388 bulletin: museum of comparative zoology. 



advance of winter. And the data show that such was the case, for 

 on the next visit, December 4, we found that the water was not only 

 appreciably colder at all depths, but very nearly uniform from sur- 

 face to bottom, the surface temperature having fallen to 6.6°, with 

 46.1° at 20 fathoms and at the bottom (30 fathoms). Probably the 

 slight excess of heat at the surface over the deeper layers was the result 

 of diurnal warming, the day being sunny and calm, with an air temper- 

 ature at noon of 46°. The salinity was 32. 56%^ on the surface and at 

 25 fathoms, 32.61%o on the botton, i. e., practically the same as at the 

 preceding station (November 20). The density at the surface (at 

 the temperature in situ) was 25.38, at 25 fathoms 25.39, at 38 fathoms 

 (bottom) 25.42. The fact that the surface water was slightly less saline 

 than the subsurface layers is no doubt to be explained as the result 

 of recent rains. 



The next station was made on December 23, on a bright sunny day, 

 with a brisk northwest wind, and air temperature, in the shade, at 

 noon, of 36°. Considerable cooling of the water proved to have taken 

 place during the three weeks since our last visit, and the fact that this 

 was the first station at which there was no change of temperature at 

 all with depth, the reading being 4.5° from surface to bottom, shows 

 that convectional overturning, together with tidal currents, now kept 

 the water thoroughly mixed. The water samples proved especially 

 interesting, for while the salinity, like the temperature, was uniform 

 from surface to bottom, it was considerably higher than any previous 

 reading in Massachusetts Bay, i. e., 32.74%o, good evidence that there 

 must have been an accession of salt offshore water, the origin of which 

 is discussed (p. 400). 



On January 16, 1913, at the same locality, the water had cooled to 

 41.7° at the surface; 41.5° at 25 fathoms; 42.1° at 38 fathoms, an in- 

 structive series for the fact that the lowest temperature was at the 

 mid-level shows that the convectional overturning, now a constant 

 phenomenon only interrupted by diurnal warming of the surface, was 

 most active in the upper 25 fathoms, foreshadowing the time when cool- 

 ing would be so rapid at the surface that the latter would be constantly 

 cooler than the bottom. The slight excess of warmth (.2°) of the 

 surface over the 25 fathom reading, was no doubt the result of diurnal 

 warming during the preceding two or three days, which were unseason- 

 ably warm. The salinity was 32.81%o at the surface; 32.86%o at 25 

 fathoms; 32. 94%^ at 38 fathoms (bottom); a considerable rise since 

 the previous stations. The difference in salinity between surface and 

 bottom was probably in part evidence of an inshore flow of salt bottom 



