Let us assume that production is doubled by the investment of £5 instead of £2½ in each of the soils A, B, C and D, i.e., a total of £20 instead of £10, and that the relative fertility remains unchanged. This would be tantamount to cultivating 2 instead of 1 acre of each of these kinds of soil at the same cost. The rate of profit would remain the same; also its relation to surplus-profit or rent. But if A were now to yield 2 qrs, B — 4, C — 6, and D — 8, the price of production would nevertheless remain £3 per quarter because this increase is not due to doubled fertility with the same capital, but to the same proportional fertility with a doubled capital. The two quarters of A would now cost £6 just as 1 qr cost £3 before. The profit would have doubled on all four soils, but only because the invested capital was doubled. In the same proportion, however, the rent would also have been doubled; it would be 2 qrs for B instead of 1, 4 qrs for C instead of 2, and 6 for D instead of 3; and correspondingly, the money-rent for B, C and D would now be £6, £12, and £18 respectively. Like the yield per acre, the rent in money per acre would be doubled, and, consequently, also the price of the land whereby this money-rent is capitalised. Calculated in this manner, the amount of rent in grain and money increases, and thus the price of land, because the standard used in its computation, i.e., the acre, is an area of constant magnitude. On the other hand, calculated as rate of rent on invested capital, there is no change in the proportional amount of rent. The total rental of 36 is to the invested capital of 20 as the rental of 18 is to the invested capital of 10. The same holds true for the ratio of money-rent from each type of soil to the capital invested in it; for instance, in C, £12 rent is to £5 capital as £6 rent was formerly to £2½ capital. No new differences arise here between the invested capitals, but new surplus-profits do, merely because the additional capital is invested in one of the rent-bearing soils, or in all of them, with the same proportional yield as previously. If this double investment took place, for example, only in C, the differential rent between C, B and D, calculated with respect to capital, would remain the same: for when the amount of rent obtained from C is doubled, so is the invested capital.