864 



BULLETIN OP THE BUREAU OF FISHERIES 



The directions and velocities given on the chart (fig. 173) for the stations off 

 Cape Cod and in the region of Nantucket Shoals are copied direct from the Coast 

 Pilot (1912, chart to face p. 9; based on observations taken by the U. S. Coast 

 and Geodetic Survey). A south-southeasterly drift of about 12 miles a day at a 

 station 7 miles off Nauset Light illustrates the general tendency toward a south- 

 erly movement of the water along Cape Cod, mentioned in the Coast Pilot. Obser- 

 vations taken at the Pollock Rip lightship and at Round Shoal lightship, at the 

 entrance to Nantucket Sound, from June 20 to September 14, 1911, have also brought 

 out dominant drifts toward the southeast at rates, respectively, of 9 to 10 and 2 to 3 

 miles per 24 hours. By this evidence, corroborated by bottle drifts (p. 886), the sur- 

 face water sets southerly across and out of the eastern end of Nantucket Sound, not 

 into the latter. This is corroborated by an east-southeasterly set of about 7 miles per 

 24 hours, recorded at a station 4 miles within the sound (2 miles south of Handker- 

 chief Shoal lightship). 



Sets of varying duration, taken by the Coast and Geodetic Survey at 11 stations 

 in the general region of Nantucket Shoals, show a general dominant set between 

 south and east, roughly paralleling the chief axis of the shoal ground, at rates 

 varying from about 2 miles per day to about 14 miles (average about 3 mUes). 

 However, this is complicated by evidence of subsidiary eddying movements, such 

 as might be expected over this uneven bottom, where strong tidal currents are 

 compMcated by rips and deeper channels. 



Earlier studies pointed to the conclusion that the tidal currents at a point about 

 16 mileis to the eastward of Nantucket light vessel are not only rotary but run at 

 an equal velocity at all hours (Coast Pilot, 1912, p. 10); and it seems to have been 

 taken generally for granted that there is no dominant set at the lightship, which is 

 situated about 10 miles to the southward of the 40-meter contour of the shoals and 

 42 miles SSE. from Nantucket Island (lat. 40° 37', long. 69° 37'), but that the 

 currents there are purely tidal. This, however, is contradicted by 19 sets of current 

 measurements, each of 29 days' duration, taken at this lightship by the United States 

 Coast and Geodetic Survey in the months of June, July, August, September, October, 

 November, December, February, March, AprU, and May of the years 1911, 1912, and 

 and 1914, tabulated below.^' In 13 cases a dominant set results toward the north 

 and west; a set toward the south and east in four; and one series showed no appreciable 

 set in either direction, as tabulated. 



Dominant set at Nantucket lightship for various months 



Month and year 



June, 1914 



June-July, 1914 



June-July, 1911 



July, 1914 



July, 1911 



August, 1914 - 



August, 1911-- 



August-September, 1911- 

 September, 1U14 , 



Direction of domi- 

 nant set 



N. 46° W. 

 N. 55° W. 

 N. 6°E.. 

 N. 63° W. 

 N. 26° W. 

 N. 45° W. 

 N. 53° W. 

 N. 45° W. 

 N. 74° W. 



Drift per 



24 hours 



2.2 



2.2 

 1. 1 

 2.7 

 1.9 

 4.8 

 3.8 

 2.4 

 7.4 



Month and year 



September-October, 1913 



Do. 



October, 1913 



November, 1913 



December, 1913,- 



February, 1914 



March, 1914.- 



April, 1914 



May,- 1914 



Direction of domi- 

 nant set 



N. 89° W- 

 N. 80° W- 

 N. 86° W- 

 S. 68° E.-. 

 S. 44°E... 

 S.51° E... 

 S. 40°E... 

 N. 76° W. 

 N. 62° W. 



Drift per 

 24 hours 



5.3 

 8.2 

 5.3 

 2.4 

 4.0 

 2.9 

 1.0 

 1.4 

 4.3 



" Data supplied by the U. S. Coast and Geodetic Survey. 



