

Mr. F. Guthrie on Approach caused by Vibration, 347 



as in § 5, was brought to the open end of the wide tube. The 

 fork and tube had in succession the four relative positions shown 

 in fig. 2, namely : — 



(1) H c , axis of tubes perpendicular to a. 



(2) H c , axis of tubes perpendicular to b. 



(3) H a or H 6 , axis of tubes perpendicular to c. 



(4) The same as (3), but having one prong of the fork thrust 

 as far as possible into the tube T. In none of the cases did the 

 smoke show any tendency to escape through the tube t ; nor was 

 fresh air drawn in. 



§ 7. Experiment 3. — The cork and tube t of Experiment 2 

 were withdrawn and replaced by a film bubble of glycerine-soap 

 water. The combinations of position of Experiment 2 were re- 

 peated. 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 penetrating the surrounding air in unclosed 

 paths. 



§ 9. The superficial whirlwinds examined by Mr. Faraday 

 may be supposed 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 neigh- 

 bourhood of the resonant fork, some experiments supplementary 

 to those of Mr. Faraday were found necessary. 



§ 10. In general, when a light powder is strewn upon a 

 smooth surface and a vibrating fork is brought near the surface, 

 the arrangement of the particles of the powder will depend upon 

 (1) the superficial whirlwinds, and (2) the nodal lines of the sur- 

 face which vibrates by induction. Accordingly, and to get rid 

 of the latter class of phenomena, my first experiments in this 

 direction were made by employing an anvil with a smooth sur- 

 face to receive the powder. However, as experiment speedily 

 showed me that the arrangement of the powder on the surface 

 was the same upon a sheet of glass, cardboard, or paper as upon 

 an anvil, I adopted the first of these substances, on account of 

 its smoothness, and because it enabled me to examine the mo- 

 tions of the particles from below. 



§ 11. Experiment 4. — Freshly baked carbonate of magnesium 

 was evenly strewn upon a horizontal sheet of plate glass. The 

 fork A, screwed into its sounding-box, was so adjusted that when 

 at rest (H a ) the surface a was parallel to the plate. The bow 

 was drawn across/. When the fork was in full vibration (0*0025 

 metre amplitude) the glass plate was raised vertically. 



2 A2 



