iron tubes, three inches in
diameter, and between six and seven feet long; but sometimes brass tubes of
similar dimensions are employed. When brass tubes are employed, the use of
ferules driven into the ends of the tubes is sometimes employed to keep
them tight; but when the tubes are of malleable iron, of the thickness of
Russell's boiler tubes, they may be made tight merely by firmly driving
them into the tube plates, and the same may be done with thick brass tubes.
The holes in the tube plate next the front of the boiler are just sensibly
larger in diameter than the holes in the other tube plate, and the holes
upon the outer surfaces of both tube plates are very slightly countersunk.
The whole of the tubes are driven through both tube plates from the front
of the boiler,--the precaution, however, being taken to drive them in
gently at first with a light hand hammer, until the whole of the tubes have
been inserted to an equal depth, and then they may be driven up by degrees
with a heavy hammer, whereby any distortion of the holes from unequal
driving will be prevented. Finally, the ends of the tubes should be riveted
up so as to fill the countersink; the tubes should be left a little longer
than the distance between the outer surfaces of the tube plates, so that
the countersink at the ends may be filled by staving up the end of the tube
rather than by riveting it over; and the staving will be best accomplished
by means of a mandril with a collar upon it, which is driven into the tube
so that the collar rests upon the end of the tube to be riveted; or a tool
like a blunt chisel with a recess in its point may be used, as is the more
usual practice.

385. _Q._--Should not stays be introduced in substitution of some of the
tubes?

_A._--It appears expedient in all cases that some of the tubes should be
screwed at the ends, so as to serve as stays if the riveting at the tube
ends happens to be burned away, and also to act as abutments to the riveted
tube--or else to introduce very strong rods of about the same diameter as a
tube, in substitution of some of the tubes; and these stays should have
nuts at each end both within and without the tube plates, which nuts should
be screwed up, with white lead interposed, before the tubes are inserted.
If the tubes are long, their expansion when the boiler is being blown off
will be apt to start them at the ends, unless very securely fixed; and it
is difficult to prevent brass tubes