1 870.] Mr. F. Guthrie on Approach caused by Vibration, 35 



Weekly Returns of Births and Deaths in each of the Districts of the City 

 of Dublin. Yol. VI. Nos. 23-48, 50-52 ; Vol. VII. Nos. 1-12, 14, 

 15, 17, 19-22. 8vo. Dublin 1869-70. Quarterly Return of the Mar- 

 riages, Births, and Deaths registered in the Divisions and Districts 

 of Ireland, Nos. 21, 22, 24. 8yo. Dublin 1869. 



The Registrar General for Ireland. 



The Society adjourned over the Long Vacation to Thursday, Novem- 

 ber 17. 



ff On Approach caused by Vibration." By Frederick Guthrie, 

 B.A. Communicated by Prof. G. G. Stokes, Sec. R.S. Re- 

 ceived August 26, 1869*. 



§ 1 . The chain of experiments which I have to describe arose from the 

 endeavour to explain an observation that a delicately suspended piece of 

 cardboard moves, from a considerable distance, towards a vibrating tuning- 

 fork. It will be preferable to detail the experiments, not in the order in 

 which they occurred to me and were actually performed, but in the order 

 in which I conceive them to form a logical sequence. 



§ 2. The experiment of Clement shows that when a continuously renewed 

 current of air passes between two parallel disks from the common axis to- 

 wards the circumference, the disks are urged together. Consequently, in 

 seeking to explain the fact observed in § 1, it was necessary to examine the 

 air surrounding the resonant fork in order to ascertain whether air-currents 

 existed in its neighbourhood ; and further, to distinguish between such 

 currents as might be found to move in closed curves forming whirlwinds in 

 the immediate neighbourhood of the fork, and such as might radiate in un- 

 closed paths from the fork through the air. 



§3. In 1831 Mr. Faraday f, in tracing the cause of the accumulation of 

 light particles on the internodal points and lines of vibrating bodies, came 

 to the conclusion that such accumulation was due to minute whirlwinds, and 

 not, as had been held by M. Savart J, to the existence of secondary nodes. 

 A general conclusion at which Mr. Faraday arrived was this : whenever the 

 different parts of a surface are vibrated to different degrees, there is always 

 a tendency for the air to flow along the surface of the vibrating body to- 

 wards the more violently agitated portions from the less agitated. 



§ 4. It is clear that, before examining the possible connexion between 

 these superficial whirlwinds and the fact mentioned in § 1, it is necessary 

 to examine into the existence of air-currents of unclosed paths. 



* Head Dec. 17, 1868. See abstract, vol. xvii p. 106. 

 f Phil. Trans. 1831, p. 299. 



X Ann. de Chim. et de Pbys. t. xxxvi. pp. 187, 257* 



D 2 



