Mr. R. Mallet on Volcanic Energy. 145 



remarks here that, upon the basis of the heat annually dissipated 

 from our globe being equal to that evolved by the melting of 777 

 cubic miles of ice at zero to water at the same temperature, and 

 of the experimental data contained in his paper, he had demon- 

 strated, in terms of mean crushed rock, the annual supply of heat 

 derivable from the transformation of the mechanical work of con- 

 traction available for volcanic energy, and had also estimated the 

 proportion of that amount of heat necessary to support the annual 

 vulcanicity now active on our globe ; but, from the want of 

 necessary data, he had refrained from making any calculation as 

 to what amount in volume of the solid shell of our earth must be 

 crushed annually, in order to admit of the shell following down 

 after the more rapidly contracting nucleus. This calculation he 

 now makes upon the basis of certain allowable suppositions, where 

 the want of data requires such to be made, and for assumed thick- 

 nesses of solid shell of 100, 200, 400, and 800 miles respectively. 



From the curve of total contraction (plate x. Phil. Trans, part i. 

 1873) obtained by his experiments on the contraction of slags, 

 he has now deduced partial mean coefficients of contraction for a 

 reduction in temperature of 1° Pahr., for intervals generally of 

 about 500° for the entire scale, between a temperature somewhat 

 exceeding that of the blast-furnace and that of the atmosphere, 

 or 53° Pahr. And applying the higher of these coefficients to the 

 data of his former paper, and to the suppositions of the present, 

 he has obtained the absolute contraction in volume of the nuclei 

 appertaining to the respective thicknesses of solid shell above 

 stated. In order that the shell may follow down and remain in 

 contact with the contracted nucleus, either its thickness must be 

 increased, its volume remaining constant, or the thickness being 

 constant, a portion of the volume must be extruded. The former 

 supposition is not admissible, as the epoch of mountain-building 

 has apparently ceased ; adopting the second, the author calculates 

 the volume of matter that must be crushed and extruded from the 

 shell in order that it may remain in contact with the nucleus. 

 He tabulates these results for the four assumed thicknesses of 

 shell, and shows that the amount of crushed and extruded rock 

 necessary for the heat for the support of existing volcanic action 

 is supplied by that extruded from the shell of between 600 and 

 800 miles thickness, and that the volume of material, heated or 

 molten, annually blown out from all existing volcanic cones, as 

 estimated in his former paper, could be supplied by the extruded 

 matter from a shell of between 200 and 400 miles in thickness. 



On data which seem tolerably reliable the author has further 

 been enabled to calculate, as he believes for the first time, the 

 actual amount of annual contraction of our globe, and to show 

 that if that be assumed constant for the last 5000 years, it would 

 amount to a little more than a reduction of about 3*5 inches on 

 the earth's mean radius. This quantity, mighty as are the effects 

 it produces as the efficient cause of volcanic action, is thus shown 

 to be so small as to elude all direct astronomical observation, and, 



Phil. Mag. S. 4. Vol. 49. No. 323. Feb. 1875. L 



