WHY THE SEA IS SALT. 277* 



diately occur. The lighter water will be lifted by the surround- 

 ing heavier waters till there is no difference in pressure between 

 its lower boundary and the surrounding waters at the same 

 depth ; but, as its pressure at all levels above this lower boundary 

 will now have become greater than that of the surrounding heav- 

 ier waters, it will instantly begin to displace and overflow them. 

 This movement of the lighter water will require considerably 

 more time than the movement of the heavier water by which itr 

 was lifted and continues to be lifted as its level sinks by lateral 

 diffusion, because the sum of the differences of pressure which 

 caused the lifting of the lighter water was, in the first place, 

 greater than the sum of the differences that caused its lateral 

 diffusion. Secondly, the differences of pressure that caused the 

 first movement must extend all the way to the bottom, whereas 

 those which cause the latter extend no deeper than the lighter 

 stratum itself, and, even within the extent of that, have their chief 

 effect confined to the superficial strata. 



On the other hand, when the equilibrium of a mass of water is 

 disturbed by causes that do not diminish the specific gravity, the 

 disturbance must extend down to the bottom, and the differences 

 of pressure at all levels beneath the surface must be equal. The 

 equilibrium is then restored by a general movement of the whole 

 mass, which movement is sensible in inverse proportion to the 

 mass that is set in motion. This is the essential cause for the dif- 

 ference in strength between the currents observed in salt and fresh 

 waters, for, of all the current-producing causes which act in fresh 

 waters, only the one resulting from variations of temperature can 

 sensibly affect the specific gravity, while the specific gravity of 

 sea water, besides being much more affected by variation of tem- 

 perature, is still further influenced by the fresh water which rains 

 upon the surface of the ocean. If the whole basin of the ocean 

 were filled with fresh water and exposed to the most extreme 

 meteorological influences, the currents produced would not be near- 

 ly equal either in size or strength to those now observed in the 

 waters of the ocean. 



So the saltness of the sea is involved in all the great subjects 

 into which the ocean currents enter. Having contributed to the 

 growth of the continents, it has in a like degree peopled them by 

 influencing human migrations through the streams of the ocean 

 upon which the race of man was spread to the distant archipelagoes 

 at a time when there were only rudimentary means for struggling 

 against the forces of Nature. Besides its influences in geology and 

 anthropology, it is concerned to a marked extent in the climate of 

 the earth and of the sea, and in their botany and zoology. 



