as to distribute the pressure, and prevent the
disposition to heat which would otherwise exist. The cross head journals
also should be long and large; for as the tops of the side rods have little
travel, the oil is less drawn into the bearings than if the travel was
greater, and is being constantly pressed out by the punching strain. This
strain should therefore be reduced as far as possible by its distribution
over a large surface. In the rules which are contained in the answers to
the ten preceding questions (358 to 367) the pressure on the piston in lbs.
per square inch is taken as the sum of the pressure of steam in the boiler
and of the vacuum; the latter being assumed to be 15 lbs. per square inch.

CHAPTER VII.

CONSTRUCTIVE DETAILS OF BOILERS.

* * * * *

LAND AND MARINE BOILERS.

369. _Q._--Will you explain the course of procedure in the construction and
setting of wagon boilers?

_A._--Most boilers are made of plates three eighths of an inch thick, and
the rivets are from three eighths to three fourths of an inch in diameter.
In the bottom and sides of a wagon boiler the heads of the rivets, or the
ends formed on the rivets before they are inserted, should be large and
placed next the fire, or on the outside; whereas on the top of the boiler
the heads should be on the inside. The rivets should be placed about two
inches distant from centre to centre, and the centre of the row of rivets
should be about one inch from the edge of the plate. The edges of the
plates should be truly cut, both inside and outside, and after the parts of
the boiler have been riveted together, the edges of the plates should be
set up or caulked with a blunt chisel about a quarter of an inch thick in
the point, and struck by a hammer of about three or four pounds weight, one
man holding the caulking tool while another strikes.

370. _Q._--Is this the usual mode of caulking?

_A._--No, it is not the usual mode; but it is the best mode, and is the
mode adopted by Mr. Watt. The usual mode now is for one man to caulk the
seams with a hammer in one hand and a caulking chisel in the other, and in
some of the difficult corners of marine flue boilers it is not easy for two
men to get in. A good deal of the caulking has also sometimes to be done
with the left hand.

371. _Q._--Should the boiler be proved after caulking?

_A._--The boiler should be filled with water and caulked afresh in any
l