MAY BE SEEN AT SEA. 33 



prolong-ed to the surface of the aerial ocean, it fol- 

 lows that the farther we remove from the object 

 the less also becomes the difference between the 

 light of the surromiding atmosphere and that of the 

 strata of air placed before the momitain. For this 

 reason, when summits of low elevation begin to ap- 

 pear above the horizon, they are of a darker tint 

 than tliose more elevated ones which we discover at 

 very great distances. In like manner, the visibility 

 of mountains which are only negatively perceived 

 does not depend solely upon the state of the low 

 regions of the air, to which our meteorological ob- 

 servations are confined, but also upon its transpa- 

 rency and physical constitution in the most elevated 

 parts; for the image is more distinctly detached, 

 the more intense the aerial light which conies from 

 the limits of the atmosphere has originally been, or 

 the less it has lost in its passage. This in a certain 

 degree accounts for the circumstance that the Peak 

 is sometimes visible and sometimes invisible to 

 navigators who are equally distant from it, when 

 the state of the thermometer and hygrometer is pre- 

 cisely 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 light as the snow with which the summits of 

 the Andes are covered ; but, on the contrary, make 

 the mountain, when seen from a great distance, be- 

 come more obscurely detached, and assume a brown 

 tint. They contribute, as it were, to equalize the 

 portions of aerial hght, the variable difference of 

 which renders the object more or less distinctly vis- 

 ible. Bare calcareous mountains, summits covered 

 with granitic sand, and the elevated savannas 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 



