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. Joseph
Goold, and shown in Fig. 169. It consists of--(1) a long pendulum, free to
swing in all directions, suspended from the ceiling or some other suitable
point. The card on which the figure is to be traced, and the weights, are
placed on a platform at the bottom of this pendulum. (2) A second and
shorter free pendulum, known as the "deflector," hung from the bottom of
the first.

This form of harmonograph gives figures of infinite variety and of extreme
beauty and complexity. Its chief drawback is its length and weight, which
render it more or less of a fixture.

Fortunately, Mr. C. E. Benham of Colchester has devised a Miniature Twin
Elliptic Pendulum which possesses the advantages of the Goold, but can be
transported easily and set up anywhere. This apparatus is sketched in Fig.
170. The main or platform pendulum resembles in this case that of the
Rectilinear Harmonograph, the card platform being above the point of
suspension.

Value of the Harmonograph.--A small portable harmonograph will be found
to be a good means of entertaining friends at home or elsewhere. The
gradual growth of the figure, as the card moves to and fro under the pen,
will arouse the interest of the least scientifically inclined person; in
fact, the trouble is rather to persuade spectators that they have had
enough than to attract their attention. The cards on which designs have
been drawn are in great request, so that the pleasure of the entertainment
does not end with the mere exhibition. An album filled with picked designs,
showing different harmonies and executed in inks of various colours, is a
formidable rival to the choicest results of the amateur photographer's
skill.

Practical Instructions for making Harmonographs.

Pendulums.--For the Rectilinear type of harmonograph wooden rods 5/8 to
3/4 inch in diameter will be found very suitable. They cost about 2d. each.
Be careful to select straight specimens. The upper pendulum of the
Miniature Twin Elliptic type should be of stouter stuff, say a broomstick;
that of the Goold apparatus stouter still.

All pendulums on which weights are slid up and down should be graduated in
inches and fractions, reckoning from the point of suspension as zero