_General Interest as Rent._--If you compute the net income of all tools, machines, and other like things in the world, add the amounts, and get the grand total of them all, you have the entire income from this part of the capital of the world in the form of net rent. If then you compute the value of all this class of instruments and see how large a part of this value the net rent is, you translate this total rent into the form of interest, and therefore net rent and interest are the same income regarded in two different ways.[3] [3] In computing both of these values for comparison one should use a labor-cost standard, and we shall later see under what limitations such a standard may legitimately be used. _Stocks of Made Instruments graded in Quality as is Land._--It is necessary to notice the fact that the permanent series of tools, buildings, and other active capital goods shows forever the same gradations of quality that are found in the case of land. There are always to be found some instruments which are producing a large amount--that is, they are adding a large amount to the product of the labor and the further capital that are combined with them in production. A given amount of labor and capital creates much more wealth when working with a machine of the highest class than it would if distributed in marginal positions; and this is equivalent to saying that such an instrument is itself highly productive. Other instruments are to be found which are creating less, and there is never wanting a grade of no-rent instruments which are adding nothing to the marginal product of the other agents. It would be as well for the labor that used them if it should drop them and add itself to the force which is working with good instruments. Any one manufactured instrument begins its career as a maximum-rent instrument and ends it as a no-rent one. The ship is at its best when it starts on its first voyage, and the mill is at its best in the first year of its running. Each instrument goes gradually downward in the scale till it reaches a stage in which it really produces nothing, since it adds nothing to what would be produced without it. The _permanent series_ of instruments never thus deteriorates. All the depreciation of particular things is made good by the repairing and the replenishing which go on. In the series as a whole there are forever present grade number one, grade number two, grade number three, etc., exactly as in the case of land. If we wish, we can reckon the income that is to be gotten from each part of the series according to the old-time formula that is familiarly used in the case of land, "What labor and capital create by the use of this piece of ground in excess of what they would create if they were applied to the poorest land in use." For a grade of land read a grade of the self-perpetuating series of artificial instruments, and it will appear that each grade above the poorest yields, with the labor and