190 Lord Bayleigh [Jan. 20, 



will slide upon a stand here arranged for it. When put in this 

 position the flame is very little affected ; the place is what I call a 

 node — a place where there is great pressure variation, but no vibra- 

 tory velocity. If I move the glass back, the flame becomes vigorously 

 excited ; that position is a loop. Move it back still more and the 

 flame becomes fairly quiet ; but you see that as the plate travels gradu- 

 ally along, the flame goes through these evolutions as it occupies in 

 succession the position of a node or the position of a loop. The 

 interest of this experiment for our present purpose depends upon this 

 — that the distances through which the glass plate, acting as a 

 reflector, must be successively moved in order to pass the flame from 

 a loop to the next loop, or from a node to the consecutive node, is in 

 each case half the wave length ; so that by measuring the space 

 through which the plate is thus withdrawn one has at once a measure- 

 ment of the wave length, and consequently of the j)itch of the sound, 

 though one cannot hear it. 



The question of whether the flame is excited at the nodes or at 

 the loops, — whether at the places where the pressure varies most or 

 at those where there is no variation of pressure, but considerable 

 motion of air — is one of considerable interest from the point of view 

 of the theory of these flames. The experiment could be made well 

 enough with such a source of sound as I am now using ; but it is made 

 rather better by using sounds of a lower pitch and therefore of greater 

 wave-length, the discrimination being then more easy. Here is a 

 table of the distances which the screen must be from the flame in 

 order to give the maximum and the minimum efi'ect, the minimum 

 being practically nothing at all. 



Table of Maxima and Minima. 



Max. Min. 



1-1 



4-5 



7-5 



10-3 



130 



3-0 



5-9 



8-9 



11-7 



14-7 



15-9 



The distance between successive maxima or successive minima is 

 very nearly 3 (centims), and this is accordingly half the length of the 

 wave. 



But there is a further question behind. Is it at the loops or is 

 it at the nodes that the flame is most excited ? The table shows 

 what the answer must be, because the nodes occur at distances from 

 the screen which are even multiples, and the loops at distances 

 which are odd multiples ; and the numbers in the table can be 

 explained in only one way — that the flame is excited at the loops 



