466 PHILOSOPHICAL TRANSACTIONS. [aNNO I76O. 



frequently take their rise from the sea. According to the description of the 

 structure of the earth before given, any combustible stratum must lie at greater 

 depths in places under the ocean than elsewhere; hence far more extensive fires 

 may subsist there than where the quantity of matter over them is less; for any 

 vapour raised from such fires, having both a stronger roof over it, and being 

 pressed by a greater weight, beside the additional weight of the water, will not 

 only be less at liberty to expand itself, and consequently of less bulk, but it will 

 also be easily driven away towards the parts round about, where the superincum- 

 bent matter is less, and therefore lighter. On the other hand, any vapour raised 

 from fires, where the superincumbent matter is lighter, finding a weaker roof 

 over it, and being not so easily driven away under strata that are thicker and 

 heavier, will be very apt to break through, and open a mouth to a volcano; and 

 it must necessarily do this long before the fires can have spread themselves suffi- 

 ciently to be nearly equal to those which may subsist in places that lie deeper. 

 All this seems to be greatly confirmed by the situation of volcanos, which are 

 almost always found on the tops of mountains, and those often some of the 

 highest in the world. 



If then the largest fires are to be supposed to subsist under the ocean, it is no 

 wonder that the most extensive earthquakes should take their rise from thence: 

 the great earthquake of Lisbon has been shown to have done so; and that the 

 cause of it was also at a greater depth than that of many others, appears from 

 the greater velocity with which it was })ropagated. 



The great earthquake that destroyed Lima and Callao, in 17 46, seems also to 

 have come from the sea; for several of the ports on the coast were overwhelmed 

 by a great wave, which did not arrive till 4 or 5 minutes after the earthquake 

 began, and which was preceded by a retreat of the waters as well as that at 

 Lisbon. Against this it may perhaps be alleged, that there were 4 volcanos 

 broke out suddenly in the neighbouring mountains, when this earthquake hap- 

 pened, and that the fires of these might be the occasion of it. This however 

 is not very probable; for, to omit the argument of the wave and previous re- 

 treat of the waters, already mentioned, it is not very likely that more than one 

 fire was concerned: besides, the vapour, opening itself a passage at these places, 

 could not well be supposed, if it took its rise from thence, to spread itself far; 

 especially towards the sea, where it is manifest that the strata over it were of 

 great thickness, as appears from the gre^t velocity with which the earthquake 

 was propagated there; the shocks also continued with equal, or nearly equal, vio- 

 lence, for some months after the openings were made; whereas, if thpse fires 

 had been the cause of them, they must immediately have ceased on the fires 

 finding a vent, as it has hapjjcned in otlier cases. It is therefore much more 

 probable that a very large quantity of vapour, taking its rise from some far 



