200 ANNUAL OP SCIENTIFIC DISCOVERY. 



tity of coloring matter to tinge so immense a body of water : wherefore 

 Sir Humphrey Davy's supposition of an admixture of iodine cannot he 

 admitted, for its presence is barely traceable under the most careful analy- 

 sis. Those who contend for there being no color at all, may remind us 

 that the blue rays are the most refrangible, ?<nd that, being reflected in 

 greatest quantity by the fluid, (which, because of its density and depth, 

 causes them to undergo a strong refraction,) they cause a tint which is only 

 apparent. Be that as it may, seamen admit of one conclusion namely, 

 that a green hue is a general indication of soundings, and indigo-blue of 

 profound depth." Admiral Smyth on the Mediterranean. 



ON TRANSPARENCY OF THE OCEAN. 



A communication on the above subject was made to the American 

 Scientific Convention, at the Washington meeting, by Captain Glynn, 

 U. S. N. : 



Philosophers ashore and philosophers of the forecastle, said Capt. Glynn, 

 have wondered in all times as to the causes and extent of the color of the 

 sea, and queried how far into it our vision could penetrate. Capt. 

 Wilkes advanced the opinion that the transparency of the sea varied quite 

 directly with its temperature. To this his observations did not allow him 

 to assent. In order to obtain correct observations, the surface of the sea 

 must be perfectly tranquil and smooth. There must not be a ripple on it. 

 So essential is this point, that, during a cruise of four years, he only suc- 

 ceeded in making sixteen observations that proved worth saving. Of these 

 sixteen, in only one the water was ruffled by a slight breeze. The next 

 trouble was to discover what object would be most likely to reflect all the 

 rays of light what would be longest visible. First we tried an iron pot, 

 painted white. When we looked for it for the second experiment, the 

 white pot was a black one again. Next we tried a sphere of hoops, cov- 

 ered with white cotton cloth. Before it was called for the second time, it 

 was smashed into a cocked hat. Next we tried a mere hoop, covered with 

 a canvas. It was laid away on some old spikes ; and when needed next, it 

 came up sound enough, but of a bright yellow. At last we took a com- 

 mon white dinner-plate. It was good enough. It was the brightest 

 object we could find, was always handy, and was always clean. It was 

 slung so as to lie in the water horizontally, and sunk by an iron pot, with 

 a line. The observations were taken wherever we could get them ranging 

 over 200 of latitude, in different oceans, in very high latitudes, and near 

 the Equator. I have to assume, what doubtless I may, that they do not 

 differ from what they would if taken all in the same place. At every 

 station, we noted in the connection, the sun's latitude, the velocity of the 

 current, the temperature of both the air and the water, and the number of 

 fathoms at which the plate was visible below the siu-face. We took these 

 observations from a boat, bringing the line on to its shady side then lean- 

 ing over, with faces almost touching the water, and eyes shaded from the 



