of the point type. The
weights scale 5 lbs. each. The platform pendulum is provided with a second
weight, which can be affixed above the suspension to slow that pendulum for
2:3, 4:5, 7:8, and higher harmonies.

The baseboard is plain, and when the apparatus is in action its ends are
supported on boxes or books laid on two tables, or on other convenient
supports. The whole apparatus can be taken to pieces very quickly for
transport. The total cost of materials used did not exceed 3s. 6d.

The Twin Elliptic Pendulum of Fig. 170 is supported on a tripod base made
of three pieces of 1-1/2 x 1-1/2 inch wood, 40 inches long, with ends cut
off to an angle of 72 degrees to give a convenient straddle, screwed at the
top to an oak head 3/4 inch thick, and braced a foot below the top by
horizontal crossbars 2 inches wide and 1/2 inch thick. For transport this
stand can be replaced by a flat baseboard similar to that of the
Rectilinear Harmonograph described in the last paragraph.

The main pendulum is a straight ash rod, 33 inches long and 1-1/4 inches
in diameter, suspended 13-1/2 inches from its upper end. Two weights of
4-1/2 lbs. each, made of rolled sheet lead, are provided for this pendulum.
According to the nature of the harmony, one only, or both together below
the suspension, or one above and one below, are used.

The weight of the lower pendulum, or deflector, is supported on a disc,
resting on a pin passing through the bottom of a piece of brass tubing,
which is provided with an eye at its upper end. This eye is connected by a
hook with several strands of silk thread, which are attached to the upper
pendulum by part of a cycle tyre valve. The stem part of the valve was cut
off from the nut, and driven into a suitably sized hole in the end of the
main pendulum. The screw collar for holding the valve in place had a little
brass disc soldered to the outside, and this disc was bored centrally for
the threads to pass through. The edges of the hole had been rounded off
carefully to prevent fraying of the threads. (Fig. 177.) The over-all
length of the pendulum, reckoning from the point of suspension, is 20
inches. The weights of the lower pendulum are several in number, ranging
from l lb. to 3 lbs.

[Illustration: FIG. 177.--Suspension for lower weight of Twin Elliptic
Harmonograph.]

Working the Harmonograph.--A preliminary remark is needed here. Harmonies
are, as we have seen, a question of ratio of swing periods. Th