130 



" In order, then, that a steam engine should work with a 

 pressure in the cylinder differing from that in the boiler only 

 by the small fraction n of the latter, the velocity of the piston 

 should not exceed the value determined for that particular 

 engine by the equation (3), 



" Whatever the pressures in the boiler and cylinder may 

 be, if the velocity, and therefore the pressures, be uniform, we 

 must have the relation 



^a —j— = ^, ; (4) 



/ « + qP 



which is, in fact, a statement, in algebraic form, that the vo- 

 lume of cylinder open for the reception of steam during each 

 unit of time is equal to the volume of steam under the pres- 

 sure P\ furnished by the quantity S of water evaporated in 

 the same time, and is one of the fundamental equations of 

 De Pamhour's Theory of the Steam Engine, 



" If the engine is working at full pressure, as defined 

 above, we may put P for P in (4), and then 



^' + ^ ^ 

 ^« —J- = 5 5 (5) 



I m + qP ^ ' 



and substituting for P the greatest value (fl), which the 

 boiler of a given engine will bear, we have for the lowest ve- 

 locity at which it can work, without loss of steam, the equa- 

 tion 



I ^c S 



va —— = -. (6) 



For any velocity between the highest limit given by equation 

 (3), and the lowest given by (6), the engine will work at 

 ' full pressure,' and the value of the pressure corresponding 

 will be given by equation (5). 



" The velocity of full pressure,' then, is not ajixed velo- 

 city, but in a given engine has assignable limits ; a higher 

 limit depending on the area of the steam pipe, ami a lower, 

 determined by the strength of the boiler. 



