The Tides 



79 



It must, however, be stated beforehand that your almanack 

 or a local tide-table will show you that high tide by no means 

 exactly coincides with the arrival of the moon on the meridian. 

 In preparing these tables astronomical data can furnish the 

 time at which on each day the moon will cross the meridian, 

 and also the position of the sun at that time relative to the 

 earth and moon. There are, however, other conditions — such 

 as the conformation of the coast line and the depth and 

 character of the bed of the sea, round and over which the 

 tide travels — which must be taken into account before any 

 correct calculation can be accomplished. But — to say nothing 

 of their intricacy — many of these conditions are beyond our 

 ken, and cannot, therefore, be brought into calculation. The 

 only resource left is to note empirically, on the spot, what 

 interval actually elapses between the occurrence of the moon 

 on the meridian and the arrival of high tide. The difference 

 thus ascertained is then allowed for in the local table, and this 

 allowance is called the " Establishment of the Port." You 

 may by observation " establish the port " for the place where 

 you chance to be staying. 



We are writing of Tides as they may be observed on the 

 sea coast. If we pass up an estuary such as that of the 

 Humber, which gradually narrows to a river, certain important 

 differences will be noted and these become greater if we 

 ascend the rivers themselves. On the open coast the tide 

 rises slowly and it needs close looking to tell when its rise 

 begins. This is because there are no impediments. The banks 

 of the estuary or river by narrowing the stream constitute an 

 impediment which increases the higher you go. Thus the 

 beginning of the tidal flow is retarded until the force of the 

 risen ocean having accumulated drives up the water with con- 

 siderable and sudden impetus. Thus the beginning of the rise 

 of the tide at Cleethorpes, Grimsby or Spurn Point, will be 

 very gradual, but at Goole or Selby sudden and violent. At 

 these places a sort of wave, or wall of water, rushes up the 

 river which may break ships from their moorings or even 



