THE STEAM-ENGINE. 145 



about Ytfths of that value, or about 41^, mating a total of 

 gross, from which, if we deduct the atmospheric back pressure 

 of 15, we get a work of 86 J, or if we used the whole steam we 

 should have got an effect of 165 per cent, instead of an effect 

 of 100. Diagram 5 shows this further : if we cut off 120 Ibs. 

 above zero of steam, at Jthand employ only 15 parts of steam 

 instead of 120, we get by expansion increments of 10J down 

 to quarter-stroke, 10 ^ again between quarter and half stroke, 

 and 10J again between the half and the bottom, making 46 

 gross, from which we have to deduct the 15 back pressure, 

 leaving 31 as the net work done. Thus ^th of the weight of 

 steam gives 29 1 per cent, of the work that would have been 

 given by the full steam, or if we were to employ full steam 

 we should get 236 of work instead of 100. 



I will now come to the advantage of condensation when not 

 expanding. This is illustrated by Diagrams 6 and 7. Suppose 

 again that we take 120 of steam to work the whole stroke, 

 and that we take non-condensed vapour in the cylinder as 

 equal to 2, we then get 1 1 8 of work done. Now formerly we 

 found that without condensation we should have to deduct 

 the whole atmosphere and get, therefore, only 105 work done. 

 The difference therefore between 105 and 118 is 12 J per cent., 

 the gain one has got by condensation without expansion. 

 That gain of course will become very much greater as the 

 pressures become lower, because with the 60 Ibs. steam, as 

 I showed you, we get only 45 without expansion and con- 

 densation, but we get 58 with condensation, or a gain of 

 28-8 per cent. With 30 Ibs. steam we got only 15, and 

 with condensation we get 28, or a gain of 86 '6 per cent., and 

 with 18 Ibs. steam where we only get 3 non- condensing we 

 now get 16, and, therefore, gain 433 per cent., showing the 

 advantage of condensation applied to a non-expansive engine. 



The last kind of engine we have to consider is the expan- 

 sive condensing engine, and Diagram 8 shows us its advantages. 

 If we use 120 initial pressure, and cut off at a half, the weight 

 of steam used in the cylinder will be 60 ; the useful effect, 

 if non-condensing, would be 86^, and the useful effect con- 

 densing 99 J ; the percentage of gain being 15. If we cut 

 off at a quarter we shall use 30 Ibs. weight of steam, we 

 shall get an effect of 56^ without condensation, or 69 J- with 

 condensation, and the gain will be 23 per cent. If we cut 

 off at Jth we shall use but 15 Ibs. of steam, and we shall 



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