1860.] WJiy the Indus-flood was probably caused by a wave. 281 



a wave by the pressure-action, though not so large as the previous 

 one, because some forcé will have been destroyed by impact and 

 friction. 



8. If the barrier, causing the accumulation of waters, occurred 

 on the main-stream, it might be objected, that, owing to the long 

 stoppage of the supply, there could not have been water enough 

 below the barrier for the descending mass to impinge upon and 

 produce the wave. In this case the mass would rush down the dry 

 or almost dry channel, and as soon as it carne to a part of the river 

 where (from its tributaries) the depth of water was sufficient, the 

 sudden influx of the flood would by its weight press downwards and 

 cause the wave to spring np ahead and run down the stream as 

 already described, exactly as the tidal-wave is formed. 



9. The reasons which favour the hypothesis of the wave-explan- 

 ation are these : 



(1.) Captain Henderson, who appears to have been the only 

 European who observed the disturbance of the river, inclines to a 

 velocity which accords more with the notion of a wave of water 

 than with that of the water itself rushing down at such a speed : see 

 Journal, 1859, p. 207. 



(2.) In his account he says (p. 208) " at first it [the water] carne 

 welling up quietly, but very rapidly." This looks much more like 

 the uplifting of the surface by a pressure from below, than the rush 

 of water down the river. 



(3.) He tells us in his account (p. 208) that four hours after 

 the rise began, and three hours and a half before the máximum rise 

 was attained, he crossed the river in a boat. This he hardly could 

 have done had the waters of the swollen river been moving down 

 bodily at the wave's velocity. 



10. Mr. Obbard in his paper attributes the low state of the river 

 at Attock before the flood carne, to the hollow which precedes a wave, 

 like the tidal-wave in the Hooghly, and he takes the existence of this 

 depression to be an argument in favour of his explanation. But this 

 would rather appear to have arisen from the stoppage of the full 

 supply of water in consequence of the dam being formed : and it is 

 evident that there was no cause producing a hollow in the process 

 explained above by which the wave was generatecl. 



2 p 



