350 Mr. F. Guthrie on Approach caused by Vibration. 



thrown across the middle line parallel to/, remain on the fork 

 for a longer time. The apparent effect produced is assisted by 

 the retention of images on the retina ; and the appearance is as 

 though a nebulous band of powder rested along the middle of 

 the c face parallel to/. On account of the greater amplitude of 

 the vibrations at the edges of the prong, a particle there will be 

 hurled forward with greater force than one nearer to the central 

 line. If the particle projected happen to alight upon the fork 

 when the latter is moving in the same direction as the particle, it 

 will be hurled forwards again, and perhaps be precipitated over 

 the edge parallel to f. If it alight when the fork is moving in 

 the opposite direction it will be thrown back again. 



§ 19. It appears, then, from §§ 11-18 that when the fork is 

 vibrating in a plane parallel to b, it is only on face a that any 

 appreciable air-circuits are formed, and that there such circuits 

 do not extend sensibly beyond at most 0*006 metre. 



§ 20. We shall see that the existence of such air-circuits, con- 

 fined as they are to the immediate vicinity of the fork, are quite 

 insufficient to account for the class of phenomena which have to 

 be described, and which are similar to the fundamental fact men- 

 tioned in § 1. 



§ 21. Experiment 7. — To one end of a splinter of wood, 0*5 

 metre long, a card 0*08 metre 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 unspun silk 

 (fig. 9) and counterpoised. The tuning-fork A was set in vibra- 

 tion as before, and was brought towards the card in the three re- 

 lative positions corresponding to those of § 6 ; namely : — 



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



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



(3) (H a or Ha). 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 metre at least,— a distance quite beyond the 

 direct influence of the superficial whirls which exist in position (1) 

 (on face a). 



§ 22, There is perhaps nothing essentially contrary to reason 

 in the conception of two bodies in space free to move, so related 

 to one another that while the first has no tendency to move to- 

 wards the second, the second has a tendency to move towards 

 the first. But if the tendency of the one to move is caused by 

 the condition of the medium between the two, it seems inevitable 

 that the tendency shall be mutual. Thus, if that tendency result 

 from a general diminution in the tension of an elastic medium 



