120 Proceedings of the 



of the groove on each side. Being now placed with the groove 

 downwards 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 by a knob, 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 its 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 con- 

 sisted 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 was 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 ex- 

 pansion and contraction, as Professor Leslie and Mr. Trevellyan have 

 done generally; but he then 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 also 

 heated and therefore expanded, and form two hills ; when one side 

 of the rocker is raised, the point relieved from its contact is instantly 

 cooled by the neighbouring portions of lead ; the expansion ceases, 

 and the hill falls. When the rocker, therefore, is left free, the 

 raised side descends through a greater space than that through 

 \vhich it was lifted ; and also to a lower level than the other side : in 

 consequence of which a momentum is given to it, which carries its 

 centre of gravity beyond the point to which it would pass if there 

 had been no alteration in the heights of the sustaining points. It is 

 this additional force which acts as a maintaining power; it recurs 

 twice in each vibration, i. e. once on each side. The force is 

 gained by the whole rocker being lifted bodily by the point on 

 which it is for the time supported, and comes into play by the side 



