s to waste, as in
the feeding apparatus of land engines.

409. _Q._--Suppose that the engineer should shut off the feed water from
the boilers while the engine was working, what would be the result?

_A._--The result would be to burst the feed pipes, except for a safety
valve placed on the feed pipe between the engine and the boilers, which
safety valve opens when any undue pressure comes upon the pipes, and allows
the water to escape. There is, however, generally a cock on the suction
side of the feed pump, which regulates the quantity of water drawn into the
pump. But there must be cocks on the boilers also to determine into which
boiler the water shall be chiefly discharged, and these cocks are sometimes
all shut accidentally at the same time.

410. _Q._--Is there no expedient in use in steam vessels for enabling the
position of the water level in the boiler to determine the quantity of feed
water admitted?

_A._--In some steam vessels floats have been introduced to regulate the
feed, but their action cannot be depended on in agitated water, if applied
after the common fashion. Floats would probably answer if placed in a
cylinder which communicates with the water in the boiler by means of small
holes; and a disc of metal might be attached to the end of a rod extending
beneath the water level, so as to resist irregular movements from the
motion of the ship at sea, which would otherwise impair the action of the
apparatus.

411. _Q._--How is the proper level of the water in the boiler of a steam
vessel maintained when, the engine is stopped for some time, and the boiler
is blowing off steam?

_A._--By means of a separate pump worked sometimes by hand, but usually by
a small separate engine called the Donkey engine. This pump, by the aid of
suitable cocks, will pump from the sea into the boiler; from the sea upon
deck either to wash decks or to extinguish fire; and from the bilge
overboard, through a suitable orifice in the side of the ship.

LOCOMOTIVE BOILERS.

412. _Q._--Will you recapitulate the general features of locomotive
boilers?

_A._--Locomotive boilers consist of three portions (see fig. 29): the
barrel E, E, containing the tubes, the fire box B, and the smoke box F; of
which the barrel smoke box, and external fire box are always of iron, but
the internal fire box is generally made of copper, though sometimes also it
is made of iron. The tubes are sometimes of iron, but generally of brass
fixed