I860.] 



Inundation of the Indus in 1858. 



275 



* 





1. In fig. 1 suppose that A E 

 is a surface of still-water, in a 

 canal closed at one end and ex- 

 tending indefinitely to the left. 

 P is a gigantic plug, supposed to 

 be thrust down vertically into 

 the water. As the plug descends, 

 pressure will be continually com- 

 municated through the water so 

 as to lift up the surface of the 

 water in the canal. As the plug 

 descends successively to a, b, c, 

 d, e (omitted by the engraver) 

 the surface will be raised up 

 into the curves at A, B, C, D, E. 

 The greatest rise at any instant 

 will be close to the plug, where 

 the pressure has been acting 

 longest ; and the elevation of the 

 surface in each curve will be less 

 and less in passing down the 

 canal, because the pressure has 

 been acting for a shorter and 

 shorter time. At the instant the 

 plug reaches the bottom, the sur- 

 face will have been elevated into 

 half a convex wave L E, its 

 length depending upon the rapi- 

 dity with which the pressure 

 has been communicated. The 

 amount of water in this elevated 

 half- wave will be equal to the 

 volume of water displaced by the 

 plug. It is evident, that during 

 the formation of this half-wave 

 the several particles of water 

 beneath its surface have received 

 a slight upward and forward mo- 



