The principal test of the efficiency of a boiler is the quantity of water (generally expressed either in pounds or gallons), which it will evaporate with the consumption of One of coal. Of course this varies very much with the quality of the fuel, but with good pit coal (not dross), a Cornish boiler should evaporate 6 to 8 lbs. of wafer per lb. coal, and a multitubular boiler, such as fig. 2, about 10 or 11 lb. per lb. coal. The best rate of combustion on the grate varies with the construction of the boiler, from 10 to 18 or 20 lb. per sq. ft. of grate surface per hour.
Boilers are almost invariably made of wrought-iron plates riveted together. The parts most exposed to the action of the flame are made of the best quality of iron, and the other parts of inferior qualities, according to their position in reference to the flame. Occasionally boilers are made of steel, where lightness is the chief requisite, but makers have not yet sufficient confidence in steel plates to use them very largely. Copper is often used iu the fire-boxes of locomotives, but seldom in any other description of boiler. Brass boiler tubes are often seen, and on account of its better conducting qualities, brass is to be preferred to iron, but its dearness prevents its superseding iron in the great majority of cases.
Every boiler has, to render. It complete and workable, a number of fittings or mount, ing8, of which the following are the principal: A glass gauge to show the level of the water inside the boiler, and gauge-cocks for the same purpose; a gauge to show the pres sure of the steam; a cock for admitting water; a cock at the bottom for emptying or "blowing off;" a valve for the discharge of the: team; one or two safety valves, weighted so that, when the pressure of steam in the boiler reaches a certain height, they open and allow the steam to rush into the air; a door by which a man can get in to clean the boiler, etc.
In order to induce a current of air through the furnace so that a proper combustion may be maintained, stationary boilers are generally provided with a chimney of consid erable height, and made of brick or sheet-iron, to which the products of combustion are conducted after they have left the boiler. In locomotive boilers, and in some other cases where a sufficiently tall chimney cannot be made use of, a very powerful current is made by the ejection of the waste steam through a blast-pipe with a contracted nozzle at the base of the chimney. To prevent loss of heat by radiation, and the consequent waste of fuel, boilers should always be covered, in all parts exposed tothe atmosphere, with felt or some non-conducting composition.
For further details see also BOILDTG, MANOMETER, SAFETY-VALVE, STEAM, STEAg ENGINE, STEAM-CRANE.