1'30 B. Mallet— Reply to Mr. Poulett-Scrope. 



we assume his fissures full of water, with a copious surface supply, — then would his 

 solution signally fail to account even for such differences of increments as can be 

 traced to the differences in conductivity alone. But Hopkins, in one of the most 

 valuable of his papers, has shown that there are differences of increment not trace- 

 able to conductivity alone, and for which he was unable to offer any explanation. 

 Facts^ also have been observed since his time which cannot be accounted for upon 

 Mr. Scrope's notions : such as a continued increment, and then a decrement of 

 hypogeal temperature, again becoming an increment, in the very same shaft ; two 

 shafts, alike as to wetness, not far apart, in the very same formation, extending to a 

 vast depth below, and extending for miles all around, and yet showing great dis- 

 parity of increment ; two shafts not far apart, in different rocks, extending far 

 around in every direction, but in the line joining them, and yet with differences of 

 increment the very reverse of those due to conductivity alone. For these and other 

 like cases Mr. Scrope's views, even if physically well founded, would offer no 

 solution whatever. It is, however, as I have already briefly remarked, one of the 

 striking confirmations of the truth in nature of my theory that it does, as a 

 collateral consequence, offer a complete and consistent solution of this previously 

 unsolved puzzle — viz. the immense inequalities in hypogeal increments. 



I have shown in my paper (Phil. Trans, part i. vol. for 1873, p. 168, par. 

 ^9 to 75) that these inequalities are due to the different amounts of work expended 

 m the horizontal compression of strata, varying in compressibility or resistance 

 when transformed partially into heat ; work which is now going on more or less 

 within every part of the superficial crust of our globe. The heat thus produced 

 may in places be so slight as scarcely to affect the thermometer ; while at others, 

 as my theory "declares, it may rise to the highest tempei-atuve of volcanic action. 

 The source to which I attribute volcanic heat and energy is not a mere local 

 phenomenon existing alone along the lines of volcanic vents, but is a great cos- 

 mical condition pervading every part of the thick and solid crust of our globe, and 

 varied only in degree at one place or at another, dependent upon the amount of 

 work expended at any point, and transformed into heat in the unit of time. 



Mr. Scrope suspects that I have but an imperfect acquaintance with the phe- 

 nomena of volcanoes in eruption, and then proceeds to say, ' ' or he would not 

 speak as he does of the expenditure of heat in the explosions of steam from a 

 volcano in eruption as 'not resembling that which takes place in a steam engine, 

 but rather that of powder exploded in a cannon, the loss from which is shown to 

 be much smaller. ' The contrary is really the case ; the explosions from a volcano 

 in activity resembling precisely in character (and apparently in cause) those of a 

 Perkins steam-cannon fed by a continuous escape of steam from a boiler." Mr. 

 Scrope has here made a giant of his own, founded upon an almost ludicrous 

 mistake as to my meaning. I have not said one word in any part of my paper, or 

 in paragraphs 189-190, about the resemblances to either steam engine or cannon 

 of the explosive efforts seen at volcanic vents in eruption. I am, in the passage 

 quoted, referring simply to the amount of waste of heat that should be allowed for 

 as most probable in estimating the total amount of heat annually expended to 

 produce the existing volcanic phenomena of our globe. The waste of heat due to 

 lifting action in enaptions by steam blown off uselessly is, no doubt, as Mr. Scrope 

 says, immense ; and I have made an immense allowance for it by assuming that 

 every three units of heat thus expended only do the lifting work of one. I have, 

 as I conceive, made a most ample allowance in my estimate of the annual heat 

 required for existing volcanic action, for that which is usefully consumed or wasted 

 in the three great operations of melting the solid ejecta, vaporising the gaseous 

 ones, and lifting the whole to the height ejected. But whether my figures be ac- 

 cepted as exact or not, I have also shown that the heat annually lost by radiation 

 from our globe (which, as Mr. Scrope calls it, is i\i& primu?7t mobile) is so vastly in 

 excess of that which is demanded to account for all existing volcanic action, that 

 Mr. Scrope may, if he pleases, increase by some hundreds of times the expenditure 

 I have assigned, without the result affecting the validity of my argument. 



As to Mr. Scrope's suspicion that I have but an imperfect acquaintance with the 

 natural phenomena of active volcanoes, my own field of personal observation has been, 

 I believe, not very far from co-extensive with his own ; but besides my own personal 

 observations, which have neither been few nor unsystematic, I have made diligent 

 use of the eyes and observations of others, and readily acknowledge the debt I owe 



