ENGINEERING STRUCTURES 147 



the phenomenon was due to air compression alone. If more 

 gradually applied, so as to facilitate displacement of air by 

 the incoming water, a very different state of affairs was 

 indicated. Fig. 6, b, shows a diagram obtained under such 



Times 



pressure 

 equivalent 



u 8 



to - feet 

 20 



of water 



Times 



FIG. 6 



circumstances. The initial rise in pressure is no longer gradual 

 but is as instantaneous as that accompanying a sudden valve 

 closure, and this is followed by rapid negative and positive 

 alternations of pressure, substantially of the form indicated 

 by theory, on the assumption of hammer action. The experi- 



