If we use the same method in measuring the rent of land which we used in measuring the wages of labor and the returns of capital, we shall represent the rent of a given piece of land as the sum of a series of differential amounts. In the accompanying figure the vertical belts bounded by lines rising from the letters _A_, _B_, _C_, etc., represent the products realized by applying successive increments of labor and capital to a given piece of land; and the horizontal lines running toward the left from _A'_, _B'_, _C'_, etc., separate the wages and interest from the amounts that are successively added to rent. When one composite unit of labor and capital is working, its product and its pay is measured by the belt between the line _AA'_ and the line _NN'_. A second composite unit produces the amount represented by the area between _AA'_ and _BB'_, and that is the amount which each unit separately considered will produce and get as its pay. This leaves the area between the horizontal line running from _B'_ and the section of the descending curve as the rent of the land. A third unit of labor and capital produces what is represented by the area between _BB'_ and _CC'_, and this becomes the standard of pay for all units, leaving the enlarged area above the horizontal line at _C'_ as rent. In the end there are ten units of labor and capital. Their total earnings are expressed by the area of the rectangle below the horizontal line running from _J'_, and the sum of all the areas above that line is rent. [Illustration] _The Intensive Margin of Cultivation._--The extensive margin of cultivation is the land that is adjacent to an imaginary boundary line separating the grades of land that are good enough to be used from those that are too poor to be used. There is, however, what may be called the intensive margin of cultivation. A given bit of land is said to be cultivated more and more intensively when more and more labor and capital are used on it. Land is subject to what is called the law of diminishing returns. _Law of Diminishing Returns._--The more labor and capital you employ on a given piece of land, the less you will get as a product for each unit of these agents. What the last unit of labor adds to the antecedent output is less than was added by any of the other units, and the same is true of the last unit of capital. As we continue the process of enlarging the working force and adding to the working appliances, we reach a point at which it is better to cease putting new men with their equipment at work on this piece of land and to set them working on a bit of land so poor that it was not formerly utilized at all. We may assume here that what a man needs, in the way of auxiliary capital, goes with him, whether he joins a force that is working on good land or migrates to a less productive region. He will go if it will pay him to do it. In this way we make a sort of dual unit of labor and capital and apply a series of such units to land.