3. _A rise in rents due to social changes may be relatively permanent or temporary._ Business conditions sometimes change quickly. An urgent demand for special machinery raises quickly its rent and value. It is said that lace machinery is sometimes thrown out of use for several years, until a sudden renewal of the demand for lace causes the rental to equal, in two years, more than the original cost. At such times the value of factories increases greatly, but after a few years of prosperity business again collapses. Such prosperous periods are the opportunity of the business man and of the promoter to sell the factory at its highest price. Machinery adapted only for a special product will not sell as readily when less needed for its special use, as that which, like a turning-lathe, can be used for many purposes; but the more special the appliances needed for a certain product, the higher, more abnormal will be their temporary value when they are suddenly needed. Land near the site of an exposition takes on a very great value and again falls after the exposition is over. During the Boer War horses and mules rose in price in the United States on account of British purchases. [Sidenote: Cause efforts to increase the supply of agents] A rise in the value of any agent at once causes an attempt to duplicate it or to find a substitute for it; this attempt, if successful, puts a check or sets a limit to the rise. In this search for new devices the man who can see most quickly and clearly has a key to wealth. Some kinds of agents, as rare minerals or tools that can be produced only by highly skilled labor, cannot be increased rapidly in number and remain high in price for a long period; and favorably located building sites illustrate the same principle. In some cases, it is true, the demand may be due to some temporary cause, as in a period of unsound land speculation, but usually the growing value of location is due to a steady and abiding change in population or business. [Sidenote: Franchises guard the growing rents from the influence of substitution]