CONSTITUENTS OF COAL. — PROCESS OF COMBUSTION. — HEAT EVOLVED IN IT. — FORM AND STRUCTURE OF BOILER. — WAGGON BOILER. — FURNACE. — METHOD OF FEEDING IT. — COMBUSTION OF GAS IN FLUES. — CONSTRUCTION OF GRATE AND ASH-PIT. — MAGNITUDE OF HEATING SURFACE OF BOILER. — STEAM-SPACE AND WATER-SPACE IN BOILER. — POSITION OF FLUES. — METHOD OF FEEDING BOILER. — LEVEL GAUGES. — SELF-REGULATING FEEDERS. — STEAM-GAUGE. — BAROMETER-GAUGE. — INDICATOR. — COUNTER. — SAFETY-VALVE. — FUSIBLE PLUGS. — SELF-REGULATING DAMPER. — SELF-REGULATING FURNACE. — POWER AND DUTY OF ENGINES. — HORSE-POWER OF STEAM ENGINES. — EVAPORATION PROPORTIONAL TO HORSE-POWER. — SOURCES OF LOSS OF POWER. — ABSENCE OF GOOD PRACTICAL RULES FOR POWER. — COMMON RULES FOLLOWED BY ENGINE MAKERS. — DUTY DISTINGUISHED FROM POWER. — DUTY OF BOILERS. — PROPORTION OF STROKE TO DIAMETER OF CYLINDER. — DUTY OF ENGINES. — CORNISH SYSTEM OF INSPECTION. — ITS GOOD EFFECTS. — HISTORICAL DETAIL OF THE DUTY OF CORNISH ENGINES. (145.) The machinery which has been explained in the preceding chapters, consisting of the cylinder with its passages and valves, the piston-rod, parallel motion, beam, connecting-rod and crank, together with the condenser, air-pump, and other appendages, having no source of moving power in themselves, must be regarded as mere instruments by which the mechanical effect developed by the furnace and the boiler is transmitted to the working point and so [Pg252] modified as to be adapted to the uses to which the machine is applied. The boiler is at once a magazine in which the moving power is stored in sufficient quantity to supply the demands of the engine and an apparatus in which that power is fabricated. The mechanical effect evolved in the conversion of water into steam by heat, is the process by which the power of the steam-engine is produced, and space is provided in the boiler, capacious enough to contain as much steam as is necessary for the engine, besides a sufficient quantity of water to continue that supply undiminished, notwithstanding the constant drafts made upon it by the cylinder: even the water itself, from the evaporation of which the mechanical power is produced, ought to be regarded as an instrument by which the effect of the heat of the combustible is rendered mechanically efficient, inasmuch as the same heat, applied not only to other liquids but even to solids, would likewise be productive of mechanical effects. The boiler and its furnace are therefore parts of the steam-engine, the construction and operation of which are entitled to especial attention.