Page:Popular Science Monthly Volume 13.djvu/75

Rh the combustion of fuel, into mechanical power, using steam as the receiver and conveyer of that heat.

The problem embodies two distinct and equally important inquiries: The first, What are the scientific principles involved in the problem, as stated? The second, How shall we construct a machine that shall most efficiently embody and accord with not only known scientific principles, but also with all well-settled principles of engineering practice?

The one question is addressed to the man of science; the other to the engineer. They can only be satisfactorily answered, even so far as our knowledge at present permits, after studying with care the scientific principles involved in the theory of the steam-engine, under the best light that science can afford us, and by a careful study of the various steps of improvement that have already taken place, and of accompanying variations of structure, analyzing the effect of each change and tracing the reasons therefor. The theory of the steam-engine is too important and too extensive a subject to be treated in even the space available for a complete course of college lectures; and we can only here attempt an exceedingly concise statement of the principles, pointed out by science, as those applicable in the endeavor to increase the economic efficiency of the steam-engine.

The teachings of science indicate that, in the modern steam-engine: Success in economically deriving mechanical power from the energy of heat-motion will be the greater as we work between more widely-separated limits of temperature, and as we more perfectly provide against losses by dissipation of heat in directions in which it is unavailable for the production of power.

Scientific research has proved that, in all varieties of heat-engines, a very great loss of effect is unavoidable from the fact that we cannot reduce the lower limit of temperature, in working, below a point that is far above the absolute zero of temperature: the point corresponding to the mean temperature of the surface of the earth in our latitude is now practically our lower mean limit of temperature. The higher the temperature of the steam, however, when it enters the engine, and the lower the temperature at which it leaves the cylinder, and the more thoroughly we provide against waste of heat by conduction and radiation, and of power by friction, the greater will be our success.

Now, looking back over the history of the steam-engine, we may rapidly note the prominent points of improvement and the most striking changes of form; and we may thus obtain some idea of the general direction in which we are to look for further advance.

Beginning with the machine of De Caus, at which point we may first take up an unbroken thread, it will be remembered that we there found a single vessel performing the functions of all the parts of a modern pumping-engine; it was at once boiler, steam-cylinder, and condenser, as well as both a lifting and a forcing pump.

The Marquis of Worcester, and, still earlier, Da Porta, divided the