§ 12. Durative character of hydraulic power sites. The sources from which man has as yet successfully obtained power are domestic animals, winds, falling waters, the tides, and heat producing materials (wood, coal, oil, etc.). The winds, while inexhaustible sources, are too irregular to be of the greatest importance. Waterfalls are of increasing use with the progress in the art of transmitting power in the form of electricity. The maintenance of water-power plants in efficient condition calls for much labor on the banks of the millstream, or for the building and repairing of dams and reservoirs, of pipes and of water wheels. With this care, waterfalls are durative in a high degree. The supply of power from water is capable of enormous increase through the construction of reservoirs, the building of canals, and the economizing of great sources now going to waste. The water fall as a whole is permanent, perennially renewed by rains; but the energy liberated by the falling water is consumed each moment. Because of this natural renewal of the power, a continuing usufructuary value adheres in the site of land whose possession gives control over the falling water. A simi lar view is to be taken of the rare sites where tidal power can be economically employed.
§ 13. Goods varying in increasableness. It has long been customary for economists to talk of economic goods that could be increased indefinitely (meaning infinitely or, in any event, without any limit ever appreciable to man) without any in crease in the cost or scarcity. This class of goods was con sidered to be very large. There is no such class of economic goods; it is impossible that there should be ; if they are "scarce," increasing demand must make them scarcer, except as discoveries and improvements increase the supply. All
kinds of wealth are, so far as it is economical to do so, thus increased, even land surface. Many kinds in the course of time are very greatly increased with little or no direct effort, but the supply of all alike can be secured in larger amount at any given moment with the known methods and tools only with increasing difficulty. The different forms of wealth may be ranged on a scale according to the ease with which they can be increased by effort. They may, therefore, be classed as relatively fixed and relatively increasable. Some natural resources belong at one end, and some at the other end of this scale, and, necessarily, the tools and appliances made from these materials must likewise range between the ex tremes. Except as form and place changes are thus limited by elemental materials and natural sources of power, the out look is that form and place change will grow constantly more easy, and elementary materials constantly more difficult, to obtain. No hard and fast line divides the different kinds of goods, but the difference in degree of increasableness is a fact of great social importance, affecting the direction in which industry can and must progress.
The difference in increasableness of the various forms of wealth is of importance in considering various social ques tions, such as the effects of an increase of population, and the kinds of taxation most equitable and most favorable to the progress of society. Account must be taken of the fact, for instance, that the number of bricks can be increased more easily than the amount of land ; but there must not be over looked the possibility of increase in any of these forms of wealth, nor the limits to the increase of any one of them.