450 Prof. FORBES on the Vibrations which take place 



space removed. The sudden changes of note before alluded to 

 have been very satisfactorily accounted for by Mr ROBISON, as 

 arising from a sudden movement of the bar, which, by changing 

 its points of bearing, of course alters the velocity of vibration. 

 The rise of tone which is usual towards the end of the experi- 

 ment, depends on the diminished impulse received at each stroke, 

 and consequent diminution of the arc of vibration. 



42. Indeed the success of the whole experiment depends 

 mainly upon the careful exclusion of adhesion between the two 

 bodies. When merely tried under the most favourable circum- 

 stances, as when copper vibrates on lead, the experiment can 

 hardly fail to succeed. With metals less distant from one an- 

 other on the scale, more delicacy is requisite, and it is then abso- 

 lutely necessary to avoid any extent of contact in regard to the 

 length as well as breadth of the bar. The form employed origi- 

 nally by Mr TREVELYAN was well adapted for this effect, though 

 the cause seemed not to be attended to : the bar AB (Fig. 8.) 

 rested upon an obtuse angle of the lead-block C : had the bar 

 been so inclined as to have touched the whole plane d e, in many 

 cases no vibration would have taken place, and I have always 

 been at pains to place the bar so that the angle g ef should be 

 nearly equal to h e d. If, as has been sometimes the case, I used 

 a block of hard metal, with an angle much more acute than that 

 shewn at e, and placed a bar of lead upon it, the effect was less 

 favourable than when the angle was more obtuse, and the con- 

 tact might seem to be greater. The truth, however, was, that 

 in this case the lead, from its softness, was cut by the harder 

 metal, and a new adhesion produced, as in the action of a wedge. 



43. These and many other experiments have proved to me, 

 that, to facilitate the vibrations as much as possible, we must have 

 a minimum of adhesion ; thus their frequency will be increased 

 and the note raised. Mr TREVELYAN states, that if the surfaces 

 in contact of the two metals be highly polished, no vibration will 



