460 THE POPULAR SCIENCE MONTHLY. 



celebrated engineer, Werner Siemens, cites, as opposed to Sir W. 

 Thomson's theory, the results of his experiments at the glass-works 

 of his brother, Fr. Siemens, at Dresden. When the melted vitreous 

 mass commenced to cool, contraction was at first very rapid, then more 

 gradual as it attained a pasty consistency, and at the time of solidifica- 

 tion it even seemed to expand slightly. From this M. Siemens con- 

 cludes that the contraction resulting from the solidification of the 

 fused silicates occurs during the change from the liquid to the pasty 

 state; and, by Sir W. Thomson's reasoning, it would seem all the more 

 probable that the central portions of the globe have already attained a 

 pasty consistency.* 



On the supposition that the solid crust has but a slight thickness, 

 many phenomena are explained, notably the ascent of lava in volcanic 

 vents, which might thus be due to the hydrostatic pressure caused by 

 the weight of masses of rock. This same cause may even have con- 

 tributed to the elevation of mountains, by forcing the lighter solid 

 masses above the level of a sea of heavier lava. Again, the slow 

 changes of level of the land, as seen in the changes in certain coast- 

 lines, indicate a mobility of the solid crust, which would naturally 

 experience oscillations in consequence of a secular displacement of its 

 center of gravity, and this displacement may result from modifications 

 of the exterior surface by the action of water, and of the inner surface 

 by the action of lava. Indeed, do not earthquakes whose cause may 

 be found as well in the falling of masses of rock, or the action of sub- 

 terranean waters, as in purely volcanic phenomena constantly show 

 that great changes are occurring in the depths of the ground ? 



Sir George Airy has lent the weight of his great authority to the 

 hypothesis of a liquid nucleus, in his recent interesting address at 

 Cockermouth, before an audience of miners and others. The illus- 

 trious astronomer royal holds the opinion that the earth's crust is 

 formed of more or less compact rocks that float on a mass of fluid or 

 semifluid lava. The heaviest of the rocks form the ocean-beds ; 

 lighter ones the continents ; and the mountains are composed of the 

 portions that project the farthest into the lava, in exactly the same 

 way that large ships draw more water than small ones. It follows 

 from this that beneath the mountains a considerable volume of rela- 

 tively dense lava has been displaced by lighter masses, which would 

 account for the slight effect produced by certain ranges the Hima- 

 layas, for example on the plummet. 



Again, it is on the hypothesis of an internal fire that such theories 

 of the elevation of mountains as that of M. Elie de Beaumont are 

 founded. The earth's crust in cooling undergoes a contraction, caus- 

 ing ruptures on the arcs of great circles ; the lava, as it is compressed 

 by the contracting solidified crust, is forced through these fissures, 



* " Physikalisch-mechanische Betrachtungen " (" Monatsbericht der Acad, der Wiss. 

 zu Berlin," 1878). 



