ever. The land, thus increased in productiveness, bears a value in the market, proportional to the increase: and hence it is usual to consider the capital which was invested, or sunk, in making the improvement, as still existing in the increased value of the land. There must be no mistake, however. The capital, like all other capital, has been consumed. It was consumed in maintaining the labourers who executed the improvement, and in the wear
