105 



the subtended angle be under twenty-five se- 

 conds. 



As the visibility of an object, which detaches 

 itself in a brown color, depends on the quantities 

 of light which the eye meets with on two lines, 

 one of which ends at the mountain, and the other 

 reaches on to the surface of the aerial ocean, it 

 follows that the farther we remove from an object, 

 the smaller the difference becomes between the 

 light of the surrounding atmosphere, and that of 

 the strata of air placed before the mountain. It 

 is on this account, that, when less elevated sum- 

 mits begin to appear above the horizon, they pre- 

 sent themselves at first under a darker tint, than 

 those we discover at very great distances. In 

 the same manner, t he visibility of the mountains, 

 which are seen only in a negative manner, does 

 not depend solely on the state of the lower re- 

 gions of the air, to which our meteorological ob- 

 servations are limited, but also on it's transpa- 

 rency and physical constitution in the most ele- 

 vated parts ; for the image detaches itself better 

 in proportion as the aerial light, which comes 

 from the limits of the atmosphere, has been ori- 

 ginally more intense, or rather has undergone less 

 loss in it's passage. This consideration explains 

 to a certain point, why, under a perfectly serene 

 sky, the state of the thermometer and the hy- 

 grometer, being precisely the same in the air 

 which is nearest the Earth, the peak is sometimes 



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