HARMONY IN MUSIC. 59 



portions stand before the holes of the box, the air cannot escape at 

 all. On turning the disc rapidly, the vent-holes of the box are alter- 

 nately opened and closed. During the opening, air escapes ; during 

 the closure, no air can pass. Hence the continuous stream of air from 

 the bellows is converted into a series of discontinuous puffs, which, 

 when they follow one another with sufficient rapidity, gather them- 

 selves together into a tone. 



Each of the revolving discs of this instrument (_which is more 

 complicated in its construction than any one of the kind hitherto 

 'uade, and hence admits of a much greater number of combinations 

 of tone) has four concentric circles of holes, the lower set having 

 8, 10, 12, 18, and the upper set 9, 12, 15, and 16 holes respectively. 

 The series of holes in the covers of the boxes are precisely the same 

 as those in the discs, but under each of them lies a perforated ring, 

 which can be so arranged, by means of the stops i i i i, that the 

 corresponding holes of the cover can either communicate freely with 

 the inside of the box, or are entirely cut off from it; We are thus 

 enabled to use any one of the eight series of holes singly, or com- 

 bined two and two, or three and three together, in any arbitrary 

 manner. 



The round boxes, h h and hj h n of which halves only are drawn 

 in the figure, serve by their resonance to soften the harshness of the 

 tone. 



The holes in the boxes and discs are cut obliquely, so that when 

 the air enters the boxes through one or more of the series of holes, 

 the wind itself drives the discs round with a perpetually increasing 

 velocity. 



On beginning to blow the instrument, we first hear separate im- 

 pulses of the air, escaping as puffs, as often as the holes of the disc 

 pass in front of those of the box. These puffs of air follow one an- 

 other more and more quickly, as the velocity of the revolving discs 

 increases, just like the puffs of steam of a locomotive on beginning to 

 move with the train. They next produce a whirring and whizzing, 

 which constantly becomes more rapid. At last we hear a dull drone, 

 which, as the velocity further increases, gradually gains in pitch and 

 strength. 



Suppose that the discs have been brought to a velocity of 33 re- 

 volutions in a second, and that the series with 8 holes has been 

 opened. At each revolution of the disc all these 8 holes will pass 

 before each separate hole of the cover. Hence there will be 8 puffs 

 for each revolution of the disc, or S times 33, that is, 20-i pulls in a 



