34 DISTANCE AT WHICH MOUNTAINS 



ciiAi'. u ill what Bouguer calls a negative manner, or because 

 Cia<sificrttioii ^^^*-'y intercept the light transmitted from the extreme 

 oiiiioiuiuiiis. limits of tlie atmosphere ; and we perceive their ex- 

 istence only by means of the difference of intensity that 

 subsists between the liglit which surrounds them, and 

 that reflected by the particles of air placed between the 

 object of vision and the, observer. In receding from 

 Teneriffe, the Sugar-loaf is long seen in a positive 

 Tlic sugai- manner, as it reflects a whitish light, and detaches 

 luaf. itself clearly from the sky ; but as this terminal cone is 



only 512 feet high, by 250 in breadth at its summit, it 

 hiis been questioned whether it can be visible beyond 

 the distance of 139 miles. If it be admitted that the 

 mean breadth of tiie Sugar-loaf is G395 feet, it will still 

 subtend, at the distance now named, an angle of more 

 than three minutes, which is enough to render it visible ; 

 and were the height of the cone greatly to exceed its 

 basis, the angle might be still less, and the mass yet 

 make an impression on our organs ; for it has been 

 jiruved by micrometrical observations, that the limit of 

 vi.>ion is one minute only when the dimensions of objects 

 are the same in all directions. 

 EffcTtof As the visibility of an object, which detaches itself 



ros'iilm" from the sky of a brown colour, depends on the quan- 

 tities of light the eye meets in two lines, of which one 

 ends at the mountain and the other is prolonged to the 

 surface of tlie aerial ocean, it follows that the farther 

 we remove from the o])ject, the less also becomes the 

 ditt"erence between tlie light of the surrounding atmos- 

 phere and that of the strata of air placed before the 

 niountain. For this reason, when summits of love 

 elevation begin to appear above the horizon, they are of 

 a darker tint than those more elevated ones which we 

 discover at very great distances. In like manner, the 

 vi-<ibility of mountains which are only negatively per- 

 ceived, does not depend solely upon the state of the low 

 regions of the air, to which our meteorological observa- 

 tions are confined, l)ut also upon its transparency and 

 phvbicul coubtitutiou in the most elevated parts ; for the 



