REFRACTION AND REFLECTION. 
43 
originally more intense, or lias undergone less loss in its 
passage. This consideration explains to a certain point, why, 
under a perfectly serene sky, the state ol the thermometer 
and the hygrometer being precisely the same in the air 
nearest the earth, the peak is sometimes visible, and at other 
times invisible, to navigators at equal distances. It is even 
probable, that the chance of perceiving this volcano would 
not he greater, if the ashy cone, at the summit of which is 
the mouth of the crater, were equal, as in Vesuvius, to a 
quarter of the total height. These ashes, being pumice-stone 
crumbled into dust, do not reflect as much light as the snow 
of the Andes ; and they cause the mountain, seen from afar, 
to detach itself not in a bright, but in a dark hue. The ashes 
also contribute, if wc may use the expression, to equalize the 
portions of aerial light, the variable difference of which 
renders the object more or less distinctly visible. Calcareous 
mountains, devoid of vegetable earth, summits covered with 
granitic sand, the high savannahs of the Cordilleras,* which 
arc of a golden yellow, are undoubtedly distinguished at small 
distances better than objects which arc seen in a negative 
manner; but the theory indicates a certain limit, beyond 
which those last detach themselves more distinctly from the 
azure vault of the sky. 
The colossal summits of Quito and Peru, towering above 
the limit of the perpetual snows, concentre all the peculiari- 
ties which must render them visible at very small angles. 
The circular summit of the peak of Taneriffe is only a hun- 
dred toises in diameter. According to the measures I made 
at Eiobamba, in 1803, the dome of the Chimborazo, 153 
toises below its summit, consequently in a point which is 
1300 toises higher than the peak, is still 673 toises (1312 
metres) in breadth. The zone of perpetual snows also forms 
a fourth of the height of the mountain ; and the base of this 
zone, seen on the coast of the Pacific, fills an extent of 3137 
toises (6700 metres). But though Chimborazo is two-thirds 
hi oh or than the peak, we do not sec it, on account of the 
curve of the globe, at more than 38 miles and a third farther 
distant. The radiant brilliancy of its snows, when, at the 
port of Guayaquil, at the close of the rainy season, Chinnbo- 
* Los Pajonules, from }nija, straw. This is the name given to tin 
region of the gramina, winch encircles the zone of the perpetual snows. 
