VOL. XLVI.] PHILOSOPHICAL TRANSACTIONS. 113 



purposes, it is plain there is room for them to pass. Besides, as Mons. de 

 Buffoii remarks, naturalists have observed perpendicular and oblique clifts, in all 

 kinds of layers of earth, not only among rocks, but also among all kinds of 

 earth, that have not been removed, as is observable wherever the earth is open 

 to any depth. Now these clifts are caused by the drying of the several horizontal 

 layers of the earth ; and will also be considerably the wider in long dry hot sea- 

 sons, which arc usually the preparatory forerunners of earthquakes, and the ex- 

 plosion of the sulphureous vapours may probably widen them more. 



" It is very observable, in the opinion of Borelli, and other naturalists, that 

 Volcanos begin first to kindle near the surface or top of the mountains, and not 

 in the caverns in the lower parts of them. Mons. de BufFon says, that earth- 

 quakes are most frequent where there are Volcanos ; sulphureous matter abound- 

 ing most there : but that, though they continue burning long, yet they are not 

 very extensive. But that the other sort of earthquakes, which are not caused by 

 a Volcano, extend often to a great distance. These are much longer east and 

 west, than broad north and south ; and shake a zone of earth with different de- 

 grees of force in different parts of their course ; viz. in proportion to the dif- 

 ferent quantities of explosive sulphureous matter in different places. These 

 kinds of earthquakes are observed to be progressive, and to take time to extend 

 to the great distances sometimes of some thousands of miles. They are an in- 

 stantaneous explosion in every place, near the surface of the earth ; and therefore 

 do not produce mountains and islands, as volcanos sometimes do. 



" The earthquake in London, March 2, was thought to move from eastward 

 to westward. M. Buffon mentions an earthquake at Smyrna, in the year 1 688, 

 which moved from west to east ; viz. because the first kindling probably began 

 on the western side ; and in the earthquake at London on the eastern side. And 

 accordingly it was observed, that the reddish bows in the air, which appeared 

 several days before that earthquake, arose in the east, and proceeded westward. 

 It was observed, after the earthquake at Smyrna, that the castle-walls, which 

 run from east to west, were thrown down ; but those from north to south stood ; 

 and that the houses on rocks stood better than those on the earth. 



" M. de Buffon relates, that the vibrations of the earth, in earthquakes, have 

 commonly been from north to south ; as appears by the motion of the lamps in 

 churches : which makes it probable, that though the progress of the earthquake 

 at Smyrna was from west to east, yet the vibrations of the earth might be from 

 north to south ; and thereby occasion the falling of the castle walls, which run 

 from east to west, but not those which run from north to south. A probable 

 argument that, as the freest passage, so the greatest explosions were made in 

 the clifts of the earth which run east and west ; which would make the vibra- 

 tions north and south, '^j 



VOL. X. Q 



