[ 345 ] 



XLV. On Approach caused by Vibration. 



By Frederick Guthrie. 



[With a Plate.] 



To the Editors of the Philosophical Magazine and Journal, 



Gentlemen, 



IN the Proceedings of the Royal Society of London, vol. xix. 

 p. 35, there is published a considerable portion of the ac- 

 companying paper " On Approach caused by Vibration." The 

 chief part thence omitted is that relating to the modifications 

 which Mr. Faraday's " surface-currents " undergo when excited 

 in the neighbourhood of a rigid plane. I beg to submit the 

 complete series of experiments to your readers, because I believe 

 that the examination of these currents has a distinct bearing 

 upon the chief question — not in regard to its ultimate solution, 

 but in regard to the method by which that solution is obtained. 



Yours truly, 



Frederick Guthrie. 

 September 14, 1870. 



§ 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 continu- 

 ously renewed current of air passes between two parallel disks 

 from the common axis towards the circumference, the disks are 

 urged together. Consequently, in seeking to explain the fact 

 observed, § 1, it was necessary to examine the air surrounding 

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

 isted in its neighbourhood — and, further, to distinguish between 

 such currents as might be found to move in closed curves, form- 

 ing whirlwinds in the immediate neighbourhood of the fork, and 

 such as might radiate in unclosed paths from the fork through 

 the air. 



§ 3. In 1831 Mr. Faraday*, in tracing the cause of the ac- 

 cumulation of light particles on the internodal points and lines 

 of vibrating bodies, came to the conclusion that such accumula- 

 tion was due to minute whirlwinds, and not, as had been held by 

 M. Savartf, to the existence of secondary nodes. By fastening 



* Philosophical Transactions, 1831, p. 299. 



t Ann. de Chim. et de Phys. vol. xxxvi. pp. 187,257. 



Phil. Mag. S. 4. Vol. 40. No. 268. Nov. 1870. 2 A 



