NUMBER OF VIBRATIONS OF MUSICAL NOTES 353 



c' d' e f f cf a' V c" 

 264 297 330 352 396 440 495 528 



Another mode of producing a series of rapid pulsa- 

 ions of air is to hold one corner of a playing-card some- 

 -hat inclined against any of the series of holes. When 

 le disc is made to rotate, the corner of the card is 

 lightly bent between every two successive holes, but 

 sturns each time to its original shape by its elasticity; 

 ;ius a note is produced which has the same pitch as 

 aat produced by blowing through the tube, but is 

 omewhat harsher. The card vibrates to and fro as 

 ften as one of the holes passes the corner, and pro- 

 aces each time a pulsation of the air, the rapid succes- 

 on of which causes a musical note. 

 If instead of the perforated cardboard a disc of metal 

 ith a milled edge be fixed to the whirling-table, very 

 igh notes may be produced by holding the corner of a 

 laying-card against the projecting ridges. Thus sup- 

 3se the edge to have 150 ridges ; the handle can be 

 irned quickly enough to give 28 revolutions in a second 

 > the disc, we should thus have 28 times 150 = 4,200 

 brations, or nearly the very high note c v . 



The cardboard for the perforated disc should be smooth, rather 

 ff, and not more than I 13 "" thick. From the centre of the disc c, in 

 i 213, draw five circles with the radii 8'5, 9'5, 10'5, 11 -5, and 

 cm ; divide the whole into four parts by two diameters perpendicu- 

 \ to eacli other, a b and d e ; and from a, b, d, e, as centres, and 

 >th radius 12 cm draw arcs on both sides, thus determining the points 

 1 7, /i, i y fc, Z, m, n. Join the opposite points / and k, g and Z, h and 

 i 1 i and n, by straight lines. Each circle is now divided into 

 telve equal parts. Divide with the compasses, by trial, each 

 t elfth part of the second circle (counting from the centre of the disc) 

 i:0 five equal portions ; this gives the position of the holes for the 

 circle. Now bisect each twelfth part of the outer circle, 

 A A 



