NATURAL PHILOSOPHY. 181 



the stronger attraction is at the equator or at the poles, since the flow of 

 water is determined, not by the intensity of the gravity at the place where 

 the water is, or anywhere else, but by the direction of this gravity in relation 

 to the surface at that place. 



Again, Mr. Mann speaks of the centre of gravity of the earth, and says 

 that the waters of the Mississippi are constantly approaching this centre of 

 gravity. But why is it that the Nile moves northward? Does that also 

 approach constantly the same centre of gravity ? The whole argument from 

 the centre of gravity of the earth is fallacious, for the earth has no fixed 

 centre of gravity. There is a new centre of gravity to the earth for every new 

 spot of surface which an attracted body visits. "Water could not flow in any 

 direction without approaching some of these centres of gravity, and deserting 

 others. And, in fact, the waters at the mouth of the Mississippi are further 

 from the centre of gravity which belongs to the geographical situation of the 

 mouth, than the waters of the sources of the river are from the centre of 

 gravity which belongs to the position of these sources. In the case of the 

 Xjle, exactly the reverse of this is true. 



What, then, is the true mechanical principle which is applicable to these 

 cases ? It is this : the mutual attraction of the particles of matter upon each 

 other, which, if undisturbed, would mould a yielding earth into the form of a 

 perfect sphere, has been so modified by the centrifugal force, resulting from 

 the planet's rotation, as to make the figure of an ellipsoid, in which the largest 

 radius exceeds the shortest by thirteen miles, the true figure of equili- 

 brium. Cohesion enables the solid land to hold out to a limited extent against 

 these moulding influences. But the free waters yield readily to their plastic 

 touch, and are at rest only so long as the figure of equilibrum 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 hah' 

 miles more distant from the earth's centre 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 up to the proper level. The source of the 

 Nile ought to be about two and a half miles more distant from the earth's cen- 

 tre 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 pole 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 mouth, where the em- 

 bankment 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 sufficient 

 water has already gone to make the figure perfect now. Inasmuch 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 equilibrium is such that the resultant of 

 gravity and the centrifugal force must be everywhere normal to its surface, the 



