[footnote continues] The relative depth reached at Mount Massi, in Tuscany, south of Volterra, amounts, according to Matteuci, to only 1253 feet. The boring at the new salt-works near Minden is probably of about the same relative depth as the coal-mine at Apendale, near Newcastle-under-Lyme, in Staffordshire, where men work 725 yards below the surface of the earth. (Thomas Smith, 'Miner's Guide', 1836, p. 160.) Unfortunately, I do not know the exact height of its mouth above the level of the sea. The relative depth of the Monk-wearmouth mine, near Newcastle, is only 1496 feet. (Phillips, in the 'Philos. Mag.', vol. v., 1834, p. 446.) That of the Liege coal-mine, 'l'Esperance' at Seraing, is, according to M. Gernaert, Ingenieur des Mines, 1223 feet in depth. The works of greatest absolute depth that have ever been formed are for the most part situated in such elevated plains or valleys that they either do not descend so low as the level of the sea, or at most reach very little below it. Thus the Eselchacht, at Kuttenberg, in Bohemia, a mine which can not now be worked, had the enormous absolute depth of 3778 feet. (Fr. A. Schmidt, 'Berggestze der oter Mon.', abth. i., bd. i., s. xxxii.) Also, at St. Daniel and at Geish, on the Rorerbubel, in the 'Landgericht' (or provincial district) of Kitzbuhl, there were, in the sixteenth century, excavations of 3107 feet. The plans of the works of the Rorerbubel are still preserved. (See Joseph von Sperges, 'Tyroler Bergwerksgeschichte', s. 121. Compare, also, Humboldt, 'Gutachten uber √∫erantreibung des Meissner Stollens in die Freiberger Erzrevier', printed in Herder, 'uber Herantreibung des Meissner Stollens in die Freiberger Erzrevier', printed in Herder, 'uber den jetz begonnenen Erbstollen', 1838, s. cxxiv.) We may presume that the knowledge of the extraordinary depth of the Rorerbuhel reached England at an early period, for I find it remarked in Gilbert, 'de Magnete', that men have penetrated 2400 or even 3000 feet into the crust of the Earth. ("Exigua videtur terrae portio, quae unquam hominibus spectanda emerget aut eruitur; cum profundinus in ejus viscera, ultra efflorescentis extremitatis corruptelam, aut propter aquas in magnis fodin, tanquam per venas scaturientesaut propter seris salubrioris ad vitam operariorum sustinendam necessarii defectum, aut propter ingentex sumptus ad tantos labores exantlandos, multasque difficultates, ad profundiores terrz' partes penetrre non possumus; adeo ut quadrigentas aut [quod rarissime] quingentas orgyas in quibusdam metallis descendisse, stupendus omnibus videatur connatus." -- Guilielmi Gilberti, Colcestrensis, 'de Magnete Physiologia nova'. Lond., 1600, p. 40.)