BY COLOURED MEDIA. 



4 8 7 



(12.) The disked tuning fork is a most instructive in- 

 strument, and I shall not quit it until I have availed my- 

 self of its properties to exemplify the easy propagation of 

 vibrations, of a definite pitch, through a system compara- 

 tively much less disposed to transmit those of any other 

 pitch. Take two or more forks in unison, and furnish 

 each of them with a single disk of the size of a large 

 wafer, looking outwards. Having struck one of them, 

 let its disk be brought near to that of the other, 

 centre opposite to centre, and it will immediately 

 set the other in vibration, as will be evident by the 

 sound produced by it when 

 the first fork is stopped, as 

 well as by its tremors, sensible 

 to the hand which holds it. 

 The communication of the 

 vibration is much more power- 

 ful and complete when a 

 small loop of fine silver wire 

 is fixed to one of the forks, 

 and brought lightly into con- 

 tact with the other, with its 

 looped or convex side. Ima- 

 gine now a series of such 

 forks and loops arranged as in the annexed figure, and 

 let the first, A, be maintained in vibration by any excit- 

 ing cause, as, for instance, by sounding a musical note 

 opposite to its disk, A, in unison with its pitch. The 

 vibrations so excited will, as is evident, run along the 

 whole line, though with diminishing intensity, to the last 



