CHANNEL-CURRENTS : SUN's ATTRACTION. 27 



fall, and is (at least in the open sea) affected with 

 only a slight current-motion towards the west ; a 

 motion which only acquires strength when the 

 wave is heaped up, as it were, by obstacles to its 

 progress, as happens to it over the shallow parts 

 of the sea, on the coasts, in gulfs, and in the 

 mouths of rivers, &c. If, for instance, the first 

 wave advancing meets in its course with the resist- 

 ance of the coasts on the two sides of any narrow 

 sea-channel, it is forced to rise, by the pressure of 

 of the following w r aves, whose motion is not at all 

 retarded, or certainly less so than that of the first 

 wave ; and thus an actual current of the water is 

 set up in straits and narrow channels. Hence the 

 periodical currents of the flood and ebb which are 

 met with in all straits, gulfs, and mouths of rivers 

 in which in general this effect of the tide is felt. 



The sun too, quite independently of the moo^ 

 by its daily apparent motion round the earth, brings 

 about a twofold change of tide. Its strength how- 

 ever is not more than half as great as that of the 

 moon-flood, because the far greater distance of the 

 sun from the earth renders the difference of its 

 attractions on different parts of the earth far less 

 than in the case of the moon. Now, after what I 

 have said above on the cause of the tides in general, 

 you will at once understand, that, at the times of 

 new moon and full moon, that is when the moon 

 stands between the sun and the earth, and when 



