30 THE BIOLOGY OF THE SEA-SHORE 



heap up or depress it many feet, especially if of long con- 

 tinuance. In the North Sea the maximum effect is produced 

 by a N.N.W. wind, and it is recorded that such a gale in 

 1905 caused an extra rise on the Suffolk coast of 6 feet 

 3 inches. Water is warmer when the wind blows toward 

 land, and the reverse also holds good ; on-shore winds pile 

 the warmer surface waters up against the coast, and the 

 colder bottom waters escape seaward (Johnson, op. cit.). 



According to Johnson (19 19) the tides may best be 

 considered as great waves which combine some of the 

 features of both oscillatory waves and waves of translation 

 (see " Wave Action," p 32). There is a shoreward move- 

 ment of the water particles until the time of high tide, 

 after which a seaward movement takes place. In water 

 30 metres in depth a tidal rise of 3 metres should result in 

 currents of 17 knots ; with 4*5 to 6 metres it would attain 

 over 3 knots. In the Channel between Scilly and Hastings 

 the tidal velocity is 2 miles an hour ; in the N. Wash, 4 ; 

 off Ushant, 6 to 7 ; through Petite Passage, S.W. of Digley 

 Gut, Nova Scotia, not less than 8. In the Severn estuary 

 the speed is 6-12 miles (according to Sollas), and Kurmmel 

 cites 8-10 between Orkney and Shetland, 11 in the dreaded 

 " Roost " of Pentland Skerries. The transporting and 

 eroding power of such currents is enormous. A velocity 

 of but "4 knot will drive ordinary sand along the bottom, 

 while fine gravel will be moved at i knot ; shingle about 

 I inch in diameter at 2*5 knots, and angular stones i| inches 

 in diameter at 3*5 knots. As the currents continue for many 

 miles in the same direction, they must play a very important 

 role in the transportation of shore debris. Tidal currents 

 do not always, or even commonly, act in a direction which 

 the lie of the shore-line would indicate as normal. Along 

 the sides of a bay or headland whose axis is in the line of tidal 

 advance, the currents may be parallel to the shore. Shore- 

 line irregularities will deflect the tidal waters, giving longshore 

 currents in all possible directions. It is also a well-known fact 

 that a narrowing bay compresses a tidal wave into smaller 

 space, and constrains it to rise higher. " In bays and sounds 



