THE HEAT OF THE EARTH. 37 



low specific gravity and underlying the upheaved mountain 

 chains. 4 In a work devoted to cosmical phenomena no 

 question should be overlooked on which actual observations 

 have been instituted, or which may seem to be elucidated by 

 close analogies. 



b. The Existence and Distribution of Heat in the interior of 



our Globe. 



(Expansion of the Delineation of Nature, Cosmos, vol. i., 



p. 1G8-17G.) 



Considerations regarding the internal heat of our earth, 

 the importance of which has been greatly augmented by the 

 connection which is now generally recognized to exist be- 

 tween it and phenomena of upheavals and of volcanic action, 

 are based partly upon direct, and therefore incontrovertible 

 measurements of temperature in springs, borings, and sub- 

 terranean mines, and partly upon analytical combinations 

 regarding the gradual cooling of our planet, and the influence 

 which the decrease of heat may have exercised in primeval 

 ages upon the velocity of rotation and upon the direction 

 of the currents of internal heat.f The figure of the com- 

 pressed terrestrial spheroid is further dependent upon the 

 law, according to which density increases in concentric su- 

 perimposed non-homogeneous strata. The first or experi- 

 mental, and therefore the more certain portion of the inves- 

 tigation to which we shall limit ourselves in the present 

 place, throws light only upon the accessible crust of the 

 earth, which is of very inconsiderable thickness, while the 

 second or mathematical part, in accordance with the nature 

 of its applications, yields rather negative than positive results. 

 This method of inquiry, which possesses all the charm of 

 ingenious and intellectual combinations of thought^ leads 

 to problems, which can not be wholly overlooked when we 

 touch upon conjectures regarding the origin of volcanic 

 forces, and the reaction of the fused interior upon the solid 

 external crust of our earth. Plato's geognostic myth of the 

 Pyriphlegethon,§ as the origin of all thermic springs, as well 



* See Petit sur la latitude de T Observatoire de Toulouse, la densite 

 moyenne de la chaine des Pyrenees, et la probability quHl existe un vide 

 sous sette chaine, in the Comptes rendus de I Acad, des Sc, t. xxix., 1849, 

 p. 730. f Cosmos, vol. i., p. 176. 



% Hopkins, Physical Geology, in the Report of the British Association 

 for 1838, p. 92; Philos. Transact., 1839, pt. ii., p. 381, and 1810, pt. 

 i., p. 193; Hennessey (Terrestrial Physics'), in the Philos. Transact., 

 1851, pt. ii., p. 504-525. § Cosmos, vol. i., p. 237. 



