BIGELOW: EXPLORATIONS IN THE GULF OF MAINE. 



45 



sion to return in our discussion of vertical circulation; and off Cape 

 Ann the temperature during this same period ranged from 60°-66° 

 (day and night) ; usually r)3°-65° in the day time. On our return we 

 crossed Massachusetts Bay twice (August 28th-31st). On the first 

 passage the surface temperature ranged from 60°-62°, the mean being 

 about 61°; on the second, two days later, from 59°-61°, the mean 

 being nearly 61°; and on August 29th, off Cape Cod, the temperature 

 range was from 60° to 62°, with a mean of 61°. These observations 

 show that by the end of August an appreciable cooling of the surface 

 water had taken place in and near Massachusetts Bay, from the annual 

 maximum, which must be reached about the first of August. 



Satisfactory data as to diurnal warming can be obtained only when 

 the vessel lies at one spot for considerable periods, so our information 

 on this point is not very extensive. But we made some observations 

 which suggest an unusually great diurnal warming under certain con- 



Fig. 4. — Air and surface temperatures, off Cape Ann, July 15, 1912. 



ditions. On July loth we ran eastward from Massachusetts Bay to 

 Station 7, and then westward again in the evening, being continu- 

 ously within an area of weak tides, with clear sky and moderate 

 breezes. Surface and air temperatures for each hour from 7 A. M. 

 to 12 midnight are shown (fig. 4). The surface temperature, which 

 was 60°, near Boston Light-ship, rose rapidly to 63° at 10 a. m. 

 It then remained constant until 2 p. m., when there was an irregular 

 rise, culminating, at 7 p. m., with 66°. After this the temperature 

 fell reaching 60° once more at midnight. Obser^•ations made during 

 the rest of the night are not comparable with the foregoing, because we 



