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BULLETIN OF THE BUREAU OF FISHERIES 
water, however, which evaporates somewhat more rapidly than salt water under 
equal conditions of temperature, humidity, etc. According to Mazelle (1898), the 
evaporation of salt water averages about 81 per cent that of fresh at Trieste, while 
Okada (1903) found it averaging about 95 per cent that of fresh over a 7-year period 
in Japan. As Okada’s measurements were taken open to the sky, Mazelle’s under a 
roof, the former simulate more the conditions at sea. 38 
As a rough approximation, the evaporation of salt water from the surface of the 
Gulf of Maine may, then, be set at about 27 to 28 inches, or about 71 centimeters, 
annually. 
DEEP STRATUM 
SLOPE WATER 
The scources so far mentioned contribute chiefly to the superficial stratum of 
the Gulf of Maine. We must next consider the comparatively warm and highly 
saline water that drifts intermittently inward along the trough of the Eastern Channel 
to form the bottom water of the gulf. The high salinity of this makes its offshore 
origin clear enough. As certainly, however, it is not a direct and unmixed indraft from 
the mid depths of the Atlantic Basin. Two reasons warrant this confident assertion. 
In the first place, neither the temperature nor the salinity of the bottom water of 
the Eastern Channel, or of the gulf basin within, is high enough to accord with such 
an origin. In the second place, profiles enough have now been run by various 
expeditions to make it certain that a broad band, intermediate in temperature and in 
salinity between the coastal water, on the one hand, and the tropic Atlantic water, 
on the other, always separates the latter from the edge of the continent from Georges 
Bank to the Grand Banks. 
The “ cold wall” of the earlier oceanographers — the source of this band — has been 
the subject of much discussion, with upwelling from the ocean abyss and currents 
from the north most freqently invoked to explain its low temperature as contrasted 
with the “Gulf Stream” on its seaward side. Recent explorations, however, have 
made it clear that this “cold wall” is simply the product of the mixture that is 
constantly taking place between the tropic water, on the one hand, and the coastal 
water, on the other (or Arctic water in the Grand Banks region), at their zone of 
contact along the slope of the continent. “Slope water,” as defined by Huntsman 
(1924), is therefore a better name for it than “cold wall,” and as such it is referred 
to repeatedly in the preceding pages. 
It is the presence of a continuous zone of this slope water right across the mouth 
of the gulf at all times of year which effectively bars unadulterated oceanic or tropic 
water from entering the Eastern Channel. It is because the most saline bottom 
water of the gulf draws from this source that members of the bathypelagic plankton 
of the Atlantic Basin occur only as the rarest of stragglers within the gulf (Bige- 
low, 1926, p. 67). 
Explorations by the Canadian Fisheries Expedition (Bjerkan, 1919; Sandstrom, 
1919; and Huntsman, 1924) have similarly proven that the high salinity (34.5 to 
34.7 per mille) and comparatively high temperature (4° to 5°) of the deepest stratum 
J® For further discussion of evaporation see Krummel, 1907, p. 244. 
