PHYSICAL OCEANOGKAPHY OF THE GULF OF MAINE 



779 



currents. Unfortunately, no stations have been occupied there at the more tide- 

 swept localities, where sahnity, Uke temperature (p. 605), is probably kept nearly 

 homogeneous vertically throughout the summer. A difference of 0.41 per mille of 

 salinity between the surface (31.73 per mille) and the bottom (32.14 per mille, depth 

 30 meters) was recorded on the southwestern edge of the shoals on July 25, 1916 

 (station 10355), with about this same vertical range at a station close to Nantucket 

 Lightship on July 9, 1913 (station 10060; salinity 32.63 per mille at the surface, 

 32.04 per mille at 46 meters) . A vertical distribution of this same sort has prevailed 

 in shallow water off Marthas Vineyard in July and August (stations 10356 and 10357, 

 July 26, 1916; 10258 and 10263, August 25 and 27, 1914), the water as usual saltest 

 on bottom. 



Farther out on this sector of the shelf, where the vertical distribution varies at 

 any given locality and date according to what overflow of oceanic water has recently 

 taken place and at what level, the mid depths may be less sahne than either the 



Fig. 143.— Vertical distribution of salinity on the outer part of the continental shelf o£E Nantucket and 

 Marthas Vineyard. A, August, 1914 (station 10260); B, August 26, 1914 (station 10262); C, July 

 24, 1916 (station 10351); D, July 10, 1913 (station 10061) 



surface or bottom, as was the case at station 10259 on August 25, 1914. However, 

 there is every reason to suppose that such a state is exceptional and probably transi- 

 tory, and that the vertical distribution is usually of the same type there (freshest 

 at the surface, saltest on the bottom; fig. 143) as it is nearer the land and within 

 the Gulf of Maine. 



Our summer stations outside the edge of the continent, whether abreast of the Gulf 

 of Maine or a few miles to either side of the meridians bounding the latter, have aU 

 shown a very rapid increase in salinity with increasing depth in the superficial stra- 

 tum (fig. 144), though with wide differences in the actual values from station to 

 station. In part these differences depend on whether the oceanic water lies far out 

 from or close in to the banks at the time, but also on the precise location of the sta- 

 tions in question, because the transition from banks to ocean is so abrupt along this 

 zone that a difference of half a dozen miles in geographic position may be accompa- 

 nied by a very wide difference in the salinity of the surface water as well as in its 

 temperature (p. 605). 



