One cause of the great heat of many of our deep mines, which appears to have been entirely lost sight of, is the chemical action going on upon large masses of pyritic matter in their vicinity. The heat, which is so oppressive in the United Mines, is, without doubt, due to the decomposition of immense quantities of the sulphurets of iron and copper known to be in this condition at a short distance from these mineral works. The heat which man is enabled to measure beneath the earth’s surface, appears to be alone due to the conducting powers of the rocks themselves; it has been observed that the line of equal temperature follows, as nearly as possible, the elevations and depressions which prevail upon the surface, and the diminishing rate of increase beyond this line, certainly is such as would arise, was all the heat so measured, the result of the passage of the heat by conduction through the crust of rocks. Whether or not the subterranean bands of equal heat have any strict relation, upon a large scale, to the isothermic lines which have been traced around most portions of our globe, is a point which has not yet been so satisfactorily determined as to admit of any general deductions. The Oriental story-teller makes the inner world a place of rare beauty--a cavern temple, bestudded with self-luminous gems, in which reside the spiritual beings to whom the direction of the inorganic world is confided. The Philosopher, in the height of his knowledge, has had dreams as absurd as this; and amid the romances of science, there are not to be found any more strange visions than those which relate to the centre of our globe. At the same time it must be admitted, that many of the peculiar phenomena which modern geological researches have brought to light, are best explained on the hypothesis of a cooling sphere, which necessarily involves the existence of a very high temperature towards the centre.