88 Proceedings of Royal Society of Edinhurgli, [jan. 6, 
diameter of this tube is about 1 mm. It is such tubing as is used 
for the fall tubes in a Sprengel pump, t and t' are two stop-cocks ; t 
being a tbree-way stop-cock, connecting together the volume gauge, 
the manometric gauge, and the pressure pump ; and t' is a stop-cock 
used for adjusting the quantity of liquid in the volume gauge. 
The object of the two cylindrical reservoirs h and h' in the volume 
gauge is to give space into which the air in the bulb a may expand 
during heating, or in which a supply of the air may be kept during 
the cooling of the thermometer. The tube d is very small in capa- 
city in comparison with the bulb ; and were it not for these reser- 
voirs, a very small change in temperature would cause the air to be 
driven out round the bend of the U, or the liquid in the bend to 
be drawn over into the bulb, unless the observer were incessantly 
on the watch to prevent this occurring by regulating the pressure. 
The U of the volume gauge is filled so full of liquid that the 
equilibrium reading is taken at the points pp of the tubes d and d ' ; 
and both in the selection of the tubes c and d, and in the glass- 
