TIDES. 35 



to return towards B is equal to the tendency of the earth's rotation to 

 carry it towards c. The crests of the tide will under these circum- 

 stances remain at ;// and //, while the earth continues to rotate ; and 

 of course they will to us, inhabitants of the earth, seem to travel 

 westward, just as the sun and stars seem to travel westward. The 

 solar tide thus assumes the appearance of an immensely broad and 

 excessively flat pair of waves travelling once round the earth from 

 east to west every day. 



In the same way the attraction of the moon causes the lunar 

 tide. The moon is so close to the earth at a distance only i-4ooth 

 part of the distance of the sun that, although the whole force of 

 the moon's attraction is small in comparison with the sun's, there is 

 a more sensible difference in the degree in which a body is attracted 

 by the moon, according as the body is on the side of the earth next 

 the moon or on the farthest side. And by going through the com- 

 putation, it appears that the moon, on this account, produces a tide 

 about two and a half times the height of the solar tide. 



Such is a picture of what would happen if the earth were covered 

 everywhere by a deep ocean. But the phenomena are complicated 

 upon the earth by the obstacle which continents present to the even 

 advance of the tidal waves, and by the modifications which they 

 undergo in shallow or narrow seas, in which they rise to an exag- 

 gerated height. 



The height of the tide varies in the different regions of the globe 

 according to local circumstances. The eastern coast of Asia and the 

 western coast of Europe are exposed to extremely high tides ; while in 

 the South Sea Islands, where they are very regular, they scarcely reach 

 the height of twenty inches. On the western coast of South America, 

 the tides rarely reach three yards ; on the western coast of India 

 they reach the height of six or seven ; and in the Gulf of Cambay it 

 ranges from five to six fathoms. This great difference makes itself 

 felt in our own and adjoining countries : thus, the tide which at 

 Cherbourg is seven and eight yards high, attains the height of fourteen 

 yards at Saint Malo, while it reaches the height of ten yards at 

 Swansea, at the mouth of the Bristol Channel, increasing to double 

 that height at Chepstow, higher up the river. In general, the tide 

 is higher at the bottom of a gulf than at its mouth. 



The highest tide which is known occurs in the Bay of Fundy, 

 which opens up to the south of the isthmus uniting Nova Scotia and 

 New Brunswick. There the tide reaches forty, fifty, and even sixty 

 feet, while it only attains the height of seven or eight in the bay to 



