168, use a gimbal for the platform pendulum, and design it so
that the upper suspension gives a motion at right angles to the pen
pendulum. The use of two little wedges will then convert the apparatus in a
moment from semirectilinear to purely rectilinear.
Weights.--The provision of weights which can be slipped up and down a rod
may present some difficulty. Of iron and lead, lead is the more convenient
material, as occupying less space, weight for weight, and being more easily
cast or shaped. I have found thin sheet roofing lead, running 2 lbs. to the
square foot, very suitable for making weights, by rolling a carefully
squared strip of the material round the rod on which it will have to move,
or round a piece of brass tubing which fits the rod. When the weight has
been rolled, drill four holes in it, on opposite sides near the ends, to
take nails, shortened so that they just penetrate all the laps but do not
enter the central circular space. These will prevent the laps sliding over
one another endways. A few turns of wire round the weight over the heads
makes everything snug.
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