the vibration of their
anterior and posterior membranes; while the sound is modified by the
dampers as they are opened more or less widely.

The actual source of the sound is elsewhere, and is somewhat difficult
for a novice to find. On the outer wall of either chapel, at the ridge
formed by the junction of back and belly, is a tiny aperture with a
horny circumference masked by the overlapping damper. We will call this
the window. This opening gives access to a cavity or sound-chamber,
deeper than the "chapels," but of much smaller capacity. Immediately
behind the attachment of the posterior wings is a slight protuberance,
almost egg-shaped, which is distinguishable, on account of its dull
black colour, from the neighbouring integuments, which are covered with
a silvery down. This protuberance is the outer wall of the
sound-chamber.

Let us cut it boldly away. We shall then lay bare the mechanism which
produces the sound, the _cymbal_. This is a small dry, white membrane,
oval in shape, convex on the outer side, and crossed along its larger
diameter by a bundle of three or four brown nervures, which give it
elasticity. Its entire circumference is rigidly fixed. Let us suppose
that this convex scale is pulled out of shape from the interior, so
that it is slightly flattened and as quickly released; it will
immediately regain its original convexity owing to the elasticity of the
nervures. From this oscillation a ticking sound will result.

Twenty years ago all Paris was buying a silly toy, called, I think, the
cricket or _cri-cri_. It was a short slip of steel fixed by one end to a
metallic base. Pressed out of shape by the thumb and released, it
yielded a very distressing, tinkling _click_. Nothing else was needed to
take the popular mind by storm. The "cricket" had its day of glory.
Oblivion has executed justice upon it so effectually that I fear I shall
not be understood when I recall this celebrated device.

The membranous cymbal and the steel cricket are analogous instruments.
Both produce a sound by reason of the rapid deformation and recovery of
an elastic substance--in one case a convex membrane; in the other a slip
of steel. The "cricket" was bent out of shape by the thumb. How is the
convexity of the cymbals altered? Let us return to the "church" and
break down the yellow curtain which closes the front of each chapel. Two
thick muscular pillars are visible, of a pale orange colour; they join
at an angle, for