Nile back to the Mountains of the Moon. My own answer is, that this excess of attraction at the equator does not exist, and therefore neither carries the Mississippi towards its mouth, nor tends to carry the Nile back from its mouth. To many, the assumption will seem a plausible one, that the excess of matter at the equator should be accompanied with a redundancy of attraction there. They forget that the whole earth attracts everywhere. And calculation proves that the attraction of the whole earth upon a body at the surface is greater the nearer this body is to the poles; and for this obvious reason. The excess of equatorial matter operates to the prejudice of equatorial gravity, by keeping the rest of the earth at an unusually large distance. Moreover, it is of no importance to the flow of the Mississippi whether 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 attract ed 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 farther 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 Nile, 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 yield. ing earth into the form of a perfect sphere, have 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 equilibrium. Cohesion enables the solid land to hold out to a limited extent against these moulding influences. But the free waters yield readily to their plas 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 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 it 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 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 mouth, 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 sufficient 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 equilibrium is such that the resultant of gravity and the centrifugal force must be everywhere normal to its surface, the direction and the velocity 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 centrifugal force. If the earth did not rotate, and the sphere were the figure of equilibrium, the Nile would flow in direction as it now does, but much more rapidly. Under existing circumstances, the same centrifugal 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 down 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 the attraction is greatest, then also the Mississippi runs up. We cannot say, with Mr. Mann, that down means towards the earth's centre of gravity, because the earth has no single centre of gravity. His definition of up and down, therefore, is without any meaning, and is not, as he says, based upon the only philosophical idea we can have of these terms. The only standard level of altitude is the surface of equilibrium. If we understand by down below the surface of equilibrium,' and by up above the surface of equilibrium,' then our definitions will be as broad as nature's laws, and will lead to no paradoxes, all of which nature abhors more than a vacuum: then all the rivers will be found flowing downward. On a small scale, and in local mechanics, an inclined plane is one which is inclined to the local plumb-line. But on a large scale, such as will take in the whole length of a great river, every plane surface is inclined to every plumb-line but one, and the surface which is not inclined, and on which, therefore, a body has no tendency to slide, is a surface which is everywhere perpendicular to the plumb-lines which intersect it; that is, it is the earth's surface of equilibrium. This is the only true, broad, and universal standard of level. "It may be concluded from what has been said, that the new hydrostatic paradox is of man's invention, and that nature is in no way responsible for it. Science abounds in such paradoxes; and men of science are too prone to array the merest truisms in paradoxical language which catches the popular ear, though at the sacrifice of making science itself vulgar. Moreover, if the explanation which I have given of the paradox under consideration is beyond the knowledge or above the comprehension of a child, then the question which involves it is unfit to be addressed to him.” Professors W. B. Rogers and D. Treadwell offered some remarks upon the subject, and expressed their concurrence in the view taken of it by Mr. Lovering. DONATIONS TO THE LIBRARY, FROM OCTOBER, 1854, TO APRIL, 1856. Academy of Natural Sciences of Philadelphia. Proceedings, Vol. VII., Vol. VIII. No. 1. 8vo. Philadelphia. Francis Wayland. The Elements of Intellectual Philosophy. By Francis Wayland, President of Brown University. 1 vol. 12mo. Boston. 1854. Academy of Sciences, Arts, and Belles-Lettres, Dijon. Mémoires pour Années 1852-53. 2° Série. Tome II. 8vo. Dijon. Academy of Sciences, Arts, and Belles-Lettres, Caen. Rapport sur les Travaux de l'Académie des Sciences, Arts, et Belles-Lettres de Caen, lu dans Séance publique du 24 Novembre, 1853. Par M. 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