Mr. J. Croll on the Physical Cause of Ocean- currents. 241 



moisture than they give out ; and the result is that, in intertro- 

 pical regions, evaporation is much in excess of precipitation; and 

 as fresh water only is taken up, the salt being left behind, the 

 process, of course, tends to increase the saltness of the inter- 

 tropical seas. Again, in polar and extratropical regions the re- 

 verse is the case ; precipitation is in excess of evaporation. This 

 tends in turn to diminish the saltness of the waters of those 

 regions. See on these points §§ 31, 33, 34, 37, 179, 517, 526, 

 and 552. 



In the system of circulation produced by difference of tempe- 

 rature, as we have already seen, the surface-currents flow from 

 the equator to the poles, and the under or return currents from 

 the poles to the equator ; but in the system produced by differ- 

 ence of saltness, the surface-currents flow from the poles to the 

 equator, and the return undercurrents from the equator to the 

 poles. That the surface-currents produced by difference of salt- 

 ness flow from the poles to the equator, Maury thinks is evident 

 for the two following reasons : — 



(1) As evaporation is in excess of precipitation in intertropical 

 regions, more water is taken off the surface of the ocean in those 

 regions, than falls upon it in the form of rain. This excess of 

 water falls in the form of rain on temperate and polar regions, 

 where, consequently, precipitation is in excess of evaporation. 

 The lifting of the water off the equatorial regions and its deposit 

 on the polar tend to lower the level of the ocean in equatorial 

 regions and to raise the level in polar ; consequently, in order to 

 restore the level of the ocean, the surface-water at the polar 

 regions flows towards the equatorial regions. 



(2) As the water taken up at the equator is fresh, and the 

 salt is left behind, the ocean, in intertropical regions, is thus made 

 salter and consequently denser. This dense water, therefore, 

 sinks and passes away as an undercurrent. This water, evaporated 

 from intertropical regions, falls as fresh and lighter water in 

 temperate and polar regions ; and therefore not only is the level of 

 the ocean raised, but the waters are made lighter. Hence, in 

 order to restore equilibrium, the waters in temperate and polar 

 regions will flow as a surface-current towards the equator. 

 Undercurrents will flow from the equator to the poles, and sur- 

 face or upper currents from the poles to* the equator. Difference 

 in temperature and difference in saltness, therefore, in every re- 

 spect tend to produce opposite effects. 



That the above is a fair representation of the way in which 

 Maury supposes difference in saltness to act as a cause in the 

 production of ocean-currents will appear from the following 

 quotations : — 



u In those regions, as in the trade-wind region, where evapo- 



