91. Q.—­Will you describe the construction of the boilers used in steam vessels?

A.—­These are of two classes, flue boilers and tubular boilers, but the latter are now most used.  In the flue boiler the furnaces are set within the boiler, and the flues proceeding from them wind backwards and forwards within the boiler until finally they meet and enter the chimney.  Figs. 6, 7, and 8 are different views of the flue boilers of the steamer Forth.  There are 4 boilers (as shown in plan, Fig. 6), with 3 furnaces in each, or 12 furnaces in all.  Fig. 7 is an elevation of 2 boilers, the one to the right being the front view, and that to the left a transverse section.  Fig. 8 is a longitudinal section through 2 boilers.  The direction of the arrows in plan and longitudinal section, will explain the direction of the smoke current.

[Illustration:  Fig. 7.]

[Illustration:  Fig. 8.]

92. Q.—­Is this arrangement different from that obtaining in tubular boilers?

A.—­In tubular boilers, the smoke after leaving the furnace just passes once through a number of small tubes and then enters the chimney.  These tubes are sometimes of brass, and they are usually about 3 inches in diameter, and 6 or 7 feet long.

[Illustration:  Fig. 9.]

[Illustration:  Fig. 10.]

[Illustration:  Fig. 11.]

Figs. 9, 10, and 11 represent a marine tubular boiler; fig. 9 being a vertical longitudinal section, fig. 10 half a front elevation and half a transverse section, and fig. 11 half a back elevation and half a transverse section near the end.  There is a projecting part on the top of the boiler called the “steam chest,” of which the purpose is to retain for the use of the cylinder a certain supply of steam in a quiescent state, in order that it may have time to clear itself of foam or spray.  A steam chest is a usual part of all marine boilers.  In fig. 9 A is the furnace, B the steam chest, and C the smoke box which opens into the chimney.  The front of the smoke box is usually closed by doors which may be opened when necessary to sweep the soot out of the tubes.

The following are some forms of American boilers: 

Figs. 12 and 13 are the transverse and longitudinal sections of a common form of American marine boiler.

Figs. 14 and 15 are the front and sectional elevation of one of the boilers of the U.S. steamer Water Witch.

[Illustration:  Fig. 12.]

[Illustration:  Fig. 13.]

[Illustration:  Fig. 14.]

[Illustration:  Fig. 15.]

Fig. 16 is a longitudinal section of a boiler of the drop flue variety.  For land purposes the lowest range of tubes is generally omitted, and the smoke makes a last return beneath the bottom of the boiler.

Figs. 17 and 18 are the transverse and longitudinal sections of a tubular boiler, built in 1837 by R.L.  Stevens for the steamboat Independence.