rigidly the internal piston, L, with the hammer, G. When the main piston
is raised by the rocking lever, the air in the space, X, between the
main and internal pistons, is compressed, and forms an elastic medium
for lifting the hammer; when the main piston is moved down, the air in
the space, Y, is compressed in its turn, and the hammer forced down to
give the blow. Two holes drilled in the side of the hammer renew the air
automatically in the spaces, X and Y, at each blow of the hammer.

Figs. 4 to 6, on the next page, represent the medium size forging
hammer, for making forgings in dies, swaging and tilting bars, and
plating edged tools, etc.

The hammer weighs 1 cwt., has a stroke variable from 4 in. to 141/2 in.,
and gives 200 blows per minute; the compressed air space between the
main piston and the hammer is sufficiently long to admit forgings up to
3 in. thick under the hammer.

To make forgings economically, it is necessary to bring them into the
desired form by a few heavy blows, while the material is still in a
highly plastic condition, and then to finish them by a succession of
lighter blows. The heavy blows should be given at a slower rate than the
lighter ones, to allow time for turning the work in the dies or on the
anvil, and so to avoid the risk of spoiling it. In forging with the
steam hammer the workman requires an assistant, who, with the lever
of the valve motion in hand, obeys his directions as to starting and
stopping, heavy or light blows, slow or quick blows, etc; the quickest
speed attainable depending on the speed of the arm of the assistant.
In the movable-fulcrum forging hammer the operations of starting and
stopping, and the giving of heavy or light blows, are under the complete
control of one foot of the workman, who requires therefore no assistant;