166 COSMOS. 



pose that the periodic elevations and depressions of the molten 

 jnass under the already solidified strata must have caused 

 inequalities in the vaulted surface from the force of pressure. 

 The amount and action of such oscillations must, however, be 

 gmall ; and if the relative position of the attracting cosmical 

 bodies may here also excite " spring tides," it is certainly not 

 to these but to more powerful internal forces that we must 

 ascribe the movements that shake the Earth's surface. There 

 are groups of phenomena to whose existence it is necessary to 

 draw attention, in order to indicate the universality of the 

 influence of the attraction of the Sun and Moon on the external 

 and internal conditions of the Earth, however little we may be 

 able to determine the quantity of this influence. 



According to tolerably accordant experiments in Artesian 

 'wells, it has been shown that the heat increases on an average 

 about 1 for every 54' 5 feet. If this increase can be reduced to 

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

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



: * See The Introduction. This increase of temperature has been 

 found in 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 of 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 pro- 

 posed by Arago, in the year 1821 (Annuaire du Bioreau 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 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 5177 feet. 

 (Poisson, Theorie math, de la Chaleur, pp. 415 and 462.) In the 

 course of the last 17 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 permea- 

 tion of water, these errors are less injurious to the accuracy of conclusions 

 "than those resulting from currents of cold air, which are almost always 

 present in mines. The general result of Reich's great work on the tem- 

 perature of the mines in the Saxony mining districts, gives a some- 

 what slower increase of the terrestrial heat, or 1 to 76*3 feet. (Reich, 

 JSeob. iiber die Temperatur 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 Monkwearmouth, near Newcastle, in which, 



