HOW IT WORKS.

Fig. 6. —Diagrammatic sketch of a locomotive type of boiler. Water indicated by dotted lines. The arrows show the direction taken by the air and hot gases from the air-door to the funnel.

THE MULTITUBULAR BOILER.

Fig. 7. —The Babcock and Wilcox water-tube boiler. One side of the brick seating has been removed to show the arrangement of the water-tubes and furnace.

To save room, boilers which have to make steam very quickly and at high pressures are largely composed of pipes. Such boilers we call multitubular. They are of two kinds—(1) Water-tube boilers; in which the water circulates through tubes exposed to the furnace heat. The Babcock and Wilcox boiler (Fig. 7) is typical of this variety. (2) Fire-tube boilers; in which the hot gases pass through tubes surrounded by water. The ordinary locomotive boiler (Fig. 6) illustrates this form.