Mr. F. Guthrie on Approach caused by Vibration, 37 



drawn and replaced by a film-bubble of glycerine-soap-water. The com- 

 binations of position of experiment 2 were repeated. In none of the cases 

 did the bubble show any variation from the vertical plane. 



§ 8. Hence I conclude that when a tuning-fork is in a state of plane 

 vibration, no permanent true air-currents are formed; that is, no air- 

 currents could be detected departing from any side of the fork and pe- 

 netrating the surrounding air in unclosed paths. 



§ 9. The superficial whirlwinds examined by Mr. Faraday may be sup- 

 posed to be greatly modified when they are excited in the immediate 

 neighbourhood of a solid body ; and as the ''attraction" which formed 

 the starting-point of the present examination (§1) is exerted upon a solid 

 body in the neighbourhood of the resonant fork, some experiments, sup- 

 plementary to those of Mr. Faraday, were found necessary. 



§ 10. Numerous experiments, which need not be here detailed, showed 

 (1) that Mr. Faraday's surface-currents, as exhibited on a freely vibrating 

 fork, are very much modified when the fork vibrates in the immediate 

 neighbourhood of a rigid plane, and (2) that the effects of any currents 

 produced by vibration do not extend sensibly beyond 0'006 m. from the 

 fork's face, and only even to this extent near the a face. 



§11. We shall see that the existence of such air-circuits, confined as 

 they are to the immediate vicinity of the fork, are quite insufficient to ac- 

 count for the class of phenomena which have to be desciibed, and which 

 are similar to the fundamental fact mentioned in § 1 . 



§ 12. Experiment 4. — To one end of a -p- ^ 



splinter of wood, 0*5 m. long, a card 

 0'08 m. square was fastened in such a way 

 that the plane of the card was vertical, 

 and contained the line of the splinter. 



The whole was hung from a fibre of un- 



spun silk (fig. 3) and counterpoised. The 

 tuning-fork A was set in vibration as be- ' 



fore, and was brought towards the card in the three relative positions cor- 

 responding to those of § 6, namely : — 



(1) (H c ). The face a parallel to the card. 



(2) (H c ). The face b parallel to the card. 



(3) (H a or H 4 ). The face c parallel to the card. 



In all three Cases the card moved towards the fork. The rate at which 

 the card moved was greatest when the fork was sounding loudest. In all 

 three cases it was possible to draw the card from a distance of 0"05 m. at 

 least, — a distance quite beyond the direct influence of the superficial 

 whirls which exist in position (1) (on face a). 



§ 13. There is perhaps nothing essentially contrary to reason in the con- 

 ception of two bodies in space free to move, so related to one another that 

 while the first has no tendency to move towards the second, the second 

 has a tendency to move towards the first, But if the tendency of the one 



