ENGINEERING STRUCTURES 149 



(2) Jet played into vertical pipe. 



The diminution in the magnitude of the maximum pressures 

 given in these tables, as the length of the passage is increased, 

 is undoubtedly due partly to the reduction in the velocity of 

 impact which is produced by the greater resistance to flow 

 and the greater effect of the entrapped air in the longer 

 passages, and partly to the fact that while the multiplying 

 factor has been deduced on the assumption that the length I 

 of the impinging column is identical with that of the passage, 

 this can only be approximately true in the shortest of the 

 passages. Unfortunately the true length I cannot be ascer- 

 tained with any degree of accuracy, and while it is certain 

 that the maximum pressures attained are, except in the 

 shortest of the passages, greater than are given above, no 

 very close approximation to their actual value can be obtained. 

 Under favourable circumstances they may, however, be 

 expected to approach the maximum values obtained for the 

 shortest passages, and for safety such values should be 

 considered possible. 



It is evident, moreover, that with values of I less than the 

 least of those experimented upon, the effect of the entrapped 

 air would become increasingly small, and the results appear 

 to show that with an infinitely small value of I the pressure 

 would ultimately approximate to the value corresponding to 

 sudden stoppage of a column moving with the velocity of 

 impact. In these experiments this would be 87,500 Ibs. per 

 square foot, or approximately six times the pressure obtained 

 when Us 1 inch. 



