an Agent of Geological Change. 339 



produced by the action of the sun and moon the particles 

 move in an excessively elongated ellipse, the shorter axis of 

 which is vertical. 



Sir John TIerschel clearly shows " that all the water that 

 goes to form the elevated portion of the tide-wave must be 

 brought from the depressed area ; and this can only take place 

 by a lateral approach of the vertical sections of the sea when 

 the water is rising, and their recess from each other when 

 falling, which is only another way of expressing an alternating 

 backward and forward movement/'' * It is evident, from these 

 considerations, that we may practically consider the particles 

 of water in tidal movement to be differentiated in velocity 

 only by frictional retardation on the sea-bottom, in other 

 words, by work done on the materials constituting it. 



It is necessary to explain that, in the free ocean, the total 

 height of the tide-wave between high and low water is but 

 small, and the advance and recess consequently not rapid ; but 

 by the concentration of forces the movement of an immense 

 mass of water, though at first slow, advancing up a gradually 

 constricted channel increases rapidly in velocity as the sectional 

 area diminishes, and a derivative wave is created. 



With this bare statement of principle we will proceed to 

 question recorded phenomena, and see if it is borne out by the 

 facts of nature. 



Evidences of Bottom-velocity. — Between Glas island and 

 Sgeir-i-Noe, in the Little Miuch off the west coast of Scotland, 

 the flood-stream often takes the buoys of the long lines down; 

 " and it is a remarkable circumstance, indicative of the great 

 depth of the tidal-stream here, that the buoys, though anchored 

 in 70 or 80 fathoms, are taken completely to the bottom ; 

 star-fish and other marine animals being found attached to 

 them." f As in fine weather the surface-velocity here is only 

 1^ knots per hour, it seems pretty evident the bottom-velocity 

 in this case must exceed that at the surface. 



In the Gulf of Coirebhreacain, between Scarba and Jura, 

 the tide runs with a velocity, by actual test, of 9^ miles an 

 hour. The principal stream of flood enters from the eastward 

 from the Sound of Jura, and runs out in a dangerous race 

 about N.W. for about halfway towards Colonsay, forming- 

 eddies from the island of Scarba on its western side. 



During the time that the stream of flood runs westward 

 through the middle of the Gulf there are counter-streams 



* Physical Geography, p. 63. 



f ' Sailing Directions for the West Coast and Islands of Scotland from 

 the Mull of Cantyre to Cape Wrath,' p. 119. 



