20 



ANIMAL MECHANICS. 



tion with the standard CCC, and the beats counted ifitbenot 

 in unison. 



Let m denote the number of vibrations per second in the 

 standard pipe, n the corresponding number in the tuned su- 

 surrus pipe, or vice versa, so that m shall be greater than n y 

 and let k be the number of beats per second counted on play- 

 ing the two pipes together ; then it is easily shown that 



li-m — n. (6) 

 As examples of this principle I take the following observa- 

 tions : — 



Dr. William Stokes, Regius Professor of Physic in the 

 University of Dublin, kindly aided me in my investigations 

 upon one occasion, by tuning the wooden pipe to the susurrus 

 observed in himself, leaning both his elbows on a table, with 

 the muscles of the fore-arm contracted, and making the sound 

 of the pipe coincide with the susurrus when heard by both 

 ears together. 



On comparing the note he fixed on, with the standard CCC, 

 it was found to be higher than it to such an extent as to pro 

 duce 50 beats in 15 seconds. 



Hence, 



5° 



— = m- ; or 

 3 J = m- 3 2; 



and, finally, 



m== 35s vibrations per second. 

 This is DDD natural, a little flat, for DDD corresponds to 

 thirty-six vibrations in the second. 



In July, 1863, Professor Czermak, of Prague, assisted me 

 at an acoustical experiment, and found his susurrus to corre- 

 spond with a position of the moveable piston below CCC, such 

 as to give 40 beats in 20 seconds. 



