28 THE BIOLOGY OF THE SEA-SHORE 



on its axis the moon is moving in the same direction round 

 the centre of gravity of the earth and the moon, and the 

 earth has consequently to make a Httle more than a complete 

 revolution before a particular meridian is again beneath the 

 moon. 



The differences in height of tide are due to the greater 

 or lesser degree in which the attractions of the sun and 

 moon coincide or are in opposition. With the moon in the 

 first or last quarter, moon and sun form a right angle with 

 the earth's axis and, the attraction of the one tending to 

 neutralise that of the other, neap tides result. With new 

 and full moon, all three bodies are in the same straight 

 line and, the effects of sun and moon coinciding, we get 

 spring tides. The great tidal wave produced in the way 

 described sweeps round the earth with a velocity of 480 to 

 660 miles per hour. The proximity of land reduces the 

 velocity of the oceanic tidal wave, and local interference 

 (due to local coastal configuration) causes calculations to 

 become in many instances a matter of some uncertainty. 



It is important for us to follow the course of the tidal 

 wave in British waters. The wave reaches the British Isles 

 from the west, and on meeting Ireland divides into two. 

 Of these, the southern portion is again divided by the Cornish 

 peninsula, one branch going up the Irish Sea and Bristol 

 Channel, the other travelling up the English Channel. 

 The northern portion (after sending a small offshoot to the 

 Irish Sea) has travelled round the north of Scotland and 

 down the east coast to mingle, in the southern part of the 

 North Sea, with the Channel wave of the next tide, having 

 taken, owing to its longer course, exactly twelve hours 

 more to reach this particular spot than the Channel wave. 

 Reference to the co-tidal chart will make this clear. 



Where the tide can enter a sea from both ends there may 

 be various modifications of the normal tidal rhythm. For 

 instance, along the Hampshire coast there are four tides a 

 day, owing to tidal waves entering the stretch of water 

 between the Isle of Wight and the mainland from both the 

 east and west ends. Similarly one tide may serve to neutralise 



