136 WAVE IMPACT ON 



the case, the latter is sufficiently porous to allow of the escape 

 of air under pressure, a further and indefinite reduction of 

 pressure will follow. 



Possibilities of water-hammer action If, during the advance 

 of the impinging column, there should be any direct impact of 

 water on water in a confined space, of the nature indicated in 

 Fig. 1, c, the possibilities of water-hammer action become 

 obvious, and the authors' experiments have been mainly 

 devoted to ascertaining whether any such action takes place, 

 and, if so, the magnitude of the resultant effects. 



As has been previously pointed out, experiments on the 

 impact of jets on plane surfaces pierced with orifices com- 

 municating with the pressure-measuring device, show a sudden 

 rise in pressure of magnitude v 2 + 2g feet, with no hammer 

 action. 



Further apparent confirmation of this is afforded by the 

 results of the first experiments made by the authors. In 

 these, a closed cylindrical cast-iron box, six inches in internal 

 diameter and six inches deep, was provided with a cast-iron 

 cover pierced with a series of orifices respectively J, J, f- , J, f , 

 |, and 1 in. diameter. An indicator cock mounted on the 

 side of the box, carried a M'Innes Dobbie steam-engine in- 

 dicator with a ^ spring. All the orifices but one being plugged, 

 the box was filled with water and the jet from a 1-in. diameter 

 pipe, discharging with velocities up to approximately 20 feet, 

 per second, was suddenly directed on to this orifice. In no 

 case was any appreciable vibration of the indicator pencil, 

 such as would indicate an action of the nature of water- 

 hammer, noticed, the pressure rising suddenly to a value 

 slightly less than that corresponding to the velocity of impact 

 (v z -r 2g feet of water). 



A little consideration, however, shows that under circum- 

 stances such as obtained in these experiments and in those 

 previously noted, the absence of any indication of water- 

 hammer action by the pressure recorder does not necessarily 



