101 



at nine miles farther. It was in September, to 

 wards the evening, and in very damp weather. 

 Reckoning fifteen feet for the elevation of the 

 eye, I find, that to render an account of this 

 phenomenon, we must suppose a refraction equal 

 to 0*158 of the arch, which is not very extraor- 

 dinary for the temperate zone. According to the 

 observations of General Roy, the refractions vary 

 in England from one twentieth to one third ; and 

 if it be true, that they reach these extreme limits 

 on the coast of Africa, which I much doubt, the 

 peak, in certain circumstances, may be seen on 

 the deck of a vessel as far off as 61 leagues. 



Navigators who have much frequented these 

 latitudes, and who can reflect on the physical 

 causes of the phenomena, are surprised that the 

 peaks of Teyde and of the Azores* are sometimes 

 visible at a very great distance, though at other 



* The height of this peak, according to Fleurieu, is 1100 

 toises ; to Ferrer, 1238 toises j and to Tofino, 1200 toises : 

 but these measures are only approximative estimations. The 

 captain of the Pizarro, Don Manuel Cagigal, proved to me, by 

 his journal, that he observed the peak of the Azores at the 

 distance of 37 leagues, when he was sure of his latitude 

 within two minutes. The volcano was seen at S. 4* E., so that 

 the error in longitude must have an almost imperceptible in- 

 fluence in the estimation of the distance. Nevertheless, the 

 angle which the peak of the Azores subtended was so great, 

 that Mr. Cagigal thinks this volcano must be visible at more 

 than 40 or 42 leagues. The distance of 37 leagues supposes 

 an elevation of 1431 toises. 



