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 63°-65° in the day time. On our return we 
crossed Massachusetts Bay twice (August 28th-3lst). 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- 
Fia. 4.— Air and surface temperatures, off Cape Ann, July 15, 1912. 
ditions. On July 15th 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. Observations made during 
the rest of the night are not comparable with the foregoing, because we 
