124 A TEXTBOOK OF OCEANOGRAPHY 



from the North Sea, with a resulting backing up of the 

 water. 



There are minor peculiarities, such as prolonged high- 

 water at Havre (for three hours), or prolonged low-water at 

 Portland (for four hours, locally known as the Gulder), but 

 these are due to the bottling up of the water in a comparatively 

 large basin and its difficulty of escaping through a narrow 

 passage. 



In the Irish Sea the tidal wave as it approaches develops 

 two heads to its convex front, one for the Bristol Channel and 

 the other for the Irish Sea. 



The first wave reaches Lundy at 5.30 G.M.T. on full and 

 change days, and passes up the Bristol Channel, being to a 

 certain extent reflected from one side to another, so gradually 

 adding to the height of the crest, which reaches 42 feet at 

 Portishead at springs. 



The second wave reaches Carnsore and the Smalls Light- 

 house at 6 hours 25 minutes G.M.T. on full and change days, 

 with a crest of 9 feet at springs at Carnsore, and 21 feet at the 

 Smalls. 



The Irish Sea being open to the northward, this same 

 tidal undulation enters through the North Channel between 

 Rathlin Island and Cantyre, also at 6 hours 25 minutes, thus 

 making it high-water simultaneously at each opening of the 

 Irish Sea, while at the same time it is low-water, or nearly so, 

 in the Dee, Mersey, Ribble, Morecambe Bay, and Solway 

 Firth. 



The southern undulation, running up St. George's Channel, 

 causes high-water all along the west coasts of Wales, England, 

 and Scotland, as far as the Mull of Galloway. 



To the westward of the British Isles, in the deep-water 

 channel between them and Rockall, the tidal undulation 

 assumes the shape of waves caused by a steamer travelling 

 at great speed, with bow and lateral waves. These lateral 

 waves on the east side become eventually almost parallel to 

 the coast, so that it is high-water on the west coasts of Ireland 



