f all.

Care must be taken not to confound the addition that is made to the
product in consequence of the additional working force with the amount
which, after the enlargement of the force, is created by the last unit
of labor _and its pro rata share of the capital_. When the tenth unit
of labor is working, it is using a tenth of the capital and the two
together create a tenth of the product. This is more than the amount
which is _added_ to the product by the advent of the tenth unit of
labor. That addition is merely the difference between the product of
all the capital and nine units of labor and that of all the capital
and ten units of labor. This extra product can be attributed entirely
to the increment of labor.

It is also carefully to be noted that when the units are all working
together, their products are equal and the particular one which
happened to arrive last is not less productive than the others. Each
one of them is _now_ less productive than each one of the force of
nine _was under the earlier conditions_. In like manner each unit of
the nine is less productive than was, in the still earlier period,
each unit of the force of eight. At any one period, all units produce
the same amount. At any one period, then, what any one unit of labor
produces by the aid of its _pro rata_ share of the capital is a larger
amount than what each can be regarded as producing by itself. Though
one of ten units creates, with the aid of a tenth of the capital, a
tenth of the product, of itself it creates less; for we can only
regard as its own product what it adds to the product that was
creating before it arrived on the scene. It is the bare product of a
unit of labor alone that we are seeking to distinguish from other
elements in the general output of the industry, and that consists in
the difference between what nine units of labor and all the capital
can produce, and what ten units of labor and all the capital can
produce.

We will consider the amount of capital fixed and let the amount of
labor increase along the line _AE_, and we will let the product of
successive units of labor be measured by the vertical distance from
the points on the line _AE_ to the descending curve _CD_. _AC_ is the
product of the first unit of labor. The product of later units is
measured by lines to the right of _AC_ and parallel with it, which
grow shorter as the number of units increases. _ED_ is the product of
the last unit. In each case we