40 PUYSICAL GEOGRAPITY OF TIE SEA. 
of the moon, that we must look for the “ chief cradle of the 
tides.” 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 corner 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 parts 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 
