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



at one place and another can give rise to currents only through 

 the influence of the earth's gravity. All currents resulting 

 from difference of specific gravity can be ultimately resolved into 

 the general principle that the molecules that are specifically 

 heavier descend and displace those that are specifically lighter. 

 If, for example, the ocean at the equator be expanded by heat or 

 by any other cause, it will be forced by the denser waters in 

 temperate and polar regions to rise so that its surface shall 

 stand at a higher level than the surface of the ocean in these 

 regions. The surface of the ocean will become an inclined plane, 

 sloping from the equator to the poles. Hydrostatically, the 

 ocean, considered as a mass, will then be in a state of equili- 

 brium; but the individual molecules will not be in equilibrium. 

 The molecules at the surface in this case may be regarded as 

 lying on an inclined plane sloping from the equator down to the 

 poles, and as these molecules are at liberty to move they will 

 not remain at rest, but will descend the incline towards the 

 poles. When the waters at the equator are expanded, or the 

 waters at the poles contracted, gravitation makes, as it were, a 

 twofold effort to restore equilibrium. It in the first place sinks 

 the waters at the poles, and raises the waters at the equator, in 

 order that the two masses may balance each other ; but this very 

 effort of gravitation to restore equilibrium to the mass destroys 

 the equilibrium of the molecules by disturbing the level of the 

 ocean. It then, in the second place, endeavours to restore equi- 

 librium to the molecules by pulling the lighter surface-water at 

 the equator down the incline towards the poles. This tends not 

 only to restore the level of the ocean, but to bring the lighter 

 water to occupy the surface and the denser water the bottom of 

 the ocean ; and when this is done, complete equilibrium is re- 

 stored, both to the mass of the ocean and to its individual mole- 

 cules, and all further motion ceases. But if heat be constantly 

 applied to the waters of the equatorial regions, and cold to those 

 of the polar regions, and a permanent disturbance of equilibrium 

 maintained, then the continual effort of gravitation to restore 

 equilibrium will give rise to a constant current. In this case, 

 the heat and the cold (the agents which disturb the equilibrium 

 of the ocean) may be regarded as causes of the current, inas- 

 much as without them the current would not exist; but the 

 real efficient cause, that which impels the water forward, is the 

 force of gravity. But the force of gravity, as has already been 

 noticed, cannot produce motion (perform work) unless the thing 

 acted upon descend. Descent is implied in the very conception 

 of a current produced by difference of specific gravity. 



But Maury speaks as if difference of specific gravity could 

 give rise to a current without any descent. 



