It does worse, for even its customary annual produce requires to be extracted by a greater and greater expenditure of labor and capital. As a mine does not reproduce the coal or ore taken from it, not only are all mines at last exhausted, but even when they as yet show no signs of exhaustion they must be worked at a continually increasing cost; shafts must be sunk deeper, galleries driven farther, greater power applied to keep them clear of water; the produce must be lifted from a greater depth, or conveyed a greater distance. The law of diminishing return applies therefore to mining in a still more unqualified sense than to agriculture; but the antagonizing agency, that of improvements in production, also applies in a still greater degree. Mining operations are more susceptible of mechanical improvements than agricultural: the first great application of the steam-engine was to mining; and there are unlimited possibilities of improvement in the chemical processes by which the metals are extracted. There is another contingency, of no unfrequent occurrence, which avails to counterbalance the progress of all existing mines toward exhaustion: this is, the discovery of new ones, equal or superior in richness. Professor Jevons has applied this economic law to the industrial situation of England.(139) While explaining that the supply of cheap coal is the basis of English manufacturing prosperity, yet he insists that, if the demand for coal is constantly increasing, the point must inevitably be reached in the future when the increased supply can be obtained only at a higher cost. When coal costs England as much as it does any other nation, then her exclusive industrial advantage will cease to exist. In the United States the outlying iron deposits of Lake Superior, Lake Champlain, and Pennsylvania, so geologists tell us, will find competition arising from the new grades of greater productiveness in the richer deposits of States like Alabama. In that case we shall be going from poorer to better grades of iron-mines, but after the change is made a series of different grades of productiveness will be established as before. To resume: all natural agents which are limited in quantity are not only limited in their ultimate productive power, but, long before that power is stretched to the utmost, they yield to any additional demands on progressively harder terms. This law may, however, be suspended, or temporarily controlled, by whatever adds to the general power of mankind over nature, and especially by any extension of their knowledge, and their consequent command, of the properties and powers of natural agents. Chapter X. Consequences Of The Foregoing Laws. § 1. Remedies for Weakness of the Principle of Accumulation.