*V^ Questions Jbr Solution relating to Meteorology, 



reflected exteriorly from the surface of the sea. If the latter 

 were sixty times more intense than the former, they would to- 

 tally conceal the effect. The shoal would not even be suspect- 

 ed, for it has been proved by the experiments of Bouguer, often 

 repeated since, that the most experienced eye is not sensible of 

 an augmentation of light of -^-q. If there be a small proportion 

 between these two lights, the appearance of the shoal will not be 

 entirely lost ; but it will be very feeble. When it is remember- 

 ed that the atmospheric rays sent to the eye from the sea, have 

 a greater degree of splendour if reflected under an acute angle, 

 every one will perceive that two diff*erent causes concur to ren- 

 der a submarine object less apparent in proportion as the visual 

 line approaches the surface of the sea, namely, on the one hand, 

 the progressive and real weakness of the rays emanating from 

 the object, which have to form its image in the eye ; and, on 

 the other, a rapid augmentation in the intensity of the light re- 

 flected from the exterior surface of the waters, or rather, if I 

 may be allowed the expression, in the luminous curtain to 

 which the rays issuing from the shoal must communicate their 



light. 



On the supposition that the comparative intensities of the two 

 superimposed pencils are, as every thing leads us to believe, the 

 only cause of the phenomenon which we are now analyzing, 

 we have it in our power to point out a better and more easy 

 means of detecting submarine shoals, than has been hitherto fol- 

 lowed. This means is very simple ; it consists of looking at the 

 sea, not with the naked eye, but through a plate of tourmaline 

 cut parallel to the edges of the prism, and placed before the 

 pupil in a certain position. A few words will render the mode 

 in which the crystalline plate acts evident. 



Let us assume that the visual line is inclined to the surface 

 of the sea 37°. The light which is reflected from the outer sur- 

 face of the sea under this angle, is completely polarized. Po- 

 larized light, as every natural philosopher knows, does not tra- 

 verse plates of tourmaline suitably placed. A plate of tourma- 

 line, then, may keep off entirely the rays reflected by the water, 

 which, in the direction of the visual line, were mingled with the 

 light emanating from the shoal, either obstructing it entirely, or 

 at least greatly weakening it. When this effect is produced, the 



