[Illustration: Diagram of apparatus as described in text]
The construction of a hydraulic organ. Let A B C D (fig. 76), be
a small altar of bronze containing water. In the water invert a
hollow hemisphere, called a _pnigeus_, E F G H, which will allow of
the passage of the water at the bottom. From the top of this and
communicating with it let two tubes ascend above the altar; one of
them, G K L M, bent without the altar and communicating with a box, N
X O P, inverted, and having its inner surface made perfectly level to
fit a piston. Into this box let the piston R S be accurately fitted,
that no air may enter by its side; and to the piston attach a rod, T U,
of great strength. Again, attach to the piston rod another rod, U Q,
moving about a pin at U, and also working like the beam of a lever on
the upright rod W Y, which must be well secured. On the inverted bottom
of the box N X O P let another smaller box, Z, rest, communicating with
N X O P and closed by a lid above: in the lid is a hole through which
the air will enter the box. Place a thin plate under the hole in the
lid to close it, upheld by means of four pins passing through holes in
the plate, and furnished with heads so that the plate cannot fall off:
such a plate is called a valve. Again, let another tube, F I, ascend
from F G, communicating with a transverse tube, A´ B´, on which rest
the pipes A, A, A, communicating with the tube, and having at
the lower extremities small boxes, like those used for money; these
boxes communicate with the pipes, and their orifices B, B,
B, must be open. Across these orifices let perforated lids slide, so
that, when the lids are pushed home, the holes in them coincide with
the holes in the pipes, but, when the lids are drawn outwards, the
connexion is broken and the pipes are closed. Now, if the transverse
beam U Q be depressed at Q, the piston R S will rise and force out the
air in the box N X O P; the air will close the aperture in the small
box Z by means of the valve described above, and pass along the tube
M L K G into the hemisphere: again it will pass out of the hemisphere
along the tube F I into the transverse tube A´ B´, and out of the
transverse tube into the pipes, if the apertures in the pipes and in
the lids coincide, that is, if the lids, either all, or some of them,
have been pushed home.
In order that, when we wish any of the pipes to sound, the
corresponding holes may be opened, and closed again when we wish the
sound to cease, we may employ the following contrivance. Imagine one
of the boxes at the extremities of the pipes, C D, to be isolated, D
being its orifice, E the communicating pipe, R S the lid fitted to it,
and G the hole in the lid not coinciding with the pipe E. Take three
jointed bars F H, H M, M M^2, of which the bar F H is attached to the
lid S F, while the whole moves about a pin at M^3. Now, if we depress,
with the hand, the extremity M^2 towards D the orifice of the box, we
shall push the lid inwards, and, when it is in, the aperture in it will
coincide with that in the tube. That, when we withdraw the hand, the
lid may be spontaneously drawn out and close the communication, the
following means may be employed. Underneath the boxes let a rod, M^4
M^5, run, equal and parallel to the tube A´ B´, and fix to this slips
of horn, elastic and curved, of which M^6, lying opposite C D, is one.
A string, fastened to the extremity of the slip of horn, is carried
round the extremity H, so that, when the lid is pushed out, the string
is tightened; if, therefore, we depress the extremity M^2 and drive
the lid inwards, the string will forcibly pull the piece of horn and
straighten it, but, when the hand is withdrawn, the horn will return
again to its original position and draw away the lid from the orifice,
so as to destroy the correspondence between the holes. This contrivance
having been applied to the box of each pipe, when we require any of the
pipes to sound we must depress the corresponding key with the fingers;
and when we require any of the sounds to cease, remove the fingers,
whereupon the lids will be drawn out and the pipes will cease to sound.
The water is poured into the altar that the superabundant air, (I
mean, of course, that which is thrust out of the box and forces the
water upwards,) may be confined in the hemisphere, so that the pipes
which are free to sound may always have a supply. The piston R S, when
raised, drives the air out of the box into the hemisphere, as has been
explained; and when depressed, opens the valve in the small box Z. By
this means the box is filled with air from without, which the piston,
when forced up again, will again drive into the hemisphere. It would
be better that the rod T U should move about a pivot at T also, by
means of a single [loop,] R, which may be fitted into the bottom of the
piston, and through which the pivot must pass, that the piston may not
be drawn aside, but rise and fall vertically.