_A''_, and _A'''_. A certain other part of the outflow of _A'''_'s goes also to capitalists as interest on that other permanent fund which is maintained in the form of tools, machines, and buildings, such as must everywhere be used in the series. A third part of the flow of _A'''_'s is wages of labor in this group; and a final portion is what we have called the sinking fund, the amount that is given over as an income to the producers in another group, not here represented, who keep the stock of buildings, tools, etc., intact. These four withdrawals of income constitute the process by which the stock of passive goods is depleted, and the grand resultant of all industry is to atone for that depletion. _Labor and the Obtaining of its Product, in Static Industry, Synchronous._--One function of the permanent series of _A_'s is to enable labor everywhere to get its virtual product without waiting, and that too in the form in which it needs it for use. The labor that converts _A''_ into _A'''_ supplies the waste of tissue that takes place at that end of the line by withdrawal of an _A'''_. The labor that turns _A'_ into _A''_ replaces the waste that takes place at that point when an earlier _A''_ becomes an _A'''_. The labor at _A'_ replaces the waste at that point, and that at _A_ replaces the waste at still another point. They are all at work keeping the stock of _A_'s unimpaired, and one of them does as much toward keeping up the perpetual flow of _A'''_'s as any other. If we pump water in at one end of a full reservoir, we instantly cause it to overflow at the other end; and every worker in such a series as we have described may be thought of as putting something into the permanent reservoir of capital and so causing a corresponding overflow. He gets his reward day by day as the work proceeds. Wherever a laborer may be in such a series, his work creates a ripened product as it goes on. He has not to wait for it. His work and its fruit are synchronous. _Differences between Land and Made Instruments Apparent in Dynamic Conditions._--A point that has great theoretical interest is the nature of the difference between land and other productive instruments. In a static society the difference would be comparatively unimportant, but it is brought into prominence by the changes which constitute a dynamic state. The static hypothesis requires that capital should not increase or diminish in quantity, and that it should not change its forms. The equipment of every mill and of every ship is kept unimpaired but not enlarged or improved. There is a fixed number of spindles in the cotton mill, of lathes in the machine shop, of sewing machines in the shoe factory, etc., and this fact removes the most striking difference which, in a dynamic society, actually distinguishes land from other things.