128 PHYSIOGRAPHY. [CHAP. 



be quoted. The density of this water was found to be 

 1027 : — 



COMPOSITION OF WATER OF THE BRITISH CHANNEL IN GRAINS 

 PER GALLON OF 70,000 GRAINS. 



Every tide brings this sea water into contact with the 

 fresh water of a tidal river, like the Thames, and the two 

 kinds thus become mixed. On going down the Thames 

 from London Bridge, it is found that the water gradually 

 loses its freshness. A little below Gravesend, it begins to 

 acquire a saltish flavour, and this saltness increases until the 

 water becomes decidedly brackish and undrinkable. Going 

 still farther out into the estuary, the saltness becomes more 

 pronounced ; and, by the time Whitstable is reached, the 

 water is hardly to be distinguished from that of the sea 

 itself 



The fresh water brought down by a river does not how- 

 ever immediately mix with the salt water, but rather tends 

 to float upon its surface. For, since the sea water is rich in 

 solid matter, its density is proportionally high ; that is to say, 

 sea-water must weigh considerably more than fresh water 

 when equal bulks are compared. A gallon of water from 

 the Nore weighs rather more than a gallon of water from 

 the Thames at Teddington. If a given measure of pure 



