The machine-driven rivet is marked a, and b represents the hammered rivet.

It will be observed that the machine rivet fills the hole completely, while the hand rivet is very imperfect.  This experiment was tried several times, with similar results each time.

The hand rivet, it will be observed, filled up the hole very well immediately under the head formed by the hammer; but sufficient 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 high tension before the boiler is put in actual use.  It is, I hope, rapidly becoming a thing of the past.