14 THE STORY OF LIFE IN THE SEAS. 



lakes, and this causes it to be very much heavier. 

 If a tumbler be half filled with sea-water, upon 

 which some fresh water is slowly and care- 

 fully poured, there will be for some time very 

 little mixture of the two fluids, the heavier 

 sea- water remaining at the bottom, and the 

 lighter fresh water at the surface. Now the 

 density of the sea-water, or in other words the 

 amount of salts in solution, is not the same over 

 the whole world, and the differences that may 

 be observed in this respect are due, in most 

 cases, to the simple physical principle just 

 enunciated. If we could imagine a river pouring 

 its waters into a perfectly calm, tideless sea, we 

 should be able to trace the fresh river water far 

 away from the coast, for it would simply float on 

 the heavier sea-water without mixing with it to 

 any appreciable extent. In most cases, however, 

 the tidal- waves, rushing up and down the river 

 estuaries, stir up the fresh and salt water to- 

 gether, and cause a very considerable mixture, 

 so that the water becomes either distinctly salt 

 or brackish. Where very large quantities of 

 fresh water are poured into the ocean, as, for 

 example, at the mouth of the Amazon or the 

 Mississippi, the surface water remains so fresh 

 that the salt taste can hardly be appreciated at 

 a distance of some miles from the coast. This 

 fact sufficiently indicates the influence of great 

 rivers upon the density or saltness of the sea- 

 water in their neighbourhood, and the reader 

 will be prepared for the statement that many 

 inland seas, such as the Black Sea, are appreciably 

 less salt than the great oceans. 



