St?'ess and Strain on the Properties of Matter. 97 



block at D, and is then passed over the pulley at C, so as to be 

 stretched to any required extent by placing weights on the scale-pan S. 

 When the required stress is attained the wire is clamped at E, and 

 the part to the right of E having been detached from the pulley and 

 scale-pan, is drawn on one side and rested on some non-conductor of 

 sound, such as baize or flannel. In several cases, before the last-men- 

 tioned adjustment had been completed, the blocks of wood at E were 

 shifted backwards or forwards, until the note given out by the longi- 

 tudinally rubbed wire was in unison with a Koenig's tuning-fork, but 

 in others the pitch of the note was determined with the syren. If 

 the clamps at D and E had been secured to perfectly rigid supports, 

 the number of vibrations obtained when the wire was clipped in the 

 centre would have been exactly double the number when the wire was 

 free, except at both ends, but in consequence of lack of rigidity of 

 the supports at D and E, the note given out in the former case had 

 less than double the number of vibrations of the note in the latter. 

 Now Lord Rayleigh has proved for transverse vibrations* that when, 

 as in the present instance, the mass at each end is large compared with 

 the force of the spring which urges the extremity attached to the 

 mass towards the position of equilibrium, any slight yielding of the 

 supports will cause a rise in pitch, and will produce the same effect 

 as if the wire had been shortened in the ratio of 1 : 1—h/n 2 , where h 

 is a constant, if we experiment with the same length of the same wire 

 under the same conditions as regards the nature of the supports, and 

 n is the tone of the wire. Lord Rayleigh's mathematical reasoning 

 <3an be equally applied to longitudinal vibrations, and it is obvious 

 that by obtaining the number of vibrations of the wire when free, 

 except at both ends, and then when clipped in the centre, we may 

 determine the amount by which the yielding of the supports heightens 

 the pitch of the note. For the sake of greater accuracy the number 

 of. vibrations yielded when the wire was clipped one-third of its whole 

 length from one end was in some cases also ascertained. The next 

 experiments will sufficiently illustrate the mode of proceeding. 



lengthening the wire slightly it was very much improved, though of course of a 

 different pitch. The very marked want of clearness was presently found to arise 

 from synchronism between the time taken by a pulse to pass from one block to the 

 other through the ivood and the time taken to pass from end to end of the wire and 

 back again. When the blocks were insulated from the box the want of clearness 

 vanished and the pitch of the note rose 6 or 7 per cent. 

 * ' Theory of Sound,' vol. 1, § 135. 



