Air-Pressures used in Playing Reed Instruments. 271 



The values o£ otWl/u for the different values o£ X are 

 given in the following table, also the corresponding values of 

 V'W :- 



X. 



aWl 



u 



Y'2 



Co 2 ' 



P 



in volts. 



40 

 50 



10" 4 

 3xl0~ 4 



640 



705 



23-7 

 26 



70 



1-3X10 -3 



750 



28 



90 



3-3X10" 3 



790 



29 



If V be the velocity acquired by an electron in travelling 

 freely between two points differing in potential by P volts, 



it is easy to show that — r = 27P. 

 J Co 2 



The values of P thus obtained are given in the last column 

 of the above table, the determination corresponding to the 

 lowest value of X being the most reliable. 



It thus appears that the energy required to produce 

 ionization by collision in air is about 23e/300, e being the 

 charge on the ion. This estimate of the energy is in agree- 

 ment with that previously obtained by considering the larger 

 values of <x. 



It is remarkable that large alterations in the numbers 

 given in the second column have very little effect on the 

 values of P. It is only necessary, therefore, to know the 

 values of a, I, W, and u approximately, but the value of k 

 must be known accurately, as an error in k would produce a 

 proportional error in P. 



XXX. Air-Pressures used in Playing Peed Instruments. 

 By Charlton W. H. Foord, Mem. Royal Soe. of Arts* . 



[Plate III.] 



IN Dr. W. H. Stone's work 'Elementary Lessons on 

 Sound' (Macmillan 1891) a table is given showing the 

 maximum and minimum wind-pressures used in playing 

 various wind instruments. The investigation of these 

 pressures has been carried further by Dr. E. H. Barton and 



* Communicated by the Author, 



