The greatest depth to which man has excavated is nothing when compared with the radius of the earth. The Eselschacht mine at Kuttenberg in Bohemia, now inaccessible, which is 3778 feet below the surface, is deeper than any other mine. Its depth is only 150 feet less than the height of Vesuvius, and it is eight times greater than the height of the pyramid of Cheopos, or the cathedral of Strasburg. The Monkwearmouth coal-mine near Sunderland, descends to 1500 feet below the level of the sea, so that the barometer stands there at 31·80, which is higher than anywhere on the earth’s surface.[99] The salt-works of New Saltzwerk in Prussia are 2231 feet deep, and 1993 feet below the level of the sea; and various other mines, such as the Liege coal-mine of Esperance, and that of Mont Massi, in the Maremma of Tuscany, do the same. Mines on high ground may be very deep without extending to the sea-level: that of Valenciana, near Guanaxuato in Mexico, is 1686 feet deep, yet its bottom is 5960 feet above the surface of the sea; and the mines in the higher Andes must be much more. For the same reason the rich mine of Joachimsthal in Bohemia, 2120 feet deep, has not yet reached that level. The fire-springs at Tseu-lieu-tsing in China are 3197 feet deep, but their relative depth is unknown.[100] How insignificant are all the works of man compared with nature!—A line 27,600 feet long did not reach the bottom of the Atlantic Ocean. The metals are very profusely diffused over the earth. Few countries of any extent do not contain some of them. A small number occur pure, but in general they are in the form of ores, in which the metal is chemically combined with other substances, and the ore is often so mixed with earthy matter and rock that it is necessary to reduce it to a coarse powder in order to separate the ore, which is rarely more than a third or fourth part of the mass brought above ground. Gold is found in almost every country, but in such minute quantities that it is often not worth the expense of working. It is almost always in a native state, and in the form of crystals, grains, or rolled masses. Sometimes it is combined with silver. It is exhausted in several parts of Europe where it was formerly found. The united produce of the mines in Transylvania, Hungary, the north-western districts of Austria, and the bed of the Danube, is nearly 60,000 ounces annually. Gold is found in small quantities in Spain, in the lead-hills in Scotland, and the Wicklow mountains in Ireland.