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



to move be 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 be- 

 tween the two, they will be urged towards one another. To test the reci- 

 procity of the motive tendency in the case under consideration the follow- 

 ing experiment was tried. 



§ 14. Experiment 5. — The tuning-fork A was fastened to the end of a 

 rod 1*0 m. long; the other end of the rod was counterpoised, and the 

 whole was hung from a silk tape. If the vertical plane passing through 

 the rod be called V, then the rod and fork received in succession the rela- 

 tive 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 m. 

 square was brought near the face a (in 1), b (in 2), and c (in 3). In all 

 cases the suspended 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. 



§ 15. Experiment 6. — 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 , b', c of the fork B 

 were held in succession parallel to the three faces a, b, c of the fork A, 

 that is parallel to Y when the faces a, b, c were in each of the three posi- 

 tions described in § 14. There were thus nine combinations effected. Iu 

 every case the suspended fork approached the stationary one. Hence, to 

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



§ 16. 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 undulations are generated ? Though this 

 question has received very great attention from theoretical physicists, it 

 has not been approached, as far as I am aware, from the side of experiment 

 in the manner to be described. 



§ 17. Experiment 7. — The fork A was fixed in an upright position in 

 its sounding-box (fig. 4). One of its prongs was enclosed in a glass tube 

 T, 0*4 m. long, and 042 m. internal diameter, carrying a cork through 

 which the prong passed. The upper end of T also carried a cork, through 

 which passed a narrow tube t, bent twice at right angles, and dipping into 

 water. The internal diameter of t was 0*0035 m. The corks of the tube 

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

 by warming it with the hand, so that when the atmospheric temperature 

 was regained, the water stood at some distance up the tube t. The tube t 



