278 DYNAMICAL THEORY OF SOUND 



Metal pipes are richer in harmonics than wooden pipes of 

 the same dimensions. This may be partly due to the greater 

 fineness of the lip, which introduces a greater degree of abrupt- 

 ness in the action of the jet, and so favours the amplitude of 

 the terms of higher order in the Fourier series which expresses 

 the driving force. Another source of the contrast in quality 

 may be found in the smaller rigidity and imperfect elasticity 

 of the walls of a wooden pipe, which may tend to absorb the 

 energy, especially in the case of the higher harmonics. 



The " speaking " of a resonator of any kind, when a jet of 

 air is blown across its mouth, is to be explained on the same 

 principles. In a resonator of the usual type the normal modes 

 after the first are far removed in pitch from the fundamental, 

 and are not sensibly excited by the essentially periodic impulse. 

 The note obtained is therefore a pure tone. 



The vibrations of a column of air may also be excited by the 

 periodic application of heat, as in the well-known experiment 

 of the " singing flame," where a jet of hydrogen burns within an 

 open cylindrical pipe. For the maintenance of the vibration it 

 is necessary that heat should be supplied at a moment of con- 

 densation, or abstracted at a moment of rarefaction. To explain 

 how the adjustment is effected, it would be necessary to take 

 account of the fact that the vibrating system includes the gas 

 contained in the supply tube of the jet, as well as the column 

 of air in the pipe. The matter is thus somewhat intricate, 

 but a satisfactory theory has been made out, which accounts 

 clearly for the several conditions under which the experiment 

 is found to succeed or to fail*. 



90. Theory of Reed-Pipes. 



The mechanism of the reed stops of the organ is quite 

 different. The current of air issuing from the wind-chest is 

 made intermittent by its passage through a rectangular 

 aperture in a metal plate, which is periodically opened and 

 closed (partially) by a vibrating metal tongue, or " reed." The 

 period is accordingly determined mainly by the elasticity and 

 inertia of the tongue itself. The vibrations of the latter were 



* Lord Rayleigh, Theory of Sound, 322 h. 



