312 On Sounds from heated Metals. [1831. 



side. Beingnow placed with the groove down ward a upon a table, 

 and shaken, it rocks to and fro, and is in right condition for the 

 experiment. It is convenient to fasten a brass wire, terminated 

 byaknob, to one end of this rocker, so as to act as a prolongation 

 of an axis : it renders the whole arrangement steady and regular 

 in action. When this piece of metal is used instead of the 

 poker, musical sounds are almost always produced. The surface 

 of the lead upon which it rests should be clean. 



The peculiar effects exhibited in these experiments depend 

 upon the occurrence of isochronous vibrations performed by 

 the rocker. When by loading the rocker these are rendered 

 slow, they become visible ; but when they occur with sufficient 

 rapidity, they produce the necessary result, a musical note, of 

 higher or lower pitch, as the vibrations or tappings are more 

 or less numerous. It often happens that other and extraneous 

 sounds, as those due to the ringing of the metal, the vibration 

 of the table, or subdivisions of the whole vibrating system, 

 mingle with the true sound produced by the blows of the 

 rocker ; these were referred to and illustrated, and a method 

 shown of easily distinguishing the latter from the former. It 

 consisted in pressing perpendicularly with a small stick or 

 pointed metal rod on the back of the rocker, exactly over the 

 groove, so as to make the vibrations quicker, but not to disturb 

 their regularity; the true sound of the beats of the rocker 

 immediately rises in pitch, and may be sometimes made to pass 

 through an octave or more at pleasure, falling again as the 

 pressure is removed. 



As the sound is evidently due to the rapid blows of the rocker, 

 the only difficulty was to discover the true cause of the sustaining 

 power by which the rocker was continued in motion, whilst any 

 considerable difference of temperature existed between it and 

 the block of lead beneath ; this Mr. Faraday referred to the 

 ultimate expansion and contraction, as Professor Leslie and 

 Mr. Trevelyan have done generally ; but he gave a minute 

 account of the manner in which, according to his views, such 

 expansion and contraction could produce the effect. When 

 the heated rocker is reposing upon a horizontal ridge of lead, 

 it touches at two points, which are heated and expanded, and 

 form, as it were, two hills ; when one side of the rocker is raised, 

 the point relieved from its contact is instantly cooled by the 



