Geology and Natural History. 141 
A vertical circulation is maintained in the Strait of Gibraltar 
by the excess of evaporation ‘in the Mediterranean over the 
t 
a) 
the two, so that a deep inflow must take place to restore the equi- 
ae assumed to be the case in the Bo 
sam 
phorus and Dardanelles. 
A vertical circulation must, on the same principles, be main- 
tained between polar and equatorial waters by the difference of 
their temperatures; the level of polar water being reduced, and 
Ww 
lve, since it extends to the whole depth ot the water, while the sec- 
ond effects, in any considerable degree, only the superficial stra- 
tum.) Thus a movement will be imparted to the upper stratum of 
Oceanic water from the equator toward the poles, while a move- 
ead 
not m 
Which cannot be accounted for in any other way than by an under- 
= of polar water toward the equator. Further, under particu- 
br circumstances, a yet greater degree of cold is brought by gla- 
Cial currents into the Temperate zone: thus giving ‘inet indi- 
Cation of a general movement of deep water from the poles toward 
ua 
oO . 
toward it being not so much propelled into it by the Gulf Stream, 
48 drawn into it from an area of which the or ; 
oo. if at all, above the normal. On the other hand, the Gulf 
