thick: it is usually of the same diameter as the
cylinder, but is better smaller, and must not stand more than 14 ft. high
above the level of the rails.

417. _Q._--Are locomotive boilers provided with a steam chest?

_A._--The upper portion of the external fire box is usually formed into a
steam chest, which is sometimes dome shaped, sometimes semicircular, and
sometimes of a pyramidical form, and from this steam chest the steam is
conducted away by an internal pipe to the cylinders; but in other cases an
independent steam chest is set upon the barrel of the boiler, consisting of
a plate iron cylinder, 20 inches in diameter, 2 feet high, and three
eighths of an inch thick, with a dome shaped top, and with the seam welded
and the edge turned over to form a flange of attachment to the boiler. The
pyramidical dome, of the form employed in Stephenson's locomotives,
presents a considerable extent of flat surface to the pressure of the
steam, and this flat surface requires to be very strongly stayed with angle
irons and tension rods; whereas the semiglobular dome of the kind employed
in Bury's engines requires no staying whatever. Latterly, however, these
domes over the fire box have been either much reduced in size or abandoned
altogether.

418. _Q._--Is any beneficial use made of the surplus steam of a locomotive?

_A._--To save the steam which is formed when the engine is stationary, a
pipe is usually fitted to the boiler, which on a cock being turned conducts
the steam into the water in the tender, whereby the feed water is heated,
and less fuel is subsequently required. This method of disposing of the
surplus steam may be adopted when the locomotive is descending inclines, or
on any occasion where more steam is produced than the engine can consume.

419._Q._--What means are provided to facilitate the inspection and cleaning
of locomotive boilers?

_A._--The man hole, or entrance into the boiler, consists of a circular or
oval aperture of about 15 in. diameter, placed in Bury's locomotive at the
apex of the dome, and in Stephenson's upon the front of the boiler, a few
inches below the level of the rounded part; and the cover of the man hole
in Bury's engine contains the safety valve seats. In whatever situation
this man hole is placed, the surfaces of the ring encircling the hole, and
of the internal part of the door or cover, should be accurately fitted
together by scraping or grinding, so that they need only the i