itself adds
to the output of the other agents in the combination. The true
conception of rent is that of the specific addition which land makes
to the product of other agents used in connection with it. There are
various ways of measuring this addition, but the method just used will
at least show that the presence of the good land is the cause of the
excess of product which given amounts of labor and capital secure over
what they could create on land of the poorest quality.

_Rent as a Differential Product._--In the early statements of the rent
law it was not said that the rent of a piece of land is the product
specifically attributable to it. If it had been, the chances are large
that a much broader and more scientific use of the rent formula would
have resulted. The law of rent, as it was actually stated, made it
consist of a differential amount. It was what a given amount of labor
and capital would produce under one set of conditions minus what they
would produce under another. Since it is the presence or the absence
of the productive land which makes the only difference between the two
conditions, rent, even as it is thus defined, is really the amount of
product specifically attributable to the land. It is what is created
when the land is used in excess of what would be created if it were
not used and if the cooeperating agents did the best they could without
it. We may use, as the most general formula for the rent of land, the
contribution which land itself makes to the product of social
industry.

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