attraction of the planet.  A movement of rotation occurred to his mind as constituting the principle of stability, and he deduced the necessary velocity from this consideration.  The velocity thus found was exactly equal to that which Herschel subsequently derived from a series of extremely delicate observations.  The two parts of the ring, being at different distances from the planet, could not fail to be given different movements of precession by the action of the sun.  Hence it would seem that the planes of both rings ought in general to be inclined toward each other, whereas they appear from observation always to coincide.  It was necessary then that some physical cause capable of neutralizing the action of the sun should exist.  In a memoir published in February, 1789, Laplace found that this cause depended on the ellipticity of Saturn produced by a rapid movement of rotation of the planet, a movement whose discovery Herschel announced in November of the same year.

If we descend from the heavens to the earth, the discoveries of Laplace will appear not less worthy of his genius.  He reduced the phenomena of the tides, which an ancient philosopher termed in despair “the tomb of human curiosity,” to an analytical theory in which the physical conditions of the question figure for the first time.  Consequently, to the immense advantage of coast navigation, calculators now venture to predict in detail the time and height of the tides several years in advance.  Between the phenomena of the ebb and flow, and the attractive forces of the sun and moon upon the fluid sheet which covers three fourths of the globe, an intimate and necessary connection exists; a connection from which Laplace deduced the value of the mass of our satellite the moon.  Yet so late as the year 1631 the illustrious Galileo, as appears from his ‘Dialogues,’ was so far from perceiving the mathematical relations from which Laplace deduced results so beautiful, so unequivocal, and so useful, that he taxed with frivolousness the vague idea which Kepler entertained of attributing to the moon’s attraction a certain share in the production of the diurnal and periodical movements of the waters of the ocean.

Laplace did not confine his genius to the extension and improvement of the mathematical theory of the tide.  He considered the phenomenon from an entirely new point of view, and it was he who first treated of the stability of the ocean.  He has established its equilibrium, but upon the express condition (which, however, has been amply proved to exist) that the mean density of the fluid mass is less than the mean density of the earth.  Everything else remaining the same, if we substituted an ocean of quicksilver for the actual ocean, this stability would disappear.  The fluid would frequently overflow its boundaries, to ravage continents even to the height of the snowy peaks which lose themselves in the clouds.