Hydrography^ and the Art of'JSfavtgntioiu 63 



eye placed behind the plate, will receive only one kind of rays, 

 those which emanate from submarine objects ; instead of the two 

 superimposed images, only a single image will be formed on the 

 retina ; and the visibility of the object which this image repre- 

 sents will thus be found greatly facilitated. 



The entire and absolute obstruction of the light reflected from 

 the surface of the water, cannot possibly take place but under 

 an angle of 37°, because it is under this angle alone that it is 

 completely polarised ; but under angles from 10° to 12° greater 

 or less than 37°, the number of polarised rays which the tour- 

 maline can arrest, is still so considerable, that the same means of 

 observation cannot fail to be attended with very advantageous 

 results. 



By engaging in the trials which we now propose to them, the 

 officers of the Bonite will throw light on a curious question of 

 photometry ; they will probably confer on navigation a means of 

 observation which may prevent many shipwrecks; and by in- 

 troducing polarization into the nautical art, they will afford an 

 additional proof of what those individuals expose themselves to, 

 who unceasingly collect experiments and theories without any 

 practical application of them, meeting every remonstrance with 

 a contemptuous cui bono ? 



Water-Spouts. — Has electricity any influence in producing 

 water-spouts ? A distinct and categorical answer to this question 

 would be possessed of great interest. The officers of the Bonite 

 ought therefore to attempt to discover, when this phenomenon 

 presents itself to them, whether it produces thunder and lights 

 ning. 



Depressions of the Horizon, — The rather distinctly defined 

 blue line, forming the apparent separation between the sky and 

 the sea, to which sailors refer the position of the stars, is not m 

 the mathematical horizon ; but the degree at which it appears 

 below it, and which is called the depressioji, may be calculated 

 exactly, since it depends merely on the height of the observer's 

 eye, and the dimensions of the earth. It is unfortunately not 

 so easy to appreciate the effects of atmospheric refractions. It 

 must even be said, that in the calculations of the tables of de- 

 pression usually employed, the meati refraction only is taken 

 into account, relative to a certain state of the thermometer and 



