NATURAL HISTORY. ?$' 



where the air penetrates, which must necessarily ex- 

 tend the conflagration and augment the action of the 

 fire, to the point in which we see it when it produces 

 the terrible effects we have taken notice of. 



There are two kinds of earthquakes, the one occa- 

 sioned by the action of subterraneous fires, and the ex- 

 plosion of volcanoes, which are only felt at small dis- 

 tances, and at the time when volcanoes act, or before 

 they open. When the matters which form subterran- 

 eous fires, ferment, heat and inflame, the fire makes an 

 effort on every side, and if it does not find a natural 

 vent, it raises the earth and forces a passage for itself 

 by throwing it out, which produces a volcano, whose 

 effects are repeated, and last in proportion to the quan- 

 tity of inflammable matters. If the quantity of mat- 

 ters which take fire is not considerable, a commotion 

 or an earthquake may ensue, without a volcano being 

 formed. The air produced and rarefied by the subter- 

 raneous fire, may also find small vents, by which it will 

 escape, and in this case there will be only a shock with- 

 out any eruption or volcano. But when the inflamed 

 matter is in a great quantity, and confined by solid and 

 compressed matters, then a commotion and volcano a- 

 rises : but all these commotions form only the first 

 kind of earthquakes, and can enly shake a small space 

 of ground. A very violent eruption of mount ^tna 

 will occasion, for example, an earthquake through the 

 whole island of Sicily ; but it will never extend to the 

 distance of three or four hundred leagues. When any 

 new mouth is formed in mount Vesuvius, there are 

 earthquakes at Naples and in the neighbourhood of the 

 volcano : but these earthquakes have never shook the 

 Alps, and are not communicated to France or to other 

 countries remote from the source of the phenomenon. 

 Vol. I. I 



