960 Transavtioxs of the American Institute. 



horse power C(5st three pounds of coal per hour, under these 

 circumstances, it will cost a little loss tlian five pounds in an engine 

 using 100 pounds of steam, without expansion, while it will cost 

 about six and one-half pounds in an engine using only twenty-five 

 pounds of steam, Avitli the most favorable degree of expansion. (Mr. 

 Emery here illustrated Ins remarks by diagrams on the blackboard).' 

 In condensing engines the differences in economy for the various 

 steam pressures are not so great, but the fact is equally true that 

 economy is due as much to the higher steam pressure as to the expan- 

 sion. The reason of this arises, in a great measure, to the fact that 

 the back pressure forms a smaller percentage of a high steam pressure 

 than it does of, a low one ; but another reason is that a high tempera- 

 ture is necessary in order to prevent the steam from liquifying, or 

 becoming greatly condensed during expansion. It is now determined, 

 beyond dispute, that the great enemy to the expansion of steam is 

 the internal condensation in the cylinders. Steam may enter the 

 cylinder at 300* and leave it at 212*^, or even less, in a condensing 

 engine. The metal walls of the cylinder during the exhaust stroke 

 are cooled to the temperature last named, and of course, until 

 reheated, form surface condensers for the live steam which flows in 

 upon them, during the return stroke. The live steam has therefore 

 two offices to perform, first that of heating, the surrounding walls, 

 and second that of keeping up tlie pressure, and performing mechan- 

 ical work. Now,' in some cases, it takes as much steam to do the ■ 

 one thing as the other, and as the steam used for heating is con- 

 densed, it helps very little to fill the cylinder and the engine was in 

 fact two cylinders full of steam wliere the volume of one only is 

 required. The condensation is rarely as extreme as this, but it 

 always has more or less influence. This condensation must be separ- 

 ated entirely from tliat which is due to external refrigeration. The 

 internal refrigeration is not so great in non-condensing engines, but 

 they are subject to the greater loss due to the high back pressure at 

 all steam pressures below 100 pounds. The internal condensation , 

 can be lessened in various ways, as, for instance, by steam jacketing, 

 superheating the steam and using compound engines. 



In reply to tlie remarks of tlie last speaker, I will say that in the 

 best steam engines, the largest proportion of the heat must go off 

 with the exhaust steam, in order to keep it in the state of steam ; 

 that is, it must contain its latent heat, which is the principal portion. 

 For this reason, the best steam engines now in use, utilize but one-niuth 



