with wood is illustrated in Figure 40. The base of the machine
is 7 feet long, 2.5 feet wide at the centre, and weighs 3,500
pounds. Two upright columns, each 8 feet long, act as guides for
the striking head. At the top of the column is the hoisting
mechanism for raising or lowering the striking weights. The
power for operating the machine is furnished by a motor set on
the top. The hoisting-mechanism is all controlled by a single
operating lever, shown on the side of the column, whereby the
striking weight may be raised, lowered, or stopped at the will
of the operator. There is an automatic safety device for
stopping the machine when the weight reaches the top.

[Footnote 59: For description of U.S. Forest Service automatic
and autographic impact testing machine, see Proc. Am. Soc. for
Testing Materials, Vol. VIII, 1908, pp. 538-540.]

[Illustration: FIG. 40.--Impact testing machine.]

The weight is lifted by a chain, one end of which passes over a
sprocket wheel in the hoisting mechanism. On the lower end of
the chain is hung an electro-magnet of sufficient magnetic
strength to support the heaviest striking weights. When it is
desired to drop the striking weight the electric current is
broken and reversed by means of an automatic switch and current
breaker. The height of drop may be regulated by setting at the
desired height on one of the columns a tripping pin which throws
the switch on the magnet and so breaks and reverses the current.

There are four striking weights, weighing respectively 50, 100,
250, and 500 pounds, any one of which may be used, depending
upon the desired energy of blow. When used for compression tests
a flat steel head six inches in diameter is screwed into the
lower end of the weight. For transverse tests, a well-rounded
knife edge is screwed into the weight in place of the flat head.
Knife edges for supporting the ends of the specimen to be
tested, are securely bolted to the base of the machine.

The record of the behavior of the specimen at time of impact is
traced upon a revolving drum by a pencil fixed in the striking
head. (See Fig. 41.) When a drop is made the pencil comes in
contact with the drum and is held in place by a spring. The drum
is revolved very slowly, either automatically or by hand. The
speed of the drum can be recorded by a pencil in the end of a
tuning fork which gives a known number of vibrations per second.

[Illustration: FIG. 41.--Drum record of impact bendin