PEOGRESS OF TIDAL WAVES. 105 



The sinuous line whicli unites all tlie points in tlie ocean where 

 the full tide occurs exactly at the same hour, has received from 

 Whewell the name of cotidal line ; it indicates the curve which the 

 crest of the tidal wave forms at any one moment on the surface of 

 the sea. It is around the British Isles that these lines of simul- 

 taneous swelling or of equal establishment^ have been most care- 

 fully traced. By calculation and direct observation, that part of the 

 oscillation on the mobile and almost always agitated surface of the 

 sea, which is to be referred to the phenomena of ebb and flow, has 

 been detected ; and much more exact maps of these swellings and 

 depressions, which are invisible on the open sea, have been drawn, 

 than of the vast continental regions which are at present but little 

 known. Thanks to the labours of Whewell, Airy, Lubbock, and 

 Beechy, one can now follow the whole series of cotidal lines which 

 succeed one another from hour to hour around these two great 

 islands, from the crest coming in from the open sea, at the entrance 

 of the English Channel and the Irish Sea^ 4 hours after the passage 

 of the moon over the meridian, to the swelling, which 19 hours 

 later reaches to the south of the German Ocean and penetrates into 

 the funnel of the Straits of Dover, where it meets the other tidal 

 wave coming directly by the Channel. The general form of these 

 curves demonstrates in a striking manner that the speed of pro- 

 pagation of the tide is in proportion to the depth of the seas. Every- 

 where we see the cotidal lines develope their convex part above the 

 deeper valleys of the marine bed ; everywhere we see the wave 

 slacken its speed in the neighbourhood of shallow rocks and shores. 

 One could even by an inspection of these lines of equal intumescence 

 indicate exactly those parts where the lead would descend lowest ; 

 so intimate is the connection of cause and efiect between the depth 

 of the sea and the progress of the tide. 



