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238 THE TECHNICAL PROCEDURES IN FILLING TEETH. :

the ball struck. But it gives us no infonnation regarding the
stress between the balls at the instant of contact. The Ballistic
pendulum affords a fine opportunity to study the force exerted
by rifle balls, but this. again is registered in motion i^roduced
in the iDendulum and does not give the pressure between the rifle
ball and the weight forming the penduliun at the instant of
contact.
While a gentleman was trying a 22-caliber modern high-
power ri&e, he happened upon a child's house-building block
about two inches square and a quarter-inch thick, made of some
very light wood. He set it on a gate post and stepped off a short
distance and fired at it. The effect apparent at the moment
was to move the block slightly, but it remained standing on edge
on the post. Upon examination, he found a hole through it near
its center. To the tremendous momentum of the rifle ball, the
cutting of the hole through the block was scarcely a feather's
weight in retardation, and that was all that was left to distui'b
the inertia of the block. The ball cut the hole without communi-
cating more than this in motion.
A man was handling a pile-driver, setting very long logs into
loose sand. In letting down his steam hammer to drive one of
these, he happened to let it, wth its accompanying steam cylinder,
fall some twenty feet and strike the end of the log. The effect
of this tremendous blow was not to drive the log deep into the
sand, but the whole of the up^jer j^art was broken into thousands
of splinters. The force of the blow was spent in destroying the
log instead of driving it into the sand. A blow has the nature
of an explosion in which all the force generated by weight and
velocity, meeting overpowering resistance, is discharged in an
instant of time ; or, a blow represents the accumulated force of
weight, velocity and resistance concentrated in an explosive dis-
charge instead of being spread over seconds of time as measured
in foot pounds.
In considering the effect of blows there are four elements
(1) weight of the mallet; (2) velocity of the mallet; (3) the
character of the resistance. Upon these three the pounds force
of the blow is determined. (4.) Area of surface over which the
effect of the blow is expended is a secondary but important factor
in determining the work done by the blow upon any substance
struck. In this we reckon the force per sijuare millimeter.
In filling teeth we depend upon the force of tiie blow at the
moment of contact. In order to produce the results desired,
blows must be carefully fitted to the conditions and the work to