52 HARBOUR CONSTRUCTION. 



Head, as to abruptly reduce its width by about 40 per cent. 

 The tidal water is thus diverted into the angle or " bight " on the 

 French coast, at the apex of which Avranches is situated ; and 

 the converging coasts impound this water, which is thus made 

 to heap up to the extent named. 



The effect of this sudden narrowing of the channel, and im- 

 pounding of the water, is clearly shown by comparing the tidal 

 range at Dartmouth, the islands of Guernsey (St. Peter's Port), 

 Jersey (St. Heliers), and at Avranches. 



If upon a base-line the respective positions of Dartmouth, 

 Guernsey, Jersey, and Avranches be marked, and the tidal 

 ranges at these several places plotted viz. Dartmouth 14J feet, 

 Guernsey 26 feet, Jersey 31 feet, and Avranches 39 feet a line 

 joining these respective heights will be found to give an almost 

 uniform gradient of 3 inches per mile. The periods of high 

 water at these several places occur within about 20 minutes of 

 each other, and are therefore nearly synchronous. 



Normal Rise of Tides. The observed or compound tides rise 

 and fall respectively twice in each lunar day, one being the 

 positive or upper tide, and the other the negative or under tide. 



The normal rise of tides in the open ocean, at the equator, 

 where it is obviously greatest, has been variously calculated ; but 

 it is generally assumed that the rise of spring tides is from 2 feet 

 to 3 feet, and that of neap tides from 10 inches to 15 inches. 

 Airy calculated (" Tides and Waves," p. 355) that, in the open 

 ocean, the height of the solar tidal wave above the normal water- 

 level would be 0'61 foot, and that of the lunar wave 1*34 foot; 

 thus giving a height of 0'61 + 1'34 = T95 foot at spring tides, 

 and 1'34 - 61 = 073 foot at neap tides. Newton, on the other 

 hand, calculated that spring tides would rise about 6| feet, and 

 neap-tides 2f feet. Observations made on islands in the open 

 ocean seem, however, to endorse the accuracy of Airy's calculation. 



In the Mediterranean Sea except at the extreme western 

 end, where the tides are influenced by those from the Atlantic 

 the range of tide is very small, by reason of the tidal wave being 

 throttled at the Straits of Gibraltar. 



The tidal range, due to the direct tide-producing action of the 

 sun and moon upon the waters of small seas, bays, and estuaries, 

 is so small as to be altogether unworthy of notice. Even in the 

 Mediterranean it is insignificant, only amounting to two or three 

 inches ; nevertheless, this scarcely perceptible range is increased 



