Mr. J. Croll on the Physical Cause of Ocean-currents. 245 



their motion. The greater saltness of the waters, to which he 

 attributes so much, can in no way be regarded as a cause of 

 motion. Its effect, so far as it goes, is to stop the motion of 

 the stream rather than to assist it. 



But, again, although Maury maintains that difference of salt- 

 ness and difference of temperature are both causes of ocean- 

 currents, yet he appears actually to admit that temperature and 

 saltness neutralize each other so as to prevent change in the 

 specific gravity of the ocean, as will be seen from the following 

 quotation : — 



" It is the trade-winds, then, which prevent the thermal and 

 specific-gravity curves from conforming with each other in 

 intertropical seas. The water they suck up is fresh water; and 

 the salt it contained, being left behind, is just sufficient to coun- 

 terbalance, by its weight, the effect of thermal dilatation upon the 

 specific gravity of sea-water between the parallels of 34° north 

 and south. As we go from 34° to the equator, the water grows 

 warmer and expands. It would become lighter ; but the trade- 

 winds, by taking up vapour without salt, make the water Salter, 

 and therefore heavier. The conclusion is, the proportion of 

 salt in sea-water, its expansibility between 62° and 82°, and 

 the thirst of the trade-winds for vapour are, where they blow, so 

 balanced as to produce perfect compensation ; and a more beau- 

 tiful compensation cannot, it appears to me, be found in the 

 mechanism of the universe than that which we have here stum- 

 bled upon. It is a triple adjustment : the power of the sun to 

 expand, the power of the winds to evaporate, and the quantity 

 of salts in the sea — these are so proportioned and adjusted that 

 when both the wind and the sun have each played with its 

 forces upon the intertropical waters of the ocean, the residuum 

 of heat and of salt should be just such as to balance each other 

 in their effects ; and so the aqueous equilibrium of the torrid zone 

 is preserved" (§ 436, eleventh edition). 



" Between 35° or 40° and the equator evaporation is in excess 

 of precipitation; and though, as we approach the equator on 

 either side from these parallels, the solar ray warms and expands 

 the surface-water of the sea, the winds, by the vapour they 

 carry off, and the salt they leave behind, prevent it from making 

 that water lighter " (§ 437, eleventh edition) . 



" Philosophers have admired the relations between the size 

 of the earth, the force of gravity, and the strength of fibre in 

 the flower-stalks of plants ; but how much more exquisite is the 

 system of counterpoises and adjustments here presented between 

 the sea and its salts, the winds and the heat of the sun I" (§ 438, 

 eleventh edition) . 



How can this be reconciled with all that precedes regarding 



