430 Prof. FORBES on the Vibrations which take place 



left on its lower surface, which is bevelled away on either side. 

 The narrower this ridge, of course the more easily the equili- 

 brium is disturbed. No sooner is the bar of copper, iron, or 

 other hard metal, placed upon the lead-block, the former being 

 heated to a moderate temperature, than 'visible vibrations com- 

 mence, the bar oscillating upon its horizontal axis. Musical 

 notes are not always produced, but generally under the circum- 

 stances shortly to be noticed. Soon after Mr TREVELYAN'S 

 communication to the Royal Society of Edinburgh, the subject 

 was taken up by Professor LESLIE and Mr FARADAY, both of whom 

 explained the vibrations upon recognised principles, and did not 

 conceive that any new mode of action was concerned in their 

 production. Doubts which I ventured to entertain as to the 

 conclusions of these eminent individuals, led me to investigate 

 the subject experimentally, by which these doubts were strongly 

 confirmed. Facts increased in number, and I was forced to 

 abandon several successive hypotheses. The real difficulty of 

 the subject, and the singular conclusions deducible from several 

 of my experiments, led me to delay putting together the facts 

 which I had accumulated, into the form of a paper. Nearly two 

 years having now elapsed since the commencement of my expe- 

 riments (which were almost all made in the summer of 1831), 

 and no one else having taken up the investigation, I have re- 

 solved to publish the conclusions at which I have arrived, though 

 such as are purely theoretical I offer with all the diffidence which 

 a speculation connected with some of the most unexplained pro- 

 cesses in natural science requires. 



5. I propose to divide this paper into three sections, first, 

 on the Phenomena of Sound, as those which earliest presented 

 themselves, and the consideration of which will pave the way to 

 farther inquiries ; second, on the Phenomena of Vibration ; and, 

 third, on the Theory of these Phenomena. 



