190 OF THE CONSTRUCTION OF [SECT. vi. 



and during the first descent the cock F should be open, but afterwards closed : the 

 injection being stopped, and the slide B moved to close the passage to the con- 

 denser, on opening that for the steam the pistons will again ascend, and the air 

 and water of condensation will be expelled at the valve Q : the alternate opening 

 and closing of the passages and the injection cock are required to continue the 

 action. 



401. This engine may be regulated by closing the valve B at any period of 

 the ascent, and the cock I at any period of the descent ; and as the application will 

 be limited to raising water, the velocity in feet per minute should be 98 times ' the 

 square root of the length of the stroke, (art. 342.) ; the length of the cylinder 

 twice its diameter ; the area of the steam passages to the area of the cylinder, as 

 the velocity in feet per minute is to 4800, (art. 154.) The air pump should be 

 one-fourteenth of the capacity of the cylinder, (art. 349. note) ; or making the 

 stroke of the air pump half that of the steam piston, the diameter of the pump 

 should be three-eighths of the diameter of the cylinder. The quantity of steam is 

 found by multiplying the area of the cylinder in feet, by half the velocity in feet, 

 with the addition of one-fifth for loss by cooling and waste (art. 161.) : the result 

 divided by 1480 will give the quantity of water per minute required to supply the 

 boiler, and 24 times that quantity will be that required for injection, (art. 284.) 

 The diameter of the aperture for the injection should be one thirty-sixth of the 

 diameter of the cylinder, and the injection pipe one-ninth. 



402. The power of this atmospheric engine will be the difference between the 

 pressure of the atmosphere on the piston, and the retarding force multiplied by half 

 the velocity. 



The pressure of the atmosphere being - 1 '000 

 The retarding forces are, 



1. The resistance of the uncondensed steam, temperature 125, = '134 



2. The force to expel it through the passages, (art. 154.) - -007 



3. The loss by cooling in the cylinder, &c. (art. 161.) - '067 



4. The friction of the piston, (art. 474.) '050 



5. The force required to open the valve, raise water for injection, 



and overcome the friction of the axes - '100 



6. The force required to work the air pump, (art. 354.) - '100 



Sum of the retarding forces - 0'45S 



Effective pressure in parts of the atmosphere - 0*542 



1 For ordinary pumping engines the velocity should not exceed 40 times the square root of the 

 length of the stroke. (See note p. 167.) ED. 



