NOTES. XKX1X 



consequently so many years ago this object must already have had an exist- 

 ence in the sidereal heavens, in order to send out those rays by which we now 

 perceive it." William Herschel, in the Phil. Trans, for 1802, p. 498. Sir 

 John Herschel, Astron. 590. Arago, Annuaire, 1842, p. 334, 359, and 

 382385. 



( 122 ) p. 145. From the beautiful sonnet, " Freiheit und Gesetz," by my 

 brother, Wilhelm von Humboldt : Gesammelte Werke, Bd. iv. S. 358, No. 25. 



( 123 ) p. 145. Otfried Muller, Prolegomena, S. 373. 



(* 24 ) p. 149. In speaking of the greatest depths reached by mining and 

 boring operations, we must distinguish between the absolute depth, or that 

 below the surface of the earth at the point where the work began, and the 

 relative depth, or that below the level of the sea. The greatest relative depth 

 which has yet been obtained is probably that of the salt-works of Neu-Salzwerk, 

 near Mindeu, in Prussia : in June 1844, its exact depth was 607'4 metres 

 (1993 English feet) ; the absolute depth was 680 metres (2231 English feet). 

 The temperature of the water at the bottom was 32'7 Cent, or 90'8 Fab..; 

 which, assuming the mean temperature of the air at 9 '6 Cent. (49 Fah.), 

 gives an increase of 1 Cent, for 29'6 met., or 1 Fah. for 53'8 feet. The 

 absolute depth of the Artesian well of GreneUe is only 1683 feet (1794 Engl. 

 feet). According to the accounts of the missionary, Imbert, the depth of our 

 Artesian wells is much exceeded by that of the fire-springs (Ho-tsing) in 

 China, which are sunk for the purpose of obtaining hydrogen gas to be em- 

 ployed in salt-boiling. In the Chinese province Szu-tschuan, the fire-springs 

 very ordinarily reask a depth from 1800 to 2000 French feet : it is even said, 

 that at Tseu-lieu-tsiug (place of continual Cow), there is a Ho-tsing 3000 feet 

 deep (3197 Engl.), which was bored with a rod in the year 1812 (Humboldt, 

 Asie centrale, T. ii. pp. 521 and 525. Annales de 1'Association de la Pro- 

 pagation de la Foi, 1829, No. 16, p. 369). The relative depth reached at 

 Monte Massi, in Tuscany, south of Volterra, amounts, according to Matteucci, 

 only to 382 metres (1253 Engl. feet). The boring at Neu-Salzwerk has pro- 

 bably very nearly the same relative depth as the coal mine at Apendale, near 

 Newcastle-under-Line (Staffordshire), where men work at a depth of 725 

 yards below the surface. (Thomas Smith, Miner's Guide, 1836, p. 160.) 

 Unfortunately, I do not know the exact height above the level of the 

 sea. The relative depth of the Monkwearmouth mine, near Newcastle, is 

 1496-5 Engl. feet (Phillips, Phil. Mag. Vol. v. 1834, p. 446); that of 

 the Liege coal mine of Esperance, at Seraing, is 1271 (1355 Engl.) 

 teet, according to M. von Dechen, the director j and the old coal mine 



