Now when the tooth of the key is raised by one of the studs of the barrel passing under it, the other end of the key is depressed, and drawing down the piece g, shifts the lever e into the position shown by the dotted lines, and thus reverses the position of the slide. When the stud has passed the key, the spring c returns the key to its former position, and the piece g in rising is thrown by the spring h against the stud in the upper arm of the lever e. The compound pedal stops are a most admirable invention of Mr. Robson's, by means of which the performer is enabled to throw off instantaneously a number of instruments, and to bring into play a number of others, as the varying nature of the music may require, with an effect and precision equal to that of the best appointed orchestra. The mechanism is extremely ingenious, as well as beautifully simple. The arrangement will be easily understood by referring to the annexed cuts, which represent one set of pedal levers, Fig 5 being a side view, and Fig. 6 an end view.

On the upper axis a is a number of arms b b acting upon slits in a corresponding number of slides c c; on the lower axis d is likewise one or more arms e acting on one or more slides f; on the axis d is a tooth g acting upon a similar tooth h on the axis a; the length of each tooth being proportional to the length of the arms b and d. k is an arm, connected by wires and bell-cranks to the pedal beneath the keyboards Now upon depressing this arm, it will be seen that the arms b b and d d will move over equal spaces in opposite directions; one set of slides will therefore open the communication between the wind grooves and the instruments to which they belong, and the other set will close the passages to which they belong, and thus throw out of play the corresponding instruments; and there being five of these compound pedal stops, an amazing variety of changes may be obtained by varying the combinations of the slides. The whole number of the keys acted upon by the cylinders is about 250. There are upwards of 1900 pipes, 45 draw-stops, and 2 kettle-drums. To the musical amateur the following list of the stops will doubtless prove interesting.

On the first cylinder, -

Fig. 1.

Apollonicon 90

Fig. 2.

Apollonicon 91

Fig. 3.

Apollonicon 92

Fig. 4.

Apollonicon 93

Fig. 5.

Apollonicon 94

Fig. 6.

Apollonicon 95

1. Open Diapason.

2. Ditto ditto. 9. Stop ditto.

4. Principal.

5. Twelfth.

6. Fifteenth.

7. Flute.

8. Sesqui Altera.

9. Cornet - Trumpet.

On the second cylinder are two octaves of wooden pipes, of large dimensions, termed double diapason pedal pipes: the largest is 24 inches long, and 23 o square; this is 8 feet longer than the corresponding pipe in the great organ at Haarlem. The range of the scale is from G G G to G. On the third cylinder are the following stops, -

Diapason, or Corni Stop. Stop Diapason. Violoncello. German Flute. Wood Fifteenth.

Trumpet.

Cremonas.

Flutes.

Vox hum ana.

Octave Flute.

Hautboys. Piccolais. Trumpets. Diapason. Principal, etc.

The mechanism is enclosed in a case, 20 feet broad by 18 feet deep, and 24 feet high. The front is divided into three compartments by pilasters of Grecian Doric, surmounted by others of the Ionic order. Between the upper pilasters are three paintings; that in the centre representing Apollo, and those on the sides the Muses, Clio and Erato, somewhat larger than life, which do much credit to the artist (Mr. John Wright) by whom they were painted. The mechanical action of the apollonicon was first exhibited to the public in June, 1817, when the overtures to Anacreon and to the Clemenza di Tito were performed by the cylinders in a style that called forth the most marked approbation from large and scientific audiences. From that period to the present time it has maintained its well-deserved popularity, and continues an object of interest alike to the musician and the mechanist, offering to the former some of the grandest combinations of harmony, and to the latter some of the most curious and complicated specimens of his art.