ENGINEERING STRUCTURES 141 



square foot, 1 and as the velocity of propagation of pressure 

 waves through such a column of fluid is given very approxi- 



yif'f, 

 -^rf-i this velocity, for the experimental pipe 



line, is 3400 feet per second. 



Although valve closure was not instantaneous, it may 

 readily be shown that if closure is complete before the dis- 

 turbance produced reaches the open end of the pipe, i.e. if the 

 time is less than 1 4- V p or ^ second, the pressures produced 

 are the same as would accompany instantaneous stoppage. 

 The fact that up to a discharge of 25*7 Ibs. per minute 

 (v='60 f.s.) the pressure is equal to 48 v Ibs. per square inch 

 for all velocities, shows that up to this point the time of 

 closure was less than this, and indicates that for this discharge 

 and corresponding valve opening, the time was approximately 

 ^ second. 



Impact of a moving on a stationary column of water When 

 a column of water which is confined laterally, impinges on a 

 stationary column of the same fluid, the magnitude of the 

 hammer pressure may be shown to be one-half that attending 

 sudden stoppage by means of a valve. On playing a jet into 

 the open end of pipe ' C ' Fig. 2, except for the effect of the 

 entrapped air a sudden rise in pressure, of magnitude 24 v Ibs. 

 per square inch, would be attained throughout both the 

 moving and the stationary columns. This pressure is indepen- 

 dent of the length of either of these columns. Assume, as 

 will be commonly the case in a sea-wall, that the length of the 

 stationary column of water filling a cavity which may extend 

 for some considerable distance in the interior of the mass, is 

 greater than that of the impinging column, which is largely 

 governed by the length of the crevice piercing the face. As 

 in the experimental coil, a mass of water hurled at the open 

 end forms a column which will, except as modified by the 



1 Water-Hammer, ante cit., p. 16. 



