198 OF THE CONSTRUCTION OF [SECT. vi. 



419. Double engine acting expansively. The motion of a double engine acting 

 expansively ought to be equalized by a fly or some other method, (see Sect, vui.) 

 otherwise the effect cannot be perfectly obtained. To determine the point of the 

 stroke at which the steam should be cut off, we have this proportion : 



As the whole force of the steam in the boiler is to 1, so is "368 times that force, 

 (art. 416.) added to the resistance of the uncondensed steam, to the part of the 

 stroke to be made before the steam be cut off. 



Thus, if the force in the boiler be 35 inches of mercury, and the resistance of 

 the uncondensed vapour 3 '7 inches, we have 



35 : (35 x -368) + 3-7 : : 1 : -473= -L o f the stroke. 



30*1 



420. To find the mean pressure on the piston of an expansive engine, the 

 part of the stroke at which the steam is cut off being .divide 2-3 times the 



common logarithm of n by n, and multiply the quotient by the whole force of the 

 steam in the boiler in Ibs. per circular inch ; the result will be the mean moving- 

 force on the piston per circular inch. 



Example. Suppose the steam to be cut off at ^ of the stroke, then n = 2'l, 

 and the logarithm of 2-1 is '32222 ; 



,, 2-3 x -32222 

 consequently, jrrr = ' 



and as the pressure corresponding to this point of cutting off the steam is 35 inches, 

 or 13'5 Ibs. per circular inch, we have 13'5 x '354 = 4-8 Ibs. per circular inch, 

 the mean pressure. 



421. The velocity should be found by art. 336 or 343. and the quantity of 



steam will be - - part of that required when the engine works at full pressure ; 

 therefore the water for steam, the fuel, injection water, will be less in the same 

 proportion in regard to the dimensions of the cylinder ; but the passages, pumps, 

 boiler, and other proportions should be found by the rules in art. 415. in order 

 that the engine may work either at full pressure or expansively, as circumstances 

 may render desirable. 



422. Taking the dimensions and force of steam of the engine given as an 

 example in art. 417. its power as an expansive engine would be 4'8 x 24 2 x 215 

 = 594432 Ibs. raised 1 foot high per minute, or 



594432 

 33Q0Q- =18 horse power. 



