5,400,000 horse power was being developed from water falls, whereas
about 37,000,000 primary horse power[6] was available; but, by
the storage of flood waters so as to equalize the flow, at least
100,000,000 horse power, and possibly double that amount, could be
developed. As it requires ten tons of coal to develop one horse power
a year in a steam engine by present methods, there is here a potential
substitute for coal equal to two to four times our present annual use
of coal (about 500,000,000 tons in 1912).

But this does not mean that it would be economical, at present costs
of mining coal and of building reservoirs, to make this substitution
now. To determine when, how far, and by what methods to develop this
water power from lakes and rivers for the use of the people and to
make this substitution, is another of our great economic problems.

Petroleum and natural gas, of which our original reservoirs were
perhaps the richest in the world, are being rapidly exhausted. These
may be merely mentioned as being related to coal in the source
of their supply, in the nature of their uses, and in the economic
problems to which they give rise.

§ 10. #Transportation agencies#. First to mention among the means of
transportation are the navigable waters--oceans, lakes, rivers, and
canals, with the necessary equipment of dredged inlets, harbors,
docks, locks, and lighthouses. Few of these appear in the total of
"capitals," for they are not in private possession. Yet a good system
of natural waterways may be greater wealth to one nation than costly
additional railroads are to another. Good natural harbors on the
waterways leading out to the oceans are a most important kind
of national wealth, as are the navigable great lakes within the