292 Lord Rayleigh on the Production 



Thus it' t'o, j)^ rej)resent the volume and pressure of air in 

 its natural (atmospheric) condition, v, p the corresponding 

 quantities under compression, so that according to Boyle's 

 law pv=PqVq, then the work (W) of compression is given by 



^y=PoVii\og(p/p,), (6) 



or, if the compression be small, 



W=^>o^-o^ 0) 



Po 



In c.G.s. measure ^^o (the atmospheric pressure) will be 10^ 

 and if Vq be measured in cubic centimetres, W will be ex- 

 pressed in ergs. If in (7) Vq be understood to mean the 

 volume compressed per second of time, W will be given in 

 ergs per second, of which 7*4:6 x 10^ go to the horse-power. 



The first example is that of a small horn (without valves) 

 blown by the lips. It resonates to e^ of my harmonium, and 

 the pitch when sounded is about e' flat. From one inspira- 

 tion I can blow it for about 30 seconds with a pressure (in 

 the mouth) of IJ inch (3'8 cm.) of mercury. The contents of 

 the lungs may be taken at 1200 c.c, giving 40 c.c. per second 

 as the wind consumption. This is the value of Vq? ^^^ 

 {p — Po)IPo is 2^0. Hence W in ergs per second will be 

 2 X 10^, or in horse-power 



W = -00027 H.P. 



The sound from this very small horse-power is unpleasantly 

 loud when given in a room of moderate dimensions. 



In the case of the harmonium reed e' the wind consumption 

 w^as 220 c.c. per second, and the pressure 2 inches of water, 

 so that (j9— po)/i^o=2 0d- Hence 



W = -00015 H.P. 



A small hand fog-signal of Holmes' pattern, known as the 

 " Little Squeaker/' consumed a horse-power calculated on 

 the basis of similar measurements to be "03. For the very 

 effective " Manual " of the Trinity House Service the horse- 

 power was about 3*0. 



These examples may all be classed under the head of reeds, 

 the harmonium reed being " free " and probably in conse- 

 quence less efficient, and the others " striking." To them 

 may be added the case of a whistle of high pitch *, for which 

 the wind consumption represented 1-8 X 10^ ergs per second, 

 or -00024 horse-power, practically the same as for the small 

 horn above. The latter was certainly the more powerful of 

 the two, considered as a source of audible sound. 



* Proc. Roy. Soc. xxvi. p. 248 (1877) ; Scientific Papers, i. p. 329. 



