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.

[Illustration: Fig. 16.]

[Illustration: Fig. 17.]

[Illustration: Fig. 18.]

Fig. 19 is a longitudinal section of a common wood-burning locomotive.

[Illustration: Fig. 19.]

THE ENGINE.

93. _Q._--The steam passes from the boiler through, the steam pipe into the
cylinder of the engine?

_A._--And presses up and down the piston alternately, being admitted
alternately above and below the piston by suitable valves provided for that
purpose.

94. _Q._--This reciprocating motion is all that is required in a pumping
engine?

_A._--The prevailing form of the pumping engine consists of a great beam
vibrating on a centre like the beam of a pair of scales, and the cylinder
is in connection with one end of the beam and the pump stands at the other
end. The pump end of th