So far we have considered the figures obtained by two pendulums swinging in
straight lines only. They are beautiful and of infinite variety, and one
advantage attaching to this form of harmonograph is, that the same figure
can be reproduced exactly an indefinite number of times by releasing the
pendulums from the same points.
[Illustration: FIG. 169.--Goold's Twin Elliptic Pendulum Hamonograph.]
But a fresh field is opened if for the one-direction suspension of pendulum
B we substitute a gimbal, or universal joint, permitting movement in all
directions, so that the pendulum is able to describe a more or less
circular path. The figures obtained by this simple modification are the
results of compounded rectilinear and circular movements.
[Illustration: FIG. 170.--Benham's miniature Twin Elliptic Pendulum
Harmonograph.]
The reader will probably now see even fresh possibilities if both
pendulums are given universal movement. This can be effected with the
independent pendulums; but a more convenient method of obtaining equivalent
results is presented in the Twin Elliptic Pendulum invented by Mr.
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