90 Proceedings of Royal Society of Edinburgh. [jan. 6, 
used for these connections require to be strengthened with canvas to 
resist the pressure. 
The form of volume gauge shown in fig. 3 is designed for use at 
very high temperatures. It is made in two parts, which are con- 
nected together at the cup e (shown enlarged, fig. 4). The bulb and 
tubes c, 6^, and h, are made of hard Bohemian glass; the remaining 
part of the gauge is of German glass or English flint glass. The 
stopper of the cup e is made to fit the throat of the cup closely, and 
just below the throat an enlargement / is 
blown out, through which the elongated 
point of the stopper passes. The stopper 
Is fastened air-tight into the cup with 
German “ Siegel wachs ” ; and the object 
of the enlargement is to furnish a cushion 
of air which prevents the liquid of the 
volume gauge from coming in contact 
with the cement. The making of this 
joint is a little troublesome, and it 
requires to be protected against radiation 
from the hot source. There are various 
stoppers and joints well known, which 
prevent leakage inwards from without; 
but it is much more difficult to find an 
efficacious stopper which will act against 
pressure from within outwards. 
The thermometer bulb is filled with per- 
fectly pure dry air, and it is desirable to 
have the bulb filled with such a quantity 
of air that the pressure is approximately that of a normal atmosphere 
when the temperature is freezing. For, if the quantity of air be con- 
siderably greater than corresponds with this condition, there is a loss 
of range in the instrument ; whereas, if there be but a small quantity 
of air, there is a tendency for the liquid of the volume gauge to be 
drawn over into the bulb when the temperature of the room comes down 
(as in winter it may) to about the freezing point, unless the instru- 
ment be left with the three-way stop-cock closed and the air under 
diminished pressure. For special circumstances the quantity of 
air may be made to suit the conditions; for, as Kegnault has 
