232 PROCEEDINGS OF THE AMERICAN ACADEMr 



tic touch, and are at rest only so long as the figure of equilibrium is 

 unruffled, and always move in such a way as to restore it when it is 

 disturbed. Water everywhere flows from places which are above the 

 surface of equilibrium to places which are below it. The mouth of the 

 Mississippi is two and a half miles more distant from the earth's cen- 

 tre of figure than the source. But it ought to be three miles. It is, 

 therefore, below the surface of equilibrium. And the water flows south 

 to fill it up to the proper level. The source of the Nile ought to be 

 about two and a hal^ miles more distant from the earth's centre than 

 the mouth of that river. But the excess of distance is more than two 

 and a half miles. Hence the source is above the figure of equilibrium, 

 and the waters flow as they do. The same mechanical causes, which 

 originally swept the two oceans from the poles to the equator in order 

 to build up that great equatorial embankment of water thirteen miles 

 high, and thus give the earth a stable figure, are now carrying the 

 Mississippi to its mouthy where the embankment is not yet high enough, 

 and the Nile from its source, where the liquid embankment is too 

 high. And here I may answer Mr. Mann's inquiry, ' Why does not 

 the centrifugal motion of the earth drive the waters of the Atlantic 

 and Pacific Oceans towards the equator ? ' It did once. But suffi- 

 cient water has already gone to make the figure perfect now. Inas- 

 much as the earth's waters flow so as to restore the ideal figure of 

 equilibrium wherever it is lost, and inasmuch as this figure of equi- 

 librium is such that the resultant of gravity and the centrifugal force 

 must be everywhere normal to its surface, the direction and the ve- 

 locity of the flow are intimately connected with the centrifugal force. 

 Without a rotation, and the centrifugal force which rotation produces, 

 the earth's figure of equilibrium would be a sphere. In this event, 

 the Mississippi would flow northward. Its southern direction, under 

 existing circumstances, may therefore be fairly attributed to the cen- 

 trifucral force. If the earth did not rotate, and the sphere were the 

 fio-ure of equilibrium, the Nile would flow in direction as it now does, 

 but much more rapidly. Under existing circumstances, the same cen- 

 trifugal force which accelerates the flow of the Mississippi retards the 

 flow of the Nile. 



" If the inquiry be made whether the Mississippi runs up hill or 

 down, I reply that this is simply a question of definition. If doicn 

 means towards the earth's centre of figure, then the Mississippi runs 

 up. If down means towards that part of the earth's surface where 



