The Earth’s Heat.
Those who believe that the centre of the earth is exceedingly hot, so that all the substances composing it are molten, if not even vaporised by intensity of heat, may find their faith in some degree shaken by the results of deep-mining at Stawell, in Victoria. According to the rate at which the temperature has been observed to inorease with depth in European mines, the lower part of the shaft sunk by the Magdala Quartz Company to a depth of 2100 ft ought to be so hot that the miners would barely be able to work. But the manager reports tliat when the thernometer was close to the bottom the mercury stood only at 82deg Fahrenheit, when suspended 17ft from the bottom at 80deg, and in the chamber at 84deg ; in the western drive, 82deg in the face : in the rise, where no air pipes carry air, at 83deg, and on the same afternoon the same glass showed the temperature inside the office, on the surface of the ground, to be 80deg. Science will have considerably to modify her interpretation of the great heat at the bottom of many European mines of no greater depth. If the whole central mass of the earth were hot, an approach towards the centre by so many feet would produce about as much effect in one region as in another (excepting, of course, certain volcanic districts). The theory of local subterranean fires, other than those finding visible outlets, may remove this special difficulty, but introduces others scarcely less serious. If the increase of heat in European mines were limited to coal mines, the difficulty might perhaps be removed in another way, but this is not the case. In two shafts in coal mines in England, each about 2000 ft deep, the temperature was found to increae by Ideg Fsduienheit for every increase of depth 'ufum 65ft to 70ft, and about exactly the same increase has been noticed in the principal lead and silver mines of iSaxouy. A thermometer fixed in the rock of the Dalcoath mine in Cornwall, at a depth of 1308 ft, showed a mean temperature for the year 18deg below that of the surface. Some observers estimate the rate of increase at Ideg for 45ft (M. Cordier’s estimate) and others at so much as Ideg in 39ft. But taking only Ideg in 70ft, the temperature at the bottom of the Stawell shaft should be 30deg hotter than at the surface, whereas the difference is but a small fraction of this amount. Probably the true explanation of the matter is that, as Charles Lyell long since suggested, 1 ‘ vast reservoirs of molten matter exist beneath the surface, but such, nevertheless, as may hold a very subordinate place in the earth's crust.
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