AT HIGH PEESSURES BY OPTICAL METHODS. 
123 
into a softer steel surface, in the centre of which a capillary pressure-conducting 
channel enters. These steel edges have to he very carefully hardened, yet nuist he 
only slightly liarder than the supporting surface. If tliey ai'e too liard tiiey easily 
break, and in any case a very much harder edge soon spoils tlie supporting steel 
surface. A joint which once has been got into good working order keeps ])erfect]y 
tight for any lengtli of time, and at any pressure it vdll stand without Innaking. 
The oil used in compressors and pump was castor oil. 
All four manometers have been standardized at the works of Schidfer and 
Budenberg in Magdeljurg-Buckau by means of a hydraulic plant of a similar type to 
the Amagat absolute manometer. The manometers were standardized l)y tlie direct 
application of weights, the high-pressure ones up to 4,000 kg./cm.^ (the limit to 
which the testing plant can he used) and the smaller pair to 600 kg./cm.“. Tlie 
accuracy of the large manometers is about 5 kg./cm.^, and that of the smaller ones 
about kg./cm.^ The one large manometer is used as a standard whereby the three 
working manometers may he checked from time to time, to secure tliat tlieir 
indications are constant. As all three working manometers communicate up to 
pressures of 600 kg./cm.^, the two smaller ones give the exact value of the correction 
for the zero-point of the large one. If the indications of a single liigh-jiressure 
manometer are not in some Interval of pressure checked liy another manometer grave 
errors may result with regard to the real position of the zero-point. 
2. The Pressure-Boriih in wldcli the Optical Investigation is carried out and the 
The) 'mostat. 
The pressure-bomh fits into a U-shaped iron support, into which it can lie firmly 
fixed by means of a screw. The U-piece is by a strong screw-bolt fixed to another 
inverted U-piece, which again is firmly screwed to an iron plate 3 feet long and 
1 foot 2 inches wide. The connecting screw-bolt rnns througli the l)ott(.)m of a 
water-jacket, and the joints are rendered tight by screw nuts and packings. There 
are also mica packings between tlie screw-holt and the U-pieces in order to lessen 
the conduction of heat as much as possible. The water-jacket is of a squai'o shape, 
flat, and only a few millimetres wider than the pressure-homb. It holds about 
2 litres. Opposite the two windows in the pressure-bomh are two holes in the sides 
of the water-jacket, and a short piece of brass tulie with an outer, flat, ring-shaped 
edge can be screwed on to the window part of the pressure-lioml), pressing the 
side-plate of the water-jacket tightly against the pressure-boml). These joints 
are rendered perfectly tight by inserting a sufficient munljer of rings of sheet 
lead 1 mm. thick on both sides of the water-jacket plate liefore the Ijrass-tuhe caps 
are screwed on. In this way the pressure-liomli is kept fixed fiiinly inside the 
water-jacket, and a free view secured tlirough the glass windows of the pressiire- 
R 2 
