438 MR. GEORGE W. WALKER ON THE DEPENDENCE OF THE 
Ihe ari’angements for altering the pressure in the tubes of gas and measuring the 
differences are shown in fig. 4. The glass tube D connected the brass tube J/ with 
one limb of the manometer. The branch F led to a tap L, througli which different 
gases could be passed into the apparatus. 
The liranch H led to the second brass tube N, and from II the branch G led to the 
second limlj of the manometei'. The branch E was connected to a glass reservoir A 
of about I litre capacity, the tidie being drawn to a fine ca| 3 illary just before entering 
the'reservoir. A was connected with a second reservoir by a rubber tube. On 
lowering B, mercury ran from A to B, and thus the pressure could be altered. The 
tap A being shut, a difiei'ence of pressure in the two brass tubes was produced, which 
could be measui'ed on tlie manometer. C is a glass bulb introduced to keep the 
pressure in N more nearly constant than it would otherwise be. The manometer was 
over an inch in diameter in the wide portions, and thus capillary error was avoided. 
A steel scale S was hung between the two limbs and the level of the mercury, read 
by means of a telescope of a cathetometer placed in front of the manometer and about 
150 centims. from it. 
Wherever it was possible, the glass joints were made hy means of a blow-pipe, and 
the only other joints were at the I'eseiwoir A and at the small tubes connected with 
31 and H. These were made with thick rubber tube, wired, and painted with black 
enamel. J he glass taps A and A were fitted with mercury seals. Very great care 
was taken in testing to see that the whole apparatus was absolutely air-tight. 
