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Rh when that computation has been reached it must finally be squared with an ideal standard involving time. Suppose, for example, that engineers conclude that the present fuels are being exhausted at a certain rate; that barring new discoveries industry will have to enter a phase of contraction at some definite time in the future. We have then to determine how much thrift and self-denial we will use, after all feasible economies have been exercised, in order not to rob posterity. But what shall we consider posterity? Our grandchildren? Our great-grandchildren? Perhaps we shall decide to calculate on a hundred years, believing that to be ample time for the discovery of alternative fuels if the necessity is made clear at once. The figures are, of course, hypothetical. But in calculating that way we shall be employing what reason we have. We shall be giving social time its place in public opinion.

Let us now imagine a somewhat different case: a contract between a city and a trolley-car company. The company says that it will not invest its capital unless it is granted a monopoly of the main highway for ninety-nine years. In the minds of the men who make that demand ninety-nine years is so long as to mean "forever." But suppose there is reason to think that surface cars, run from a central power plant on tracks, are going out of fashion in twenty years. Then it is a most unwise contract to make, for you are virtually condemning a future generation to inferior transportation. In making such a contract the city officials lack a realizing sense of ninety-nine years. Far better to give the company