REVIEW OF FEED- WATER TESTS. 267 



steam, this steam would have contained 1% of moisture, cor- 

 responding in round numbers to, say, 20 of superheating, 

 the difference in the quality of the steam in the two cases, 

 expressed as superheating, is about 60. According to this 

 calculation, therefore, the effect of the superheating is to re- 

 duce the feed- water consumption 1/ for a superheating of 

 60, or a trifle over 1% for each 10; and this practically 

 corroborates the evidence furnished by the tests on Engine 

 No. 1. 



The compound engine which shows the highest economy of 

 any in the list, is one which is supplied with superheated steam ; 

 and although this fact may be considered as one reason for 

 the high result, there were other conditions which were favor- 

 able, and the exact effect of the superheating is a matter of 

 conjecture. 



Incidentally, it should be noted that superheating produces 

 a marked effect in the character of the expansion line of the 

 indicator diagram. In Engine No. 1 this is clearly revealed by 

 a comparison of the steam accounted for by the indicator at 

 cut-off and release. In test No. 1 B, where the engine was 

 running with ordinary steam, the proportion accounted for at 

 cut-off is .839, and that at release is .861, which is an increase 

 of .022. On the other hand, on test No. 1 C, where the steam 

 was superheated 82, the proportion accounted for at cut-off 

 was .947, and at release .900, there being a reduction here 

 of .047. This change is evidently due to the reduced con- 

 densation produced by the superheating, and the consequent 

 reduction in the amount of re-evaporation during expansion. 



VI. RELATIVE ECONOMY OF SIMPLE, COMPOUND AND TRIPLE 

 EXPANSION ENGINES. 



In comparing the Economy of a compound or other multiple 

 expansion engine with that of a simple engine, the question may 

 be raised, What should be the conditions of the comparison ? 

 One method of comparing the two would be to select those run- 

 ning under the same boiler pressure and quality of steam, and 



