Energy and Apparent Intensity of Sounds of different Pitch. 383 



was found in following out Topfer's method of observation with 

 accuracy, until the minutiae which follow were attended to. The 

 object is to observe the time of descent of the bellows under dif- 

 ferent circumstances. A pendulum of bullet and string was 

 employed to observe the time, by counting the oscillations ; the 

 eye was thus occupied. The points of full and empty were de- 

 termined by the feel of the bellows-handle, as follows : — The 

 excess-valve is placed between the bellows and one of the feeders ; 

 so that, when the bellows is overfull, the pressure of the wind 

 comes on the feeder, and destroys the equilibrium of the handle, 

 causing a downward pressure, which is sustained by the hand. 

 At the moment of closing of the excess-valve this pressure dis- 

 appears, and thus the moment of departure of the observation is 

 distinctly announced. A similar sudden change of pressure in- 

 dicates the emptying of the bellows and close of the observation. 

 Until this method was adopted it was impossible to obtain the 

 accuracy required ; but with it the accuracy was great, so that, 

 if the same element was observed repeatedly, the number came 

 almost always the same. 



As a preliminary experiment, I observed some time ago the 

 wind-consumption of the five C pipes ; the results were as 

 follows : — 



g . . 95 59 641 40-0 



-M-4 

 -1-6 



+0-2 



d" . . 96 91 57 57 



The first column contains the name of the key depressed, a is 

 the number of oscillations of the pendulum counted during de- 

 scent of bellows with stop closed, b is the number of oscillations 



during descent of bellows, pipe sounding. - and t are conse- 

 quently the fractions of the content of the bellows consumed in one 



oscillation in the two cases, and 7 is the fraction of the con- 



a 



tent of the bellows consumed by the pipe alone in one oscillation of 



the pendulum ; and this I have taken as the measure of the work 



W converted into sound during one oscillation of the pendulum. 



— , where t is referred to the periodic time of c'" as unity, is 



the next column ; it should be constant according to Topfer's 



law, and is the measure of intensity according to what precedes. 



The number for c is seen to depart widely from the result of 



Cp . 



. 92 



69 



406 



50-7 



c' . 



. 93 



79 



191 



47-7 



c" . 



. 100 



91 



99 



49-5 



