410 
originally existing between the volumenometer and the piezometer 
had sunk to a value that was insignificant. The reliability of this 
method is shown by the ecaleulations published by W. J. pr Haas 
in Comm. No. 127. For assistance rendered in the application of 
this method and for further help given in the course of this research 
we should like to express our indebtedness to Mrs. pr Haas-LORENTZ. 
Readings were taken with a very fine Societé Genevoise catheto- 
meter with three telescopes '), each with a micrometer eyepiece and 
level. A seale with very accurate subdivisions (Comm. N°. 60) was 
used for the readings with the micrometer eyepieces. 
Communication between the volumenometer and the piezometer 
(see the right hand portion of the diagram) was obtained through 
the tap %,, the glass 7'-piece (closed on the other side by #,) over 
which a connecting tube is cemented, a copper capillary g, (to give 
a certain elasticity to the connections), the steel taps /;,,4,, a steel 
capillary g, and a glass capillary f,. The steel taps 4,, 4,,4,, were 
provided with selected cork packing and were kept for about half 
an hour at a pressure of 50 atm. They closed perfectly. Connections 
between steel and glass capillaries were also made with the greatest 
care. This connection was made by means of a brass screw soldered 
to the glass capillary, the capillary being very slightly rounded and 
projecting about ‘/, mm. beyond the serew; this joint sustained a 
high vacuum for a long time. The rounded end of the glass capillary 
was covered with a packing ring made of fibrous plate, and could 
be screwed with force into the brass nut soldered to the steel 
capillary. 
The diagram does not show the wool with which all the principal 
parts of the apparatus were wrapped. The barometer was wrapped 
with the greatest care in wool, and was, moreover, surrounded by 
a double layer of paper so as to eliminate all convection currents. 
1) Compare the similar adjustments of Comm. No. 95e, Table I. 
The difference between the levels of the top and the edge of the meniscus and 
between the top of the meniscus in one of the necks and the central line on a 
screen (Cf. Comm No. 84, Pl. IL, Proc. March 1903) can be obtained with sufficient 
accuracy and more quickly from the catethometer scale than with the standard 
scale and level and the micrometer eyepiece. In the majority of cases it is sufficient 
and much simpler still to estimate these differences of level from the standard scale 
without focussing the micrometer upon the divisions of the standard scale at all. 
lor an error of 10/, in the determination of the height of the meniscus leads to 
an error of 1°/) in the capillary depression ; and an error of 1 mm. in the estima- 
tion of the height of a line on the sereen induces an error of only 16 or 17 mm}, 
in the volume, which makes a difference of only 1 in 60,000 in the volume of 
gas usually employed. ‘ 
