INTERNAL HEAT OP THE EARTH. 37 



Grenelle ; it sinks 377 feet deeper below the surface of the 

 sea, and the temperature of its waters is 9. 18 F. higher. 

 The increase of the heat at Paris, is about 1 F. for 59 

 feet, and therefore scarcely T ] 7 th greater. I have already else- 

 where drawn attention to the fact that a similar result was 

 obtained by Auguste de la Rive and Marcet, at Bregny, near 

 Geneva, in investigating a boring which was only 725 feet 

 in depth, although it was situated at an elevation of more 

 than 1600 feet above the Mediterranean Sea. 35 



If to these three springs, which possess an absolute depth 

 varying between 725 feet and 2285 feet, we add another, that 

 of Monkwearmouth, near Newcastle, (the water rising 

 through a coal mine which, according to Phillips is worked 

 at a depth of 1496 feet below the level of the sea,) we 

 shall find this remarkable result, that at four places widely 

 separated from one another an increase of heat of 1 F. 

 varies only between 54 and 58.6 feet; 36 such a coincidence 

 in the results cannot, however, be always expected to occur 

 when we consider the nature of the means which are em- 

 ployed for determining the internal heat of the earth at 

 definite depths. Although we may assume that the water 

 which is infiltrated in elevated positions through hydrostatic 

 pressure as in connected tubes, may influence the rising of 

 springs at points of great depth, and that the subterranean 



35 Cosmos, vol. i, p. 166, and Memoires de la Societe d'ffist. Naturelle de 

 Geneve, t. vi, 1833, p. 243. The comparison of a number of Artesian 

 wells in the neighbourhood of Lille, with those of Saint Ouen and 

 Geneva would, indeed, lead us to assume, if we were quite certain as to 

 the accuracy of the numerical data, that the different conductive powers 

 of terrestrial and rocky strata exert a more considerable influence 

 than has generally been supposed (Poisson, Theorie Matliematlgue de 

 la Chaleur, p. 421). 



36 In a table of foxirteen borings, which were more than one hundred 

 yards in depth, and which were situated in various parts of France, 

 Bravais, in his very instructive encyclopaedic memoir in the Patria, 

 1847, p. 145, indicates nine in which an increase of temperature of 

 1 F. is found to occur for every 50 70 feet of depth, which would 

 give a deviation of about 10 feet in either direction from the mean 

 value given in the text. See also Magnus in Poggen. Ann. Bd. xxii, 1831, 

 B. 146. It would appear, on the whole, that the increase of tempera- 

 ture is most rapid in Artesian wells of very inconsiderable depth, 

 although the very deep wells of Monte Massi in Tuscany, and Neuffen 

 on the north-west part of the Swabian Alps, present a remarkable ex- 

 ception to this rule. 



