Page 45 - My FlipBook

eter, it will require one hundred and seventy pounds to pene-
trate the paper. If I place this mallet, weighing three troy
ounces, on this standard and let it fall three feet, how many
pounds pressure do you suppose it will develop. By actual
trial, and also by calculation, it will develop 599>^ pounds;
i. e., when falling upon a boxwood block on the anvil. Now
mind you, the pound force of that blow depends directly
upon how quickly the momentum is arrested, how quickly
it is stopped, just as much as it depends upon the momentum.
If I let this fall upon a cushion of any kind the pounds blow
would be lessened in proportion to the cushion. I have a
cushion of sole leather in one end of this mallet, as hard as
I could make it, and yet it requires to fall about twice as far
to produce the same result upon the sole leather as it does
upon the steel, but we get nearly the same impulse with the
sole leather that we do with steel.
Now, a question has come before all your minds as to
the solidity of the anvil of the tuptodynamometer. I will
speak of that and then I will be through for this morning.
I will use a large instrument point, because I only want to
get at the impulse in this experiment. Now I will let the
weight fall and you see the pencil fly. (At this point Dr. Black
explained the mechanism and working of the instrument.)
Now, you saw the impulse; you see it again. Now I will
repeat that, and will put my finger on this lever that strikes
the pencil and the impulse is just the same. Apparently
neither the anvil nor the lever moves at all (a student holds
The
the lever) you will feel just a slight jar and that is all.
impulse is propagated through the steel just as the impulse
is propagated from one billiard ball to another ; you know
the one billiard ball will stop in the place of the one struck
and the one struck will fly forward. While that anvil seems
to be apparently solid, there is enough of spring in the steel
to give the difference which I have stated, the difiference of
fifteen to twenty-eight, between this and the actually solid
anvil, and we get that kind of a difference in the pounds force
in the blow on the teeth that we fill.
Now, just let me speak of this: You are condensing
your gold, your gold gives under your plugger point, and you
suddenly step of? onto the enamel of the tooth ; the patient
starts ; by simply stepping ofif of that gold that yields onto

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