5i6 THE POPULAR SCIENCE MONTHLY. 



of the globe ; although the celebrated catastrophe at Lisbon on the 

 1st of November, 1755, extended over some 17° or 18°, into Africa 

 and the two Americas, or over a surface equal to about four times 

 that of Europe. 



The detailed examination of many earthquakes has enabled us to 

 determine the center of the shocks as well as the contours of the dis- 

 turbed areas. From the manner in which the latter surfaces agree 

 with the lines of pre-existing dislocations, several of the most distin- 

 guished geologists, including Mr. Dana, M. Suess, and Albert Heim, 

 have considered the shocks in question as connected with the forma- 

 tion of chains of mountains, of which they may be a kind of continu- 

 ation. 



In fact, the crust of the earth everywhere shows the enormous 

 effects exercised by the lateral pressures that have been in operation 

 at all epochs. The strata, bent and bent over again many times through 

 thousands of metres of thickness, as well as the great fractures that 

 traverse them, are the eloquent witnesses of these mechanical actions. 

 Notwithstanding the apparent tranquillity now reigning on the surface 

 of the globe, equilibrium does not exist in the earth, and commotions 

 have not been arrested in its depths. The proof of this is found, not 

 only in earthquakes, but also in the slow movements of the soil, of 

 elevation and depression — a kind of warping, which has continued to 

 manifest itself within historical times in all parts of the globe. It is 

 conceivable that slow actions of this kind, after more or less prolonged 

 strains, may end in abrupt movements, as !filie de Beaumont supposed. 

 We can see, also, in experiments intended to imitate the bending of 

 strata, how gradual inflections lead all at once to fractures and out- 

 bursts. Simple cavings-in, in deep cavities, have also been regarded 

 as possibly giving rise to earthquakes ; and this opinion has been 

 adopted by M. Boussingault after the well-known observations he 

 made in the Andes. There is, in fact, nothing to prove that disturb- 

 ances of these different kinds do not take place in the interior of the 

 globe ; but we may certainly consider them as the general cause of 

 earthquakes. These shocks are, however, most commonly in evident 

 connection with volcanoes ; and it is in the neighborhood of the latter 

 that they are especially frequent. As is well known, every volcanic 

 eruption is announced by precursory earthquakes, the violence of 

 which is stilled when an outlet is opened for the vapor of water which 

 is successively the cause of the subterranean agitations and the pro- 

 jecting agent of all the eruptions. The tension of the vapor in the 

 volcanic reservoirs must be very high. Thus, that pressure which 

 forces the lara up to more than 3,000 metres above the sea, to the top 

 of Etna, can not be less than a thousand atmospheres. 



An attentive study of the phenomena confirms the attribution of 

 the cause of the shocks, however violent they may be, to the vapor of 

 water. It is sufficient for this to be the case for vaporization to take 



