40 PHYSICAL GEOGRAPHY OF THE SEA. 



of the moon, that we must look for the " chief cradle of the 

 tides. 1 " From this starting point they flow on all sides to the 

 northward, progressing like any other wave that arises on a 

 small scale in a pond from a gust of wind, the throwing of 

 a stone, or any other cause capable of producing an undulating 

 movement on the surface of the waters. 



The tide-wave, which ultimately reaches our shores, arrives 

 at the Cape of Good Hope thirteen hours after it has left 

 Van Diemen's Land, and thence rolls onward in fourteen or 

 fifteen hours to the coasts of Spain, France, and Ireland. It 

 penetrates into the North Sea by two different ways. One of 

 its ramifications turns round Scotland and thence flows onwards 

 to the south, taking nineteen or twenty hours for the passage 

 from Galway to the mouth of the Thames. A tide-wave, for 

 instance, which appears at five in the afternoon on the west 

 coast of Ireland, arrives at eight near the Shetland Islands, 

 reaches Aberdeen at midnight, Hull at five in the morning, and 

 Margate at noon. 



The other ramification of the same tide-wave, taking the 

 shorter route through the Channel, had meanwhile preceded 

 it by twelve hours, having reached Brest about five o'clock of 

 the afternoon (at the same time that the northern branch 

 appeared at Galway), Cherbourg at seven, Brighton at nine, 

 Calais at eleven, and the mouth of the Thames at midnight. 



Thus, in this southern comer of the North Sea, two tide- 

 waves unite that belong to two successive floods; the Scotch 

 branch having started twelve hours sooner from the great 

 Southern Ocean than the Channel branch, which thus results 

 from the next following tide. The meeting of the two branches 

 naturally gives rise to a more considerable rising of the waters, 

 so that this circumstance, by allowing large ships to sail up 

 the Thames, may be considered as one of the fundamental 

 causes of the grandeur of London. 



In other parte of the North Sea, where the two tide-waves 

 appear at different times, the contrary takes place, for the 

 ebb of the one coinciding with the rising of the other, they 

 naturally weaken or even neutralise each other. This occasions 

 the low tides on the coast of Jutland, in Denmark, where they 

 are scarcely higher than in the Mediterranean, and explains 

 the otherwise startling fact of there being a space in the North 



