1. _The continued rent of indirect agents is dependent on the continual repair of certain parts necessary for their efficiency._ All earthly things wear out or decay. Whenever man's hand is withheld, nature takes possession of his work, regardless of his purposes. Dust gathers on unused clothes, and moths burrow in them. Shut up a house, and windows are shattered, roofs leak, and vermin swarm. To close a factory is to hasten the time when buildings and machinery will be piled upon the rubbish heap. The most magnificent and solid works of man have crumbled under the finger of time. The earth is strewn with ruins of gigantic engineering works, aqueducts, canals, temples, and monuments, whose restoration would be no less a task than was their first building. Everywhere vigilance and repairs are the conditions of continued uses of wealth. Some works of nature, such as waterfalls, may appear to have a continued use without repair, but they bear rent only when used with other things that must be constantly mended. A certain amount of labor on the banks of the mill-stream, and certain repairs on the dam, the water-wheel, and the gates are necessary. By a fiction in business contracts the waterfall may be dealt with apart from those conditions to its use, and may be rented, as a field is, with the agreement that the tenant keep up the repairs. The efficiency of land as mere standing-room usually does not seem to be dependent on repairs. But here again the land yields rent in connection with other rent-bearing agents (such as houses and other agents above ground), which must be repaired. Standing-room on land is not a complete indirect agent; it is but one of the conditions for carrying on an industry, and even it often requires repairs to make it usable. Ranging from these extreme cases of stableness and durability, indirect agents vary to the extremes of fragility and ephemeralness. [Sidenote: The fertile lands of large regions have lost their usefulness]