and on Reed Organ-Pipes. 257 



was obliged to alter the length of the portevent, or tube con- 

 veying- the wind from the bellows to the "reed, as there appeared 

 to be some given length of portevent for each reed, which com- 

 pletely prevented the reed from speaking, but it did not appear 

 that these lengths followed any law. As this phenomena ap- 

 peared to be of the same nature, as that I have just described, 

 I set about to investigate it with the following apparatus. 



By way of portevent I took two brass telescope tubes AB, 

 CD, Fig. 16, sliding tightly over each other, and each a foot in 

 length, the internal diameter of CD being .5 inches, at ^ was a 

 socket for the reception of the reed R. Two additional tubes 

 (as EF), with sockets, served to increase the length, so that by 

 combining the use of these with the slide BC, I could try the 

 effect of a portevent of any length from one foot to four. 



For shorter lengths than this, I made use of a pipe AB, 

 Fig. 13, to the end of which was attached a piston BV, carrying 

 a reed R, and fitting air-tight in the portevent TTC, which is 

 represented as standing on the wind-chest of the organ. 



The note of the first reed I tried was = 4.7 inches by the 

 pitch-pipe, and upon drawing the piston BF up by means of 

 its attached pipe AB, the reed refused to speak, when T^=3.4in. 

 and sounded again when TF=b.b 



Now according to the former experiments, this should have 

 begun to happen when TV was nearly equal to the half wave ; 

 but the note of the reed I employed = 4.7 inches by the pitch- 

 pipe, which, corrected by Bernouilli's Table, gives about 5.3 for 

 the half wave, so that the portevent was much too short for ray 

 theory*. The cause of this anomaly appeared to lie in the short 

 passage efg already described, which conveys the wind from the 



* Especially as the waves in these experiments always come out rather longer than 

 the corrected length of the pitch-pipe, (See Note A.) 

 Vol. III. Part I. K It 



