SECT. XIII. THEORY OF THE TIDES. 93 



The periodic motions of the waters of the ocean, on the hypo- 

 thesis of an ellipsoid of revolution, entirely covered by the sea, 

 are, however, very far from according with observation. This 

 arises from the great irregularities in the surface of the earth, 

 which is but partially covered by the sea, from the variety in 

 the depths of the ocean, the manner in which it is spread out on 

 the earth, the position and inclination of the shores, the currents, 

 and the resistance which the waters meet with : causes impos- 

 sible to estimate generally, but which modify the oscillations of 

 the great mass of the ocean. However, amidst all these irregu- 

 larities, the ebb and flow of the sea maintain a ratio to the forces 

 producing them sufficient to indicate their nature, and to verify 

 the law of the attraction of the sun and moon on the sea. La 

 Place observes, that the investigation of such relations between 

 cause and effect is no less useful in natural philosophy than the 

 direct solution of problems, either to prove the existence of the 

 causes or to trace the laws of their effects. Like the theory of 

 probabilities, it is a happy supplement to the ignorance and 

 weakness of the human mind. 



Since the disturbing action of the sun and moon can only 

 become sensible in a very great extent of deep water, the Ant- 

 arctic Ocean is the origin and birthplace of our tides. A succes- 

 sion of tidal waves from that source follow one another in a 

 north-westerly direction down the Pacific and Atlantic Oceans, 

 modified as they proceed by the depth of the water and the 

 form of the coasts. For when the sun and moon are in the same 

 meridian, and pass over the mass of waters lying east from Van 

 Diemen's Land, New Zealand, and the South Pole, the resulting 

 force of their combined attraction, penetrating to the abyss of the 

 deep and boundless circuit of the Southern Ocean, raises a vast 

 wave or ridge of water, which tends to follow the luminaries to 

 the north and west, and continues to flow in that direction long 

 after the bodies cease to act upon it ; but it is so retarded by the 

 rotation of the earth and by the inertia of the water, that it does 

 not arrive at the different parts of the coasts till after the 

 moon's southing (N. 156). When this tidal wave leaves the 

 Antarctic Ocean and enters the Pacific, it rushes along the 

 western coast of America to its farthest end, but it is so much 

 obstructed by the number of islands in the middle of that ocean 

 that it is hardly perceptible among them ; while on the east it 



