7 o THE POPULAR SCIENCE MONTHLY. 



culty, although not certainly impossible ; ' and the author has proposed 

 a new type of steam-engine, in which the water of condensation and 

 the steam rejected from the engine shall be separated and returned, by 

 pumps of proper proportion and construction, to the boiler. The return 

 of the water demands the expenditure of an insignificant amount of 

 power. To return the rejected steam with its charge of heat which 

 usually forms so large a proportion of the total heat generated by the 

 combustion of the fuel, assuming all transfer of heat to the exhaust by 

 the operation of internal condensation and reevaporation to have been 

 prevented demands the expenditure of precisely the amount of power 

 which has been developed by its expansion. In an ideal engine of this 

 type, therefore, the efficiency is perfect, and all heat-energy is utilized 

 by transformation into mechanical energy ; but the engine cannot de- 

 velop as much power as an engine of the common type of the same size. 

 The size of engine will be nearly inversely proportional to the " effi- 

 ciency of the fluid " under similar conditions in this and the ordinary 

 type of engines. The heat rejected from the cylinder has been de- 

 graded so low on the scale of temperature as to be no longer available 

 for the production of power ; nevertheless, restored to the boiler, it 

 serves with perfect efficiency as a basis upon which to " pile up a new 

 stock of utilizable energy " in the form of heat derived from the furnace, 

 and at a higher temperature. 



The obstacles to the realization of this theoretically perfect type of 

 engine are those which make it so difficult to reduce internal condensa- 

 tion and reevaporation, and those conditions of practice which make 

 the engine of this type exceptionally bulky and mechanically inefficient. 



Whether this type of heat-engine can ever be made of practical 

 value will be determined by the rate of condensation of steam expand- 

 ing against a resisting piston ; the extent to which high pressures and 

 great expansion can be practically carried; the extent to which internal 

 transfer of heat, without doing w 7 ork, can be reduced ; the practical 

 limit of engine-speed ; and the perfection attainable in the engine con- 

 sidered as a piece of mechanism. All these conditions remain to be 

 experimentally determined, and it is only by their determination that it 

 can be known whether the " Steam-Engine of the Future " will greatly 

 exceed the engine of to-day in efficiency, and whether this newly-pro- 

 posed type may ultimately succeed. 



That the changes in practice already indicated may go on almost 

 indefinitely seems unquestionable. That this latter modification of 

 the steam-engine will ever actually take place, and become generally 

 adopted, cannot be as positively asserted. We may, at least, hope 

 that it may. 



We have seen that the most important problem offered the engineer 



1 " On a New Type of Steam-Engine," etc., by R. II. Thurston, Journal of the Franklin 

 Institute, October, November, December, 1877. "Proceedings of the American Associa- 

 tion for the Advancement of Science," 1877. 



