FROM A VIBRATING- BODY TO A SITRROIJNDINa GAS. 
463 
post, the ordinary comparatively bass sound with which we are so familiar was heard, 
appearing to emanate from the post. On receding from the post the bass sound became 
feebler, and midway between two posts was quite inaudible. Nothing was then heard 
but the peculiar high-pitched sound, which appeared to emanate from the wires over- 
head. It had a peculiar metallic ring about it which the ear distinguished from the 
whistling of the wind in the twigs of a bush. Although the 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 ques- 
tion, 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 number of different vibrations, having different 
periodic times, coexisted. As regards the vibrations of comparatively long period, the 
air around the wires behaved nearly like an incompressible fluid, and no sonorous 
vibrations of sensible amount were produced. These vibrations of the wires were, 
however, communicated to the posts, which being broad acted as sounding-boards, and 
thus sonorous vibrations of corresponding period were indirectly excited 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 T66 inch; and if we take the C below 
the middle C of a piano for the representative 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 X=50x2~ 5 , giving in the former case mc=’ 01043, and in the latter 
mc =’ 3338 The former of these values is so small that we may take I=(mc)~ 2 ; in the 
latter case the formula (32) gives for I a value a little less than We find in the 
two cases 1=9192 and I=7'202 respectively, so that in the former case the sound is 
more than 9000 times feebler than 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 by a very simple experiment. Take a tuning-fork, and holding 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 near to the fork as conveniently 
may be without touching. If the plane of the obstacle 
coincide with either of the planes of symmetry of the fork, 
as represented in section at A or B, no effect is produced ; 
but if it be placed in an intermediate position, such as C, the 
sound becomes much stronger. 
mdccclxviii. 3 T 
