PHYSICAL OCEANOGRAPHY OP THE GULF OP MAINE 807 



Island (station 10495), where the salinity increased only from 32.6 per mille at the 

 surface to 32.77 per mille at 75 meters. 



Local freshening of the surface, just described (p. 806), was then responsible for 

 the very considerable vertical range of 2.6 per niille in water only 30 meters deep 

 between Cape Ann and the Merrimac River, with differences of 0.8 to 1.4 per mille 

 between the surface and the 75 to 100 meter level off Cape Elizabeth and off Cape 

 Ann (stations 10488, 10489, 10492, and 10494). 



It is certain, however, that as the surface continued to cool during that winter 

 the decrease in vertical stability was accompanied by a progressive equalization of 

 salinity in the upper 100 meters; for the surface and the 100-meter level differed by 

 less than 0.2 per mille in salinity at five out of seven of the stations for the follow- 

 ing March (stations 10505 to 10511). Thus, the seasonal cycle was fundamentally 

 the same in this respect in 1920-21 as in 1912-13, except that it was more tardy in 

 its early progression. 



No general survey of the salinity of the gulf has yet been attempted during 

 the last half of January or the first half of February — on the whole the coldest 

 season (p. 655) . However, periodic observations taken in Massachusetts Bay during 

 this period of 1913, hydrometer readings taken at 15 stations by the Fish Hawk in 

 its southern side on February 6 and 7, 1925, and Mavor's (1923) winter records for 

 the Bay of Fundy in 1916 and 1917 show that no very wide change is to be expected 

 in the salinity of the gulf during the last half of the winter. 



These Fish Hawk determinations ranged from about 32.3 per mille to about 33.3 

 per mille, according to the precise locality, averaging lowest in the hook of Cape Cod, 

 where the surface was about 32.3 to 32.4 per mille, and highest in the center of the 

 bay (whole column close to 33 per mille, surface to bottom) . The maximum differ- 

 ence in salinity between surface and bottom was then only 0.4 per mille (average 

 difference about 0.2 per mille) , with the water virtually homogeneous, surface to 

 bottom, at the two deepest stations (about 70 meters deep). 



It is interesting to find the salinity of the deeper part of the bay for February 

 7, 1925, almost exactly reproducing the values recorded off Gloucester on the 13th 

 of the month in 1913 (station 10053, surface 32.83 per mille, bottom 32.84 per mille) ; 

 evidently neither of these winters, as contrasted with the other, can be described as 

 "fresh" or "salt" in the bay. In both 1913 and 1925 the water away from the 

 immediate influence of the shore line was equally homogeneous in salinity from 

 top to bottom by these dates; but the data for the two years combined bring 

 out a decided regional difference in this respect, with the surface continuing 0.3 to 

 0.4 per mille less saline than the deeper strata along the northern and southern mar- 

 gins of the bay, no doubt because of land drainage. 



Although we have made no offshore stations in the gulf between the middle of 

 January and the last week of February, some knowledge of the ebb and flow of the 

 slope water over that period is obtainable from the seasonal progression from Feb- 

 ruary to March in the deeper parts of Massachusetts Bay, and from the salinity of 

 the basin off Cape Ann for March 5, 1921 (station 10510), compared with the pre- 

 ceding December and January (stations 10490 and 10503). 



In 1913 the salinity rose to about 32.8 per mille at the surface, to 32.9 per mille 

 on bottom in 70 meters, at the mouth of the bay by January 16 — a mean increase of 



