PHYSICAL OCEANOGKAPHY OP THE GULF OF MAINE 
677 
and virtually all of this residue was absorbed sboaler than 50 meters. Grein’s exact- 
ing measurements, therefore, confirm Knott’s (1904) conclusion that a. m. and p. m. 
temperatures taken by the “Pola” at 16 pairs of stations, with thermometers grad- 
uated to 0.1° C., showed no evidence of the penetration of direct solar radiation 
deeper than about 20 meters. 
In more turbid northern seas we may expect the solar radiation to be absorbed 
in a still shoaler surface stratum, depending largely on the character and abundance 
of the plankton at the time. In Puget Sound, for example, Shelford and Gail (1922) 
found the first meter of water absorbing about 20 per cent of the visible light that 
actually penetrates below the surface, with only 8 to 10 per cent of even the shorter 
wave lengths reaching a depth of 10 meters under average illumination. 
In the English Channel, Poole and Atkins (1926) found the illumination at 20 
meters to be about 5.5 per cent as strong as just below the surface; while in the 
Bay of Fundy, according to Klugh (1925), onty about 1.5 per cent of the illumination 
recorded just below the surface penetrates to 10 meters in August in bright sunlight. 
In Lake Seneca, New York (probably still more turbid), Birge and Juday (1921) 
found that only 15 per cent of the solar energy that entered the water penetrated to 
a depth of 2 meters, 5.4 per cent to 5 meters, and only 1 per cent to 10 meters. Per- 
haps as striking an example as any in nature of the absorption of the sun’s heat by 
the uppermost stratum of water is afforded by certain oft-quoted salt-water basins 
along the west coast of Norway, in which the salinity is very low at the surface but 
so high from the depth of 1 meter downward that the water is in extremely stable 
equilibrium. Here solar radiation in summer induces temperatures as high as 20° 
to 30° in the upper 2 meters of water but hardly affects the temperature deeper 
than about 5 meters. (See Helland-Hansen, 1912a, p. 65, for a discussion of these 
“Polls,” as they are named locally.) 
Judging from the similarity in latitude and in general hydrographic conditions, 
the penetration of solar radiation is probably of about the same order of magni- 
tude in the open Gulf of Maine as in Puget Sound. If, then, the water of the gulf 
were entirely without motion, and if heat were conveyed downward by no other means 
than direct solar radiation, more than 90 per cent of such of the sun’s radiant energy 
as penetrated the water at all would be expended within 10 meters of the surface, 
something like 98 per cent within 25 meters of the surface, and all but a fraction of 
1 per cent at a depth of 100 meters. At times of year when the water was particu- 
larly turbid — spring, for example, during the active flowerings of diatoms — the solar 
radiation would be absorbed still more rapidly. 
We must also bear in mind that that part of the sun’s insulation which is inter- 
cepted by the superficial stratum of water does not act solely to warm the latter, but 
that a part of its energy is expended directly in evaporating water vapor from the 
surface (p. 680). 
Under the conditions existing in the gulf it seems that if direct solar radiation 
warms the surface by 20° at any given locality in the gulf, the 10-meter level would 
certainly warm by only about 2°, very probably the 50-meter level would warm 
by no more than 0.2°, and the 100-meter level would not suffer change sufficient for 
our most delicate deep-sea thermometers to record during the part of the year when 
the water is gaining heat, unless this heat were carried downward into the deeps by 
8951—28 44 
