FIG. 1.—HYDRAULIC ORGAN.
Let Β Δ be an altar[3] of bronze containing water. Let there be in the latter an inverted hollow hemisphere, Ε Ζ Η (called a damper), that allows the water to pass all around its bottom, and from the top of which rise two tubes that communicate with the interior. One of these tubes, Η Κ, is bent in the interior and communicates with a small inverted box,[4] Ν Η, the aperture of which is at the bottom, and the interior of which is bored out so that it may receive a piston, Ρ Ι, which should fit very accurately so as to allow no air to pass. To this piston is fixed a very strong rod, Τ γ, with which is connected another rod, γ Φ, movable around a pin at γ.[5] This lever moves upon a fixed vertical rod, Ψ Χ. Upon the bottom of the box, Ν Π, is placed another box, Ω, which communicates with the first, and which is closed at the upper part by a cover that contains an aperture to allow of the passage of the air into the box, Ν Π. Under the aperture of this cover, and in order to close it, there is arranged a thin disk, held by means of four pins which pass through apertures in the disk, and are provided with heads in order to hold it in place. This disk is called a platysmatim (Fig. 2). The other tube, Ζ Ζ′, is carried by the hemisphere, Ε Ζ Η, and ends in a transverse tube, Α Α′,[6] upon which rest pipes communicating with it and having at their extremities glossocomiums[7] that communicate with these pipes, and the orifices, Β′, of which are open. Across these orifices, covers provided with holes[8] slide in such a way that when they are pushed toward the interior of the organ their holes correspond to the orifices of the pipes (and to those of the tube Α Α′), and that when they are pulled back, the pipes are closed, since there is no longer any correspondence.
[3] Altars were cylindrical or square pedestals, characterized by a cavity in the upper platform, in which a fire was lighted.
[4] This box performs here the office of a pump chamber.
[5] The figure shows another arrangement.
[6] Called a wind-chest in modern organs.
[7] Flute mouths.
[8] Registers.