ROCA DEL OESTE. 31 



deavoured to get out again by the strait which sep- 

 arates Alegranza from Montana Clara ; but, the wind 

 having fallen, the currents drove them close upon a 

 rock marked in old charts by the name of Infierno, 

 and in modern ones under that of Roca del Oeste, 

 a basaltic mass which has probably been raised by 

 volcanic agency. Tacking during the night between 

 Montana Clara and this islet, they were several 

 times in great danger among shelves towards which 

 they were drawn by the motion of the water ; but 

 the wind freshening in the morning, they succeeded 

 in passing the channel, and sailed along the coasts 

 of Lancerota, Lobos, and Forteventura. 



The haziness of the atmosphere prevented them 

 from seeing the Peak of Teneriffe during the whole 

 of their passage from Lancerota ; but our traveller, 

 in his narrative, states the following interesting cir- 

 cumstances relative to the distance at which moun- 

 tains may be seen. If the height of the Peak, he 

 says, is 12,182 feet, as indicated by the last trigono- 

 metrical measurement of Borda, its summit ought 

 to be visible at the distance of 148 miles, supposing 

 the eye at the level of the ocean, and the refraction 

 equal to 0*079 of the distance. Navigators who fre- 

 quent these latitudes find that the peaks of Teneriffe 

 and the Azores are sometimes observed at very great 

 distances, while at other times, they cannot be seen 

 when the interval is considerably less, although the 

 sky is clear. Such circumstances are of importance 

 to navigators, who, in returning to Europe, impa- 

 tiently wait for a sight of these mountains to rectify 

 their longitude. The constitution of the atmosphere 

 has a great influence on the visibility of distant ob- 

 jects, the transparency of the air being much in- 

 creased when a certain quantity of water is uni- 

 formly diffused through it. 



It is not surprising that the Peak of Teneriffe 

 should be less frequently visible at a great distance 

 than the tops of the Andes, riot being like them in- 



