194 OF THE CONSTRUCTION OF [SECT. vi. 



and (by art. 190.) 75 x 8*22 = 616*5 Ibs. of caking coal, 1 or 5 = 9-8 Ibs. 



per hour, for each horse power. 



410. The application of the single engine is limited by the nature of its 

 action, to raising water or other works admitting of an inefficient returning stroke, 

 but for these purposes it has great advantages. I would suggest, as an improve- 

 ment, that the condensation should be effected as described for the atmospheric 

 engine, (art. 400.) and that it should always act more or less by expansion : the full 

 effect of expansion cannot however be obtained, unless the action be equalized by 

 a proper arrangement of the pressures and counter-weight. 



411. Single engine acting expansively. When the single engine acts expan- 

 sively, it is necessary to determine the point of the stroke at which the steam 

 should be cut off. Now the pressure on the piston should never be less than the 

 mean moving force, otherwise it would be overpowered, and the column of water 

 would descend again ; consequently we may adopt this analogy. As the whole 

 force of the steam in the boiler is to unity, so is half the greatest effective force on 

 the piston added to the resistance from friction, &c. to the portion of the stroke at 

 which the steam should be cut off. Thus, if the force in the boiler be 35 inches 

 of mercury, and the resistance of the uncondensed steam 3'7 inches, then 3'7 + 

 35 x '402 = 17'77 inches, the loss of power from friction, &c. (art. 408.) and 



OK _ T7'77 



consequently ^ g - + 17-77 = 26-38, the pressure on the piston at the end of 



the stroke ; therefore 35 : 1 : : 26'38 : -75 = f of the stroke. The steam will 



obviously act expansively in its ascent in the same proportion ; whence a less 

 counter-weight is necessary. 



412. To find the mean pressure on the piston in an expansive engine. Let 



the portion of the stroke made, when the steam is cut off, be > 



Then the th part of the whole force in Ibs. per circular inch, of the steam in the 

 boiler, multiplied by 2-3 times the common logarithm of n, added to -3, is 

 the mean moving force or pressure ; which is to be used in the rule (art. 409.) for 

 finding the power, and also for adjusting the load. 



Example. Suppose the steam to be cut off at three-fourths of the stroke, 

 then - - = I, or n = 1-33, and its logarithm = 0-12516; the whole force being 

 35 inches of mercury, or 13'5 Ibs. per circular inch, we have, 



1 This is equivalent to raising about 17,000,000 Ibs. 1 foot by a bushel of coals, or 192,000 Ibs. 

 by 1 Ib. of coal. 



