_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 locomotive, 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.