Hyarography, and the Art of Navigation, 61 



observer is elevated above the surface of the water : thus, when 

 an experienced captain navigates an unknown sea filled with 

 shoals, he sometimes places himself on the summit of the mast, 

 in order that he may direct his vessel with greater security. 



The fact appears to us so well established, that we have no- 

 thing to require of our young navigators regarding it in a prac- 

 tical point of view ; but, by following the indications which we 

 shall here point out, they may perhaps ascertain the cause of a 

 phenomenon which affects them so nearly, and thence deduce more 

 satisfactory means than casual observation has hitherto taught 

 them to en) ploy, for the purpose of detecting the situation of 

 shoals. 



When a pencil of light falls on a diaphanous surface, what- 

 ever may be its nature, a portion passes through it, and another 

 portion is reflected. What is reflected is more intense in pro- 

 portion to the smallness of the angle formed by the incident ray 

 with the surface. This photometrical law is not less applicable 

 to the rays which emanate from a rare medium, and meet the 

 surface of a dense body, than to those which, moving in a 

 dense body, fall on the surface of separation of that body and 

 of the rare contiguous medium. 



This being the case, let us suppose that an observer on ship- 

 board wishes to perceive a shoal at a little distance — a subma- 

 rine shoal, situate at thirty metres of horizontal distance, for ex- 

 ample. If his eye is about the height of a metre above the sea, 

 the visual line by which the light emanating from the shoal can 

 reach him after issuing from the water, will form a very small 

 angle with the surface of the fluid ; if the eye, on the contrary, 

 is very much elevated, suppose thirty metres, he will see the 

 slioal under an angle of 45°. But the interior angle of inci- 

 dence, corresponding to the small angle of emergence, is evi- 

 dently less open than that which corresponds to the emergence 

 of 45°. Under small angles, as has^ been seen, the strongest 

 reflections take place; the observer, therefore, will receive k 

 more considerable portion of the light which emanates from the 

 shoal the higher he happens to be placed. 



The rays emanating from the submarine shoal are not the 

 only ones that reach the eye of the observer. In the same di- 

 rection, and confounded with them, are rays of atmospheric light 



