MUSICAL SOUNDS FROM VIBRATION OF AIR. 249 



densed at B, preserving its natural density at the middle 

 point between A and B. In the case of the spring the 

 ends A B are alternately pushed outwards and pulled 

 inwards by the spring, the end A being pushed outwards 

 in No. 2, and B pulled inwards, while in No. 3 A is 

 pulled inwards and B pushed outwards. 



That the air vijbrating in a pipe is actually in the state 

 now described may be shown by boring small holes in the 

 pipe, and putting over them pieces of a fine membrane. 

 The membrane opposite to the middle part between A 

 and B, where the particles of the air have the greatest 

 motion, will be violently agitated, while at points nearer 

 the ends A and B it will be less and less affected. 



Let us now suppose two pipes A B, B C, to be joined 

 together as in Fig. 42, and to be separated by a fixed 



N'2 



partition at B ; and let a spiral spring be fixed in each. 

 Let the spring A B be now pushed to the end A, while the 

 spring B C is pushed to C, as in No, 1, and back again, as 

 in No. 2, but always in opposite directions ; then it is 

 obvious that the partition B is in No. 1 drawn in opposite 

 directions towards A and towards C, and always with 

 forces equal to each other, that is, when B is drawn 

 slightly towards A, which it is at the beginning of the 

 motion, it is also drawn slightly towards C, and when it 

 is drawn forcibly towards A, as it is at the end of the 

 motion of the spring, it is also drawn forcibly towards C. 

 If the partition B, therefore, is moveable, it will still 



