44 
NATURAL LANDMARKS. 
razo is discerned oil the horizon, may lead us to suppose, that 
it must be seen at a very great distance in the South Sea. 
Pilots highly worthy of credit have assured me, that they 
have seen it from the rock of Muerto, to the south west of 
the isle of Puna, at a distance of 47 leagues. Whenever it 
has been seen at a greater distance, the observers, uncertain 
ot their longitude, have not been in a situation to furnish 
precise data. 
Aerial light, projected on mountains, increases the visibi- 
lity of those which are seen positively ; its power diminishes, 
on the contrary, the visibility of objects which, like the peak 
of Tenerifle and that of the Azores, detach themselves in a 
brown tint. Bouguer, relying on theoretical considerations, 
was of opinion that, according to the constitution of our 
atmosphere, mountains seen negatively cannot be perceived 
at distances exceeding 35 leagues. It is important here to 
observe, that these calculations are contrary to experience. 
The peak of Teneriffe has been often seen at the distance of 
30, 38, and even at 40 leagues. Moreover, in the vicinity of 
the Sandwich Islands, the summit of Mowna-lloa, at a season 
when it was without snows, has been seen on the skirt of the 
horizon, at the distance of 53 leagues. This is the most 
striking example wo have hitherto known of the visibility of 
a mountain ; and it is the more remarkable, that an object 
seen negatively furnishes this example. 
The volcanoes of Teneriffe, and of the Azores, the Sierra 
Nevada of St. Martha, the peak of Orizaba, the Silla of 
Caracas, Mowna-Koa, and Mount St. Elias, insulated in the 
vast extent of the seas, or placed on the coasts of continents, 
serve as sea-marks to direct tlie pilot, when he lias no 
means of determining the position of the vessel by the 
observation of the stars; everything which lias a relation to 
the visibility of these natural seamarks, is interesting to the 
safety of navigation. 
