nnel.

The open end of the tube should be provided with an inch of tubing. Clips
placed on this and on the rubber connection between tube and bottle will
prevent the escape of mercury should the apparatus be upset when not in
use.

The average blowing pressure of which the lungs are capable is about 1-1/2
lbs. per square inch; inspiration pressure without mouth suction about 1
lb. per square inch; suction pressure 2-1/2 to 3 lbs. per square inch.

Caution.--Don't ask people with weak lungs to try experiments with the
apparatus described in this chapter.

XXXI. HOME-MADE HARMONOGRAPHS.

Have you ever heard of the harmonograph? If not, or if at the most you have
very hazy ideas as to what it is, let me explain. It is an instrument for
recording on paper, or on some other suitable surface, the figures
described by two or more pendulums acting in concert.

The simplest form of harmonograph is shown in Fig. 168. Two pendulums are
so suspended on points that their respective directions of movement are at
right angles to one another--that is, pendulum A can swing only north and
south, as it were, and pendulum B only east and west. On the top of B is a
platform to carry a card, and on the upper end of A a lever is pivoted so
as to be able to swing only vertically upwards and downwards. At its end
this lever carries a pen, which when at rest lies on the centre of the card
platform.

[Illustration: FIG. 168.--Simple Rectilinear Harmonograph.]

The bob, or weight, of a pendulum can be clamped at any point on its rod,
so that the rate or "period" of swing may be adjusted or altered. The
nearer the weight is brought to the point of suspension, the oftener will
the pendulum swing to and fro in a given time--usually taken as one minute.
From this it is obvious that the rates of swing of the two pendulums can be
adjusted relatively to one another. If they are exactly equal, they are
said to be in unison, and under these conditions the instrument would trace
figures varying in outline between the extremes of a straight line on the
one hand and a circle on the other. A straight line would result if both
pendulums were released at the same time, a circle,[1] if one were
released when the other had half finished a swing, and the intermediate
ellipses would be produced by various alterations of "phase," or time of
the commencement of the swing of one pendulum relatively to the
commencement of the swing of the other.

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