pressure could not be given to the metal--or at least it could not be
transferred far enough--to affect the metal at some distance from the
driven head. So great is this difficulty that in hand riveting much
shorter rivets must be used, because it is impossible to work
effectively so large a mass of metal with hammers as with a machine.
The heads of the machine rivets are, therefore, larger and stronger,
and will hold the plates together more firmly than the smaller
hammered heads.

To drive rivets by hand, two strikers and one helper are needed in the
gang, besides the boy who heats and passes the rivets; to drive each
five-eighths inch rivet, an average of 250 blows of the hammer is
needed, and the work is but imperfectly done. With a machine, two men
handle the boiler, and one man works the machine; thus, with the same
number of men as is required in riveting by hand, five rivets are
driven each minute.

The superior quality of the work done by the machine would alone make
its use advantageous; but to this is added greatly increased amount of
work done.

The difference in favor of the riveting machine over hand riveting is
at least _ten_ to _one_.

In a large establishment a record of the number of rivets driven by
the hand-driving gang, also by the gang at the steam-riveting machine
for a long period of time, in both cases making no allowances of any
kind of delays, the rivets driven per month by each was--for the hand
driven rivets at the rate of twelve rivets per hour, and for the
machine driven rivets, 120 per hour. In the case of the hand driven
rivets the boiler remains stationary and the men move about it, while
the machine driven rivets require the whole boiler to be hoisted and
moved about at the riveting machine to bring each hole to the position
required for the dies. Notwithstanding the trouble involved in
handling and moving the boiler, it shows that it is possible to do ten
times as much work, and with less skilled labor, by the employment of
the riveting machine.

_Calking._--One great source of danger in boiler making is excessive
joint calking--both inside and out--where a sharp nosed tool is
employed, and for the reason that it must be used so close to the
inner edge of plate as to indent, and in many cases actually cut
through the skin of the lower plate. This style of calking puts a
positive strain upon the rivets, commencing distortion and putting
excessive stress upon rivets--already in h