or other, is already familiar to every student of acoustics, and
"Chladni's" figures are continually reproduced in every physical
laboratory.

[Illustration: FIG. 1. CHLADNI'S SOUND PLATE]

[Illustration: FIG. 2. FORMS PRODUCED IN SOUND]

For the lay reader the following brief description may be useful. A
Chladni's sound plate (fig. 1) is made of brass or plate-glass. Grains
of fine sand or spores are scattered over the surface, and the edge of
the plate is bowed. The sand is thrown up into the air by the vibration
of the plate, and re-falling on the plate is arranged in regular lines
(fig. 2). By touching the edge of the plate at different points when it
is bowed, different notes, and hence varying forms, are obtained (fig.
3). If the figures here given are compared with those obtained from the
human voice, many likenesses will be observed. For these latter, the
'voice-forms' so admirably studied and pictured by Mrs Watts Hughes,[1]
bearing witness to the same fact, should be consulted, and her work on
the subject should be in the hands of every student. But few perhaps
have realised that the shapes pictured are due to the interplay of the
vibrations that create them, and that a machine exists by means of which
two or more simultaneous motions can be imparted to a pendulum, and that
by attaching a fine drawing-pen to a lever connected with the pendulum
its action may be exactly traced. Substitute for the swing of the
pendulum the vibrations set up in the mental or astral body, and we have
clearly before us the _modus operandi_ of the building of forms by
vibrations.[2]

[Illustration: FIG. 3. FORMS PRODUCED IN SOUND]