68 ox THE PHYSIOLOGICAL CAUSES OF 



tions of tone) has fo^l^ 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 Avith 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 combined two and two, or three 

 and three together, in any arbitrary manner. 



The round boxes, hg h^ and hj h,, of which halves only are 

 drawn in the figure, serve by their resonance to soften the harsh- 

 ness 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 per- 

 petually increasing velocity. 



On beginning to blow the instrument, we first hear separate 

 impulses 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 another 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, Avhich 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 revolutions in a second, and that the series Avith 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 8 times 33, 

 that is, 264 puffs in a second. This gives us the once-accented c' 

 of our musical scale, [that is ' middle c,' written on the leger line 

 between the bass and treble staves.] But on opening the series 

 of 16 holes instead, we have twice as many, or 16 times 33, 

 that is, 528 vibrations in a second. We hear exactly the octave 

 above the first c', that is the twice-accented c", [or c on the third 

 space of the treble staff.] By opening both the series of 8 and 

 16 holes at once, we have both c' and c" at once, and can con- 

 vince ourselves that we have the absolutely pure concord of the 



