109 



Roa*, at a season when it was without snows, 

 was seen on the skirt of the horizon, at the dis- 

 tance of 53 leagues. This is the most striking 

 example we have hitherto known of the visibility 

 of a mountain ; and what is the more remark- 

 able, it is an object seen negatively which fur- 

 nishes the example. 



I thought it proper to bring together these 

 considerations at the end of this chapter, because 

 in treating so closely on one of the most import- 

 ant problems of optics, that of the diminution 



* The height of Movvna Roa, according to Marchand, is 

 more than 2598 toises j according to King, it is 2577 toises ; 

 but these measures, notwithstanding their accidental concord- 

 ance, are not founded on very exact measurements. It is a 

 very extraordinary phenomenon, to see a summit placed in 

 the 19th degree of latitude, and which is probably 2500 toises 

 high, entirely stripped of it's snows. The very flattened form 

 of Mowna Roa, the Mesa of the old Spanish charts, it's insula- 

 ted situation in the midst of the ocean, and the frequency of 

 certain winds, which, modified by the ascending current, 

 blow obliquely, may be the principal causes. It is difficult 

 to believe, that captain Marchand was much deceived in the 

 estimation of the distance at which he saw, on the 10th of Oc- 

 tober 1791, the summit of Mowna Roa. He had left the 

 island of Owhyhee only the 7th in the evening $ and from the 

 movement of the waters, and the lunar observations of the 

 19th, it is probable that the distance was even greater than 

 53 leagues. Besides, an experienced navigator, Mr. Fleu- 

 rieu, relates, that at a distance of 35 or 36 leagues the peak 

 of Teneriffe is visible, even in weather that is not perfectly 

 clear. Foy. de Marchand, vol. i, p. 408 and 427 ; vol. ii> p. 10 

 and 78. 



