141 



departs, often greatly, and to an unknown extent, from the actual 

 physical conditions of the problem. Prof. Rogers maintained 

 that conclusions so derived, however true as logical deductions 

 from the premises, are not to be received as demonstrated phy- 

 sical facts. Should the correspondence of Earthquake pheno- 

 mena with those of the tides, be confirmed by further compari- 

 sons, to which M. Perrey has been invited by the French Academy 

 of Sciences, it would furnish a proof of the igneous fluidity of 

 the interior of the globe, and of the yielding thinness of its 

 inclosing shell, too conclusive to be weakened by any calcula- 

 tions deduced from hypothetical data. 



Prof. Rogers then alluded to the late experiments of Hopkins 

 and Fairbairn, to determine the influence of pressure upon the 

 melting point of solids. As we know that pressure augments the 

 temperature necessary to vaporize liquids, it has become a ques- 

 tion of interest, bearing upon the internal fluidity of the globe, 

 to ascertain if it has a like efl^ect upon the temperature at which 

 solid bodies become fluid. According to Hopkins and Fairbairn, 

 such an effect actually occurs with spermaceti, wax, sulphur, and 

 stearine, but has not yet been detected in certain other very fusi- 

 ble solids experimented upon. Should it prove to be a general 

 law, applicable also to mineral masses, which is yet far from 

 being demonstrated, we should have to admit a higher internal 

 temperature than would otherwise be needed to maintain the 

 interior in a fused state; but this addition would, most probably, 

 form but a small fraction of the whole temperature. 



In regard to the influence of centrifugal force in causing earth- 

 quakes, Prof. Rogers remarked, that such effect could only arise 

 from a variation of the centrifugal force, and therefore of the 

 earth's velocity of rotation. But the uniformity of this rotation is so 

 nearly perfect, that a change amounting to even a fraction of a 

 second in a day, would be too startling an occurrence to escape 

 astronomers. Hence, any variations of centrifugal force that 

 may arise must be comprised within extremely narrow limits. 

 Even at the equator, where this force is greatest, its proportion 

 to gravity is very small, and when we consider that any admissi- 

 ble fluctuation of its intensity must be a very minute fraction of 

 the whole force, and hardly more than an infinitesimal part of 



