from a Vibrating Body to a surrounding Gas. 421 



telegraph ran for miles, it was only at one spot that the peculiar 

 sound was noticed, and even there only in certain states of the 

 wind. The wires seemed to be less curved than usual at the 

 place in question, from which it may be inferred that they were 

 there subject to an unusually great tension. 



The explanation of the phenomenon is easy after what precedes. 

 The wires were thrown into vibration by the wind, and a num- 

 ber of different vibrations, having different periodic times, coex- 

 isted. As regards the vibrations of comparatively long period, 

 the air around the wires behaved nearly like an incompressible 

 fluid, no sonorous vibrations of sensible amount were produced. 

 These vibrations of the wires, however, were communicated to the 

 posts, which being broad acted as sounding-boards, and thus 

 sonorous vibrations of corresponding period were indirectly ex- 

 cited in the air. But as regards the vibrations of extremely short 

 periodic time, the wires in spite of their narrowness were able, 

 by acting directly on the air, to produce condensations and rare- 

 factions of sensible amount. 



The diameter of the telegraph-wire was about *166 inch; and 

 if we take the C below the middle C of a piano for the represen- 

 tative of the pitch of the lower note, and a note five octaves 

 higher for that of the higher, we have in the first case X=50 

 inches nearly, and in the second \ = 50x2 -5 , giving in the 

 former case mc= '01043, and in the latter mc = *3S38. The 

 former of these values is so small that we may take 1= (mc)~ 2 ; 

 in the latter case the formula (32) gives for I a value a little less 

 than (mc) " 2 . We find in the two cases I = 9192 and I = 7*202 re- 

 spectively ; so that in the former case the sound is more than 

 9000 times as feeble as that corresponding to the amplitude of 

 vibration of the wire on the supposition of the absence of lateral 

 motion, whereas in the latter case the actual intensity is nearly 

 one-seventh of the full intensity corresponding to the amplitude. 



The increase of sound produced by the stoppage of lateral 

 motion may be prettily exhibited A 



by a very simple experiment. 

 Take a tuning-fork, and hold- 

 ing it in the fingers after it has 

 been made to vibrate, place a 

 sheet of paper or the blade of a 

 broad knife with its edge parallel 

 to the axis of the fork, and as m 

 near to the fork as conveniently H 

 may be without touching. If the 

 plane of the obstacle coincide with either of the planes of sym- 

 metry of the fork, as represented in section at A or B, no 

 effect is produced ; but if it be placed in an intermediate posi- 

 tion, such as C, the sound becomes much stronger. 



