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



trade-winds. From these two causes he derives the necessary 

 slope of 9 feet. But here a new agent is brought into play of 

 an entirely different character from that of the slope, viz. the 

 pressure of a mass of water 3 feet deep. In consequence of this 

 the weight of a column of water in the Gulf of Mexico exceeds 

 that of a column in the North Atlantic by three feet of water. 

 Each square foot of the cross section of the Gulf-stream is thus 

 subjected to a hydrostatic pressure of about 190 pounds urging 

 the water forward. The amount of work which this pressure is 

 capable of performing in a given time is proportionate to the 

 superficial area of the column ; but the superficial area of the 

 column is equal to that of the entire Gulf of Mexico ; we have, 

 therefore, in this three feet of water heaped up in the Gulf a 

 power sufficient of itself to produce a very considerable current 

 through such a narrow strait as that of Florida. But were the 

 9 feet of slope wholly the result of difference of specific gravity, 

 the waters of the Gulf and the Atlantic would be in perfect 

 equilibrium, and there would then be no pressure from behind 

 impelling the water forward through the strait. The only thing 

 which could then have any influence in producing a current 

 would be the force which gravity would exert on the mole- 

 cules at the surface, tending to cause them to roll down the 

 slope, and which in the case of a slope so small would be but 

 trifling. In the case of Dr. Carpenter's oceanic circulation, 

 this force, as we have just seen, amounts to only one five-hun- 

 dredth of a grain on a pound of water. 



But there are other causes which tend to reduce the slope 

 still further : — (1) There can be no permanent current while the 

 full slope of 9 feet remains. A permanent current requires a 

 state of constant disturbed equilibrium between the equatorial 

 and polar columns ; and this again, as has been shown, neces- 

 sarily implies a permanent reduction of slope. (2) Although the 

 polar waters are colder than the intertropical, yet they are not so 

 salt, and of course are, on this account, not so heavy. (3) In 

 intertropical regions evaporation is in excess of precipitation ; but 

 in temperate and polar regions precipitation is in excess of eva- 

 poration. This, to a certain extent, tends to lower the level of 

 the sea in intertropical regions, and to raise the level of the sea 

 in temperate and polar regions; so much water is removed from 

 the surface of the ocean in intertropical regions and poured 

 down as rain or snow upon the surface of the ocean in temperate 

 and polar regions. In short, if there be any difference of level 

 between the surface of the ocean in equatorial and polar regions, 

 it must be trifling, indeed so trifling as to be absolutely incapable 

 of producing the motion supposed. 



It cannot be urged as an objection to what has been advanced 



