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 reëvaporation 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 develop 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 "efficiency of the fluid" under similar conditions in this and the ordinary type of engines. The heat rejected from the cylinder has been degraded 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 condensation and reëvaporation, 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 expanding 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 work, can be reduced; the practical limit of engine-speed; and the perfection attainable in the engine considered 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-proposed 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
- "On a New Type of Steam-Engine," etc., by R. H. Thurston, Journal of the Franklin Institute, October, November, December, 1877. "Proceedings of the American Association for the Advancement of Science," 1877.