94 THE OCEAN. 



periodical swelling of the waters could only be attempted in modern 

 times, with the aid of the knowledge obtained by astronomers on the 

 motion of the celestial bodies, and with the powerful means of investi- 

 gation, which mathematicians have supplied them with. Kepler first 

 indicated the course to be followed; and Descartes, and then Newton, 

 each gave his theory explaining the tides, the one by pressure, the 

 other by the attraction exercised by the sun and moon on the mobile 

 waters of the sea. It is the latter theory, that of Newton, which was 

 developed later, much modified by Bernouilli, Euler, and Laplace, 

 and which Lubbock, Whewell, Chazallon, and so many other natural 

 philosophers have since compared with observations made on the 

 shores of the ocean. Eeing very satisfactory in most respects, it is 

 now very generally accepted ; but it still has eminent opponents, 

 among whom F. Boucheporn* must be named ; many of the secondary 

 facts are still to be elucidated, and many local phenomena are not yet 

 understood. To follow the tides in their progress and fluctuations 

 across the seas, it is not sufficient to know the laws of gravitation, 

 and to calculate with the most rigorous exactitude the movement 

 and position of the heavenly bodies; one must also know all the facts 

 relative to the movements of fluids, and know how to apply to all 

 their phenomena of acceleration, retardation, increase, interference, 

 and equilibrium the most complicated and most minute formulae of 

 high mathematics. It would also be indispensable to know every 

 fact respecting the form of the shore, and the inequalities of the 

 bed of the sea. 



Pteduced to its principal elements, the theor}^ of tides set forth by 

 Laplace, and generally adopted since, is very simple. The earth is not 

 an isolated body in space ; it is attracted by all the nearer heavenly 

 bodies, and it is indeed in great part this force of gravitation which 

 causes it to turn round the sun, and retains the moon as its satellite. 

 Let us imagine for an instant the earth to be covered with water over 

 all its surface, and subject to the attraction of the moon alone. 

 This superficial part of the planet would be more strongly attracted 

 than the solid portion, since it is nearer to the moon which attracts 

 it ; and owing to the facility with which liquid particles glide one over 

 the other, it would swell, so to say, towards the moon till its weight 

 would be in equilibrium with the attracting force. It would then form 

 an intumescence, the summit of which would be exactly on the ideal 

 line which unites the centre of the earth to that of the moon. On 



* Fhilosophie Naturclle, pp. 1—205. 



