Page:The fairy tales of science.djvu/332

286 It has been demonstrated by numerous observations made in mines, and by Artesian wells in various countries, that the temperature of the earth rapidly increases with the depth, but that the rate of augmentation is different at different places—in the Northumberland coal-pits, for instance, one degree Fahrenheit for every 44 feet in descent; in the lead-mines of Saxony, one degree for every 65 feet; in the copper-mines of Knockmahon, county of Waterford, one degree for every 82 feet; in the Dolcoath mine, in Cornwall, one degree for every 78 feet. Assuming the average increase of temperature to be one degree of Fahrenheit for every 60 feet of depth, and the rate of increase to remain constant, at a depth of 60,000 feet below the surface of the earth the temperature must stand at 1000 degrees Fahrenheit, which is that of low red-heat. But as the temperature will increase with the depth in an augmenting ratio, Leonhard assumes that the temperature of a low red-heat would be attained already at a depth of 35,000 feet, or double the height of Cotopaxi, the most remarkable of the Peruvian volcanoes. Descending still lower, to depths varying from 80 to 160 miles below the surface, the temperature would be found at that depth to exceed 12,000 degrees Fahrenheit—a heat sufficient to melt most of the known rocks. But considering that the dense fluid portions of the earth are most probably much better conductors of heat than the crust, it may safely be assumed that this high