the effect of momentum in his own motions, and
in the motions of every thing that strikes against him; his feelings
and experience require only proper terms to become the subject of his
conversation. When he begins to inquire, it is the proper time to
instruct him. For instance, a boy of ten years old, who had acquired
the meaning of some other terms in science, this morning asked the
meaning of the word momentum; he was desired to explain what he
thought it meant.

He answered, "Force."

"What do you mean by force?"

"Effort."

"Of what?"

"Of gravity."

"Do you mean that force by which a body is drawn down to the earth?"

"No."

"Would a feather, if it were moving with the greatest conceivable
swiftness or velocity, throw down a castle?"

"No."[21]

"Would a mountain torn up by the roots, as fabled in Milton, if it
moved with the least conceivable velocity, throw down a castle?"

"Yes, I think it would."

The difference between an uniform, and an uniformly accelerated
motion, the measure of the velocity of falling bodies, the composition
of motions communicated to the same body in different directions at
the same time, and the cause of the curvilinear track of projectiles,
seem, at first, intricate subjects, and above the capacity of boys of
ten or twelve years old; but by short and well-timed lessons, they may
be explained without confounding or fatiguing their attention. We
tried another experiment whilst this chapter was writing, to determine
whether we had asserted too much upon this subject. After a
conversation between two boys upon the descent of bodies towards the
earth, and upon the measure of the increasing velocity with which they
fall, they were desired, with a view to ascertain whether they
understood what was said, to invent a machine which should show the
difference between an uniform and an accelerated velocity, and in
particular to show, by occular demonstration, "that if one body moves
in a given time through a given space, with an uniform motion, and if
another body moves through the same space in the same time with an
uniformly accelerated motion, the uniform motion of the one will be
equal to half the accelerated motion of the other." The eldest boy,
H----, thirteen years old, invented and executed the following machine
for this purpose:

Plate I, Fig. 3. _b_ is a bracket 9 inches by 5, consisting of a back
and two sides of hard wood: two inches from the back two slits are
made