[22] Were it thought necessary to make these experiments perfectly accurate, a segment of a pulley, the radius of which is half the length of the scale-beam, should be attached to the end of the beam; upon which the cord may apply itself, and the pulley (P 3) should be raised or lowered, to bring the rope horizontally from the man's shoulder when in the attitude of drawing.
[23] The word power is here used in a popular sense, to denote the strength or efficacy that is employed to produce an effect by means of any engine.
[24] In all these experiments 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.