For all boilers three different parts, viz., fire-surface, water-space and steam-room, must be considered. Each part or division has a distinct and separate duty to perform. The fire-surface includes the furnace and combustion chamber, flues and tubes; the water-space is that part occupied by the water; and the steam-room is the reservoir which holds and supplies the steam necessary to run the engine.

For convenience, we may state that all steam-boilers are either internally or externally fired. When the fuel is burned in an iron furnace surrounded with a water-jacket or water-log, as in the case of the locomotive, marine and portable boilers, they are internally fired. Cylinder-flue, double-deck, tubulous and sectional boilers are said to be externally fired, because the fuel is burned in a brick furnace lined with fire-brick.

A perfect steam-boiler should be made of the best material sanctioned by use, and should be simple in construction. It should have a constant and thorough circulation of water throughout the boiler, so as to maintain all parts at one temperature.

There should be a combustion chamber so arranged that the combustion of the gases commenced in the furnace may be completed before the escape to the chimney.

Have all parts readily accessible for cleaning and repairs.

In every boiler there should be ample water-surface for the disengagement of the steam from the water in order to prevent foaming. There should also be a large excess of strength over any legitimate strain, and proportioned for the work to be done.

Only the very best gauges, safety-valves, fusible plugs, and other fixtures should be used.

In a water-tube boiler there should be from 10 to 12 square feet of heating surface for one horse-power; in a tubular boiler 14 to 18 square feet of heating surface for one horse-power; in a flue boiler 8 to 12 square feet of heating surface for one horse-power; a plain cylinder boiler should have from 6 to 10 square feet of heating surface for one horse-power; a locomotive boiler should have 12 to 16 square feet of heating surface for one horsepower; a vertical boiler should have from 15 to 20 square feet of heating surface for one horse-power.

When considering the heating surface of a boiler, a vertical or upright surface has only one-half the evaporative value of a horizontal surface above the flamet; that is, the sides of a locomotive fire-box are only half as effective per square foot as the flat top of the box. In flues and tubes, the effective surface, measured on the circumference, is 1% times the diameter.