ower.

"Did the horses in the mill we saw yesterday, go as fast as the horses
which are drawing the chaise?" "No, not as fast as the horses go at
present on level ground; but they went as fast as the chaise-horses do
when they go up hill, or as fast as horses draw a waggon."

"How many times do the sails of that wind-mill go round in a minute?
Let us count; I will look at my watch; do you count how often the
sails go round; wait until that broken arm is uppermost, and when you
say _now_, I will begin to count the _time_; when a minute has past, I
will tell you."

After a few trials, this experiment will become easy to a child of
eight or nine years old; he may sometimes attend to the watch, and at
other times count the turns of the sails; he may easily be made to
apply this to a horse-mill, or to a water-mill, a corn-fan, or any
machine that has a rotatory motion; he will be entertained with his
new employment; he will compare the _velocities_ of different
machines; the meaning of this word will be easily added to his
vocabulary.

"Does that part of the arms of the wind-mill which is near the
_axle-tree_, or _centre_, I mean that part which has no cloth or sail
upon it, go as fast as the ends of the arms that are the farthest from
the centre?"

"No, not near so fast."

"But that part goes as often round in a minute as the rest of the
sail."

"Yes, but it does not go as fast."

"How so?"

"It does not go so _far_ round."

"No, it does not. The _extremities_ of the _sails go through more
space in the same time_ than the part near the centre."

By conversations like these, the technical meaning of the word
_velocity_ may be made quite familiar to a child much younger than
what has been mentioned; he may not only comprehend that velocity
means time and space considered together, but if he is sufficiently
advanced in arithmetic, he may be readily taught how to express and
compare in numbers _velocities_ composed of certain portions of time
and space. He will not inquire about the abstract meaning of the word
_space_; he has seen space measured on paper, on timber, on the water,
in the air, and he perceives distinctly that it is a term equally
applicable to all distances that can exist between objects of any
sort, or that he can see, feel, or imagine.

Momentum, a less common word, the meaning of which is not quite so
easy to convey to a child, may, by degrees, be explained to him: at
every instant he feels