■200 
AND ON THE LAW OF PRESSUKE OF OASES, ETC. 
M 
sponding to the zero of pressure, or rather of pressure difference. If the 2 )ressures 
operative upon the mercury surfaces lie slightly different, the setting is disturbed ; 
and the change of reading at the screw required to re-establish the adjustment 
represents the difference of pressures. In order to interpret the result absolutely it 
is*only necessary to know further the pitch of the levelling screw, the leverage with 
which it acts, and the distance between the points to which the mercury surfaces are 
set. If the space over one mercury surface be vacuous, the change of reading at 
the levelling screw represents the absolute pressure in the space over the othei- 
mercury surface. 
The difficulty, which will at once present itself to the mind of the reader, in the 
use of a manometer on this plan, is the necessity for a flexible connection between 
the instrument aud the rest of the ajiparatus, such as tlie air-pump and the vessel in 
which the pressure is required to be known. With the aid of short lengths of 
rubber tubing this requirement could be easily met, l)ut the class of w(_)rk for which 
such a manometer is wanted would usually pre¬ 
clude the use of rubber. In my a})paratus the 
requisite flexibility is obtained Ijy the insertion of 
considerable lengths (3 metres) of glass tubing 
between the manometer and tlie parts which can¬ 
not turn with it. Altliough the adjustment was 
made by tlie screw of a levelling table as descril)ed, 
the actual readings were taken by the mirror 
method, the sup])orts of the mirror Ijeing connected 
as directly as possible with tlie })oints whose 
angular motion is to be registered. In this way 
we become independent of the rigidity of the 
glass-work, and are permitted to use wood freely 
in the levelling table and in its supports. It 
frequently happened that an adjustment left 
correct was found to be ont after an interval. 
The screw had not heen moved, but the mirror- 
reading was altered. On resetting by use of the 
screw, the original mirror-reading was recovei’ed 
within the limits of error. 
The essential parts of the manometer, as flnally 
employed, are shown (fig. 2) in elevation and plan, 
and the general scheme of tlie mounting is indi¬ 
cated in fig. 3. At A is the stalk of the glass 
fork, of such length that the mercury in the hose 
below is always at a pressure aliove atmos]ihei‘e ; 
B, B are bulbs of about 25 millims. diameter, at 
VOL CXC’VT.-A. 2 F. 
