baffles are also built between the smaller tubes, as shown in the longitudinal section. These baffles compel the products of combustion to circulate among the tubes in the direction shown by the arrows. Experience has shown that this arrangement gives a better evaporative efficiency than where the furnace gases are allowed to pass unbaffled straight up between the tubes. The boilers are usually fitted in pairs placed back to back, and one side of each is always made accessible. On this side the casing is provided with numerous small doors, through any of which a steam jet can be inserted for the purpose of sweeping the tubes. [Illustration: FIG 15.--Niclausse Boiler--transverse section.] Express boilers. A class of water-tube boilers largely in use in torpedo-boat destroyers and cruisers, where the maximum of power is required in proportion to the total weight of the installation, is generally known as express boilers. In these the tubes are made of smaller diameter than those used in the boilers already described, and the boilers are designed to admit of a high rate of combustion of fuel obtained by a high degree of "forced draught." Of these express boilers the Yarrow is of similar construction to the large tube Yarrow boiler already described with the exception that the tubes are smaller in diameter and much more closely arranged. Normand. In the Normand boiler (fig. 19) there are three chambers as in the Yarrow, connected together by a large number of bent tubes which form the heating surface, and also connected at each end by large outside circulating tubes. The two outer rows of heating tubes on each side are arranged to touch one another to nearly their whole length so as to form a "water-wall" for the protection of the outer casing. They enter the steam-chest at about the water-level. The two inner rows of tubes, which are bent to the form shown in the figure, also form a water-wall for the larger portion of the length of the boiler, and thus compel the products of combustion to pass in a definite course amongst all the tubes. In the Blechynden and White-Foster boilers there are also three chambers connected by bent tubes, the curvature being so arranged that in the former boiler any of the tubes can be taken out of the boiler through small doors provided in the upper part of the steam-chest, and in the White-Foster boiler they can be taken out through the manhole in the end of the steam-chest. Reed.