1S60.] Explanation, of the Bore in the Hooghly. 279 



and the consequent checks and friction which might materially inter- 

 fere with the motion and maintenance of the wave. It may be said, 

 however, on the other hand, that the catastrophe occurred at the 

 season of the year when the river is fullest of water ; and although 

 in 1858, even in August, the river was as low as to be only 25 feet 

 (instead of 50 feet) above winter level, nevertheless there must have 

 been a considerable amount of water in the river before the flood 

 came, sufficient very likely for the generation and propagation of the 

 wave. Here, however, is a ground of uncertainty. But even if it 

 were admitted that some impediment of the kind existed between 

 the broken barrier and Attock, yet the influx of waters would at 

 length rise over the impediment like an ordinary rush of water on a 

 much swollen river, and commence to generate a wave in the river 

 below the impediment, as the influx of the tidal water at the sand- 

 heads produces a tidal wave. 



7. We may understand how the water which the wave had raised 

 just above the impediment would get over the impediment into the 

 part of the river below it, ready to produce another wave by its pres- 

 sure, by observing the breakers of the Bore in the Hooghly . The Bore 

 is simply the flood-tide-wave moving along the river at the springs 

 at which season the influx at the sandheads is greatest. The onward 

 movement of this wave or form at the rate of 24 miles an hour is 

 accompanied (as stated in para. 1) by an upward and onward move- 

 ment of the parts of the water itself in the front of the wave, though 

 at a much smaller rate than that of the form or wave itself. Con- 

 ceive this wave coming suddenly from deep water into shallow. 

 "What will take place at the boundary line between deep and shallow 

 water ? The pressure lifts up the water on the deep side of the 

 boundary line and so forms the front of the great tidal-wave at that 

 spot, and at the same instant gives the water thus lifted up a slight 

 onward motion, which carries it on to the shallow side of the boundary 

 line between the deep and shallow parts. The pressure-action by 

 which the wave should be propagated onwards over the flat is now 

 destroyed ; for the upheaved water thus lifted up over the shallow 

 has nothing but the hard bottom to press down upon, and this 

 unyielding bottom will not communicate the pressure onwards (as it 

 would if it had been itself water) to keep up the formation of a wave 



