B16 On Heat S Cold produced by Mechanical 
One circumstance is very remarkable, that 
whether the mercury rises or falls in these in- 
stances, it is done very rapidly; whereas in the 
‘Oper air, if a thermometer be only two or three 
degrees above or below the temperature, it 
moves very slowly, This seems to have sug- 
gested to every one the idea that the elasticity 
of the glass bulb of the thermometer has a prin- 
cipal share in producing the effect, by causing 
the bulb to yield a little to the pressure of the 
air. It has however been found upon trial that 
the same effects take place whether the ther- 
mometer is sealed or not. My experiments 
accord with this, having made a thermometer 
and left it unsealed for the express purpose ; in 
all the experiments with condensed and rarefied 
air, there was no sensible difference observed to | 
arise from the inequality of pressure on the ex- 
ternal and internal surfaces of the ‘bulbs, the 
sealed and open thermometers-varying the same 
_in kind and also in degree, except from circum- 
stances to be noticed hereafter. 
It being. certain then that a real change of 
temperature takes place, it remained to determine 
the quantity and manner of that change. Hav- 
ing chosen a small and consequently sensible 
thermometer, with a scale of degrees sufficiently 
large to admit. of distinguishing one tenth of a 
