So in the case of gunpowder. It is the unity of
two or more substances, that causes the expansion
called power. The heat of the fuel converting
water into steam, is another illustration of the
unity of two or more elements, which are necessary
to produce energy.

MASS AN ELEMENT IN FLYING.--The boy who
reads this will smile, as he tells us that the power
which propelled the ball through the air came
from the thrower and not from the ball itself.
Let us examine this claim, which came from a real
boy, and is another illustration how acute his mind
is on subjects of this character.

We have two balls the same diameter, one of
iron weighing a half pound, and the other of cotton
weighing a half ounce. The weight of one
is, therefore, sixteen times greater than the other.

Suppose these two balls are thrown with the
expenditure of the same power. What will be the
result! The iron ball will go much farther, or,
if projected against a wall will strike a harder
blow than the cotton ball.

MOMENTUM A FACTOR.--Each had transferred
to it a motion. The initial speed was the same,
and the power set up equal in the two. Why this