108 THE OCEAN 



following the movement of the sun and moon 

 from east to west. The whole phenomenon 

 is further complicated by the discovery in the 

 ocean of standing and boundary-waves, similar 

 to the " seiches " which have long been known 

 in the fresh-water lakes of Switzerland and 

 Scotland. As will be seen in succeeding para- 

 graphs attempts have been made to explain 

 tidal phenomena from the analogy of seiches. 

 The rise and fall of the tide is accompanied 

 by currents, especially where obstructions are 

 encountered, and the height to which the tide 

 may rise is determined by the configuration 

 of the land ; the high tides of the Severn and 

 of the Bay of Fundy may be mentioned in this 

 connection. Tidal currents prevent the for- 

 mation of muddy deposits on the top of the 

 Wyville Thomson Ridge at a depth of 250 

 fathoms. J. Y. Buchanan measured tidal cur- 

 rents on the Dacia Bank, and R. N. Wolfenden 

 discovered similar tidal currents on the Gettys- 

 burg Bank. More recently the " Michael 

 Sars " Expedition of 1910 measured tidal 

 currents in the open ocean down to a depth of 

 400 fathoms. It was found that the currents 

 at 274 fathoms ran in the opposite direction 

 to that of the upper layers, which again ap- 

 proached that of the currents at much greater 

 depths. At certain moments the currents 

 appear to be arranged in the form of a spiral 



