i9o6] OF DEEP-SEA-CIRCULATION. 39 



antagonize the present abysmal circulation. But theoretical deduc- 

 tions are rarely sure-footed in these complex subjects. The balance 

 of influence,, whatever it may be, is probably so slight as to be 

 negligible. 



2. We cannot here attempt to follow empirically the transfers of 

 water by evaporation and precipitation, but general inspection seems 

 to indicate the nature of the average effect. The saturation point 

 of the atmosphere falls progressively from the equator to the poles, 

 and the actual humidity runs roughly parallel to it on the grand 

 average. Poleward movement of the atmosphere leads therefore to 

 a lowxr content of moisture ; equatorward movement to a higher. 

 As the acquisition of moisture lags behind the capacity to hold it, 

 it is a rather firm inference that precipitation exceeds evaporation in 

 the high latitudes and that evaporation exceeds precipitation in the 

 low latitudes, on the grand average. The bearing of observational 

 data is of the same import. The result of these ratios of precipita- 

 tion and evaporation is a raising of the ocean surface by fresh waters 

 in the polar regions and a lowering of it in the low latitudes accom- 

 panied there by concentration of saline constituents. Considered 

 alone and ideally, this should give a slight equatorward gradient 

 and a flow of fresh surface waters in that direction. These fresh 

 waters, however, mingle with the superficial sea waters and involve 

 a movement of these also toward the equator. So far as these affect 

 abysmal movement, they antagonize the present circulation. 



3. In so far as evaporation exceeds precipitation in the low lati- 

 tudes, it results in an increased salinity of the superficial waters 

 and a tendency of these to sink and flow poleward to replace the salt 

 waters carried equatorward by the fresh waters as just observed. If 

 these were the only factors it seems clear that the deep circulation 

 would be poleward. 



4. On the other hand, the lower temperatures of the high lati- 

 tudes increase the density of the water and tend to cause it to sink 

 and flow equatorward. But the low temperatures affect primarily 

 the superficial stratum which is freshened by the superior precipi- 

 tation of the high latitudes, and both computation and observation 

 show that cold fresher waters may float upon warmer saline waters. 



