2 
DR. TYNDALL ON THE VIBRATIONS AND TONES PRODUCED BY 
not knowing what Schwartz and Gilbert had observed previously, regarded the 
phenomenon as new and recommended Mr. Trevelyan to investigate it more fully. 
Mr. Trevelyan did so ; among other things he discovered the form to be given to 
the vibrating mass (the rocker) in order to obtain the effect with ease and certainty. 
The results of his numerous and well-contrived experiments were communicated to 
the Royal Society of Edinburgh, and were subsequently printed in the Society’s 
Transactions. 
On the 1st of April 1831 these vibrations and tones constituted the subject of a 
Friday evening’s lecture by Professor Faraday at the Royal Institution. The following 
extract from the Journal of the Institution, vol. ii. p. 120, informs us of the views of 
the philosopher last mentioned with respect to the cause of the tones. “ As the sounds 
were 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 kept in motion, 
whilst any considerable difference of temperature existed between it and the block 
of lead underneath. This power Professor Faraday referred to expansion and con- 
traction, as Professor Leslie and Mr. Trevelyan had done generally. But he gave 
a minute account of the manner in which, according to his views, such expansions 
and contractions could produce the effect The superiority of lead, as a cold 
metal, he referred to its great expansibility by heat, combined with its deficient con- 
ducting power, which is not a fifth of that of copper, silver, or gold ; so that the 
heat accumulates much more at the point of contact in it than it could do in the 
latter metals, and produces an expansion proportionably greater.” 
Professor J. D. Forbes was present at this lecture, and by it, apparently, he was 
induced to undertake the further examination of the subject. On the 18th of March 
and on the 1st of April, 1833, the results of his inquiries were communicated to the 
Royal Society of Edinburgh. He dissents from the explanation supported by Pro- 
fessor Faraday. The vibrations, he urges, are dependent for their existence on the 
difference of temperature of the two surfaces in contact ; if then the heat accumulate 
at the surface of the cold metal, its effect will be to bring both surfaces to a common 
temperature and thus to stop the vibrations, instead of exalting them, as supposed 
by Professor Faraday. Again, if the phenomenon be due to expansion, the greater 
the expansion the greater ought to be the effect ; but the expansion depends upon 
the quantity of heat transmitted from the hot rocker to the cold block during their 
contact, and this again upon the conductivity of the block ; so that instead of being 
a bad conductor, the block, to produce the greatest effect, ought to be the best con- 
ductor possible. The idea of an accumulation of heat at the surface being more 
favourable to the action than a rapid communication with the interior, Professor 
Forbes regards as an “ obvious oversight*.” 
Having thus, to all appearance, overturned the views previously entertained, Pro- 
fessor Forbes proceeds to found a theory of his own. His experiments have led him 
* Philosophical Magazine, Series 3, vol. iv. pages 15 and 182. 
