between Metallic Masses having different Temperatures. 185 



adhesion must therefore clog the energy of the impulse in a way 

 nearly insensible to our ordinary modes of impression : yet 

 cases are not awanting where such adhesion is abundantly 

 sensible, and especially when a metal so soft as lead is one of 

 those employed. It is not difficult to perceive how the posi- 

 tion of the groove or separation of pieces (for we have seen 

 that the effect is absolutely independent of the form of the 

 groove, provided the contact of the bar and block for a certain 

 space be avoided) is the most favourable for producing the vi- 

 brations. The separation of surfaces may either be in the 

 block, as fig. 7, or in the bar, as indicated by 

 the dotted lines at a in the same figure : the Fig. 7. 



surface of contact will thus be reduced, as 

 there shown, to about one half. If, instead 

 of this, the space between the solid angles b 

 and c had been reduced to one half, the stabi- 

 lity of the bar would have been materially 

 changed, and the requisite distance between the points d'appui 

 for producing an active vibration would have been deranged. 

 By cutting out the interior space of contact, the other condi- 

 tions remain unimpaired, and the adhesion is diminished to 

 almost any required extent; in fact, the note has been most 

 clear and steady when the two points of contact of the block 

 had almost the whole intermediate space removed. The sud- 

 den changes of note before alluded to have been very satisfac- 

 torily 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 experiment, depends 

 on the diminished impulse received at each stroke, and con- 

 sequent 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 

 circumstances, as when copper vibrates on lead, the experi- 

 ment can hardly fail to succeed. With metals less distant 

 from one another on the scale, more delicacy is requisite, and 

 it is then absolutely necessary to avoid any extent of contact 

 in regard to the length as well as breadth of the bar. The 

 form employed originally by Mr. Trevelyan was well adapted 

 for this effect, though the cause seemed not to be attended 

 to: the bar A B (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 de, 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 c f should be nearly equal to // r d. If, as 



Third Scries. Vol.4. No. 21. March 1834. 2 13 



