Page:Popular Science Monthly Volume 17.djvu/476

460 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 solidification it even seemed to expand slightly. From this M. Siemens concludes 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 contributed 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 coastlines, 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 subterranean 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 illustrious 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 relatively dense lava has been displaced by lighter masses, which would account for the slight effect produced by certain ranges—the Himalayas, 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, causing ruptures on the arcs of great circles; the lava, as it is compressed by the contracting solidified crust, is forced through these fissures,