THE HEAT OF THE EARTH. 35 



second or mathematical part, in accordance with the nature 

 of its applications, yields rather negative than positive results. 

 This method of enquiry, which possesses all the charm of 

 ingenious and intellectual combinations of thought, 31 leads 

 to problems, which cannot 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, 32 as the origin of all thermic springs as 

 well as of volcanic igneous currents, emanated from the early 

 and generally felt requirement of discovering some common* 

 ?ause for a great and complicated series of phenomena. 



Amid the multiplicity of relations presented by the 

 earth's surface, in respect to insolation (solar action) and 

 its capacity of radiating heat, and amid the great differences 

 in the capacity for conducting heat, which varies in ac- 

 cordance with the composition and density of hetero- 

 geneous rocks, it is worthy of notice, that wherever the 

 observations have been conducted with care, and under 

 favourable circumstances, the increase of the temperature 

 with the depth has been found to present for the most part 

 very closely coinciding results, even at very different lo- 

 calities. For very great depths we obtain the most certain 

 results from Artesian wells, especially when they are filled 

 with fluids that have been rendered turbid by the admixture 

 of clay, and are therefore less favourable to the passage of 

 internal currents, and when they do not receive many lateral 

 affluents flowing into them at different elevations through 

 transverse fissures. On account of their depth, we will 

 begin with two of the most remarkable Artesian wells, 

 namely that of Grenelle, near Paris, and that of the New 

 Salt Works at Oeynhausen, near Minden. We will proceed 

 in the following paragraph to give some of the most accurate 

 results which they have yielded. 



According to the ingenious measurements of Walferdin, 38 



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

 for 1838, p. 92; Philos. Transact., 1839, pt. ii, p. 381, and 1840, pt. i, 

 p. 193; Hennessey (Terrestrial Physics) in the Philos. Transact., 1851, 

 pt. ii, pp. 504525. 



3-3 Cosmos, vol. i, p. 235. 



33 The observations of Walferdin were made in the autumn of 1847, 

 and deviate very slightly from the results obtained with the same appa 



D 2 



