Penny Blood: Serial Sensation and Working-Class Entertainment — page 214: what you’re looking at

Machine-transcribed from the original scan — historical spelling and the odd misread are preserved.

& cH. xv.| COMPRESSION.AND VOLCANIC ACTION. 197 where h is the height to which the pressure ¢ allows the magma to rise in the fissure. In the most favourable case h = 0, and then, work done against gravity = 7 work of compression. It is of course to be understood that what we have spoken of as the fissure of area A and width le is in reality the aggre- gate of all the fissures which occur within the length J of the tract, each of which is under an equal, or nearly equal, tension ¢, so that the compressing action takes effect throughout length. that The height m of the centre of gravity of the elevated tract is evidently very small compared with the thickness of the crust k, for the entire mean altitude of the continents is put at only m 1000 feet, so that Ps is probably considerably less than ;}, in an extreme case. We see then that but a very small part of the available force, arising under the conditions suggested, will be expended in raising the mountains, and by far the larger part of it will be available for overcoming the molecular forces which resist distortion of the rocks. But if a fissure be open to the surface so that a massive eruption takes place through it, then the pressure ¢, statically considered, upon the side of the fissure 1s what we have found in that case to be about gp x 12:2 miles, which is less than the pressure that would arise, if it were filled with a liquid of the density of the crust. This latter obviously could not produce compression. Hence, after the lava has gained access to the surface, the lateral force may cease to be effective, which agrees with the common observation, that earthquakes cease when a volcano breaks into action. In this limited sense the saying is justified, that volcanoes are the safety valves of the globe. Now it is not probable that any considerable volume of the crust could support continuously a horizontal pressure much ex- \ ceeding the vertical pressure without becoming distorted. If these orthogonal pressures were equal, they would tend only to (CO) MICOOO SS (CO)