To remedy this, the cylinder is allowed to move endways in its bearings, a space equal to the distance between two keys; on its axis is cut a screw, containing nine threads, and the bevelled edge of a lever, called a knife, taking in one of the threads of the screw, the cylinder would be moved endways, at each revolution, a space equal to the distance between two threads; or one-ninth of the distance between two keys, at each revolution; and the ends of each key would trace a spiral line on the barrel, likewise deviating from a straight line one-ninth of the distance between two keys; thus nine revolutions of the cylinder would be made before the spiral traced by one key could be brought under the next key. Now the brackets and pins being ranged on the cylinder along these spiral lines, it is clear a different key may be moved at each corresponding part of a revolution, for nine revolutions, which renders the barrel equal to one of nine times its diameter, in which the brackets should be placed in right lines surrounding the cylinder; and as the cylinder revolves very slowly, it is sufficient for the performance of most compositions.

If it is required to repeat the performance, the key-frame is turned back upon a hinge, which raises the keys clear of the pins, and the knife being lifted out of the screw cut on the axis, the cylinder is moved endways into its original position, the knife replaced in the screw, and the key-frame again brought down to the cylinder, when the piece may be repeated. Having thus explained the principles of the mechanical action of the machine, we shall proceed to notice some of the details of the arrangement. There are three cylinders, each 2 feet in diameter, and each having a separate wind chest and key frame placed over it, furnishing wind to particular portions of the scale. The main cylinder occupies the centre of the front of the machine; it is 8 feet long, and comprises a range of five octaves: viz. from G G an octave below first G in the bass clef up to G, and eighth above G in the treble clef. In a line with this, and concentric with it, is another cylinder, 3 ft. 9 in. for the bass notes extending from G G G, or an octave below the former, up to gamut G, being two octaves. The third cylinder lies at the back of the machine, parallel to the main cylinder; it is 8 feet long, and comprises two octaves.

Below the two front cylinders extends a shaft, or axis, having two pinions, which work in two wheels, one on the end of each cylinder, and a similar shaft lies below the third cylinder, in the rear of the machine; and beneath these shafts, and at right angles to them, is another shaft, extending from the front to the back of the machine, having on it two endless screws working in worm-wheels on the two shafts; this last shaft receives its motion by a band from the driving-shaft, (which has a fly-wheel, and is turned by manual labour), and causes the cylinders to revolve. The key-frame is made in distinct pieces, to allow for the unequal contraction of the wood, and any inequalities which may exist in the cylinder; and to take the weight off the key, it is supported on the cylinder by anti-friction rollers, it remains now to explain the connexion of the finger-keys with the pallets. The key-boards are five in number, the central largest comprising a scale of five octaves, and the smaller ones disposed two on each side of the larger, these containing a scale of three octaves. These keyboards stand in front of the instrument, and detached from it, so that the performers sit with their backs to the instrument, and their faces to the audience.

From the fore end of the finger keys descend wires to the fore end of a series of levers below the floor, and to the other end of these levers are attached wires, which pass through holes in the wind chest, and are fastened to their set of pallets, and thus the depression of the finger keys draws down the hinder ends of the lower levers, and opens the pallets. Within reach of the performers at the keys, are a number of levers for moving by hand the draw-stops as are termed the slides which throw off or on any particular instrument; but, in addition to these, are a set of pedals five in number, which move a set of stops, called compound pedal stops. These are the invention of Mr. Robson, and are for the purpose of enabling the performer suddenly to throw on or off a number of instruments by a single movement, and thereby add greatly to the brilliancy of effect The operation of these stops requiring a figure for its elucidation, will be deferred until afterwards. To enable the reader more fully to comprehend the foregoing description, we have subjoined two engravings, Fig. 1 being a section of the machine parallel to the barrels, or to the front; and Fig. 2 a section across the barrels, or at right angles to the former.

In these figures, we have not strictly adhered to the actual arrangement of the parts which are used in the instrument itself, but have rather endeavoured to give a general idea of the principle of the construction, which is all that our limits will allow. In each figure the same letters denote similar parts, a a are the reservoirs immediately over the bellows; b the wind trunk; c the wind chest; d d strengthening bridges; e e pallets; f f f f wind grooves or channels; g g g the stops or slides; A A the groove board, into which are inserted the foot of the pipes j j j.; k the driving shaft, turned by a band I, from a wheel below the floor; m m endless screws working into worm wheels n n on the transverse shafts o o; p p pinions driving the cylinders q; r the knife or guide; s the keyframe; t t anti-friction rollers; u u the keys; v v stops to prevent the keys striking the cylinder; w w shifting keys with the connexion to their stops; x connexion of the pallets with the finger keys; y connexion of draw stops; x connexion of compound pedal stops.

Having thus explained the genera! mechanical arrangement of the instrument, we shall proceed to explain mora minutely the manner of throwing on or off; the different instruments or ranges of pipes, both by the shifting keys, and by the compound pedal stops. The above cuts represent one of the shifting keys, Fig. 3 being a front view, and Fig. 4 a side view, a represents a portion of the cylinder; b one of the shifting keys, kept in contact with the barrel by the spring c. On the spindle d is keyed a T-shaped lever e, in the horizontal arms of which are two projecting studs f f, with a wire passed through them from one to the other. A piece of brass g is. attached at its lower end to the key b, and the upper end passing between the lever; the wire rests by one of its shoulders on one of the projecting studs; a steel spring blade h is inserted at its lower end into a cleft in the piece g, and passes through a slit in a stud in the lower end of the lever e, and by its tendency to recover a right line in the direction of the arm, it forces the piece g out of the perpendicular, so that it shall always rest against one or other of the studs, k is an arm upon the spindle d, connected by a bar l to the lever (not shown) which works the slide or stop.