riments with the skeleton-pulley, somebody must
keep it in its proper direction; as from its structure, which is
contrived for illustration, not for practical use, it cannot retain
its proper situation without assistance.

[25] In a loom this secondary lever is called _a lamb_, by mistake,
for _lam_; from _lamina_, a slip of wood.

[26] There should be three rollers used; one of them must be placed
before the sledge, under which it will easily find its place, if the
bottom of the sledge near the foremost end is a little sloped upwards.
To retain this foremost roller in its place until the sledge meets it,
it should be stuck lightly on the road with two small bits of wax or
pitch.

[27] _Mechanical advantage_ is not a proper term, but our language is
deficient in proper technical terms. The word _power_ is used so
indiscriminately, that it is scarcely possible to convey our meaning,
without employing it more strictly.

[28] In this experiment, the boy should pull as near as possible to
the shaft, within a foot of it, for instance, else he will have such
mechanical advantage as cannot be counterbalanced by any weight which
the machine would be strong enough to bear.

CHAPTER XVIII.

CHEMISTRY.

In the first attempts to teach chemistry to children, objects should
be selected, the principal properties of which may be easily
discriminated by the senses of touch, taste or smell; and such terms
should be employed as do not require accurate definition.

When a child has been caught in a shower of snow, he goes to the fire
to warm and dry himself. After he has been before the fire for some
time, instead of becoming dry, he finds that he is wetter than he was
before: water drops from his hat and clothes, and the snow with which
he was covered disappears. If you ask him what has become of the snow,
and why he has become wetter, he cannot tell you. Give him a tea-cup
of snow, desire him to place it before the fire, he perceives that the
snow melts, that it becomes water. If he puts his finger into the
water, he finds that it is warmer than snow; he then perceives that
the fire which warmed him, warmed likewise the snow, which then
became water; or, in other words, he discovers, that the heat which
came from the fire goes into the snow and melts it: he thus acquires
the idea of the dissolution of snow by heat.

If the cup containing the water, or melted snow, be taken from the
fire, and put out of the window on a