PHYSICAL OCEANOGRAPHY OF THE GULF OF MAINE 587 
whence the limiting isotherm (18°) spreads south as well as north, to the confines 
laid down on the chart, as the summer draws to its close. 
We have invariably had surface readings higher than 18° in the outer half of 
Massachusetts Bay after the first week of August, and in Cape Cod Bay; but off 
Fiq. 46. — Normal surface temperature for mid-August, based on a combination of the recent station records with Rathbun’s 
(1887) tabulation at lighthouses, the Canadian records, Dickson’s (1901) data, and the daily surface readings, at 
Gloucester, Boothbay, and Lubec (figs. 29 to 31). (Close to Cape Sable, read < 10° for > 10° .) 
the tip of Cape Cod, where tidal currents run strong, the surface is usually cooler 
locally, as is the general rule in such locations, with readings of 17° to 18° for the 
last half of August. For this same reason the coastal belt around the western and 
