FRESH-WATER OVERLYING SALT. 413 



turned to some advantage by navigators ; for it 

 is found that, in calm weather, the fresh-water 

 overlies the salt, just as oil does in respect of 

 water: by drawing water, therefore, from the 

 surface, fresh-water may be obtained; whereas, 

 if the hose of the pump penetrates some feet 

 down, it may encounter a stratum of salt- 

 water. The saline matter of the lower stratum 

 mixes with the fresh-water by a force analagous 

 to that with which gases mix with each other 

 the force of diffusion. In the narrative of the 

 voyages of the Adventure and Beagle, Captain 

 Fitzroy remarks, in their expedition up the 

 river Santa Cruz, in Patagonia, at . a particular 

 point : " The water was fresh over the surface, 

 and sometimes it is quite fresh even into the 

 estuary ; but in filling casks, or dipping any- 

 thing into the stream for fresh-water, it is 

 advisable not to dip deep, or to let the hose, if 

 one is used, go many inches below the surface, 

 since it often happens that the upper water is 

 quite fresh, while that underneath is salt. This 

 occurs more or less in all rivers which empty 

 themselves into the sea: the fresh-water, spe- 

 cifically lighter, is always uppermost."* 



* " Voyages of the Adventure and Beagle," vol. ii. p. 340. 

 In the Bakerian Lecture delivered by Prof. Graham before 

 the Royal Society, in December, 1849, the diffusion of saline 

 fluids into each other was admirably discussed. The un- 

 equal rate in which different salts diffuse upwards into the 



