254 



EARTHQUAKES. 



cover those parts of it that were thus affected by the earthquake with 

 black crape, we shall obtain a more distinct idea of the extent of sur- 

 face shaken, than a mere verbal description can convey. This ap- 

 pears to have been one of the most severe shocks that the old con- 

 tinent had experienced for several centuries. The cause which 

 could effect a simultaneous concussion over such a vast extent, must 

 probably have been seated nearly midway between the centre of the 

 globe and its surface. 



It has been remarked, that in general, earthquakes are more se- 

 verely felt in mountainous than in low countries : this might be ex- 

 pected from the structure of the earth.* In alpine districts, the pri- 

 mary mountains are not pressed with the incumbent mass of second- 

 ary rocks; and, consequently, in such situations, the resistance to a 

 force acting from beneath will be much less, as all the weight of sec- 

 ondary rock is removed. In very violent earthquakes, the second- 

 ary strata are broken or agitated ; but proofs are not wanting, of les- 

 ser vibrations being stopped by their pressure. Humboldt says, he 

 has seen workmen hasten from the mines of Marienburgh, in Saxo- 

 ny, alarmed by agitations of the earth that were not felt at the sur- 

 face. During the earthquake at Lisbon, the miners in Derbyshire 

 felt the rocks move, and heard noises which were scarcely perceiv- 

 ed by those above. That an expansive force, acting from beneath, 

 is the proximate cause of earthquakes, can scarcely be denied ; and 

 the prodigious power of steam, when suddenly generated, seems 

 equal to their production, if the quantity be sufficiently great. It is 

 said, that a single drop of water falling into a furnace of melted cop- 

 per, will blow up the whole building. This may be an exaggerated 

 statement : but the prodigious force of steam at high temperatures is 

 well known, and there can be no difficulty in admitting, that if a cur- 

 rent of subterranean water were to find access to a mass of lava ma- 

 ny miles in extent, and most intensely heated, it would produce an 

 earthquake more or less violent, in proportion to the quantity of steam 

 generated, and its distance from the surface. When the hydrogen 

 gas exploded in a mine near Workington, in Cumberland, a shock 

 like that of an earthquake was felt by ships in the river, at two miles' 

 distance. 



The horrid crash, like the ratding of carriages, which precedes 

 earthquakes, may be occasioned by the rending of the rocks, or 

 parting of the strata through which the confined vapour is forcing a 

 passage. 



All the phenomena that accompany earthquakes indicate the inr 

 tense operation of elastic vapour, expanding and endeavouring to es- 

 cape where the least resistance is presented, and producing vibra- 



* See a paper on Earthquakes, by the Rev. Mr. Mitchell, Philosophical Trans- 

 actions, 1759. 



