"20 Prof. Forbes's Researches on the Vibrations "which take place 



That this may not be the case, as we find it is not under some 

 circumstances already alluded to, namely, where certain dif- 

 ferent metals, and at different temperatures, are employed, we 

 must admit the existence of some impulse which prolongs the 

 time during which the oscillations are kept up. This impulse 

 can only be received during the successive contacts at the two 

 bearing points of heterogeneous metals, and may safely be 

 assumed to depend in some way or other upon the propaga- 

 tion of heat, since the effect does not take place unless the 

 temperatures be different, nor is it indifferent which of the 

 two kinds of metal has the highest temperature. The impulse, 

 of whatever kind it be, resembles that derived by a pendulum 

 from the pallets of a timekeeper, which in fact is the sustain- 

 ing power of the mechanism. 



10. The arcs of vibration of course depend, other things 

 being equal, upon the intensity of the impulse communicated 

 to the bar. 



11. We have already noticed, that various circumstances 

 tend to modify materially the character of the vibrations, par- 

 ticularly as to their distinctness and duration : a little practice 

 is required to distinguish between the mere mechanical oscil- 

 lations of a bar once made to vibrate, and a true vibration de-' 

 pending on an impulse received at each successive contact. 

 There is, in general, this difference, even where the sustaining 

 power is very feeble, that whereas the oscillations arising 

 merely from gravity rapidly diminish from the very first, if 

 there be a true sustaining power, they will rather increase in 

 energy and distinctness for some time, from the accumulated 

 effect of successive though small impulses. 



12. We proceed to examine the influence of contingent cir- 

 cumstances upon the intensity of these vibrations. 



1. Relation to the Specific Characters of the Substances em- 

 ployed. 



13. The first general law of these vibrations may be consi- 

 dered to be, that they never take place between substances of the 

 same nature. This is probably quite general. An exception 

 is noticed by Mr. Trevelyan, who thinks that he observed a 

 vibration of a copper bar upon a copper block ; I am inclined 

 to think that he had mistaken the oscillations connected with 

 the simple law of gravity for a true vibration, as I have in vain 

 endeavoured to repeat the experiment. 



14. The second general law is, that both substances imtst be 

 metallic. I have never seen a single exception to this law* ; 



* I need hardly state it as an exception that I have used galena (the sul- 

 phuret of lead) as a block, with success, instead of metallic lead. 



