Polytechnic Association. 859 



here referred to is known among engineers as the Binary vapor 

 engine. In it the heat of the water from the condenser of the steam 

 engine is made to evaporate some very volatile liquid, as ether or 

 carbon-di-sulphide, which, in turn, by the expansion of its vapor, 

 develops additional mechanical power. Mechanical difficulties have 

 hitherto prevented the success of this form of engine, but it cannot 

 be pronounced unlikely that coming inventors may make the system 

 commercially valuable. 



We may now summarize the result of our examination of the 

 growth of the steam-engine thus : 



First. The process of improvement has been one, primarily, of 

 " differentiation ; " f the number of parts has been continually 



f This term, though perhaps not familiar to engineers, expresses the idea perfectly. 



increased; while the work of each part has been simplified, a sepa- 

 rate organ being appropriated to each process in the cycle of opera- 

 tions. 



Secondly. A kind of secondary process of differentiation has, to 

 some extent, followed the completion to the primary one, in which 

 secondary process one operation is conducted partly in one and partly 

 in another portion of the machine. This is illustrated by the two 

 cylinders of the compound engine and by the duplication noticed in 

 the Binary engine. 



Thirdly. The direction of improvement has been marked by a con- 

 tinual increase of steam pressure, greater expansion, provision for 

 obtaining dry steam, high piston speed, careful protection against loss 

 of heat by conduction or radiation, and, in marine engines, by surface 

 condensation. 



The direction which improvement seems now to be taking, and the 

 proper direction, as indicated by an examination of the principles of 

 science, as well as by our review of the steps already taken, would 

 seem to be : 



Working between the widest attainable Limits of Temperature. 



Steam must enter the machine at the highest possible temperature, 

 must be protected from waste, and must retain, at the moment before 

 exhaust, the least possible amount of heat. He whose inventive 

 genius, or mechanical skill, contributes to effect either the use of 

 higher steam with safety and without waste, or the reduction of the 

 temperature of discharge, confers a boon upon mankind. 



In detail : In the engine the tendency is, and may probably be 

 expected to continue, in the near future at least, toward higher steam 



