FRESH-WATER SPRINGS. 17 



from the mouth the water is fresh in the whole depth, even 

 ■during the flow of the tide, but that a little nearer the sea fresh 

 water is found on the surface, while the lower strata consist of 

 sea water. According to his observations it is between London 

 and Woolwich that the saltness of the bottom begins to be per- 

 ceptible. Thus, below Woolwich the Thames, instead of flowing 

 ■over a solid bed, in reality flows upon a liquid bottom formed 

 by the water of the sea, with which no doubt it is more or less 

 ■mixed. 



Mr. Stephenson is of opinion that, at the flow of the tide, the 

 fresh water is raised as it were in a single mass by the salt water 

 which flows in, and which ascends the bed of the river, while 

 the fresh water continues to flow towards the sea. 



Where the Amazon, the La Plata, the Orinoco, and other 

 giant streams pour out their vast volumes of water into 

 the ocean, the surface of the sea is fresh for many miles from 

 the shore ; but this is only superficial, for below, even in the bed 

 -of the rivers, the bitterness of salt water is found. 



It is a curious fact, that in many parts of the ocean, fresh- 

 water springs burst from the bottom of the sea. Thus, in the 

 G-ulf of Spezzia, and in the port of Syracuse, large jets of fresh 

 water mingle with the brine; and Humboldt mentions a still 

 more remarkable submarine fountain on the southern coast of 

 Cuba, in the Gulf of Xagua, a couple of sea miles from the shore, 

 which gushes through the salt water with such vehemence, that 

 boats approaching the spot are obliged to use great caution. 

 Trading vessels are said sometimes to visit this spring, in order 

 to provide themselves in the midst of the ocean with a supply 

 of fresh water. 



The sea is not colourless ; its crystal mirror not only reflects 

 the bright sky or the passing cloud, but naturally possesses a pure 

 bluish tint, which is only rendered visible to the eye when the 

 ■lioht penetrates through a stratum of water of considerable 

 depth. This may be easily ascertained by experiment. Take a 

 glass tube, two inches wide and two yards long, blacken it inter- 

 nally with lamp-black and wax to within half an inch of the end, 

 the latter being closed by a cork. Throw a few pieces of white 

 porcelain into this tube, which, after being filled with pure 



c 



