[ 6 7 8 ] 
thefe Clefts are caufed by the drying of the feveral ho- 
rizontal Layers of the Earth } and will alfo be con- 
fulerably the wider in long dry hot Seafons, which 
are ufually the preparatory Forerunners of Earth- 
quakes, and the Exploiton of the fulphureous Va- 
pours may probably widen them more. 
It is very obiervable, in the Opinion of Borelli,m<l 
other Naturaiifts, that Volcano’s begin firft to kindle 
near the Surface or Top of the Mountains, and not in 
the Caverns in the lower Parts of the Mountains. 
Monf. de Buffon fays, that Earthquakes are mod fre- 
quent where there are Volcano’s '■> fulphureous Mat- 
ter abounding mod there: But that, tho’ they con- 
tinue burning long, yet they are not very extenfivc. 
But that the other Sort of Earthquakes, which are 
not caufed by a Volcano , extend often to a great Di- 
ftancc. Thefe are much longer Eaft and Weft, than 
broad North and South; and fhake a Zone of Earth 
with different Degrees of Force in different Parts of 
their Courfe; viz. in proportion to the different 
Quantities of explofive fulphureous Matter in differ- 
ent Places. Thefe kind of Earthquakes are obferved 
to be progreflive, and to take Time to extend to the 
great Diftances fometimes of fome Thoufands of 
Miles. They are an inftantaneous Explofton in every 
Place, near the Surface of the Earth ; and therefore do 
not produce Mountains and Elands, as Volcano’s 
fometimes do. 
The Earthquake in London , Marche, was thought 
to move from Eaftward to Weftward. M. Buffon 
mentions an Earthquake at Smyrna , in the Year 
1688. which moved from Weft to Eaft; viz. be- 
caufe the firft Kindling probably began on the 
Weftern Side; and in the Earthquake at London on 
