44G LECTURE XLVII. 



Pacific ocean to partake of the effects of its primitive tide, and their tide, 

 being secondary, is probably for this reason a few hours later. At the 

 Alniirantes, near the eastern coast of Africa, the tide is at the sixth hour ; 

 but there seem to be some irregularities in the tides of the neighbouring 

 islands. 



The progress of a tide may be very distinctly traced from its source in 

 the ocean into the narrow and shallow branches of the sea which constitute 

 our channels. Thus the tide is an hour or two later at the Scilly Islands 

 than in the Atlantic, at Plymouth three, at Cork, Bristol, and Weymouth 

 four, at Caen and Havre six, at Dublin and Brighthelmstone seven, at 

 Boulogne and Liverpool eight, at Dover near nine, at the Nore eleven, and 

 at London bridge twelve and a half. Another portion appears to proceed 

 round Ireland and Scotland into the North Sea ; it arrives from the Atlantic 

 at Londonderry in about three hours, at the Orkneys in six, at Aberdeen 

 in eleven, at Leith in fourteen, at Leostoffe in twenty, and at the Nore in 

 about twenty-four, so as to meet there the subsequent tide coming from 

 the south. From the time occupied by the tide in travelling from the 

 mouth of the English channel to Boulogne, at the rate of about 50 miles an 

 hour, we may calculate that the mean depth of the channel is about 28 

 fathoms, independently of the magnitude of the resistances of various 

 kinds to be overcome, which require us to suppose the depth from 30 to 40 

 fathoms. In the great river of Amazons, the effects of the tides are still 

 sensible at the streights of Pauxis, 500 miles from the sea, after an interval 

 of several days spent in their passage up ; for the slower progressive motion 

 of the water no more impedes the progress of a wave against the stream, 

 than the velocity of the wind prevents the transmission of sound in a con- 

 trary direction. (Plate XXXVIII. Fig. 521.) 



Such are the general outlines of the lunar tides ; they are, however, liable 

 to a great variety of modifications, besides their combination with the tides 

 produced by the sun. When the moon is exactly over the equator, the 

 highest part of the remoter, or inferior, as well as of the nearer or superior 

 tides, passes also over the equator, and the effect of the tide in various lati- 

 tudes decreases gradually from the equator to the pole, where it vanishes ; 

 but when the moon has north or south declination, the two opposite 

 summits of the spheroid describe parallels of latitude, remaining always 

 diametrically opposite to each other. Hehce the two successive tides must 

 be unequal at every place except the equator, the greater tide happening 

 when the nearer elevation passes its meridian ; and the mean between both 

 is somewhat smaller than the equal tides which happen when the moon 

 passes the equator. This inequality is, however, much less considerable 

 than it would be if the sea assumed at once the form of the spheroid 

 of equilibrium ; and the most probable reasons for this circumstance, are, 

 first, that our tides are partly derived from the equatorial seas ; secondly, 

 that the effects of a preceding tide are in some measure continued so 

 as to influence the height of a succeeding one; and, thirdly, that the 

 tides of a narrow sea are less affected by its latitude than those of a wide 

 ocean. The height of the sea at low water is the same whatever the 

 moon's declination may be. There is also a slight difference in the tides, 



