26 THE TIDES. 



2. Obstacles which check the motion of the water towards 

 a certain point retard the time of high water, and 

 increase the height. 



If the obstacle is a complete barrier, the tide will rise as 

 long as the motion of the water is towards it, and will fall 

 as long as the motion is from it. Hence, at 45 east of 

 quadratures it will be high water on the east of such an 

 obstacle, and low water on the west of it. The influence 

 of this on the time of high water at other places will 

 extend as far as the pressure is felt. 



An obstacle not sufficient to stop motion altogether 

 will produce a similar effect, but of course much smaller, 

 in consequence of the continuity of the surface. If the 

 obstacle be such as to destroy half the velocity of the water, 

 then high water on its east side would be 30 after quadra- 

 tures. In both cases the height would obviously be 

 increased. 



It appears from this that the effect of such obstacles is 

 in both respects the reverse of that of friction. 



3. Effect of the moment of the moon's attraction on the 

 tidal prominences in an equatorial canal with the 

 moon in the equator. 



This is the way in which the retardation was supposed 

 by Delaunay to be produced, and Thomson and Taithave 

 adopted the same view.* 



* The statement that the earth rotates in a "friction collar," which 

 seems to put the matter in a nutshell, obviously assumes that the passage of 

 the tidal wave is the passage of a mass of water. But this is true only so 

 far as there is a residual westward current, which is certainly not self- 

 evident. 



