Mr. F. Guthrie on Approach caused by Vibration. 351 



between the two, they will be urged towards one another. To 

 test the reciprocity of the motive tendency in the case under con- 

 sideration, the following experiment was tried. 



§ 23. Experiment 8. — The tuning-fork A was fastened to the 

 end of a rod 1 metre long ; the other end of the rod was counter- 

 poised, and the whole was hung from a silk tape. If the ver- 

 tical plane passing through the rod be called V, then the rod 

 and fork received in succession the three relative positions: — 



(1) (H c ), V parallel to a. 



(2) (H c ), V parallel to b. 



(3) (H fl or H 4 ), V parallel to c. 



In (1) and (2) the fork was simply hung from the suspended 

 rod; in (3) it was fastened to an iron rod in the direction of its 

 axis, and the two were then attached to the suspended rod at their 

 common centre of gravity. The fork was sounded by the bow 

 as before, and a piece of card 0*05 metre square was brought near 

 the face a in (1), b in (2), and c in (3). In all cases the sus- 

 pended fork approached the card ; but, owing to the great inertia 

 of the suspended fork and counterpoise, the motion was much 

 slower and less striking than was the case when the card was 

 hung. 



§ 24. Experiment 9. — Further, instead of a card, a second 

 fork B (sounding A) was set in vibration, and brought into the 

 neighbourhood of the vibrating suspended fork A. The three 

 faces a' f V , c f of the fork B were held in succession parallel to 

 the three faces a, b, c of the fork A — that is, parallel to V when 

 the faces a, b } c were in each of the three positions described in 

 § 23. There were thus nine combinations effected. In every 

 case the suspended fork approached the stationary one. Hence, 

 to whatever cause the approach is due, the action is mutual. 



§ 25. The next question, the solution of which promised to 

 throw light upon our problem, was this : — What is the general 

 or mean condition as to tension of a medium in which undula- 

 tions are generated? Though this question has received very 

 great attention from theoretical physicists, it has not been ap- 

 proached, as far as I am aware, from the side of experiment in 

 the manner to be described. 



§ 26. Experiment 10. — The fork A was fixed in an upright 

 position in its sounding-box. One of its prongs was closed in a 

 glass tube T, 0*4 metre long and 0*042 metre internal diameter, 

 carrying a cork through which the prong passed. The upper 

 end of T (fig. 10) also carried a cork through which passed a 

 narrow tube /, bent twice at right angles and dipping into water. 

 The internal diameter of / was - 0035 metre. The corks of the 

 tube T were made tight with wax, and a little air was expelled 



