1 18 Mr. J. Croll on the Physical Cause of Ocean-currents. 



he alludes, are scarcely worthy of consideration in relation to 

 the present question, because we know nothing whatever regard- 

 ing the actual amount of force producing motion of the water 

 in these experiments, further than that it must have been enor- 

 mously greater than that derived from a slope of 1 in 1,000,000. 



Supposed argument from the tides. — Dr. Carpenter advances 

 Mr. Ferrer's argument in regard to the tides. The power of 

 the moon to disturb the earth's water, he aserts, is, according 

 to Herschel, only one 11,400,000th part of gravity, and that of 

 the sun not over one 25,736,400th part of gravity ; yet the 

 moon's attractive force, even when counteracted by the sun, 

 will produce a rise of the ocean. But as the disturbance of gra- 

 vity produced by difference of temperature is far greater than 

 the above, it ought to produce circulation. 



It is here supposed that the force exerted by gravity on the 

 ocean, resulting from difference of temperature, tending to pro- 

 duce the general oceanic circulation, is much greater than the 

 force exerted on the ocean by the moon in the production of the 

 tides. But if we examine the subject we shall find that the op- 

 posite is the case. The attraction of the moon tending to lift 

 the waters of the ocean acts directly on every molecule from the 

 surface to the bottom ; but the force of gravity tending to pro- 

 duce the circulation in question acts directly on only a portion 

 of the ocean. Gravity can exercise no direct force in impelling 

 the underflow from the polar to the equatorial regions, nor in 

 raising the water to the surface when it reaches the equatorial 

 regions. Gravity can exercise no direct influence in pulling the 

 water horizontally along the earth^s surface, nor in raising it up 

 to the surface. The pull of gravity is always downwards, never 

 horizontally nor upwards. Gravity will tend to pull the surface- 

 water from the equator to the poles because here we have descent. 

 Gravity will tend to sink the polar column because here also we 

 have descent. But these are the only parts of the circuit where 

 gravity has any tendency to produce motion. Motion in the 

 other parts of the circuit, viz. along the bottom of the ocean 

 from the poles to the equator and in raising the equatorial 

 column, is produced by the pressure of the polar column ; and 

 consequently it is only indirectly that gravity may be said to 

 produce motion in those parts. It is true that on certain 

 portions of the ocean the force of gravity tending to produce 

 motion is greater than the force of the moon's attraction, tend- 

 ing to produce the tides ; but this portion of the ocean is of in- 

 considerable extent. The total force of gravity acting on the 

 entire ocean tending to produce circulation is in reality prodi- 

 giously less than the total force of the moon tending to produce 

 the tides. 



