ANNIVEESAEY ADDRESS OF THE PEESIDEIS^T. xlix 



ture in these rocks. Therefore the rate of augmentation of tempera- 

 ture as we descend cannot be fully explained by conduction. 



As the whole crust of the earth is traversed by innumerable fissures, 

 which admit of vertical and horizontal movements of water and air, 

 it is manifest that these currents must be, in a majority of cases, 

 more effective to considerable depths than the mere flow of heat- 

 waves through solid rock. Side by side, the granite and slate of the 

 Cornish mines are of unequal temperatures, and both differ in this 

 respect from the cross-courses and veins*. 



The distribution of water in the cavities of the rocks may be con- 

 ceived to influence their temperature in three ways. Let us suppose 

 a case like that of the artesian wells of London, where water is 

 pumped up in great quantity from subterranean reservoirs, which it 

 has reached by continually descending from the hills of Surrey and 

 Herts. In this case the water is warmed by the subterranean rock, 

 which therefore it cools, as far as the current extends, and the final 

 temperature of the issuing water is less than that of the isothermal 

 due to conduction (a). But suppose no artesian well in this locality, 

 and no great current of water depending on gravitation, but only a 

 very slow circulation of water through all the fissures, and of heat 

 with it, the flow of heat outwards would in this case be only a little 

 greater than by mere conduction (h). Lastly, suppose a case like 

 that of some of the Cornish mines where the water descends from 

 the surface along veins highly inclined, and attains depths much 

 greater than those reached by the miner, to receive heat also greater 

 than that due to the depth at which the spring is cut by the miner, 

 and made the subject of thermometrical experiment by the philoso- 

 pher (c). In these cases, other things being the same, the rate of 

 increase of temperature for a given measure of descent would be 

 different — least at a — greatest at c. If we assume 6 as a mean, and 

 take averages of a and c for the extremes, we shall have nearly these 

 numbers, — 



a. 60 feet, 

 6. 50 „ 

 c. 40 „ 



Now, by Mr. Hopkins's Table of conducting powers already given, 

 the ratio for a, artesian well in chalk, being 1° for 60 feet, that for c (as 

 is seen in granite or slate) should have been about 1° for 60 x 2-5 — 

 150 feet. To augment the temperature to what it is in the Cornish 

 mines, we must increase the fraction yi^ by -^^, the heat commu- 

 nicated by convection, which, in this case, is twice and three-quarters 

 as great as that obtained by conduction. 



The important thermal influence of water circulating through the 

 Cornish mines is recognized by IVIr. Henwoodt, whose observations 

 were made by preference on the springs rather than the rocks. He 

 remarks that at equal depths the cross courses are colder than the 



* Henwood on the Metalliferous Deposits of Cornwall and Devon. 

 t Henwood on Subterranean Heat in Cornish Mines (Trans. Roy. G-eol. Soc. 

 Cornwall, vol. v.). 



