3. _A change in the outer conditions that give occasion to the use of agents may cause depreciation._ The exhaustion of materials on which machinery is employed may reduce its usefulness. A sawmill located in the midst of a forest has a high-earning power while the forest lasts, but when the forest is cut off the mill itself declines in value. Unless it can be removed to another forest and thus have its earning power renewed, it will have the value only of scrap-iron; it has become an indirect agent in the wrong place. Oil-boring machinery where a rich supply of oil is found has a high rental for a time, but when the oil-fields give out the machinery falls in value, being worth more or less than the cost of transporting it according as the next oil-field is near or far. Changes in fashions, calling for different kinds of products, cause a depreciation in the value of the old agents. Coarse salt, evaporated by the sun, was used by our fathers, but the finer product of the steam process is driving out the product of the old solar plants. As homespun went out of use, much machinery still in good physical condition was cast aside. Changes in transportation work revolutions in industrial methods. Many prosperous small forges on the country roads of Pennsylvania became valueless after the building of the railroads. New forges were built at favored points where materials and products could be shipped by rail. [Sidenote: Various grades of efficiency in rent-bearers] 4. _The agents employed in any industry range from the more efficient, high rent, down to the less efficient, low rent, grades in a more or less regular series._ It follows that as these changes are going on, the place of agents on the scale of efficiency is constantly shifting. The various agents represent all grades of efficiency. One depreciates, possibly is restored later and takes a high place, and again depreciates until finally it is thrown out of use. One loom embodies the latest improvements and corresponds to the most fertile field; another can still be made to yield a little rent; the use of a third results in certain loss. A great mass of no-rent agents lie just below the margin of utilization in every industry. Some of these are permanently abandoned; some will be taken back into use when business conditions improve. When the iron industry is dull, many forges are out of blast; but when iron is again in demand, there is a gradual taking up of the abandoned forges, factories, and machines as they are brought within the margin of profitable utilization. Many agents not actually earning a rent, may become rent-earning through a change in business conditions. § III. DESTRUCTION OF NATURAL STORES OF MATERIALS [Sidenote: Destruction of the American forests]