d make these other ships somewhat
more efficient and cause each of them to earn a little more than it
now earns. Whatever the labor and capital could, in this way, produce
furnishes the basis for computing the rent of the ship. Subtract it
from the total joint product of labor, capital, and ship, and you have
what the vessel separately earns.

_The Mode of Testing the Productive Power of a Ship._--Put the labor
and capital into the ship and set it doing its proper work of carrying
freight and passengers, and you cause a certain product to be created.
The steamship company gets an aggregate amount for the service it
renders by means of the labor, the auxiliary capital, and the ship. A
certain smaller amount would be realized if the labor and the
auxiliary capital were taken out of the ship, distributed, and used in
the way we have just described. The difference between the two amounts
is the rent of the ship, or its particular contribution to the general
product. This gives us a formula for computing the rent, not only of
land, but of buildings, tools, machines, vehicles, and every other
concrete instrument of production. The formula, indeed, is so general
that it enables us to compute the earnings of any agent whatsoever.
_The rent of any such agent is what it adds to the marginal product of
labor and capital used in connection with it._

_No-rent Instruments._--The majority of instruments that are in use
add something to the marginal product of the labor and capital used in
connection with them. Some add more and some add less, according to
their several qualities. As a rule, any tool of trade produces most
when it is new and less and less as it grows older. In the end it is
discarded because it has so deteriorated that it no longer adds
anything to the marginal product of the labor and capital that are
used in connection with it. A wagon has become so rickety that it no
longer pays to furnish a horse, a harness, and a driver for it. The
capital and labor that these represent would earn as much if they were
detached from the old vehicle and added to the equipment of some
person who has a stock of good ones. The rent of this old wagon is
nothing. As in the case of the poorest land in cultivation, it is a
matter of indifference whether certain amounts of labor and capital
are used in connection with it, or whether they are withdrawn and
employed elsewhere. This poor vehicle, like the poor land, may be used
without posi