SOUND
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which form the base of the apparatus ; after which Mr.
Neilson succeeded in bringing the speaking-boxes into small
and compact form.1 The wind is supplied to these chests
(two divisions of one box) by a Y-tube with a stopcock on
each fork, the trunk of the Y being connected with the
bellows. Each short rubber tube is furnished with a pinch-
cock. The reeds are severally mounted upon identical
bevelled wooden slides, so that any note slides into dovetails,
and forms for the time the front of its box ; and each is
mounted with a mirror of silvered glass, f-inch diameter,
attached to its free end by a small pillar of cork. After being
fitted with mirrors (which load them), the reeds are fairly
tuned—excessive accuracy is not required (see g hereafter).
The reed-boxes are adjustable round vertical axes coincident
with the vertical diameters of the mirrors, whether the boxes
are in the horizontal or perpendicular position. One retains
a perpendicular position ; the other can either be similarly
placed, or fixed in a horizontal position rectangularly to it,
being clamped in either by a screw.
As regards the action of the apparatus, if it be confined
solely to Lissajous’ figures, the pencil of light might be
reflected direct to the screen from the second mirror, as from
a pair of forks. But being desirous of projecting open
* scrolls ’ also, after Tyndall’s method, and especially in the
case of ‘ beats,’ I adopted the arrangement shown in plan in
fig. 140, Omitting all details of focussing, the pencil of light
from the lantern l is reflected from the mirror on the first
reed-box, e, to that on the second, e e, and is thence reflected
1 Dr. Mann’s arrangement was quite different. He used much larger
boxes or speaking-chambers, with open apertures for the supply of wind. The
supply-tube came direct from the bellows, with a nozzle at the end contracted
to about one-third the size of the hole in the reed-box, and ending with a free
space of half an inch between this nozzle and the hole in the box. Had I
found his arrangement described earlier, I should probably have adopted both
it, and the ingenious inventor’s conclusion that it was indispensable ; as it is
I prefer (perhaps naturally) the smaller boxes as more easily adjusted, and the
closed supply as more certain and using less wind.