678 
BULLETIN OF THE BUREAU OF FISHERIES 
some other process. The warming by direct solar radiation would therefore be vir- 
tually negligible during a single summer at depths greater than about 50 meters if 
there were no vertical circulation, this limit varying with varying states of turbidity 
and with the roughness or smoothness of the surface of the water as well as with the 
cloudiness of the sky, the haziness of the atmosphere, the percentage of foggy days, etc. 
DISPERSAL OF HEAT DOWNWARD INTO THE WATER 
With at east nine-tenths of the solar energy that enters the water of the gulf at 
all absorbed within 10 meters of the surface, and virtually all of it shoaler than 30 
to 50 meters, the importance of vertical circulation in carrying down into the deeps 
water that has been warmed at the surface, and by bringing cold water up within the 
influence of the sun from below, becomes at once apparent. 66 
The vertical circulation of the gulf is discussed in another chapter (p. 924). It 
concerns us here, however, as the factor that chiefly governs the temperature of the 
mid -stratum between the depths of, say, 25 and 100 meters. In different parts of the 
gulf and at different seasons we find all gradations from water so stable, vertically, 
and with currents so weak that virtually no interchange takes place between the 
different strata, to the opposite extreme where the whole column is kept so thoroughly 
churned by tidal currents that the heat absorbed by the surface is uniformly dis- 
persed downward. This last state characterizes nearly the entire area of the gulf 
during the first days of spring and is responsible for the fact that the whole 
upper stratum, down to 100 meters, at first warms at so nearly uniform a rate. 
The vertical uniformity of temperature that characterizes Nantucket Shoals, 
locally, too, Georges Bank, parts of the Bay of Fundy, and the coastal belt along 
the west coast of Nova Scotia, results similarly from tidal stirring so active that it 
overcomes the tendency of the water to become stable as the spring progresses. Off 
the western shores of the gulf, however, where tidal stirring is not active enough to 
counteract the increasing stability of the column induced by the warming of the 
surface, the development of a light stratum at the surface tends more and more to 
insulate the deeper strata of water from the effects of solar warming as the season 
advances. The more stable the water becomes, the more effectively are the deeper 
strata protected in this way from thermal influences from above. 
It is this obstacle, which the stable state of the water opposes to vertical circu- 
lation during the warm half of the year, which is responsible for the fact that the 
temperature rises so much more rapidly and to so much higher a value at the sur- 
face than only a few meters down, and which allows the persistence of much lower tem- 
peratures at depths of only 50 to 100 meters all summer. However, there is always 
enough vertical movement of the water everywhere in the Gulf of Maine to prevent 
this insulation of the deeper strata from becoming as effective as it is along the coast 
from New York, southward, during some springs (Bigelow, 1922). 
Observations taken during our first cruises in 1912 (Bigelow, 1914) pointed to 
local differences in the strength of the tidal currents as chiefly responsible for the 
fact that the surface is so much colder, but the bottom, depth for depth, so much 
warmer along the coast of Maine east of Penobscot Bay and in the Bay of Fundy 
*• Conduction and the radiation of heat from one particle of heat to the neit are negligible in this respect. (Wegemann, 1905; 
Kxiimmel, 1907.) 
