IN CYLINDRICAL TUBES. 253 



SECTION II. 



23. I WILL now proceed to describe the experiments which have 

 been made with a view of putting the different theories on this subject 

 to an experimental test. Sonorous vibrations are usually excited in a 

 tube, either by directing a stream of air across the open end, as in 

 blowing across the embouchure of the flute; by means of a vibrating 

 tongue, as in all reed instruments ; or by placing an open end of the 

 tube close to the surface of a vibrating body. In the two first cases it 

 seems impossible to conceive that the same disturbance can be com- 

 municated to each part of the extreme section of the air in the tube 

 where the original motion is produced, a condition which is always 

 assumed to hold at least approximately in all our mathematical investi- 

 gations of the subject. This irregularity of the motion will no doubt 

 extend to some distance within the tube, and it is impossible to say 

 how it will affect the phenomena even in those parts of the tube in 

 which the motion may become more uniform. In the second case too 

 in particular, a stream of air must constantly be passing through the 

 tube, a circumstance not contemplated in our analysis of the problem. 

 This may or may not influence materially the observed phenomena, 

 but at all events the danger of derangement from any such cause 

 must be avoided, if we would render our experiments decisive tests 

 of the truth of any theory professing to account for phenomena of so 

 delicate a nature as those which are now the objects of our investigation. 

 The third method, however, above-mentioned, is entirely free from the 

 latter objection, and may be made almost entirely so from the former, 

 and is, therefore, that which I have adopted. 



24. The apparatus is very simple. Figure I. represents it. A 

 plate of common window glass is held firmly in a horizontal position 

 by a pair of pincers at its middle point. AB is a gltiss tube, having 

 a short brass tube closely sliding within it at the upper end B, so 

 that the whole tube AB can be lengthened or shortened at pleasure. 

 Within the tube a small* brass frame M, having a delicate membrane 



* Fig. (2) represents this frame with the membrane ab, which may be tuned, or rendered 

 sensitive in different degrees, to the vibrations produced by any proposed note, either by 

 Vol. V. Paet II. K k 



