TIMBRE. SPEED OF SOUND. 189 



length, producing a compound wave of a peculiar form, 

 or else to a longitudinal vibration in the sonorous body 

 taking place at the same time as the transverse vibration. 

 M. Longet states, with greater precision, that the timbre 

 of the human voice, and of wind-instruments, results from 

 the co -existence of several sonorous waves of different 

 tone and intensity, which modify the general form of the 

 principal wave. But at last the beautiful experiments of 

 M. Helmholtz have demonstrated, that the timbre of a sound 

 depends upon the number of the harmonic notes which are 

 produced at the same time as the fundamental note, and 

 upon their relative intensity. 



When a cord of a piano giving the C, for example, is 

 struck, that note is heard, but a little attention enables the 

 ear to hear other simultaneous and weaker sounds; they are 

 the result of partial vibrations which take place in the length 

 of the cord, according to certain laws which cannot be ex- 

 plained here. The C given by the shock impressed on 

 the cord is the fundamental note, the other notes which are 

 superposed upon it are the harmonics. From their fusion 

 with the fundamental note, there results to the ear a complex 

 sound which it decomposes instinctively into simple sounds, 

 but they cause only a single sensation in the brain, that of 

 a C having a special timbre. Whether the fundamental 

 note be given by an instrument or by the human voice, 

 the same phenomena are produced, and the timbre proves 

 equally characteristic to the ear. The timbre is therefore 

 the distinctive quality of the sonorous body the form, in a 

 certain sense, of sounds. 



Sound moves more rapidly in warm air than in cold; its 

 velocity in the atmosphere is 1 1 18*45 ^ ee ^ m a second at 16 C. 

 (60 '8 F.) or 1 086 '3 7 feet at zero, according to experiments 

 made by the Bureau of Longitudes in 1822; and according 

 to those of Bravais and Martins made in 1844, it is 1092*89 

 feet at zero (Cent.) This velocity is not modified by the varia- 

 tions in the pressure of the atmosphere, and it is the same 

 whether in a horizontal, vertical, or oblique direction. It is 

 increased or diminished by the wind, according as it blows 

 in the direction of the sound or contrary to it, though the 



