6 F. L. Ekman, 



The most immediate and essential consequence of such differences 

 of specific gravity between different parts of the ocean is then differen- 

 ces of level, which differences however subsequently give rise to super- 

 ficial streams, whereby the lighter neater displaces the heavier. By these 

 streams, in consequence of the smallness of their section, the disturbed 

 equilibrium can only be restored slowly, and their continuance may the- 

 refore be considerable both as regards time and extent. If the causes, 

 which produce the change of specific gravity operate in a constant man- 

 ner, both the differences of level and the consequent streams will be 

 constant. 



When the equilibrium of a mass of water is disturbed by causes, 

 which do not alter, or at least do not diminish the water's specific gra- 

 vity, the disturbance must extend all the way to the bottom, and the 

 differences of pressure at all depths beneath the lowest level of the sur- 

 face will be equal. The equilibrium is then restored by a general mo- 

 vement of the whole mass, which will however be the less visible in 

 proportion as the mass of water set in motion is greater. We have 

 here an essential cause of the difference in the strength of the streams 

 observed in salt and fresh water. For of all the causes which can pro- 

 duce streams in the latter, there is but one, which can sensibly affect 

 the water's specific weight, viz: variations of temperature. The sp. gra- 

 vity of sea-water on the contrary, besides being much more effected by 

 variations of temperature, is still further affected by the descent of fresh 

 water upon its surface. Even if the whole basin of the ocean were 

 filled with fresh water and that water exposed to the most extreme me- 

 teorological influences, which the different tracts of the earth present, thence 

 the superficial streams produced would not be nearly equal either in 

 power or extent to those we now observe in the water of the ocean ^). 



We proceed now to examine separately each of the causes, which 

 tend to produce change in the specific gravity of sea-water, with a view 

 to ascertaining their influence on the water's motion, and for simplicity's 

 sake we suppose only one of these causes to opierate at a time. AVe have first: 



') Although the importance of specific gravity, as affecting the streams in fresh 

 and salt water, has already been carefully pointed out by Mauet, I have nevertheless 

 thought it best here to touch upon that subject, because it does not appear to me 

 that he has adduced the cause of the phœnoniena in question, which I look upon as 

 the chief. 



