1. _Even where repairs are thoroughly kept up and present rent is undiminished, future rents may be decreasing because of natural decay._ Changes go on in the substance of things which cannot be prevented by any attention to repairs. The wood in a framework will decay, the metals crystallize. There is also an unpreventable wear of parts that cannot be replaced without replacing the whole machine. It is the aim of the modern manufacturers to make machines like the wonderful one-horse shay, every part of equal durability. The development in America of the system of "interchangable parts" has greatly simplified and cheapened repairs, and has lengthened the working life of machines; nevertheless their lot is the scrap-heap at last. This general depreciation appears to be nearly avoided in large factories where there is serial replacement of the parts, but occasionally some invention or some improvement of process necessitates an almost completely new equipment. An old man once said to me: "I have lived in this house forty years: it was well built, has been repainted regularly, has never been allowed to leak a drop, and it is as good as it ever was. I see no reason why it could not be kept to eternity if always kept in repair." But the same could not be said of the house now. In general, there is finally a termination of the rent-earning power of wealth, and the whole has to be replaced. [Sidenote: Technical changes destroy the uses of agents] 2. _A change in inventions and processes may reduce the rent of agents, independently of their material condition._ Rent is dependent on the indirect relation of things to wants; that relation may be changed if some other agent is found fitted to serve these wants more directly. Not only do the materials of houses change, but fashion and engineering skill change, making the old mansions cheerless and inconvenient, and affecting their rent-earning power. At every moment, in a progressive society, many rent-earning agents are being thrown out of use. The machinery in flour-mills has been almost completely changed, parts of it repeatedly, while the roller process has been substituted for the old millstones. Water-power, because of its uncertainty, has been replaced in many places by steam-power, and in many places steam-power in turn, has been rivaled by water-power since the improvements in the generation and transmission of electricity. A change in the process of making paper threw out of use much machinery that was only in part saved by its removal and adaptation to the making of coarser grades of paper. Many minor inventions in the iron industry, still more the invention of the Bessemer process, threw out of use great numbers of the old appliances. [Sidenote: Industrial circumstances affect the uses of agents]