A certain portion of it is converted into work through the medium of the piston.
Another portion is conveyed away by the walls of the cylinder, this portion including the heat of convection and that of radiation.
Yet another portion is converted into internal work. Referring to the latter, suppose that steam is permitted to expand and its atoms will be in motion, which motion has been derived at the expense of or from the conversion of a certain quantity of heat.
The amount of the heat so converted obviously depends upon the amount of the motion. Suppose, for example, that steam is generated in a closed vessel as in a steam boiler, and that a certain pressure having been attained, the steam is permitted to pass off as fast as it is formed from the boiler, then the amount of atomic motion will remain constant, because the pressure remains constant; but suppose instead of the steam passing off, it be confined within the boiler, then the pressure will increase and there will be a greater resistance to the motion of the atoms, hence their motion will be less, and less of their heat will therefore be converted into atomic motion, and, as a consequence, more of it will exist in the form of sensible heat; hence while the pressure of steam continues to increase, its heat is increased, not only by reason of the heat it receives from the furnace, but also by reason of that abandoned by the steam, because it is prevented by the pressure, from expending it in atomic motion.
Chapter XL.—THE INDICATOR.
The indicator is an instrument which marks or draws a figure, or diagram as it is called, which shows the pressure there is in the cylinder at every point in the piston stroke, while it also shows the resistance offered by the same body of steam to the piston on its return stroke. From the form of this figure or diagram, the engineer is enabled to discover whether those parts of the engine whose operation regulates the admission of the steam to and its exhaust from the cylinder are correctly adjusted.
From the diagram the engineer may find the average or mean effective pressure of steam on the piston throughout the stroke, for use in calculating the power of the engine.
He may also locate the point of cut off, of release, the amount of back pressure, the degree of perfection of the vacuum in a condensing engine, and the amount of compression.