and the suhtenancous Lakes of Pur ace. 1 13 



had already twice observed and described this phaenomenon, in 

 the mountains of Bareuth, andnear Cracow, during a journey in 

 Poland. Can it be admitted that the successive layers which are 

 added to the central nucleus are in a state of fluidity so great that 

 the rotary motion can cause the flattening of the spheroids? 



When the barometer indicated that we were come very near 

 the limit of perpetual snow, we found the masses of suljihur 

 disseminated in imperfectly columnar trachylic rocks aug- 

 mented. This pliEenomenon struck me the more, as I knew 

 how rare sulphur is on the sides of inflamed volcanos: — a co- 

 lumn of 3^ellowish smoke and a frightful noise informed us of 

 the neighbourhood of one of the mouths {bocas) of the volca- 

 no. We had some trouble to approach its edge ; the declivity 

 of the mountain being very steep, and the crevices only covered 

 by a crust of sulphur, of whose thickness we were ignorant. We 

 believed we might rate the extent of this crust, which is often 

 interrupted by rocks, at more than 12,000 square feet. These 

 little ridges of trachy tic rocks act strongly on the magnet. I tried 

 to keep at as much distance from them as possible, to determine 

 the inclination of the needle. It was at the town of Popayan 

 (height 5825 English feet) 23°,05, centesimal division; at the 

 village of Purace (height 8671 feet) 21°,81; near the summit of 

 the volcano of Purace (height 14,542 feet) 20°,85. The in- 

 tensity of the magnetic force varied very little at Popayan and 

 at the village of Purace; and the diminution of the inclination 

 is certainly not tlie effect of the height, as is proved by so 

 many other observations which I have made on the summit of 

 the Andes, but the effect of local attractions depending on 

 certain centres of action in the trachytes. 



The mouth of the volcano of Puiace is a perpendicular 

 cleft, the visible opening of which is only 6 feet long and 3 

 broad It is covered in form of a vault by a layer of very 

 pure sulphur, which is 18 inches thick, andv.hich the forc»of 

 the elastic vapours has split on the north side. At the distance 

 of 12 feet from the mouth we felt an agreeable heat. The cen- 

 tigrade thermometer, which had kept till then at 6°,2 (43° F.) 

 (a cold not at all considerable in a time of hail, and at a height of 

 14,356 feet), rose to 15" (61° F.). Placed in such a manner as 

 not to be incommoded by the vapours, we had the pleasure of 

 drying our clothes. The frightful noise which is heard near 

 this opening has almost always the same intensity : it can 

 only b-; comptired to that which would be caused by several 

 steam-engines together, were the dense steam suffered to 

 escape from all at the same moment. We threw great stones 

 into the crevice, and we discovered on this occasion that the 



Vol. 65. No. 322. Feb. 1825. P opening 



