108 



ever it has ^een seen at a greater distance, the 

 observers, uncertain of their longitude, have not 

 been ia a situation to furnish precise data. 



*The aerial light, projected on the mountains, 

 increases the visibility of those which are seen 

 positively ; it's energy diminishes, on the contra- 

 ry, the visibility of the objects, which, like the 

 peak of Teneriffe and that of the Azores, detach 

 themselves in a brown tint. Bouguer, building 

 on theoretical considerations, found that accord- 

 ing to the constitution of our atmosphere, the 

 mountains seen negatively cannot be perceived 

 at distances which exceed 35 leagues*. It is 

 important here to observe, that these calcula- 

 tions are contrary to experience. The peak of 

 Teneriffe has been often seen at 36, 38, and even 

 at 40 leagues. Moreover, in the vicinity of 

 the Sandwich Islands, the summit of Mowna 



Ast. t. ii, p. 298, 357, and 433) the Muerto is a little less, 

 distant than Chimborazo. 



* If, according to the theory of Bouguer, (Trait£ d* Optique 

 p. 360) the intensity of the aerial colour, which is reflected 

 by the whole of the atmosphere towards the horizon in a de- 

 terminate direction, is equal to ^gL q. ; the intensity, after a 

 passage of 30 leagues, would be q. This quantity dif- 

 fers from the other a little more than one sixtieth, whilst after 

 a passage of 45 leagues, the intensity of the aerial colour is 

 already — q., which differs too little from,^„q. for the 



J 10000 * ' 10000 1 



difference to be perceived by our organs. From these data 

 we find by interpolation, that the visibility should have ceas- 

 ed at 35 leagues distance. 



