1. _Changes in the number and kind of competing resources may raise the rents of particular agents._ Rents may increase without increase in the quantity or number of a particular group of agents or without change in their technical efficiency. As changes in the conditions of society may reduce rents, so other changes may increase them. Agents of the same kind may diminish in number, either absolutely or relatively. If some of the competing machines are destroyed, the rents of the machines that remain rise, while if new supplies are found, either in nature or by improved industrial processes, the rents of the older agents fall. [Sidenote: Effect of new uses for agents] 2. _The discovery of new uses for agents or for their products raises their rents._ Farm land of the poorest kind often is found to contain valuable mineral deposits. Such a lucky find has lifted the mortgage from a farm in eastern Pennsylvania, from which, in two or three years, has been taken feldspar exceeding in value the agricultural products of the same land in the last fifty years. The discovery of building stone, coal, natural gas, or oil land may make the annual rent (or royalty) of land tenfold its former total value. Fitness to produce nettles is not ordinarily a virtue in land, but the discovery that certain fields produce a superior quality of the nettle used for heckling cloth, causes them to take on a new value. A mineral spring, because of the supposed or proved healing properties of its waters, may be as good as a mine to the owner. Peculiar fitness for the cultivation of celery may convert marsh land into a substantial source of income. Social changes are constantly causing agents to shift from lower to higher uses. As population grows and groups about new industries, farm land is used for residence lots, and in turn for business purposes. Rents therefore rise, and this rise is reflected in the higher selling value of the land. If a new demand arises for the product of any machine, its rent rises, although it may continue to turn out the same product as measured by number or quantity. For, if consumers increase, a given supply of agents becomes relatively smaller than before. [Sidenote: Sudden variations in demand]