354 PHYSICAL GEOGRAPHY. 



a flux and reflux of the sea. The tidal influence of the sun has been calculated to be 

 about three times less than that of the moon. Sometimes these bodies act in conjunction, 

 as at the seasons of new and full moon, and then the tides rise the highest, and are called 

 spring-tides ; but when the moon is in quadrature, the relative position of the two bodies 

 is such that their influence is antagonistic, and then occur the lowest or neap-tides. Let 



s be the position of the sun, and , b, c, d that of the 

 moon in four parts of her orbit. It is clear that 

 when the moon is at b and c?, her action and that of 

 the sun upon the earth are in opposite directions. 

 Hence the lunar and solar attraction counteract each 

 other to some extent, and the tides are low. But 

 when the moon is at , or it is new moon, she unites 

 her influence with the sun, and both act with the 

 greatest intensity upon the ocean. So when the 

 moon is at c, or full, the influence of the two bodies 

 is again united, for they are in the same meridian, 

 and while the sun draws away the waters on the side 

 nearest to him, the moon draws away the earth from 

 them, and while the moon attracts the waters on the side nearest to her, the sun attracts 

 the earth towards him. Thus are produced those mighty movements in the ocean which 

 aid the mariner on his course, and which, being periodical, are consistent with the se- 

 curity of life and property on the coasts. As far, then, as astronomical causes affect the 

 level of the ocean, we have an alternate rise and fall produced twice a day, with aug- 

 mented or spring tides, and neap or low tides, occurring at the times of new and full 

 moon, and when she is in quadrature. But in addition to this, we have the highest floods 

 and the lowest ebbs at the new and full moon near the equinoxes in March and Sep- 

 tember, when the moon is in perigee, of nearest the earth. 



Hitherto we have been proceeding upon the supposition of the earth being completely 

 environed with a zone of water of uniform depth. If that were the case, and the solar 

 and lunar attraction acted always upon the line of the equator, there would always be 

 high water at some point directly under the equator, and no tide whatever at the poles, 

 and the calculation might be made with precision of the elevation and depression of the 

 waters in every latitude. But various causes operate to introduce irregularity in the 

 phenomena of the tides. The positions of the sun and moon vary with reference to each 

 other and the equator, the moon being sometimes as much as 28J on each side of the 

 plane of the latter. The action of the winds also, and their varying force, with currents 

 in the ocean, affect the disturbance of its level by the solar and lunar influence. But the 

 most energetic causes of complexity in the tides, are the inequalities that mark the bed of 

 the sea, and its geographical distribution in contact with immense masses of dry land in 

 its basin. Shoals, peninsulas, and promontories, offer obstructions to the flow of its 

 waters, and vary their direction. In some narrow seas, channels, and gulfs, they are 

 prodigiously accumulated, so that the tides rise to a far greater height than in the main 

 ocean. Thus, on the coasts of the islands of the South Sea, the tides have only an ele- 

 vation of one or two feet ; and at St. Helena, a solitary islet in the heart of the South 

 Atlantic, the rise very rarely exceeds three feet; but in the Irish Sea and English 

 Channel the periodical flow attains a much greater height. At the mouth of the Wye 

 the tidal rise amounts to about fifty feet ; at the mouth of the Avon, to forty -two feet ; at 

 Milford Haven, to thirty-six feet ; at London and Beachy Head, to eighteen feet ; at the 

 Needles, off the Isle of Wight, to nine feet ; and at Weymouth, to seven. The conse- 

 quence of these extraordinary elevations is, that large tracts are alternately oceanic and 



