400 bulletin: museum of comparative zoology. 



higher than on the last visit, the 35 fathom reading being 44°. The 

 general increase in surface saUnity from west to east, of course, was to 

 have been expected from the previous records on the Bank, as well as 

 from its geographic location, and from the fact that the shore water 

 was at this time at its freshest (p. 393). But that the salinity was 

 higher just northwest of the Bank than on the Bank itself is less 

 easily explained. A bottom sample from the former would have 

 shown whether the higher surface salinity was the result of the 

 vertical mixing of a column of water nearly twice as deep, and 

 consequently with a higher mean salinity, than the water over the 

 Bank; and inasmuch as the tidal currents in this region are pro- 

 verbially violent, it is likely that this is the explanation. But it is 

 also possible that there was an indraught of salt water from the 

 south, via the broad channel between Georges Bank and Nan- 

 tucket shoals. Without more extensive data, it is impossible to 

 settle the question. 



General discussion. 



The temperature and salinity records presented in the preceding 

 pages, when added to the 1912 summer records, allow us to recon- 

 struct the seasonal changes which the w^aters of the western side of the 

 Gulf underwent from the summer of 1912 to the early spring of 1913. 

 Summer conditions were marked by high surface temperature, with a 

 rapid decline, and considerable increase in salinity, with depth. But 

 even as early as the end of August the surface had cooled appreciably, 

 while the bottom temperature, in 30-40 fathoms, had risen. And this 

 process of equalization of temperature and salinity, progressed until, 

 by the end of November, both these factors were nearly uniform from 

 surface to bottom. Meanwhile the mean temperature of the whole 

 column of water had fallen about 1°, while its mean salinity had risen 

 appreciably. During the winter and early spring there was an irregu- 

 lar rise in salinity at all depths: the bottom water being usually 

 saltest, in spite of the active vertical circulation. And the fact that 

 the bottom salinity continued to rise after the surface salinity com- 

 menced to diminish, is good evidence that the rise is in general the 

 result of an inward movement of the salt bottom water. It is evident 

 that the thorough mixing of the water which had taken place by 

 November must be caused by vertical circulation, which is no doubt 

 the result of the rather strong tidal currents, growing more and more 

 active as it is less and less strongly opposed by the vertical stability 



