Birmingham; Pullar & Sons, of Perth; Salter & Co., of West Bromwich;
Vernon Hope & Co., of Wednesbury, etc.; and also for stamps by Messrs.
Collins & Co., of Birmingham, etc.]

Until these hammers were introduced, no satisfactory method had been
devised for altering the force of the blow. The plan generally adopted
was to have either a tightening pulley acting on the driving belt, a
friction driving clutch, or a simple brake on the driving pulley, put in
action by the hand or foot of the workman. Heavy blows were produced
by simply increasing the number of blows per minute (and therefore the
velocity), and light blows by diminishing it--a plan which was quite
contrary to the true requirements of the case. To prevent the shock
of the hammer head being communicated to the driving gear, an elastic
connection was usually formed between them, consisting of a steel spring
or a cushion of compressed air. With the steel spring, the variation
which could be given in the thickness of the work under the hammer was
very limited, owing to the risk of breaking the spring; but with the
compressed air or pneumatic connection the work might vary considerably
in thickness, say from 0 to 8 in. with a hammer weighing 400lb. The
pneumatic hammers had a crank, with a connecting rod or a slotted
crossbar on the piston-rod, a piston and a cylinder which formed the
hammer-head. The piston-rod was packed with a cup leather, or with
ordinary packing, the latter required to be adjusted with the greatest
nicety, otherwise the piston struck the hammer before lifting it, or
else the force of the blow was considerably diminished. As the piston
moved with the same velocity during its upward and downward strokes,
and, in the latter, had to overtake and outrun the hammer falling under
the action of gravity, the air was not compressed sufficiently to give
a sharp blow at ordinary working speeds, and a much heavier hammer was
required than if the velocity of the piston had been accelerated to a