If land were all occupied, and of only one grade, the first installment of labor and capital produced, we will say, twenty bushels of wheat; when the price of wheat rose, and it became profitable to resort to greater expense on the soil, a second installment of the same amount of labor and capital when applied, however, only yielded fifteen bushels more; a third, ten bushels more; and a fourth, five bushels more. The soil now gives fifty bushels only under the highest pressure. But, if it was profitable to invest the same installment of labor and capital simply for the five bushels that at first had received a return of twenty bushels, the price must have gone up so that five bushels should sell for as much as the twenty did formerly; so, _mutatis mutandis_, of installments second and third. So that if the demand is such as to require all of the fifty bushels, the agricultural capital which produced the five bushels will be the standard according to which the rent of the capital, which grew twenty, fifteen, and ten bushels respectively, is measured. The principle is exactly the same as if equal installments of capital and labor were invested on four different grades of land returning twenty, fifteen, ten, and five bushels for each installment. Or, as if in the table on page 240, A, B, C, and D each represented different installments of the same amount of labor and capital put upon the same spot of ground, instead of being, as there, put upon different grades of land. The rent of all land is measured by the excess of the return to the whole capital employed on it above what is necessary to replace the capital with the ordinary rate of profit, or, in other words, above what the same capital would yield if it were all employed in as disadvantageous circumstances as the least productive portion of it: whether that least productive portion of capital is rendered so by being employed on the worst soil, or by being expended in extorting more produce from land which already yielded as much as it could be made to part with on easier terms.