THEORY OF VOLCANIC ENERGY. 477 



work expended is reckoned only for raising the centre of gravity of 

 the cone above its base ; whereas, in fact, the energy ought to have 

 been calculated upon the height to which all the dust and lapilli 

 would have been shot up into the air, on the supposition that it had 

 taken place in vacuo. To take a parallel case : — If a ball were fired 

 vertically from a cannon, no true estimate of the energy expended 

 would be obtained by multiplying the mass of the ball by its radius 

 ■ — the height of its centre of gravity after it had fallen to the 

 ground. 



Such are some of the objections which have occurred to me upon 

 a somewhat mature consideration of Mr. Mallet's now celebrated 

 theory. 



Discussion. 



Prof. Seeley regretted that Mr. Fisher had not seen his way to a 

 wider discussion of some of the interesting subjects of his paper ; 

 thus the figure of the earth might be better understood if it were 

 also considered how far such a form might result from expansion of 

 the earth arising from the equatorial heat of sunshine. He said, on 

 the hypothesis that the earth was homogeneous and formed of copper, 

 it would expand about 1 part in 35,000 for every degree of increase 

 in temperature ; and starting with the hypothesis that an equatorial 

 temperature of 30° F. had in this way been raised 50° to 80° F., and 

 that this temperature extended to the earth's centre, then an expan- 

 sion of 50 parts in 35,000 would go far towards accounting for a 

 difference between the polar and equatorial diameters of a world. 



Another point was the origin of continents. He did not think 

 that the explanations at present given exhausted the subject ; and 

 remembering that the lifting power of the moon corresponded to 

 about one 250,000th part of the earth's weight, and that the sun 

 supplemented this with a power half as great, he ventured to suggest, 

 since the earth's surface is thrown into folds which are proved, by 

 fringing reefs and atolls, to be alternately rising and falling, that 

 these movements, might be explained, in part at least, as the effect 

 of tidal movement in the earth itself. 



Mr. Peacock thought that neither Mr. Fisher nor Mr. Mallet had 

 given sufficient consideration to the force of steam. 



Prof. Ramsay said he rose not so much to criticise Mr. Fisher's 

 paper as to ask for information. He thought it might be possible to 

 arrive by calculation at the time when the earth's crust became ex- 

 ceedingly thick, but we could know nothing about a first thin crust 

 from the study of existing rocks now at the surface. In that sense 

 the Laurentian rocks are not of extreme antiquity ; for the sediments 

 of which they were formed were derived from preexisting continents, 

 composed as at present of stratified and igneous rocks. In fact we 

 know nothing positive of a time when the crust of the earth was so 

 thin as to be capable of bulging out under the influence of external 

 attraction. Prof. Ramsay referred to Mr. Hopkins's notion of the 

 fluid interior of the earth being affected by tides produced by the 



