544 
Proceedings of the Royal Society 
of the solid for the same elevation of temperature, it being supposed 
for simplicity of statement that the tube is truly cylindric from the 
upper surface to the level of the lower surface, and that the sectional 
area of the tube is the same at the two mercury surfaces. The cubic 
expansion of mercury is approximately seven times the cubic 
expansion of glass. Hence 
Hence the whole volume of the mercury is to be about 1*111 times 
the volume from its upper surface to the level of the lower surface ; 
that is to say, the volume from the lower surface in the bend to the 
same level in the vertical branch is to be ~ of the volume in the 
vertical tube above this surface. A special experiment on each 
tube is easily made to find the quantity of mercury that must be 
put in to cause the pressure to be absolutely constant when the 
surface in the lower reservoir is kept at a fixed point relatively to 
the glass, and when the temperature is varied through such moderate 
differences of temperature as are to be found in the use of the instru- 
ment at different times and seasons. 
A sheet-iron can containing water or oil or fusible metal, with 
external thermal appliances of gas or charcoal furnace, or low-pressure 
or high-pressure steam heater, and with proper internal stirrer or 
stirrers, is fitted round the bulb and manometric tube to produce 
uniformly throughout the mass of the thermometric gas the tempera- 
ture to be measured. This part of the apparatus, which will be 
called for brevity the heater, must not extend so far down the 
manometric tube that when raised to its highest temperature it can 
warm the caulking mercury to as high a temperature as 40° C., be- 
cause at somewhat higher temperatures than this the pressure of 
vapour of mercury begins to be perceptible, and would vitiate the 
thermometric use of the pure hydrogen or nitrogen of our 
thermometer. To secure sufficient coolness of the mercury it will 
probably be advisable to have an open glass jacket of cold water 
(not shown in the drawing) round the volumetric tube, 2 or 3 
centimetres below the bottom of the heater, and reaching to about 
half a centimetre above the highest position of the bottom of the 
piston. 
It seems probable that the constant-pressure hydrogen or nitrogen 
