342 HALL’S EXCURSIONS, 
the descent less difficult than we had expected for about 500 
feet, but here our progress was cut short by a perpendicular 
precipice. The point we reached is indicated by the 
extremity of the road in the Sketch of the upper part of 
Pichincha, (Tas. CXLIII.), and the section, (Tas. CXLII.) 
will show the nature of the descent, 
We had been less than half an hour on the summit when 
the prospect below became shrowded in mists. We were 
amused by observing the warm air as it ascended to the 
brink of the crater and encountered the cold stream above, 
deposit its moisture in the form of cloudy wreaths, which 
floated round us. It is this opposition of temperatures 
which renders the volcanic glen almost constantly invisible 
from above. Of several excursions made for the purpose, 
this was the first in which I had obtained a view of it. The 
sulphureous vapour, scarcely perceptible while the crater is 
clear, is strongly pungent when condensed by the damp misty 
air, and contributes to its gloomy aspect—most imposing 
when its interior is most obscure. By M. Boussingault’s 
barometrical measurement, the height of the volcano is 15,676 
feet. According to Humboldt, it is 15,976; and according 
to the Academicians, who measured it in 1739, 15,606. The 
last observation is, in these cases, generally the most correct; 
and in all the comparative measurements of Humboldt and 
Boussingault, the superiority of the barometer of the latter 
must be taken into consideration. Calculating by boiling 
water, I had, on a former occasion, estimated the height at 
15,704 feet. Water boilsat 186°. Though snow frequently 
falls, it never remains long on Pichincha: what is brought 
to Quito is not snow, but masses of hailstones frozen together 
in the clefts of the rocks. The limit of perpetual snow under 
the equator is fixed by Humboldt at 15,736 feet, by Bouguer 
at 15,608, and by Leslie, from a calculation of the increased 
capacity of rarified air for caloric, at 15,207. Pichincha is, 
therefore, barely within this limit, whichever be the measure- 
ment we assume ; but the line of perpetual congelation must 
also be presumed to admit of some local variations. The 
