cross from becoming displaced by a jar or accident. The ends of
the cross are inserted through the holes C of the stirrups, then
slipped back so the knife edges engage in the V-shaped holes of
the stirrups. The cross must be so made that the knife edges will
be in the same plane. This can be determined by placing two of the
knife edges on the jaws of a vise and then laying two rules across
the other two edges. The rules should just touch the jaws of the
vise and the two knife edges of the cross. This makes a universal
joint almost free from friction and, what is most important,
prevents the pendulum from twisting on its own axis.

The pendulum F should be made of ash or oak, 1-3/4 by 2 in., with
a length depending on the height of the ceiling. A length of 7 ft.
is about right for a 10-ft. ceiling.

A small table or platform, K, as shown in the lower part of Fig.
1, is fastened to the lower end of the pendulum as a support for
the cards on which harmonograms are made. A weight, G, of about 30
or 40 lb.-a box filled with small weights will do--is attached to
the pendulum just above the table. Another weight of about 10 lb.
is attached as shown at H. A pedestal, J, provides a means of
support for the stylus. The stylus arm should have pin-point
bearings, to prevent any side motion.

The length of the short pendulum H, which can be regulated, as
shown in Fig. 1, should bear a certain and exactly fixed relation
to the length of the main pendulum, for the swinging times of
pendulums are inversely proportionate to their lengths, and unless
the shorter pendulum is, for instance, exactly one-third,
one-fourth, one-fifth, etc., as long as the other, that is, makes