only of three plates, one for the top
and one for each side, the lower seam of the side plates being situated
beneath the level of the bars, so as not to be exposed to the heat of the
furnace. The tube plates of tubular boilers should be of the best Lowmoor,
or Bowling iron, seven eighths to one inch thick: the shells should be of
the best Staffordshire, or Thornycroft S crown iron, 7/16ths of an inch
thick.

374. _Q._--Of what kind of iron should the angle iron or corner iron be
composed?

_A._--Angle iron should not be used in the construction of boilers, as in
the manufacture it becomes reedy, and is apt to split up in the direction
of its length: it is much the safer practice to bend the plates at the
corners of the boiler; but this must be carefully done, without introducing
any more sharp bends than can be avoided, and plates which require to be
bent much should be of Lowmoor iron. It will usually be found expedient to
introduce a ring of angle iron around the furnace mouths, though it is
discarded in the other parts of the boiler; but it should be used as
sparingly as possible, and any that is used should be of the best quality.

375. _Q._--Is it not important to have the holes in the plates opposite to
one another?

_A._--The whole of the plates of a boiler should have the holes for the
rivets punched, and the edges cut straight, by means of self-acting
machinery, in which a travelling table carries forward the plate with an
equal progression every stroke of the punch or shears; and machinery of
this kind is now extensively employed. The practice of forcing the parts of
boilers together with violence, by means of screw-jacks, and drifts through
the holes, should not be permitted; as a great strain may thus be thrown
upon the rivets, even when there is no steam in the boiler. All rivets
should be of the best Lowmoor iron. The work should be caulked both within
and without wherever it is accessible, but in the more confined situations
within the flues the caulking will in many cases have to be done with the
hand or chipping hammer, instead of the heavy hammer previously prescribed.

376. _Q._--How is the setting of marine boilers with internal furnaces
effected?

_A._--In the setting of marine boilers care must be taken that no copper
bolts or nails project above the wooden platform upon which they rest, and
also that no projecting copper bolts in the sides of the ship touch the
boiler, as the galvanic acti