8I cannot here omit a brief explanation, as clearly as I can give it, of the principles on which hydraulic organs are constructed. A base of framed wood-work is prepared, on which is placed a brazen box. On the base, right and left, uprights are fixed, with cross pieces like those of a ladder, to keep them together; between which are enclosed brass barrels with moveable bottoms, perfectly round, having iron rods fixed in their centres, and covered with leather and woollen, attached by pins to the levers. There are also, on the upper surface, holes about three inches diameter, in which, near the pin-joint, are brazen dolphins with chains hanging from their mouths, which sustain the valves that descend below the holes of the barrels.
2Within the box, where the water is deposited, there is a species of inverted funnel, under which two collars, about three inches high, answer the purpose of keeping it level, and preserving the assigned distance between the lips of the wind-chest and the bottom of the box. On the neck a chest, framed together, sustains the head of the instrument, which in Greek is called κανῶν μουσικὸς (canon musicus); upon which, lengthwise, are channels, four in number, if the instrument be tetrachordal, six if hexachordal, and eight if octochordal.
3In each channel are fixed stops, that are connected with iron finger-boards; on pressing down which, the communication between the chest and the channels is opened. Along the channels is a range of holes corresponding with others on an upper table, called πίναξ in Greek. Between this table and the canon, rules are interposed, with corresponding holes well oiled, so that they may be easily pushed and return; they are called pleuritides, and are for the purpose of stopping and opening the holes along the channels, which they do by passing backwards and forwards.
4These rules have iron jacks attached to them, and being united to the keys, when those are touched they move the rules. Over the table there are holes through which the wind passes into the pipes. Rings are fixed in the rules, for the reception of the feet of the organ-pipes. From the barrels run pipes joined to the neck of the wind-chest, which communicate with the holes in the chest, in which pipes are closely fitted valves; these, when the chest is supplied with wind, serve to close their orifices, and prevent its escape.
5Thus, when the levers are raised, the piston-rods are depressed to the bottom of the barrel, and the dolphins turning on their pivots, suffer the valves attached to them to descend, thus filling with air the cavities of the barrels. Lastly; the pistons in the barrels being alternately raised and depressed with a quick motion, cause the valves to stop the upper holes: the air, therefore, which is pent, escapes into the pipes, through which it passes into the wind-chest, and thence, by its neck, to the box.
6By the quick motion of the levers still compressing the air, it finds its way through the apertures of the stops, and fills the channels with wind. Hence, when the keys are touched by hand, they propel and repel the rules, alternately stopping and opening the holes, and producing a varied melody founded upon the rules of music. I have done my utmost to give a clear explanation of a complex machine. This has been no easy task, nor, perhaps, shall I be understood, except by those who are experienced in matters of this nature. Such, however, as comprehend but a little of what I have written, would, if they saw the instrument, be compelled to acknowledge the skill exhibited in its contrivance.