MEASUREMENT OF TEMPERATURE. 
207 
FG should be adjusted so that the acid stands at zero when the pressure is the 
same on either limb of the gauge at F and K. 
In addition to the U-gauge there is another connection between the air thermometer 
C and the manometer W by way of the bulbs A, M, N, which is filled with pure dry 
mercury, and is used for observations at constant pressure, for calibrating the bulb 
with air on the method of the volumenometer, and as an open way through which the 
bulb may be exhausted and refilled with dry air. It communicates by a three-way 
tap T 3 with the manometer and P 2 ; and by T 3 with P 3 , and through a long rubber 
tube with a mercury reservoir Q, which may be adjusted at any height, and which 
communicates with the outer air through a drying tube P containing CaCl 3 and 
glass wool to keep out damp and dust. Mercury may be introduced or withdrawn to 
any extent by aid of the reservoir Q, whose height is adjusted to suit the pressure, or 
in iveighed quantities by the tubes P 3 , P 3 , which dip into weighed beakers of mercury. 
The bulbs M, N, H, J, in the figure are convenient for taking up leakage, and other 
practical details. 
To determine the pressure correction in terms of mercury corresponding to each 
reading of the scale FG, the way AMN is cleared by drawing off the mercury 
into the reservoir Q, the tap T 3 is turned off, and Th turned thus (T), and P 3 is con¬ 
nected with the open air through the drying tubes, so that the atmospheric pressure 
is communicated by way of P 3 to the F-limb of the U-gauge. Peadings of the 
manometer and the gauge FG are taken at various adjustments, from which the 
probable error of a single pressure observation and the pressure correction are easily 
deduced ; capillarity is thus eliminated. 
To dry and fill the air thermometer, the bulb is maintained at a temperature 
between 400° and 500° C., W is connected to the exhaust, and P 3 with the dry-air 
supply. By means of the three-way tap T 3 the bulb is connected alternately with 
either. Mercury is re-admitted to fill the bulb A, when the operation is considered 
complete. 
To take observations at constant pressure, mercury is allowed to run out from 
the bulb A into the weighed beaker at P 3 , till the sulphuric acid gauge indicates that 
the pressure is approximately that of the air. The scale FG gives the small varia¬ 
tions of pressure, and the weight of the mercury the auxiliary volume. The tempera¬ 
ture of the bulb A is carefully observed, and is kept steady by being surrounded with 
a water reservoir. Total volume of Bulb A from S to S' = 48 - 162 c.c. at 15° C. 
The gauge above described arrived about the beginning of August, 1886. A new 
thermometer, No. 5, was at once made of hard glass, similar to No. 3, the expansion 
of which was known. 
There are some difficulties in mounting an instrument of this description. The 
mounting of No. 5 is diagrammatically illustrated in fig. 7, Plate 12. The bulb is 
fitted with asbestos board in a brass tube AD, into which is screwed a smaller tube 
DC which carries and protects the electrodes of the spiral and the delicate capillary 
