258 ENGINE TESTS. 



It will be noticed in the table giving the quantities upon 

 which the chart of cylinder condensation and leakage is based, 

 that no distinction is made between engines which are condens- 

 ing and those running non-condensing. It is probable that the 

 transfer of heat from the steam to the metal of the cylinder, 

 under the action of the comparatively low temperature of the 

 condenser, is different from that which occurs in the non-con- 

 densing engine ; and if a suitable investigation were made, this 

 difference would appear in the percentage of . cylinder conden- 

 sation. Whatever this difference may be, it is not sufficiently 

 marked to be noticeable in the tests referred to in the chart; 

 and consequently the results are used indiscriminately, whether 

 the engines are condensing or non-condensing. 



II. EFFECT OF PRESSURE ON THE ECONOMY. 



Other things being the same, it is a well recognized princi- 

 ple in steam-engineering that the higher the pressure the more 

 economical the consumption of steam. But circumstances 

 attending its use are not always the same ; and consequently, 

 in examining the results obtained from different engines, such 

 as those here reported, it does not follow that in any individ- 

 ual case where the pressure is highest the economy is necessarily 

 the greatest. For example, two tests are reported on Engine 

 No. 18, which show practically the same economy as measured 

 by the feed-water consumed per I. H. P. per hour ; yet the 

 pressure in one case is 84 Ibs., and in the other case 59 Ibs. It 

 is evident that the difference in the cutoff which accompanied 

 the change of pressure exerted such an influence that the 

 benefit which might have been derived from a higher pressure 

 was counterbalanced. 



There are two instances among the simple engines which 

 may be examined to show the importance due to increased 

 pressure. Test No. 1 A and test No. 2 is one of these in- 

 stances. Here an increase of the pressure from 72.3 Ibs. to 

 101 Ibs., accompanied by a slight shortening of the cut-off, had 

 a marked effect in improving the economy, the consumption of 



