_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matter of indifference whether certain amounts of labor and capital are used in connection with it, or whether they are withdrawn and employed elsewhere.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 positive loss; but if it is so used, nobody gets any income from it. It has no power to enter in a really productive way into combination with labor and capital, for it cannot so combine with them as to add anything to those marginal products which the labor and capital could create if they remained detached from it. _The Universality of the Test of Rent._--This test, whether an instrument can or cannot add something to the marginal product of labor and capital, may be universally used. It may be applied to everything that is made as an aid to labor. There are no-rent buildings, locomotives, cars, tracks, ships, wagons, furnaces, engines, boilers, and, in short, instruments of every description that figure in production. Combine any one of them with labor and capital and see what you get out of the combination; then take the labor and capital away and see what they will produce as marginal labor and capital; and the difference between the two amounts, whatever it is, is the rent of the instrument. If the difference is _nil_, the instrument is at the point of being abandoned.[1] [1] Whether such an instrument should or should not be called a capital good is a question of mere nomenclature; but in this treatise we consider that every part of what we term capital produces an income, and therefore a no-rent instrument is not a capital-constituting good--otherwise termed a capital good.