lum is changed
from universal to rectilinear. If you are making a harmonograph of the type
shown in Fig. 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.

Just one caution here. The outside lap of lead should finish at the point
on the circumference where the first lap began, for the weight to be
approximately symmetrical about the centre.

An alternative method is to melt up scrap lead and cast weights in tins or
flowerpots sunk in sand, using an accurately centred stick as the core.
This stick should be very slightly larger than the pendulum rod, to allow
for the charring away of the outside by the molten metal. (Caution.--The
mould must be quite dry.)

Failing lead, tin canisters filled with metal scrap may be made to serve.
It will in this case be necessary to bore the lid and bottom centrally and
solder in a tube fitting the rod, and to make an opening through which the
weighting material can be inserted.

Adjustment of Weights.--As lead is too soft a metal to give a
satisfactory purchase to a screw--a thread cut in it soon wears out--it is
better to support a leaden weight from underneath by means of a brass
collar and screw. A collar is easily made out of a bit of tubing thickened
at the point where the screw will pass by soldering on a suitably shaped
piece of metal. Drill through the reinforcement and tubing and tap to suit
the screw used, which may well be a camera tail screw, wit