§ 11. Production of usable land surface in cities. The work of man is doing much by form changes to increase the area suitable for residence and business. Large districts on the river fronts of New York are filled land. The larger part of the most valuable lands within the city of Boston were once tidewater swamps, which have been filled and made usable by great outlays. Great hills have been dumped into the Bay of San Francisco to convert mud flats into solid earth, for railroad terminals and warehouses. In almost every city much has been done to level hillsides, to fill valleys, or to drain swamps. Along many picturesque lakes the steep banks for miles are dug with pick and shovel or blasted with dynamite, and dumped over into the water to make level sites for cottages. Wooden, stone, or cement retaining walls are built so that the debris from the streams and the sands washed up by waves may be retained to widen the solid land. Suitable places for docks, warehouses, and factories, and other needs of commerce and industry, are created on the shores of navigable waters. The engineer in tunneling mountains and building roadbeds over marshes or along swampy riversides, or in digging canals between rivers or between oceans, is making the kind of land surface suitable to the uses of transportation and trade. It is characteristic of nearly all these artificially altered spaces that they are as solid and enduring as natural formations of level land, and are subject only to the slow action of rain, streams, waves, winds, or to rare upheavals of nature. Man's works are in these cases as enduring as nature's.

§ 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 waterfall 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 similar 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 increase in the cost or scarcity. This class of goods was considered 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 extremes. Except as form and place changes are thus limited by elemental materials and natural sources of power, the outlook 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 questions, 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 overlooked the possibility of increase in any of these forms of wealth, nor the limits to the increase of any one of them.