174 COSMOS. 



to arithmetical relations, it will follow, as I have already ob- 

 served,* that a stratum of granite would be in a state of fusion 

 at a depth of nearly twenty-one geographical miles, or between 

 four and five times the elevation of the highest summit of the 

 Himalaya. 



We must distinguish in our globe three different modes for 

 the transmission of heat. The first is periodic, and affects 

 the temperature of the terrestrial strata according as the heat 

 penetrates from above downward or from below upward, being 

 influenced by the different positions of the Sun and the sea- 

 sons of the year. The second is likewise an effect of the Sun, 

 although extremely slow : a portion of the heat that has pene- 

 trated into the equatorial regions moves in the interior of the 

 globe toward the poles, where it escapes into the atmosphere 

 and the remoter regions of space. The third mode of trans- 

 mission is the slowest of all, and is derived from the secular 

 cooling of the globe, and from the small portion of the primi- 

 tive heat which is still being disengaged from the surface. 



* See the Introduction. This increase of temperature has been found 

 ia the Puits de Grenelle, at Paris, at 58*3 feet; in the boring at the new 

 salt-works at Minden, almost 53-6 ; at Pregny, near Geneva, according 

 to Auguste de la Rive and Marcet, notwithstanding that the mouth o/ 

 the boring is 1609 feet above the level of the sea, it is also 53-6 feet. 

 This coincidence between the results of a method first proposed by 

 Arago in the year 1821 (Annuaire du Bureau de$ Longitudes, 1835, p. 

 iJ.34), for three different mines, of the absolute depths of 1794, 2-231, 

 uud 725 feet respectively, is remarkable. The two points on the Earth, 

 lying at a small vertical distance from each other, whose annual mean 

 temperatures are most accurately known, are probably at the spot on 

 which the Paris Observatory stands, and the Caves de 1'Observatoire 

 beneath it: the mean temperature of the former is 51-5, and of the 

 latter 53-3, the difference being l-8 for 92 feet, or 1 for 51-77 feet. 

 (Poisson, Thiorie Math, de la Chaleur, p. 415 and 4G2.) In the course 

 of the last seventeen years, from causes not yet perfectly understood, 

 but probably not connected with the actual temperature of the caves 

 the thermometer standing there has risen very nearly 0-4. Although 

 in Artesian wells there are sometimes slight errors from the lateral 

 permeation of water, these errors are less injurious to the accuracy of 

 conclusions than those resulting from currents of cold air, which are 

 almost alwnys present in mines. The general result of Reich's greht 

 work on the temperature of the mines in the Saxony mining district* 

 {jives a somewhat slower increase of the terrestrial heat, or 1 to 76-3 

 ieet. (Reich, Beob. fiber die Temperatnr des Gestcins in versckieden en 

 Tiefcn, 1834, s. 134.) Phillips, however, found (Pogg., Annalen, bd. 

 xxxiv., B. 191), in a shaft of the coal-mine of Moiik-wearmouth, near 

 Newcastle, in which, as I have already remarked, excavations are going 

 on at a depth of about 1500 feet below the level of the sea, an ircreasa 

 of 1 to 59-06 feet, a result almost identical with lhat found by Arngo 

 in the Puite de Gienell. 



