CHAP. VIII. ] THE GULF-STREAM. 375 
the level of the polar water being reduced, and its 
density increased by the surface-cold to which it is 
subjected, whilst a downward motion is also imparted 
to each stratum successively exposed to it; and the 
level of equatorial water being raised and its density 
diminished by the surface-heat to which it is exposed. 
(The first of these agencies is by far the more effec- 
tive, since it extends to the whole depth of the water, 
whilst the second only affects, in any considerable 
degree, the superficial stratum.) Thus a movement 
will be imparted to the upper stratum of oceanic 
water from the equator towards the poles, whilst a 
movement will be imparted to the deeper stratum 
from the poles towards the equator.” 
It seems to me that the doctrine here propounded 
by my distinguished colleague, if 1 understand it 
aright, is open to the objection to which I have 
already referred in connection with the speculations 
of Captain Maury. 
If the currents flow in the direction and with the 
permanence accepted by Dr. Carpenter in the Strait 
of Gibraltar and in the Baltic Sound, if their flow 
and its direction be due to the causes to which Dr. 
Carpenter attributes them, and if there be any 
analogy whatever between the conditions of equi- 
librium of these inland seas and that of the outer 
ocean,—none of which propositions appear to me at 
all satisfactorily proved,—I should think that the vast 
equatorial region, the path of the trade-winds and 
the belt of vertical solar radiation, must, so far as 
evaporation is concerned, resemble, or rather greatly 
exaggerate, the conditions of the Mediterranean. The 
consequent accumulation of salt,—through the whole 
