84 
Proceedings of the Royal Society 
relief of pressure will cause a corresponding absorption of beat 
which will show itself in the position of the minimum index. 
Some experiments which I have made in this direction show a 
lowering of temperature of 0 '3° F. for the relief of a pressure of 2 \ 
tons per square inch, the whole rise of the maximum index having 
been 1 *8° F. 
We may, I think, be quite certain that when the minimum leg is 
the one used and the temperature low, the error caused by the effect 
of pressure on the stem is inappreciable. 
Cavendish, who invented the self-registering thermometer, foresaw 
also the most important of the uses to which it could be applied. 
Thus he suggests that the higher regions of the atmosphere might 
be investigated by attaching it to a kite — balloons not having been 
invented. With regard to deep-sea explorations, he says : “ If 
instruments of the nature above described were to be used for find- 
ing the temper of the sea at great depths, some alteration would be 
necessary in the construction of them, principally on account of the 
great pressure of the water, the ill effect of which can, I believe, be 
prevented no other way than by leaving the tube open.” 
This was written in 1757, and it was not till 1762 that Canton 
proved that liquids are compressible. Cavendish therefore hoped 
that as the pressure would not produce distortion of the glass when 
the tube was open, it would have no visible effect on the apparent 
volume of the liquid. The device of leaving his thermometer open 
at the end was adopted by Aime in some of his experiments, the 
effect of pressure on the apparent volume of the liquid being deter- 
mined independently, and a correction applied accordingly. I devised 
and constructed a mercurial thermometer, or piezometer , on the same 
principle,* but my object in admitting the water pressure to the 
inside of the instrument was to utilise it in shifting the scale of the 
thermometer as the depths changed. The thing registered in such 
instruments is always the apparent volume of the liquid, and this 
varies with the temperature and the pressure. Hence the indications 
will represent the sum of the effects of change of temperature and 
of pressure. If from any independent source we know either of 
these, we can determine the other. In a sea of uniform temperature 
throughout its depth, the apparent volume of the liquid would 
* Journal of the Chem. Soc. October 1878. 
