534 Royal Inslitutioti. 



by reference to a table exhibiting the temperatures (on Fahrenheit's 

 scale) corresponding to a variety of depths, from 50 feet to 200 miles 

 below the surface of the earth, on the average (assumed for conve- 

 nience, but corresponding closely v^rith that of observation,) of 1^ of 

 increase of heat for every 50 feet of depth. The theory of the 

 secular variation of the isothermal surfaces of the interior of the 

 globe, considered as a cause of geological phsenomena, w^as then 

 explained ; as it has been enunciated by Mr. Babbage in his paper 

 on the temple of Serapis at Pozzuoli, read before the Geological So- 

 ciety in 1834, and by Sir J. Herschel in letters addressed to Mr. Mur- 

 chison and Mr. Lyell, communicated to that Society in 1837, and 

 subsequently published by Mr. Babbage, together with his own paper, 

 in his work entitled " The Ninth Bridge water Treatise :" the ex- 

 treme generality of the terms in which Mr. Babbage's application 

 of the theory to volcanic phaenomena, properly so called, had been 

 announced, and the main object of his paper having been to explain 

 by its means the pyrometric expansion of rocks as the cause of ele- 

 vation, being assigned as reasons for his views having remained 

 comparatively unregarded, until the more recent promulgation of 

 views identical with them in their leading features, but more ex- 

 plicitly developed in their application to those phsenomena, by Sir 

 J. Herschel. The rise of the isothermal curves on the deposition 

 of fresh solid matter on the earth's surface, their conformation to the 

 sphericity of the earth in its central regions, but to the configura- 

 tion of the surface as they approach it, and Sir J. Herschel's ap- 

 plication of the theory to explain the elevation of continents, the 

 production of the metamorphic rocks, and the origin of submarine 

 volcanos and chains of volcanic vents, were severally noticed, and 

 illustrated by reference to drawings. The attention of the auditory 

 was directed, especially, to the generalization explaining the situa- 

 tion of volcanic chains along lines of coast ; Sir H. Davy, it was re- 

 marked, had assigned a reason why volcanos should only exist in the 

 vicinity of great bodies of water, but it was reserved for Sir J. Her- 

 schel to explain their actual disposition conformably to the coast lines, 

 and also why they should exist at all. 



As the temperature attained by the bottom of the newly deposited 

 mass of strata would be proportionate to the thickness of that mass, 

 and would be the same as that existing at a depth within the earth 

 equal to that thickness, (viewing the subject in the most general 

 approximate manner, agreeably to the amount of our present know- 

 ledge, and unavoidably disregarding a multitude of considerations 

 which must enter into the discussion of the problem, in order to its 

 exact solution,) the table of temperatures was now again referred 

 to ; for the purpose of indicating, on the one hand, how great a thick- 

 ness of deposited matter would be required for the original surface to 

 rise to even a moderate temperature ; but, on the other, at how insig- 

 nificant a depth (or thickness,) compared to the earth's radius, ade- 

 quate temperatures would occur ; the latter consideration being illus- 

 trated by a diagram exhibitingthe relative magnitudes of various depths 

 corresponding to some of the tcmj)eratures in the table, as referred to 

 a radius of twenty feet, representing that of the earth. Thus, at a 



