248 HYDROGRAPHICAL SURVEYING [chap. ix. 



a point at the same time as the hollow of another, the range 

 of the tide may be, if the waves are of equal height, nothing. 



To this is to be attributed the variety of the height of the 

 tide at different parts of the coasts that appear fairly open. 



Thus, in the English Channel the height of the tide varies at 

 different places on the same day from 5 feet to 25 feet ; and the 

 same phenomenon occurs in many parts of the world. 



The tidal streams are not, however, directly affected in the 

 same way, and there are many places where the rise of the 

 tide is insignificant, but the tidal streams are very strong. 



This is because the horizontal flow of the water is not deter- 

 minable by the rise at the spot, but by the rise at other places, 

 possibly at some distance, and by the fact that water once set 

 in motion is not easily arrested. 



The variations caused by interference are wholly distinct 

 from the differences in the height of tide and velocity of 

 streams caused by the conformation of the land. 



The vertical movement of the water in the deep open ocean 

 is not great, probably not more than 2 feet, and the horizontal 

 motion is practically nil, being a merely insignificant oscilla- 

 tion. It is only when the water shoals that the friction of the 

 bottom and the constriction caused by the water, which is in 

 motion tliroughout its depth, being forced into a shorter 

 column, cause the wave to become unnaturally heightened ; 

 and horizontal movements, which we know as tidal streams, 

 are set up, by reason of water flowing from the higher to the 

 lower level. 



Thus, on banks in mid-ocean regular tidal streams are 

 found, and could the height of the water be measured, it would 

 be found to vary more than in the deep water around. 



The opposition of a coast and the shape of deep bays, gulfs, 

 and channels accentuate these effects, and the height of the 

 tide and velocity of the tidal streams vary in different places 

 exceedingly. The oceanic tidal w^ave, thousands of miles in 

 length, may have the distance between its crests shortened to 

 hundreds, and the wddth of the portion that approaches the 

 shore continuously narrowed as it passes up funnel-shaped 

 passages. 



Enough has been said to show that the tidal movements are 

 exceedingly complicated, and though long-continued observa- 



