THE GROWTH OF THE STEAM-ENGINE. 67 



by both means combined ; and it has been further accompanied by 

 greater attention to the important matter of providing carefully against 

 losses by conduction and radiation, and by internal wasteful transfer of 

 heat. The use, finally, of the " compound " or double-cylinder engine 

 for the purpose of reducing friction, as well as of saving some of that 

 heat which is usually lost in consequence of internal condensation and 

 reevaporation due to great expansion, has already been considered 

 when treating 1 of the marine engine. 



It is evident that, although there is a limit, which is tolerably well 

 defined, in the scale of temperature, below which we cannot expect to 

 pass, using the now standard type of engine, a degree gained in ap- 

 proaching this lower limit is more remunerative than a degree gained 

 in the range of available temperature, by increasing the maximum tem- 

 perature. Hence, the attempt made by the French inventor, Du Trem- 

 bly, a quarter of a century ago, and by other inventors since, to utilize 

 a larger proportion of heat by approaching more closely the loicer limit, 

 was in accordance with what are now well-known scientific principles. 



The form of engine here referred to is known among engineers as 

 the Binary Vapor-Engine. In it the heat usually carried away by the 

 water delivered from the condenser of the steam-engine is made to 

 evaporate some very volatile liquid, as ether or carbon bisulphide, 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 impossible that 

 coming inventors may make the system commercially valuable. 



An important consequence of the still unchecked rise of piston- 

 speed in the modern steam-engine is the approach to a limit beyond 

 which the now standard form of "drop cut-off," or "detachable" valve- 

 gear, cannot be used. For the piston would, at that limit of speed, reach 

 the end of its stroke before the dropped valve could reach its seat, and 

 the point of cut-off and degree of expansion could no longer be deter- 

 mined accurately and invariably by the governor. This limit has prob- 

 ably already been attained in some engines ; and the engineer adopting 

 such piston-speeds as 1,000 feet per minute or more is driven back to 

 the use of the older types of " positive-motion " valve-gearing, and is 

 compelled to devise special forms of governor which shall have sensi- 

 tiveness, and yet power sufficient to control these less tractable kinds 

 of mechanism, and to invent reliable and durable forms of balanced 

 valves, and to practise every practicable expedient for making the 

 movement of the valve, and its adjustment by the regulator, perfectly 

 easy. Positive motion and ease of adjustment by the governor are, 

 therefore, evidently the requisites of a successful valve-gear for the 

 engine which will probably succeed the standard engine of to-day. 



We may now summarize the results of our examination of the 

 growth of the steam-engine thus : 



1. The process of improvement has been one, primarily, of "dif- 



