MAY BE SEEX AT SEA. OO 



image is more distinctly detached, tlie more intense ihe chap. ir. 



aerial lieht which comes from the limits of the atmos- , — 



1 1 • • 11 1 111 •, 1 1 , . . Plienomeiis 



pliere lias originally been, or the less it has lost in its of vision 



passage. This in a certain degree accounts for the cir- 

 cumstance, that the Peak is sometimes visible and 

 sometimes invisible to navigators who are equally 

 distant from it, when the state of tlie thermometer and 

 hygrometer is precisely the same in the lower stratum 

 of air. It is even probable, that the chance of perceiving 

 this volcano would not be greater, were the cone equal, 

 as in Vesuvius, to a fourth part of the whole height. 

 The ashes spread upon its surface do not reflect so much Reflected 

 light as the snow with which the summits of the Andes ''gf't- 

 are covered, but, on the contrary, make the mountain, 

 when seen from a great distance, become more obscurely 

 detached, and assume a brown tint. They contribute, 

 as it were, to equalize the portions of aerial light, the 

 variable difference of which renders the object morf 

 or less distinctly visible. Bare calcareous mountains, 

 summits covered with granitic sand, and the elevated 

 savannahs of the Andes, which are of a bright yellow 

 colour, are more clearly seen at small distances than 

 objects that are perceived only in a negative manner ; 

 but theory points out a limit beyond which the latter 

 are more distinctly detached from the azure vault of the 

 sky. 



The aerial light projected on the tops of hills increases projected 

 the visibility of those which are seen positively, but '^^'""^' ''^"'' 

 diminishes that of such as are detached with a brown 

 colour. Bouguer, proceeding on theoretical data, has 

 found that mountains which are seen negatively cannot 

 be perceived at distances exceeding 121 miles ; but ex- 

 perience goes against this conclusion. The Peak of 

 Teneriffe has often been observed at the distance of 

 124, 131, and even 1.38 miles; and the summit of 

 Mowna-Roa in the Sandwich Isles, which is probably 

 10,000 feet high, has been seen, at a period when it was 

 destitute of snow, skirting the horizon from a distance 

 of 183 miles. This is the most striking example yet 



