_The Rent of a Fund of Capital._--We saw that one unit of labor employed in connection with a given amount of capital produces more than does a second; that the second produces more than the third; and that, if we continue to supply units one at a time, the last unit in the series produces the least of all. Wages are fixed by the amount that one unit of labor produces when the working force is complete, and that is what is contributed to the general product by the unit of labor which comes last in the imaginary series by which the force is built up. Owing to the more favorable conditions under which, in their time, the earlier units worked, they were able to produce surpluses above the amount produced by the last one. When they entered the field they were supplied with excessive amounts of capital. The first one had the whole fund cooeperating with it, till it had to share it with the second; and after that each had a half of it till they had to share evenly with a third, etc. We have seen that all the surpluses appearing in connection with the earlier units are attributable in reality to capital. The area _BCD_ (page 139) represents the amount by which the presence of an excess of capital increases the products attributable to the earlier units of labor. It represents the sum of all the differences between the products of the earlier units and the product of that final one which in the end sets the standard of productivity of labor. It might be called the rent of the fund of capital. It is composed of a sum of differences exactly like those which constitute the rent of a piece of land. _The Rent of a Permanent Force of Labor._--In the figure on page 148, the working force was supposed to be fixed in amount, the capital increasing by increments, or as some earlier economists would have said, by "doses" along the line _A'E'_. The last unit of capital produces the amount _D'E'_, and all the capital produces _A'B'D'E'_, while products of the earlier units of capital, as they come successively into the field and are used by an excessively large labor force, are represented by the area _B'C'D'_. Here this area represents what may be called the rent of the force of labor, since it is a sum of surpluses that, again, are entirely akin to those that constitute the rent of a piece of land.