541 
of Edinburgh, Session 1879 - 80 . 
will be called the manometric capillary, is to be of round bore, not 
very fine, say from half a millimetre to a 
millimetre diameter. Its lower end is to 
be connected with a mercury manometer j\ 
to show if the pressure of the thermome- 
tric air is either greater or less than the 
definite pressure to which it is to be 
brought every time a thermometric mea- 
surement is made by the instrument. 
The change of volume required to do this 
for every change of temperature is made 
and measured by means of a micrometer 
screw* lifting or lowering a long solid glass 
piston, fitting easily in the glass tube, and 
caulked air-tight by mercury between its 
lower end and an iron sole-plate by 
which the mouth of the volumetric tube 
is closed. To perform this mercury 
caulking, when the piston is raised and 
lowered, mercury is allowed to flow in 
and out through a hole in the iron sole- 
plate by an iron pipe, connected with two 
mercury cisterns at two different levels by 
branches each provided with a stopcock. 
"When the piston is being raised the 
stopcock of the branch leading to the 
lower cistern is closed, and the other is 
opened enough to allow the mercury to flow up after the piston and 
* This screw is to be so well fitted in the iron sole-plate as to be sufficiently 
mercury-tight without the aid of any soft material, under such moderate 
pressure as the greatest it will experience when the pressure chosen for the 
thermometric gas is not more than a few centimetres above the external 
atmospheric pressure. When the same plan of apparatus is used for investiga- 
tion of the expansion of gasses under high pressures, a greased leather washer 
may be used on the upper side of the screw-hole in the sole-plate, to prevent 
mercury from escaping round the screw. It is to be remarked that in no case 
will a little oozing out of the mercury round the screw while it is being turned 
introduce any error at all into the thermometric result ; because the correct- 
ness of the measurement of the volume of the gas depends simply on the 
mercury being brought up into contact with the bottom of the piston, and not 
more than just perceptibly up between the piston and volumetric tube 
surrounding it. 
