comotive, the furnace, the loom, the
sewing machine, the printing press, etc., all pay for and thus
indirectly produce their own successors.

_The Net Rent of a Permanent Series of Similar Instruments._--The
first charge on the product of any instrument of this kind is the
amount necessary for replenishing the waste of it and for providing a
successor when this original instrument shall have been wholly worn
out. In like manner, the first charge on the successor is providing a
similar fund, and so on indefinitely. A part of the productive power
of every one in an endless series of similar instruments is devoted to
this type of reproduction. The series maintains itself and yields an
income besides; and that remainder of its gross rent which is left
after waste of tissue is repaired is available as a net income for the
owner. This net remainder constitutes an interest on the owner's
capital. He possesses a permanent fund of productive wealth embodied
in the endless series of these perishable instruments, and _the series
taken as a self-perpetuating whole_ yields nothing but this interest.
Each instrument, separately considered, yields interest and a sinking
fund; but the sinking fund is not available as an income, since it
must take shape as another instrument which serves to keep the series
intact. What the first instrument creates in addition to the sinking
fund is its contribution to interest, and what each instrument creates
above what is required for virtual self-perpetuation is also interest.

_Interest and Net Rent Identical._--We may therefore reduce interest
to the form of a net rent by calculating the gross rent afforded by
each instrument in such a series and by ascertaining how much of this
merely repairs waste and how much is true income. As interest is
usually expressed in the form of a percentage, we may reduce the net
rent to this form by comparing it with the cost of the first
instrument, which is the amount originally invested. The series of
instruments will yield a net return every year. We can compute the
gross return of each instrument according to the Ricardian formula for
measuring the product of the land. It will diminish from year to year
and will ultimately vanish. We can add the several annual gross
earnings of the instrument during its economic lifetime in the form of
an absolute sum, which is the total rent of the instrument. From this
we can deduct the cost of replacing this worn-out capital