,
though it is not so in the economic sense. What is really land in this
sense is not affected. Nitrates and phosphoric acid that lie in the
topmost stratum of the soil are among the destructible instruments of
agriculture. The supply of them has to be renewed, if cultivation is
continued, and they are therefore in the class with the plows, spades,
and reaping machines which also wear out. But whatever there is in the
soil that suffers no deterioration from any amount of use is the land
with which political economy has to deal.

_The Gross and the Net Rent of Land Identical._--As land does not wear
out and require renewal, all that it adds to the products of the labor
and capital that are used in connection with it may be taken by the
landlord as an income without reducing the amount of his property.
Whatever land produces at all is a net addition to the general income
of society.

_Net Rent of Artificial Instruments Smaller than Gross Rent._--It is
not safe, on the other hand, for the owner of buildings, tools, or
live stock to take for his own consumption all that these produce. If
he were to use up their gross produce as he gets it, he would find, in
due time, that a considerable part of his property had vanished. Such
instruments wear out and become worthless, and if no part of what they
produce is set aside as a sinking fund with which to purchase other
instruments to take their places, one whole genus of capital must go
altogether out of existence.

_Artificial Instruments Self-replacing._--What actually happens is
that these instruments create enough wealth to pay for their own
successors, and that, too, besides paying a net return, which,
regarded in one way, is interest. If you compute the whole product of
one of these instruments by the Ricardian formula which we have
examined, the amount of it will be whatever the instrument, during its
entire career, adds to the product of the labor and of the capital
that are used in connection with it; and that includes the fund for
renewal that has just been described, the amount, namely, which the
owners must set aside for repairing the instrument and finally
purchasing another. As the instrument itself provides this sinking
fund, it may be said to create, in an indirect way, its own successor.
The ship earns, over and above the net income which is interest on its
cost, enough to keep itself seaworthy so long as it sails and, in the
end, to build another ship. The lo