SG A VOYAGE TO BookYII. 



ill the ear<b is comparatively but small. Whence it 

 is inferred that an aperture being once made, how- 

 ever the substances in the bowels of the mountain 

 may take fire, the convulsion of the earth is seldom 

 or never felt a second time. The reason of which is, 

 that the sudden reiteration of this accident greatly 

 augments the volume of the air by rarefaction ; and as 

 it finds an easy passage without labouring in the 

 bowels of the earth for a vent, no Other concussion 

 is produced than v. hat inust follow from the eruption 

 of a great quantity of air through an aperture too 

 narrow for its volume. 



The formation of volcanoes is now well known; 

 and that they owe their origin to sulphureous, ni- 

 trous, and other combustible substances in the bowels 

 of the earth; for these being intermixed, and, as it 

 were, turned into a kind of paste by the subterraneous 

 waters^ ferment to a certain degree, when they take 

 lire; and by dilating the contiguous wind or air, 

 and also that within their pores, so that its volume 

 is prodigiously increased beyond what it was before 

 the inflammation, it produces the same effect as 

 gunpowder when fired in the narrow space of a mine ; 

 but Vv'ith this diiterence, that powder on being fired 

 immediately disappears, whereas the volcano being 

 once ignited continues so till all the oleaginous and 

 sulphureous particles contained in the mountain are 

 consumed. 



YoLCANOEs are of two kinds, contracted and di- 

 lated. TJ;e former are found where a great quan- 

 tity of inflammable matter is confined in small 

 space; the latter where these combustibles are scat- 

 tered at a considerable distance from one another. 

 The first are chiefly contained in the bowels of 

 mountains, which may lie considered as the natural 

 depositaries of these subsiances. The second may be 

 considered as ramifications which though proceeding 

 from the former^ are, however, independent, ex- 

 tending 



