FRESH-WATER SPRINGS. ae 
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 
Gulf 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 
light penetrates through a stratum of water of considerable 
depth. This may be easily ascertained by experiment. Takea 
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 
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