AMERICAN INSTITUTE. 613 



the premature release is of very little consequence. The line 

 falls very gently from the point of release, rounding the diagram 

 a little, but showing that the pressure still continues effective 

 until the end of the stroke, and that what is called the last kick 

 of the steam has force, even after the port is considerably opened. 

 The steam cannot escape instantaneously, they say; it has to 

 squeeze out, something as it had to be " rammed in," twenty 

 years ago. 



If we look at the other end of the diagram taken at high speed 

 we see a serpentine line, produced by the momentum of the 

 piston and spring of the indicator, and the reaction of the spring. 

 This vibration is considerable, although the spring is at the time 

 exerting an elastic force that balances a pressure of 60 pounds of 

 steam; but at the time of release the elastic force exerted by it 

 is equal to only 25 or 30 pounds ; and the inertia of the indicator 

 piston and its attachments is therefore far less speedily over- 

 powered. The pressure of steam must fall much faster than the 

 piston of the indicator can be driven after it by a spring so nearly 

 unbent, and the fortieth of a second is not time enough fc»r these 

 ports to be driven down, even if the exhaust were absolutely 

 instantaneous. I think that a careful attention to this point will 

 lead to the conclusion that the indicator should not be so impli- 

 citly relied upon, in these private details, however useful it may 

 be as a general test of the condition of an engine's packing and 

 valve gear. It is safer to calculate, from the known laws that 

 govern steam, what it will do when a port is open half an inch 

 or more; how long it will be in losing its pressure, that is, in 

 escaping. Such a calculation will show that the effect of steam is 

 very slight after the exhaust begins to open, much less than the 

 indicator diagranj leads some to suppose; and it will appear that 

 whatever advantage is gained from the pressure due to the diffi- 

 culty of forcing steam through an exhaust port, with its relatively 

 wide opening, is more than balanced by th e deficiency of pressure 

 due to the difficulty of its getting in through the narrow opening 

 of the steam port. 



The objection to compression is met by the advocates of the 

 link by saying, that the power expended in compressing the steam 

 remains in the steam compressed, and is used in the succeeding 

 stroke, and therefore is not lost, and that it occurs only in the 

 short admissions, when little power is required. This is a fair 

 reply to those who suppose that the power is lost; but it is the 

 work of the boiler to make steam, and of the engine to expend it 



[Am. Inst.] 40 



