^ AMERICAN INSTITUTE. 573 



throttle and steam ports and pipes; balanced valves, admitting 

 steam by rapid and equally Avide openings, while cutting oif at all 

 parts of the stroke, and at the extreme end of the cylinder; slightly 

 increasing lead; an exhaust port suddenly uncovered and held 

 open throughout the entire stroke, and large exhaust pipes and 

 nozzles. The reasons are evident. A locomotive with bh feet 

 drivers, while running between 11 and 12 miles per hour, by 

 steam which is cut off at one-quarter stroke, holds open the ports 

 only about one-eighth of a second at a time. Since, by the nar- 

 rowness of the opening, steam is reduced at least 12 lbs. below 

 the boiler pressure, at stick speed, what must be its reduction 

 when the engine in attaining the velocity of from 50 to 60 miles 

 per hour, opens its ports less that three-eighths of an inch, and 

 preserves the opening but the fortieth of a second. If the reduc- 

 tion of pressure is proportionate to this reduction of time, steam 

 with an initial force of 90 lbs. is not allowed to begin its work 

 till 40 lbs only are left, wdiile the wasted 50 must be renewed by 

 the boiler. 



Again, if the steam pressure on the valve be due only to 

 the size of the openings, it is at least 7500 lbs. in engines of 

 the larger class. How^ much of this pressure is cancelled by the 

 exhaust steam, at full stroke, may be estimated by the stripping 

 of valve stems, and the springing of rocker arms; how much is 

 relieved when steam is working very expansively, may be ima- 

 gined by listening to the exhaust, which, though it pass through 

 two inch nozzles, is hardly audible. 



Again, by reason of the premature closing of the exhaust port, 

 the maximum back pressure with the link-motion, and without 

 inside lead, is from 40 to 50 lbs. when steam expands through 

 three-quarters of the stroke. 



Each of the common forms of valve apparatus, wastes from 75 

 to 100 cubic inches of steam in the ports, twice every revolution, 

 or perhaps, 80,000 times in a day. 



The fixed independent cut off", necessitates the throttling and 

 strangling of steam, in the varying of speed and load, and both 

 the fixed and the variable, in all cases, increase the length of 

 ports, and often waste a valve box full of steam at each stroke. 



The fulfilment of some of the essential conditions, however, 

 has been approximately accomplished by long double valves, by 

 various balanced valves, by the combination of a steam and a 

 steam and exhaust valve, covering the same ports at different in- 

 tervals, by two valves covering different ports at different inter- 



