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 efiect 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 seculaT 

 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 



in the Puits de Greuelle, 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 uiouth oi 



the boring is 1609 feet above the level of the seaj 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 des Longitudes, 1835, p. 



234), for three different mines, of the absolute depths of 1794, 2231, 



and 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 I'Observatoire 



beneath it: the mean temperature of the former is 51'^"5, and of the 



latter 53°-3, the ditference being lo-8 for 92 feet, or 1° for 51-77 feet. 



(Poisson, Thiorie Math, de la Chaleur, p. 415 and 462.) 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 cuiTents of cold air, which are 



almost always present in mines. The general result of Reich's great 



work on the temperature of the mines in the Saxony mining districts 



gives a somewhat slower increase of the terrestrial heat, or 1° to 76*3 



feet. (Reich, Beob. uber die Te^nperatur des Gesteins in verschieden en 



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



xxxiv., s. 191), in a shaft of the coal-mine of Monk-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 increase 



of 1° to 59*06 feet, a result almost identical with that found by Arago 



in the Puits de Grenell. 



