and Propagation of Sound. 81 



of a cord. But although rapid revolutions of such awheel, 

 so as to make above 100 or 150 strokes in a second, do pass 

 into continuous tones which can be referred to a particular 

 pitch in the musical scale, yet the slower rotations do not pro- 

 duce a bass note, as an equal number of the vibrations of a 

 cord, but only a succession of distinct clicks. This shows, 

 I think, that the single clicks have a pitch, and that this is at 

 the point where their succession begins to form a continued 

 tone*. 



11. That single strokes have a pitch, and consist of at least 

 two vibrations (" semi- vibrations," Wheatstone), is further ap- 

 parent to an ear accustomed to distinguish between musical 

 notes, when they are made with different degrees of force : the 

 gentle strokes are obviously lower than the forcible ones. 

 The cause of this curious fact seems to be, that a forcible im- 

 pulse, by momentarily increasing the density, accelerates the 

 vibration. So also a very violent blow on a bell or bar, or a 

 forcible pull of a cord, will make the initial vibrations quicker, 

 and therefore the tone sharper, than in the proper note. 

 This is perceptible in the loud notes of the harp : but the less 

 yielding tension of metallic wires makes them still more liable 

 to this change of tone ; it is therefore most obvious in the 

 twang of those instruments with wires, which are acted on by 

 points projecting from a revolving barrel. 



12. With the exception of this effect of force (14.), the note 

 of continued sounds resulting from a rapid succession of 

 strokes, which for convenience may be called click sounds, de- 

 pends entirely on the frequency of these strokes (10.). In 

 bodies of no given tension, this rapidity is most indeterminate 

 and irregular ; but in cords or bodies of a fixed key, when 

 the series of impulses surpasses in rapidity the vibrations of 

 the fundamental note, they will pass into its upper octave, or 

 others of its higher harmonics. This is one reason why the 

 bass cords of a violoncello, when bowed by an unskilful hand, 

 give out various high and mixed notes, instead of the pure 

 rich bass which those elicit who have experimentally acquired 

 a mastery over the vibrations of the instrument. 



13. Click sounds (10. and 12.) in bodies of no given ten- 



• In attempting to excite a continued note as low as possible, M. Savart 

 was obliged to abandon his toothed wheel, and use one with long vanes, 

 which, by passing close to, but not touching, a lamina of pasteboard, pro- 

 duced in the air a series of concussions, which, if of a certain frequency, be- 

 came a continued note. It is plain that the pitch of the single strokes here 

 was exceedingly low, for the wheel was nearly five feet in diameter, and 

 with this a very powerful continued but not uniform note resulted at the 

 rate of seven or eight strokes per second. — Annates de Chimie, 1831. 



