Page:The parochial history of Cornwall.djvu/29

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21
PREFACE.

between electric energies and terrestrial magnetism, Mr. Fox has been enabled to give more than probable reasons for the extraordinary fact of some metals usually selecting, in all parts of the world, lodes or fissures running nearly east and west, and why other metals prefer rents at right angles to the former; and in respect to the fissures themselves, Mr. Fox has remarked appearances inducing him to believe that lodes of considerable breadth have not been formed by any one great and sudden rending of the earth; but that, in a manner similar to the rising or to the sinking of land, by the gradual action of causes now well known to exist, those clefts have been enlarged from time to time, and have as frequently received additional deposits, easily discriminated from each other.

Mr. Fox appears also to have settled beyond the possibility of doubt, the long-agitated question respecting the temperature of mines, by establishing a general relation between increases of heat and depth; although the ratio cannot be reduced to any definite formula, being liable to vary with the presence of more or less water, and with the different conducting power of rocks, since mines in granite and in killas differ by several degrees of heat at the same level: yet the increase corresponds so generally with greater descents into the earth, that elevation of temperature, and not the expense, nor the difficulty of exhausting water, appears likely to oppose the final limit to the progress of mines in depth.

In continuation of the same trains of reasoning