543 
of Edinburgh, Session 1879 - 80 - 
In the most accurate use of the instrument, the glass and mercury 
and oil of the manometer are all kept at one definite temperature, 
according to some convenient and perfectly trustworthy intrinsic 
thermoscope* by means of thermal appliances not represented in the 
drawing but easily imagined* This condition being fulfilled, the one 
desired pressure of the thermometric gas is attained with exceed- 
ingly minute accuracy by working the micrometer screw up or 
down until the oil is brought precisely to a mark upon the mano- 
metric capillary* 
In fact* if the glass and mercury and oil are all kept rigorously at 
one constant temperature, the only access for error is through 
irregular variations in the capillary depressions in the borders of the 
mercury surfaces. With so large a diameter as the 2 centimetres 
chosen in the figured dimensions of the drawing, the error from this 
cause can hardly amount to per cent, of the whole pressure* 
supposing this to be one atmo or thereabouts* 
Tor ordinary uses of this constant-pressure gas thermometer* 
where the most minute accuracy is not needed, the rule will still be 
to bring the oil to a fixed mark on the manometric capillary ; and 
no precaution in respect to temperature will be necessary except to 
secure that it is approximately uniform throughout the mercury and 
containing glass, from lower to higher level of the mercury. The 
quantity of oil is so small that* whatever its temperature may be, the 
bringing of its free surface to a fixed mark on the capillary secures 
that the mercury surface below the oil in the lower reservoir is very 
nearly at one constant point relatively to the glass, much more 
nearly so than it could be made by direct observation of the mercury 
surface, at all events without optical magnifying power* Now if 
the mercury surface be at a constant point of the glass* it is easily 
proved that the difference of pressures between the two mercury 
surfaces will be constant, notwithstanding considerable variations of 
the common temperature of the mercury and glass, provided a certain 
easy condition is fulfilled* through which the effect of the expansion 
of the glass is compensated by the expansion of the mercury. This 
condition is, that the whole volume of the mercury shall bear to the 
volume in the cylindric vertical tube from the upper surface to the 
level of the lower surface the ratio of (X «• J <r) to (X - a ■)* where X 
denotes the cubic expansion of the mercury and cr the cubic expansion 
