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.