154 LAPLACE. 



mere cohesion tlie continual attraction of the planet. A movement of 

 rotation occurred to bis mind as constituting the principle of stability, 

 and he hence deduced the necessary velocity. The velocity thus iound 

 was exactly equal to that which Herschel subsequently deduced Irom 

 a course of extremely delicate observations. 



Tiietwo i)arts of the ring being placed at different distances from the 

 planet, could not tail to experience, from the action of the sun, different 

 movements of rotation. It would hence seem that the planes of both 

 rings ought to be geiierally inclined toward each other, whereas they 

 api)ear from observation always to coincide. It was necessary, then, 

 that some physical cause should exist wliich would be capable of neutral- 

 izing tlie action of the sun. In u memoir i)ublished in February, 1789, 

 La[)lace Ibund that this cause must reside in the ellipticity of Saturn, 

 pioduced by a rapid movement of rotation of the planet, a movement 

 the existence of which Herschel announced in November, 1789. 



The leader cannot fail to remark how, on certain occasions, the eyes 

 of the mind can supply the want of the most powerful telescopes, and 

 lead to astrononiical discoveries of the highest imi)ortance. Let us de- 

 scend from the heavens u[)on the earth. The discoveries of Laplace will 

 appear not less inqiortant, not less worthy of his genius. 



The phenomena of the tides, which an ancient philosopher designated 

 in despair the tomb of human curiosity, were connected by Laplace 

 with an analytical theory in which the physical conditions of the ques- 

 tion figure for the tirst time. Accordingly calculators, to the immense 

 advantage of the navigation of our maritime coasts, venture in the 

 present day to predict several years in advance the details of the time 

 and height of the full tides without more anxiety resi)ecting the result 

 than if the questiou related to the phases of an eclipse. 



There exists between the different phenomena of the ebb and flow of 

 the tides and the attractive forces which the suu and moon exercise 

 upon the fluid sheet which covers three-fourths of the globe, an intimate 

 and neces^sary connection, from which Laplace, by the aid of a series of 

 twenty years of observations, executed at Brest, deduced the value of 

 the mass of our satellite. Science knows in the present day that seventy- 

 five moons would be necessary to form a weight equivalent to that of 

 the terrestrial globe, and it is indebted for this result to an attentive 

 and minute study of the oscillations of the ocean. We know only one 

 means of enhancing the admiration which every thoughtful mind will 

 entertain for theories capable of leading to such conclusions. An his- 

 torical statement, will supply it. In the year 1031, the illustrious Gali- 

 leo, as appears from his fJiuloyues, was so far from perceiving the mathe- 

 matical relations from which Laplace deduced results so beautiful, so 

 unequivocal, and so useful, that he taxed with frivolousness the vague 

 idea which Ke[)ler 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. 



