THEIR CAUSES. Ill 



Not only does the motion of the earth in its 

 annual orbit, and that of the moon in her monthly 

 course round the earth, produce an alternately 

 diminishing and increasing influence on the tides, 

 but the diurnal revolution of the earth itself 

 exercises a remarkable power. In its daily revo- 

 lution from west to east it brings every successive 

 hour one meridian after another vertically under 

 the moon, so that the point at which the greatest 

 attractive power of the moon is felt upon the 

 earth, and which is vertically beneath the moon, 

 changes hour after hour as different portions of 

 her surface are presented to the action of her 

 attendant satellite. In consequence of this the 

 attractive power of the moon and the sun is at no 

 time stationary, but is continually moving, as it 

 were, along the earth's surface, changing its posi- 

 tion with the apparent place of the bodies by 

 which it is exercised. As the moon therefore 

 moves from east to west, or, to speak more 

 correctly, as the earth revolves in the opposite 

 direction, the waters of the ocean, instead of accu- 

 mulating in one place, form a tidal wave which fol- 

 lows the course of the moon. This wave, as it moves 

 along the ocean, produces high water on the coasts 

 it visits in its flow ; but its force, direction, and 

 height, are more or less modified by circum- 

 stances already alluded to, such as the obstacles 

 in its way arising from oceanic currents, irregu- 

 larities in the figure of the land, the shape, 

 position, and breadth of the channels through 

 which it passes. The broader and deeper the 



