622 Prof. P. A. Lehfeldt on the Electrochemical 
nitric acid. The filling was done from the top by a Topler 
pump, and the vacuum tested by the discharge between a 
pair of electrodes in a side tube. 
The height is measured by a pair of glass scales by Zeiss, 
each 20 cm. long, mounted on a strip of plate-glass which 
forms the back of the instrument. This arrangement was 
sent by the manufacturers to the National Physical Labora- 
tory to have the distance between the short scales measured. 
To read the barometer the level at the top was adjusted to 
touch the glass pointer, and that at the bottom read by a 
telescope with micrometer eyepiece divided in hundredths 
of a millimetre. The mercury surface was illuminated from 
behind, through a little tissue-paper window into an adjoining- 
room, where an incandescent lamp was placed. The level of 
the pointer was found to be 87*84 mm. on the upper scale ; 
the distance between the zeros of the two scales 600*09 mm. 
at 15°*2, according to the certificate of the National Physical 
Laboratory. Hence the uncorrected reading at 15° is the 
reading of the mercury against the lower scale (5) + 600*09 
-87-84. 
From this must be subtracted the expansion of the 
mercury between 0° and 15° (on an average height of 
622 mm.), 
15 x 622 x 0*0001819 = 1*70 mm., 
and for gravity, according to the provisional value obtained 
(Lehfeldt, Phil. Mag., Nov. 1906), 
978-7-980-62 M9 1>i)S) 
x v>22 = 1 22 mm. 
980-62 
The constant of the barometer is therefore 
600-09 -87-84- 1-70 -1-22 = 509-33 at 15°. 
The temperature coefficient is 
(0*0001819-0*0000087)622 = 0*108 mm./degree. 
Hence the pressure of the atmosphere is 
5 + 509*33-0*108(^-15), 
where t is the temperature of the barometer. 
The pressure of the gas-burette could usually be adjusted 
to differ from that of the atmosphere by not more than about 
^ mm. of oil ( = 1/60 mm. mercury). 
The gas-burette was kept constantly moist by a few drops 
of water over the mercury, and the vapour-pressure of water 
at the temperature of the final adjustment was deducted from 
the observed pressure. 
