[Book VI* 



CHAP. 



EARTHQUAKES. 



Connexion between Earthquakes and Volcanoes. Earthquakes caufed 

 ky the Prugrefs of Steam between the Strata of the Earth. Signs of 

 approaching Earthquakes. Great Earthquake at Lijlon, in 1755. 

 Earthquakes in Calabria, in 1783. 



TH E ftidden explofions which take place from 

 volcanoes, probably depend on the accefs of 

 a quantity of water, which enters through fome fiffure 

 communicating with the fea, or which is derived from 

 other fources in the earth. If this mafs of water is 

 lufficiently great, it will extinguifh the volcano j if not, 

 it will be converted into fleam, the expanfive force of 

 which far exceeds that of gun-powder. The elaftic 

 fluid, thus formed, either finds vent at the mouth of 

 the volcano, or, if the fuper-inciimbent weight fhould 

 be too great, it will force a pafiage between the ftrata 

 of the earth, and occafion that undulatory but fome- 

 times violent motion which is called an earthquake. 

 From various facts demonftrative of the cohefion and 

 elafticity of bodies, we are warranted in concluding, 

 that the different ftrata of which the earth is com- 

 pofed will adhere together, and that a freer paflage 

 will be afforded to any intervening body between the 

 flrata than directly through them. If the confined 

 fire ads directly under a province or town, it will 

 heave the earth perpendicularly upwards, and the 

 mocks will be more fudden and violent. If it acts 

 at a diftance, it will raife that tract obliquely, and 

 the motion will be more oblique, undulatory, and 

 tremulous. 



The 



