( 1098 ) 
the adjusting tube can be shut off with A5, By a slight turn of the 
screws, and of the screw with which the fine adjustment of the 
height of the manometer tube is obtained, both of which are within 
the observer’s reach, the mercury surfaces are brought as near as 
possible to the two points; the height of the mid point between each 
point and its mirror image is then ascertained with the cathetometer 
provided with one of the reading microscopes of Comm. N°. 85 
(April 1905) and of Comm. N°. 95% (Sept. 1906) when the catheto- 
meter was used as a vertical comparator. 
In this way, taking account of the indication of the sensitive levels, 
heights may be measured accurately to within 0.002 mm. To elimi- 
nate the uncertainty in the correction for the refraction of light 
through the glass at the place where the point is under observation 
and that in the correction for the temperature of the equilibrating 
mercury columns (as the capillary depression is only 0.01 mm. the 
uncertainty in it may be neglected) the tap A, is introduced, and 
the spherical vessel d, forms part of the dead space *). If A5, is closed, 
and the mercury that stood in the narrow stem d, while the 
thermometer was being adjusted with A5, closed, is allowed to sink, 
there remains in the dead space only a very small and definitely 
known fraction of the total pressure, and the adjusting tube of the 
manometer must be lowered so as to bring the mercury levels once 
more to the two points. The displacement is read on a finely divided 
scale attached to the adjusting tube of the manometer, and at once 
gives in mm. of mercury the thermometer pressure for the temperature 
of the adjusting space, to which the only correction to be applied 
is that for the pressure remaining over. 
We need not stop to describe the different devices (ef. Comm. 
N°’. 60, Sept. 1900) by means of which the various points to be 
seen are so arranged that they can be brought in succession in sharp 
focus into the field of the cathetometer; the significance of the air- 
traps in the mercury filled connecting tubes is sufficiently obvious 
from the figure, as is also that of the mereury filled rubber tube 
Sa surrounding the rubber connecting tube S and its junctions with 
the other tubes. On account of the comparatively large value of 
viscosity, equilibrium is, in general, reached but very slowly between 
spaces occupied by gas at such low pressures as those obtaining in 
our thermometer reservoir and in the dead space. In the present 
1) A coupling is inserted in the capillary d> by means of which many opera- 
tions and controls are much more easily accomplished; it allows the whole ma- 
nometer part of the arrangement to be loosened, and either that or the remaining 
apparatus may be connected independently with an air-pump, etc. 
