VOL. LI.] PHILOSOPHICAL TRANSACTIONS, Ab^l 



quently repeated for some small time^ and then ceasing again for a long term 

 excepting perhaps now and then some slight shock. And this analogy between 

 earthquakes, and the effects of volcanos is so great, that he thinks it cannot but 

 appear striking to any one who will read the accounts of both, and compare them 

 together. The raging of volcanos is not one continued and uniform effect; but 

 an effect repeated at unequal interv^als, and with unequal degrees of force : thus 

 for instance, we have perhaps 2 or 3 blasts discharged from a volcano, succeeding 

 each other at the interval of a few seconds only; sometimes the intervals are of 

 a quarter of an hour, an hour, a day, or perhaps several days. And as these 

 intervals are very unequal, so is the violence of the blasts also: sometimes stones, 

 &c. are thrown by these blasts to the distance of some miles; at other times per- 

 haps not to the distance of 100 yards. The same difference is observed in the 

 intervals and violence of the shocks of earthquakes, which are repeated at small 

 intervals for some time. 



Sect. 1. The frequency of earthquakes in the neighbourhood of burning moun- 

 tains is a strong argument of their proceeding from a cause of the same kind : 

 and the analogy of several volcanos lying together in the same tract of country, 

 as well as new ones breaking out in the neighbourhood of old ones, tends greatly 

 to confirm this opinion ; but what makes it still the more probable, is that pecu- 

 liarity in the structure of the earth before mentioned. It has been already ob- 

 served that the same strata are generally very extensive, and that they commonly 

 lie more inclining from the mountainous countries than the countries them- 

 selves : these circumstances make it probable that those strata of combustible 

 materials, which break out in volcanos on the tops of the hills, are to be found 

 at a considerable depth under ground in the level and low countries near them. 

 If this should be the case, and if the same strata should be on fire in any places 

 under such countries, as well as on the tops of the hills, all vapours, of whatever 

 kind, raised from these fires, must be pent up, unless so far as they can open 

 themselves a passage between the strata; whereas the vapours raised from vol- 

 canos find a vent, and are discharged in blasts from their mouths. Now if, when 

 they find such a vent, they are yet capable of shaking the country to the dis- 

 tance of 10 or 20 miles round, what may we not expect from them when they 

 are confined? We may form some idea of the force and quantity of these vapours 

 from their effects : it is no uncommon thing to see them throw up at once such 

 clouds of sand, ashes, and pumice stones, as are capable of darkening the whole 

 air, and covering the neighbouring country with a shower of dust, &c. to some 

 miles distance: great stones also of some tons weight are often thrown to the 

 distance of 2 or 3 miles by these explosions; and Mons. Bouguer tells us, that 

 he met with stones in South America, of 8 or g feet diameter, that had been 



VOL. XI. 3 N 



