435 
of Edinburgh, Session 1879 - 80 . 
definite temperature so that the density of the mercury may be 
accurately known. 
The simple form of steam thermometer represented with figured 
dimensions in the first diagram will be very convenient for practical 
use for temperatures from freezing to 60°. Through this range the 
pressure of vapour of water, reckoned in terms of the balancing 
column of water of maximum density, increases from 6J to 202-4 
centimetres ; and for this, therefore, a tube of a little more than 2 
metres will suffice. From 60° to 140° the pressure of steam now 
reckoned in terms of the length of a balancing column of mercury 
at 0° increases from 14-88 to 271 -8 centimetres ; and for this a tube 
of 280 centimetres may be provided. For higher temperatures a 
longer column, or several columns, as in the multiple manometer, or 
an accurate air pressure-gauge, or some other means, such as a very 
accurate instrument constructed on the principle of Bourdon's 
metallic pressure-gauge, may be employed, so as to allow us still 
to use water and vapour of water as thermometric substance. 
High-pressure Steam Thermometer. 
At 230° C., the superior limit of Regnault’s high-pressure steam 
experiments, the pressure is 27 "53 atmos, but there is no need for 
limiting our steam thermometer to this temperature and pressure. 
Suitable means can easily be found for measuring with all needful 
accuracy much higher pressures than 27 atmos. But at so high a 
temperature as 140°, vapour of mercury measured by a water 
column, as shown in the diagram (fig. 3), becomes available for 
purposes for which one millimetre to the degree is a sufficient 
sensibility. The mercury-steam-pressure thermometer, with pressure 
measured by water-column, of dimensions shown in the drawing, 
serves from 140° to 280° C., and will have very ample sensibility 
through the upper half of its scale. At 280° its sensibility 
will be about 4f centimetres to the degree !. For temperatures 
above 280° sufficient sensibility for most purposes is obtained by 
substituting mercury for water in that simplest form of steam 
thermometer shown in fig. 1, in which the pressure of the steam is 
measured by a column of the liquid itself kept at a definite tempera- 
ture. When the liquid is mercury there is no virtue in the parti- 
vol. x. 3 G 
