1 8 Foster, Phenomena due to Repetitions of Stress. 



in order, as suggested by Dr. Lees, to get rid of any 

 high frequency harmonics a first approximation to the 

 shape of the ports can be made by calculation, but the 

 final adjustment would have to be made by experiment. 

 The forms so obtained would vary with the conditions of 

 working. 



It may be objected that, 



First, the pressures in the cylinder will vary very 



little owing to the high frequency and consequent 



throttling of the steam, 

 and 



Second, the pressures will not be applied steadily, that 



is, they will be equivalent to instantaneously 



applied loads. 



The first objection is not very serious as it is easily 

 possible to make the ratio of port area to cylinder volume 

 large, and moreover there are no tortuous steam passages ; 

 also it should be remembered that in high speed engines, 

 making, say, i,ooo strokes per minute, the clearance space 

 amounting to about 20 per cent, of the working cylinder 

 volume is filled with steam at the maximum pressure in 

 a very small fraction of the stroke. There will, of course, 

 be some throttling, especially just as the ports open, so 

 that the pressure will not come on the piston instanta- 

 neously, that is, not more so than any load applied at such 

 high frequencies would. Any dynamic action due to this 

 high frequency of loading would be common to all 

 methods of loading and therefore quite within the field of 

 the research. If it is thought necessary the ports can be 

 so shaped as to throttle the steam during the opening of 

 the ports and gradually to increase the port opening to a 

 maximum just before closing the ports. This would 

 ensure a gradual change of load. 



The limits of pressure can be varied by throttling the 



