INSTRUMENTS FOR EXPERIMENTS ON SOUND. 147 



reciprocity. They have been employed of late years as a 

 measure of time ; their pendular vibrations are so regular, so 

 accurate, and so easily adjusted to any one period of vibra- 

 tion, that they furnish an admirable means of measuring 

 small intervals. Here is a beautiful instrument contributed 

 by the French Conservatoire des Arts et Metiers, in which a 

 tuning-fork has been constructed for that purpose. It has 

 little stiles attached to the prongs, and as it vibrates they 

 touch a piece of blackened paper which runs slowly past 

 them. The tuning-fork makes an undulatory line upon it, 

 which is the harmonic motion as it were unfolded. Another 

 stile beside the first enables you to mark any instant of time ; 

 for example, the passage of a star across the wires of a tele- 

 scope, and to measure the exact period at which this took 

 place by counting the number of pendular vibrations which 

 the tuning-fork has made since a given period, previously 

 marked on the paper. Acoustical instruments have also 

 been found useful even for the measurement of the rapidity 

 of light ; the coarser form of vibration serving to measure 

 the quicker and more ethereal. Foucault's beautiful instru- 

 ment for measuring the rapidity of light is in the Loan 

 Exhibition, and you will find that it is worked by the instru- 

 ment which I shall speak of presently, namely, a small 

 siren. He found that the best plan to make a mirror rotate 

 at the enormous speed required, was to attach it to a small 

 turbine or siren played by steam at a high pressure ; as it 

 rotated more quickly so the note went up. The number of 

 vibrations is easily known from the pitch of the note ; and 

 he could thereby say how many times in a second the rapidly 

 rotating mirror was revolving, simply by taking the pitch of 

 the siren which was going round with it. 



When tuning-forks are struck alone, as I said, they give a 

 very feeble note, but we can alter this by combining them 

 with some resonator. The usual resonator is a box contain- 

 ing a body of air ; but Helmholtz has pointed out that a 

 string can be made to perform the same function. The 

 arrangement of the string is a little elaborate, but everybody 

 knows the plan by which tuning-forks are fastened on a 

 resonance-box, and the moment it touches it, it gives the 

 tone ; of that I shall speak again. The weak point musically 

 of tuning-forks is, the very evanescent character of their 

 sound. It is troublesome, the moment you have struck 



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