3o6 bouguer's voyage to pert/. 



roaring of the volcano of Macas, called Sangai. I was more than eighteen thoufand 

 toifes diftant from it ; notwithflanding, the noife was fo frightful, as every moment to 

 wake me. This mountain is of a conic form, wanting only the point, whofe fides are 

 perfedly regular. The people of the country all agree, that the mafs of this mountain 

 is continually diminifhing ; its aftual' height above the fea is two thoufand fix hundred 

 and fixty-four toifes. The flames ilfue from its fummit, and a ftream of inflamed mat- 

 ter often rolls down one of its fides ; a ravine at its bafe has taken the name of the Sul- 

 phur River. The roarings of the volcano are fometimes like the noife of a fliarp peal 

 of thunder ; but they quickly refume their regular periods, and rumbling founds, the 

 repetition of which fo much incommoded me. I have obferved the fame puffs of fmoke 

 iflfue out at regular intervals from Cotopaxi, and form a fort of flieaves of light. The 

 intervals between their emiflions, when I obferved them, were from forty-two to forty- 

 three feconds. The inflamed matter in the interior of the volcano doubtlefs extended 

 each time its dilatation : but this dilatation in part weakening, the inflammable parts 

 alfo a little diminiflied ; which would open a way for the exterior air to re-enter, either 

 by the upper or fome other opening. Perhaps, alfo, at this interval, other inflammable 

 matter, unlooked for, may find an eafy introduftion ; and infl:antly the eruption ac- 

 quires additional force, and re-produces a new emiflion of fmoke, or a new roar. 



The materials which take fire in the bowels of the earth, and which caufe earth- 

 quakes, muft: necelfarily be fubjed to the fame changes. When the j(ire takes in a con- 

 cavity, the dilatation of the inflammable matter and air will naturally fpread wide, and 

 be conveyed to other fubterraneous cavities communicating with the firft. The roof of 

 the vault is alfo violently forced, as are its fides, although the mafs of matter lies precifely 

 beneath. The direction of the effort will depend then upon the horizontal or inclined 

 polition of the cavity ; hence the caufe why fometimes the walls of buildings remain or 

 not, according to the manner they are fituated. The roof of the vault returns to its place, 

 after the neceflary vibrations, which are independent of the action of the fire. Their fpeed 

 mufl: depend upon the fize of the vault, its thicknefs, and the nature of the materials 

 of which it is formed. But the effort of the explofion abating a little, while the air be- 

 comes much too compreflTed in the adjacent cavities, it makes a violent reflux towards 

 the place of eruption, which opens another accefs, and gives occafion to a new and 

 more violent ftiock. Thus there muft: neceffarily be returns marked by a more excef- 

 five agitation ; and their intervals will be more fenfibly regular, till fome confiderable 

 change happens, either in the inflamed matter, or the difpofition of the cavity. The 

 weakeft fliocks are thofe from the earth already fliaken ; the ftrongeft: thofe caufed im- 

 mediately by the inflammation, which are analogous to the roarings of the volcanoes, 

 and which are repeated more or lefs frequently, according to the facility with which the 

 materials take fire, and alfo, as their volume has relation with the extent of the fpaces 

 in which they are inclofed. 



A country containing in it fo many inflammable materials, muft: be a fubjed to fur- 

 nifli many other remarks to natural hiftory. Nature has there, thus to exprefs myfelf, 

 continually in her hands the materials and implements for extraordinary operations. 

 Exhalations may produce, in certain places, the fame cafualties as in the famous Grotto 

 del Cani. Waters impregnated with mineral matters will make hot-baths, while, in 

 other places, we fliall fee them labouring at incruftations and cryftalizations. But I 

 muft: wave thefe details till another time, that I may give you a little account of the 

 part of America over which I pafled on my return. 



PART 



