id 
Ba Sa ea HS i RR Sree a ig alas aed mips SS kN a a SR Ea a Si oS era eee Sn eae 
H. Mitchell on recent seundings in the Gulf Stream. 73 
which is the usual amount for this depth in the Straits of Flor- 
a. Th r current which underlies the stream, following 
the line of maximum depression, has a temperature of less than 
At ms eoualt, it might be supposed that an obstacle in 
the track of F the stream would cause an ascension of the Polar 
current, but when we consider that this Stream Bank has not 
the nature of a bar, and that the deep channel way beyond is 
ample, there would seem to be no reason for an ascension of the 
cold waters in this neighborhood. 
Several other casts, not referred to in our table, because too far 
to the eastwasd of the section line, furnish some clue as to the 
form of the Stream Bank. It appears to be triangular in its 
general figure, presenting at its west angle a bold prow to the 
stream. As the current is here flowing with an accelerating ve- 
ocity, a depostt is impossible. This bank, it appears to us, must 
loc 
_ have, like the adjacent reef, a firm constitution. It is an inter- 
esting question whether it belongs to the mountain system of 
Cuba (as its line of least water running EH. and 8. HE. might seem 
to indicate), or whether it is an ancient reef now ch and 
crumbling away. The least depth is about that of the foot of 
the swept portion of the reef apron on the north side of the gh 
—it may indicate the true depth of the Gulf Stream itself, an 
if so, its summit is not now abraded, while its base must be 
wearing away under the action of the Polar current, A bank so 
situated must have precipitious slopes. 
Observations upon the trend of the lead line, on hauling in, 
furnish indications that “re thickness of the upper moving stra- 
tum, i.e., the depth of the Gulf Stream, is scarcely more than one 
third of the maximum oes of the channel. This stream 
water, not a profound movement. 
In the exchange between the Gulf of Mexico and the Atlantic, 
the office of the Gulf Stream appears to be the restoration of sur- 
Jace level, while the office of the counter stream, (“ Polar current”) 
below, is the restoration of equilibrium thus disturbed, between 
waters of different specific weights or densities. To illustrate 
this view of compensating currents, we may be suffered to recall 
an instance from our experience in observations at the mouth 
f Hudson River. In the dry season (July) the surface outflow 
(brackish) through the Narrows of New York harbor, occu- 
pies nine out of the twelve tidal hours, while in the lowest water 
stratum the case is more than reversed, the inflow (salt) predom- 
inates to that extent, that as a general thing it is continual along 
the bottom, although not constant in veloc The same rer 3 
tions, with ‘variable proportions, were followed some distance up 
the river. On running a line of levels from New York city to 
Albany it was found that the bed of the Hudson lies below the 
mean level of the sea for over a hundred miles; but that the sur- 
Am. Jour. Sci.—Snconp Sxr1g3, Vou. XLIII, No. 127,—Jan., 1967. 
10 
