PHYSICAL OCEANOGRAPHY OF THE GULF OF MAINE 595 



of floating debris of one sort or another. In all this, Georges Bank, in the south of 

 the gulf, agrees with the coastal belt generally in the northeast, as it does in being 

 colder at the surface than is the intervening basin where " the water moves to and fro 

 in an unbroken sheet, clear of obstruction," as Dawson (1905, p. 15) expresses it. 



Doctor Kendall's temperatures, added to readings taken by the Grampus in 

 July, 1908 (Bigelow, 1909), and from the Halcyon in the summer of 1923, show 

 that the surface is correspondingly cool (12° to 16°) in August over the shallow 

 broken bottom south of Nantucket, with similar fluctuations within short dis- 

 tances and at different stages of the tide, due to the same disturbing influence of 

 tidal mixings. Thus, the Halcyon had surface readings varying from 11.6° to 15° 

 in August, 1923, as she fished at various locations within a mile or two of Round 

 Shoal bouy; 13.3° to 16.4° over Eose and Crown Shoal; 15.5° over the slightly deeper 

 channel between Round Shoal and Rose and Crown Shoal; and 13.8° to 15.5° on 

 the Great Rip fishing ground 12 miles southeast of the island of Nantucket. Unfor- 

 tunately, it is not yet known whether this cold area is separated from the equally low 

 surface temperatures of Georges Bank by a band of warm surface water along the 

 so-called "south channel," as seems probable, or whether the cool surface forms an 

 unbroken band, west to east, from the one shoal ground to the other. 



In 1913 the surface to the seaward of the 50-meter contour off Nantucket had 

 warmed to upward of 19° by the last week in August (Bigelow, 1915, p. 350, fig. 2, 

 stations 10107 to 10112). This was true also of the whole breadth of the shelf 

 abreast of Marthas Vineyard on the 26th of the month in 1914, except close in to 

 the land (station 10263), where a surface reading of only 17.9° probably reflected 

 some tidal disturbance or other. With this same exception. Doctor Kendall likevidse 

 had 18° to 19° at every station off Marthas Vineyard early in September, 1897, 

 paralleling Libbey's (1891) record of surface warmer than 19° over this part of the 

 continental shelf during August, 1889. 



These data locate the isotherm for 18° as following the southern and western 

 edges of Nantucket Shoals around into the submarine bight west of the latter, but 

 with cool pools next the southern shores of Marthas Vineyard, as just noted. 



It is probable that the surface temperature rises higher than 20° over the outer 

 part of the continental shelf off southern New England every August, and Libbey's 

 (1891) extensive data show that in some years temperatures slightly higher than 20° 

 are to be expected within a few miles of Marthas Viaeyard. But his records also 

 show that a considerable variation in surface temperature is to be expected within 

 short periods of time over the inner half of the shelf, where a sudden cooling of the 

 surface would be the natural accompaniment of any unusual stirring of the water or 

 of the upwelUngs that so often foUow offshore winds. 



There is also considerable variation in the surface temperature off Marthas Vine- 

 yard from year to year. In 1914, for example, the isotherm for 20° included only 

 the outer half of the continental shelf on August 21 at longitude 71° (flg. 46). 



In spite of these fluctuations, it is safe to say that the surface is invariably 

 warmer than 20° along the edge of the continent in the offing of Marthas Vineyard 

 and Nantucket Island by the end of August. To flnd the surface warming to 

 upward of 22° to 23° it is only necessary to sail seaward a few miles farther. 



