178 Thos. Mudd— Analogy of Earthqualxe Phenomena. 



of the condensible vapour into the colder end of the passage from 

 the hotter, so that it may be under conditions producing condensa- 

 tion, and thus commence the water hammer action. 



That this kind of action, owing largely to the very low com- 

 pressibility of water, is capable of producing vei'y severe shocks, 

 even when the essential factors are of comparatively low value, is 

 seen from the fact that stout iron pipes of only a few inches in 

 diameter are often burst into fragments by it, even when the differ- 

 ence of pressure developed cannot be more than about 50 lbs. per 

 square inch, and where the length of pipe is not many yards. 



Given, therefore, subterranean reservoirs, as Prof. Prestwich 

 supposes, we have only to assume a communication formed between 

 these two reservoirs by a fissure, and we shall have the conditions 

 favourable for the water hammer action on such a gigantic scale as 

 appears competent to account for the phenomena of earthquakes and 

 volcanoes. Immediately a fissure is formed connecting the reservoirs 

 of water nearer the surface and at comparatively low temperature 

 with the more deep-seated reservoirs of molten rock, steam is formed, 

 which, passing along the fissure, becomes condensed, difierence of 

 pressure is set up, the water and molten rock form a fluid piston 

 or plug, and shocks proportionate to the difference of pi'essure, the 

 length and sectional area of the passage, and to the quantity of 

 matter set in motion, are thence inevitable. 



If, when this action arrives at its maximum, the water cavity is 

 incapable of sustaining the shock, and gives way, its broken walls 

 are thrown out as dust and ash, the whole body of the water, which 

 had become superheated, bursts into steam, and the molten rock is 

 poured out as lava. 



There is another analogy indirectly connected with this matter, 

 which appears to hold between the conditions beneath the crust of 

 the earth and those in a steam boiler, to which I shall only 

 make a brief reference, as the question has been dealt with in well- 

 known geological works. It bears upon the observed fact that 

 mountain ranges occur mainly along lines of thick deposit. If a 

 sediment (especially one of a loose or of a greasy character) be 

 allowed to accumulate on the outside of the fire-tube of a boilei', 

 the heat is prevented from getting through the iron plate sufficiently 

 fast to prevent overheating. The contact of the plate with the water 

 is broken, and the sediment is a bad conductor. The result is that 

 the plate becomes overheated, and the external pressure on the 

 fire-tube thrusts it out of shape, producing what is known as a 

 " collapsed furnace." 



This seems to illustrate what takes place on a large scale in areas 

 of great deposition. The thick loose sediment forms a bad con- 

 ductor, overheating takes place, and that first, of course, along 

 the line of thickest deposit, and the pressure being internal instead 

 of externa], a ridge outside is the result instead of a collapse. 



