10 On the Circulation of Air in Kundt's Tubes. [Nov. 15, 



More than twenty-five years have elapsed since Mr. Welsh made his 

 survey, and this was separated by an interval of twenty-one years from 

 that which we owe to the joint labours of Sir Edward Sabine, Sir James 

 Ross, and Mr. Fox. The instruments and methods of observation in 

 1858 were greatly superior to those of 1836-7, and hence a new 

 survey made during the approaching period of minimum sun-spot 

 disturbance, and on stations selected with careful reference to their 

 geological character, would undoubtedly afford far more accurate data 

 as to the absolute value of the magnetic elements and as to the extent 

 of secular change in this part of the world than we at present 

 possess. 



VL " On the Circulation of Air observed in Kundt's Tubes, 

 and on some Allied Acoustical Problems." By Lord 

 Rayleigh, D.C.L., F.R.S. Received October 23, 1883. 



(Abstract.) 



Experimenters in acoustics have discovered more than one set of 

 phenomena apparently depending for their explanation upon the exist- 

 ence of regular currents of air, resulting from vibratory motion, of 

 which theory has as yet rendered no account. This is not, perhaps, 

 a matter for surprise, when we consider that such currents, involving 

 as they do circulation of the fluid, could not arise in the absence of 

 friction, however great the extent of vibration. And even when we 

 are prepared to include in our investigations the influence of friction, 

 by which the motion of fluid in the neighbourhood of solid bodies 

 may be greatly modified, we have no chance of reaching an explana- 

 tion, if, as is usual, we limit ourselves to the supposition of infinitely 

 small motion and neglect the squares and higher powers of the 

 mathematical symbols by which it is expressed. 



In the present paper three problems of this kind are considered, 

 two of which are illustrative of phenomena observed by Faraday.* 

 In these problems the fluid may be treated as incompressible. The 

 more important of them relates to the currents generated over a 

 vibrating plate, arranged as in Chladni's experiments. It was dis- 

 covered by Savart that very fine powder does not collect itself at the 

 nodal lines as does sand in the production of Chladni's figures, but 

 gathers itself into a cloud, which, after hovering for a time, settles 

 itself over the places of maximum vibration. This was traced by 

 Faraday to the action of currents of air, rising from the plate at the 

 places of maximum vibration, and falling back to it at the nodes. In 



* " On a Peculiar Class of Acoustical Figures ; and on certain Forms assumed 

 by Groups of Particles upon Vibrating Elastic Surfaces." " Phil. Trans.," 1831, 

 p. 299. 



