280 The Bore in the llooghhj. [No. 3, 



ahead. Hence the water, lifted upon the shallow bottom by the 

 action of the wave moving up to the boundary line, will move on 

 over the shallow with its own proper onward motion already acquired, 

 increased by the action of gravity upon the unsupported front of the 

 mass which has found its way, as described, upon the shallow. The 

 water thus heaved up by the wave from the deep side is, so to speak, 

 poured out upon the shallow, and it rushes along over the flat in a 

 running torrent of breakers, till it covers it over with water to the 

 level of the rest of that part of the river now swollen by the flood 

 which is come in. 



The violence of this process will depend very much upon the form 

 of the bottom of the river, and the degree of abruptness of the 

 transition from deep water to shallow. If this transition is gradual, 

 the advancing wave will be reduced gradually by the increasing 

 friction of the bottom ; and the resisting pressure caused by the 

 bottom (as it inclines up and so faces the wave) will reduce the action, 

 and when the wave does break, if it break at all, it will do so 

 feebly, like ordinary waves on the sea-shore. If, however, the 

 transition be abrupt from deep water into shallow, the action will 

 be as described above in explaining the Bore. This description 

 will show why the phenomenon is so much more sensible when 

 the Hooghly is full of water, in the freshes, than in the dry season. 

 In the dry season the river lies down in the deep channel, and when 

 the accession of water at the spring tides lifts it up, the highest 

 part only of the tidal-wave rises above the flats or shallows, and 

 runs on them, therefore, without violence. But when the river is 

 full, the general level is raised higher than in the dry season and the 

 flood-wave at the springs is bodily raised up above the level of the 

 flats and falls upon them, and rushes over them with a correspond- 

 ingly greater violence. 



This digression about the Bore will serve to illustrate the action 

 of the wave in the Indus when it reaches an impediment stretch- 

 ing across its breadth, such as a fordable shallow, or a rapid 

 caused by broken rocks on a descent. The wave will break, and rush 

 over the impediment (aided in this case by the downward current of 

 the stream) in a torrent of breakers, and the mass of waters, on 

 arriving at the deeper water below the impediment, will again form 



