Proceedings of the Polytechnic Association. 1009 



sure, but not with a slide valve ; a balanced puppet valve must be 

 \ised. There is no difficulty in making an engine work at that pres- 

 sure; but he believed that from ten to twelve pounds of coal per 

 horse-power would be used, 



Mr. Charles E. Emery said that about two years ago he had occa- 

 sion to test this question of expansion. In his experiments he found 

 that the greatest loss in the steam engine is the back pressure. 

 There are fifteen pounds of air pressure to be overcome, and, if six- 

 teen pounds of steam are used, we have but one pound to w^ork over 

 the atmosphere. He had prepared a little table, which he found was 

 carried out in practice. There is little difficulty in determining 

 the steam pressures which will practically furnish the power most 

 economically in a non-condensing steam engine. In all engines 

 about nine-tenths of the heat passes away with the exhaust steam ; 

 but, in addition to this great loss, there are others, among which is 

 cliiefly the loss of power required to displace the back pressure. In. 

 non-condensing steam engines the back pressure is in round numbers 

 fifteen pounds above that of the atmosphere, or thirty pounds total 

 pressure ; one-half of the total power is required to overcome the 

 back pressure, and one-half only is utilized. By multiplying these 

 illustrations we are likely to jump at the conclusion that the higher 

 the steam pressure the greater the economy, for the reason that the 

 back pressure becomes comparatively of less value. The table spoken 

 of is strictly correct only in reference to engines using steam without 

 expansion. When the steam is used expansively, the result, so far as 

 the back pressure alone is concerned, is less favorable to the high 

 pressure than is shown ; but, in such case, some economy is pro- 

 duced by the expansion, which must be separately considered. 



[Ikst.] 64 



