Mr. J. Croll on the Physical Cause of Ocean-currents. 24<7 



trade-winds to evaporate and to counteract the dynamical force 

 of the sun, this hot and light water, by becoming hotter and 

 lighter, would flow off in currents with almost mill-tail ve- 

 locity towards the poles, covering the intervening sea with, a 

 mantle of warmth as a garment. The cool and heavy water of 

 the polar basin, coming out as undercurrents, would flow equa- 

 torially with equal velocity." 



" Thus two antagonistic forces are unmasked, and, being un- 

 masked, we discover in them a most exquisite adjustment — a com- 

 pensation — by which the dynamical forces that reside in the sun- 

 beam and the trade-wind are made to counterbalance each other, 

 by which the climates of intertropical seas are regulated, and by 

 which the set, force, and volume of oceanic currents are mea- 

 sured" (§§ 437 and 438, eleventh edition). 



The force resulting from difference of specific gravity not suffi- 

 cient to produce motion. — I shall now consider whether the forces 

 to which Maury appeals have the potency that he attributes to 

 them. Is the force derived from the difference of specific gravity 

 between the waters of the ocean in intertropical and polar 

 regions sufficient to account for the motion of ocean-currents ? 



The utter inadequacy of this cause has been so clearly shown 

 by Sir John Herschel, that one might expect that little else 

 would be required than simply to quote his words on the sub- 

 ject, which are as follows : — 



" First, then, if there were no atmosphere, there would be no 

 Gulf-stream, or any other considerable ocean-current (as distin- 

 guished from a mere surface- drift) whatever. By the action of 

 the sun's rays, the surface of the ocean becomes most heated, and 

 the heated water will, therefore, neither directly tend to ascend 

 (which it could not do without leaving the sea) nor to descend, 

 which it cannot do, being rendered buoyant, nor to move late- 

 rally, no lateral impulse being given, and which it could only 

 do by reason of a general declivity of surface, the dilated por- 

 tion occupying a higher level. Let us see what this declivity 

 would amount to. The equatorial surface-water has a tempe- 

 rature of 84°. At 7200 feet deep the temperature is 39°, the 

 level of which temperature rises to the surface in latitude 56°. 

 Taking the dilatability of sea-water the same as that of fresh, a 

 uniformly progressive increase of temperature, from 39° to 84° 

 Fahr., would dilate a column of 7200 feet by 10 feet, to which 

 height, therefore, above the spheroid of equilibrium (or above 

 the sea-level in lat. 56°), the equatorial surface is actually raised 

 by dilatation. An arc of 56° on the earth's surface measures 

 3360 geographical miles; so that we have a slope of l-28th of 

 an inch per geographical mile, or l-32nd of an inch per statute 

 mile for the water so raised to run down. As the accelerating 



