576 Proceedings of the Royal Society 
When the pressure is the same outside and inside the cylinder, we 
have 
p_ _n df_ _ n 
r 3k 5 dx 3k ’ 
and the diminution per unit volume of the interior is, as in Orsted’s 
experiment, 
IX 
To I 
For a spherical bulb the equations are reduced to 
£= -#! + («- f)li> 
and we have for external pressure II 
e = _ u_±.(l + ^l 
r a\ — al \3k 4 n 
As an external indication of the pressure (to guard against carry- 
ing it too far), a cylindrical steel bulb, screwed inside the compression 
apparatus, and filled with mercury, is used. Its indications are 
read by a glass tube inserted in its neck, which opens outwards. 
I hope by means of the apparatus I have described to be able to 
measure approximately the volumes of gases and liquids at pressures 
amounting to 15 or perhaps 20 tons on the square inch. The only 
difficulty in the case is that at these high pressures the compressibility 
of such bodies is probably of the same order of small quantities as 
that of solids. 
[ Added during printing . — To diminish the effect of temperature 
on these instruments, the interior of the cylindrical bulb is now 
nearly filled by a glass tube sealed at both ends. Thus the quan- 
tity of liquid in the bulb is not much greater than that in the 
stem.] 
