260 bulletin: museum of comparative zoology. 



over the continental shelf, in summer, because, as the profiles show 

 (p. 233), the tendency must have been just the reverse. And the very- 

 considerable difference in density between surface and deep water 

 off the coast must be a bar to upwelling, even though it may not 

 absolutely prevent it, as it does in stratified waters where the layers 

 of different densities are sharply defined (Sandstrom, 1908; Wedder- 

 burn, 1908). 



It is not to be expected that our work could conclusively settle 

 such a complex problem. But considering that the evidence of 

 temperature, salinity, and density agree, and that it is hardly con- 

 ceivable that one or other of them would not have revealed upwelling, 

 it is safe to say that no widespread vertical movement of this sort 

 was taking place over the continental shelf in July, 1913. And the 

 fact that the cold water over the shelf south of Marthas Vineyard is 

 usually separated from the cold water of the abyss by a zone of warmer 

 bottom water in summer, suggests that this conclusion holds good 

 for that season normally. It is true that during one summer, 1882, 

 the whole shelf is known to have been bathed by cold water; but it 

 is as likely that this resulted from an unusual accession of northern 

 water or from abnormal winter cooling, as from upwelling. 



Upwelling may be more important in winter, for, as Kriimmel 

 (1911) and Schott (1912) point out, the prevailing north and north- 

 west winds, which often rise to storm strength, would have more 

 tendency to produce this type of circulation, than the southwest, 

 long-shore winds of summer. Furthermore, density is not so effec- 

 tive a barrier to upwelling in winter as in summer, because its Vertical 

 range is much smaller then. Nevertheless, it is probable that upwell- 

 ing caused by off shore winds would be from a comparatively shallow 

 depth, say 100 to 200 fathoms, both because the direction of the wind 

 is not constant but often reversed, and because the abyss water must 

 be considerably heavier than coast water even in winter. And 

 gravity would similarly resist any upwelling which the effect of the 

 rotation of the earth might tend to produce along the inner edge of a 

 current moving parallel to the coast. Upwelling of this latter type 

 may play a very important part in the movements of ocean waters, 

 as pointed out by Ekman (1905a) and recently by McEwen (1912); 

 but until the movements of the bottom water of the North Atlantic 

 are better understood, discussion of this theoretic aspect of the case 

 may well be postponed. 



The real explanation of the low temperature of the coast waters 

 is to be found neither in upwelling, nor in a northern current, but 



