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times they are not seen when the distance is much 

 less, and the sky appears serene and the horizon 

 free from fogs. These circumstances are so much 

 the more worthy the attention of naturalists, as 

 several vessels returning to Europe wait impa- 

 tiently for a sight of these mountains, to rectify 

 their longitude, and think themselves much far- 

 ther off than they really are, when in fine weather 

 these peaks are not perceptible at distances where 

 the angles subtended ought to be very consider- 

 able. The constitution of the atmosphere has a 

 great influence on the visibility of distant ob- 

 jects. It may be admitted in general, that the 

 Peak of TenerifFe is seldom seen at a great dis- 

 tance, in the warm and dry months of July and 

 August, and that on the contrary it is seen 

 at very extraordinary distances in the months of 

 January and February, when the sky is slightly 

 covered, and immediately after a heavy rain, or 

 a few hours before it falls. It appears, that the 

 transparency of the air is prodigiously increased, 

 as we have already observed, when a certain 

 quantity of water is uniformly diffused through 

 the atmosphere. Independent of these observa- 

 tions, it is not astonishing, that the peak of 

 Teyde should be seldomer visible at a very re- 

 mote distance, than the summits of the Andes, 

 which were so long under my observations. This 

 peak, inferior in height to those parts of the chain 

 of Mount Atlas, at the foot of which is the city of 



