PROCEEDINGS 
OF THE 
ROYAL SOCIETY OF EDINBURGH, 
vol. ix. 1877-78. No. 102. 
Ninety-Fifth Session. 
Monday , 1 5th April 1878. 
Sir WILLIAM THOMSON, President, in the Chair. 
The following Communications were read : — 
1. On Vortex Vibrations, and on Instability of Vortex 
Motions. By Sir William Thomson. 
2. On the Theory of Vowel Sounds. By Professor 
M‘Kendrick. 
3. Preliminary Note on a Method of Detecting Fire-Damp in 
Coal Mines. By Professor George Forbes. 
The author exhibited two instruments, both founded upon the 
same principles, for measuring the quantity of fire-damp present in a 
coal mine. The first instrument consists of a tuning-fork fixed above 
the open end of a resonating tube, whose other end is closed by a 
piston whose position (read off on a scale) regulates the length of 
the resonating tube. The length of the tube, which resounds to the 
definite pitch of the tuning-fork, depends upon the nature of the 
gas with which it is filledc The more fire-damp, the longer is the 
tube. Barometric pressure has no effect upon this instrument. 
The correction for temperature is made by reading off, not a fixed 
mark upon the piston, but the top of the mercury of a thermo 
VOL. ix. 4 M 
