1391 
and p, steady, except for the small irregularities due to the regulation 
of the taps. The tube c is connected to a vessel e of about 4 I. 
placed in ice, intended for the determination of the volume. The gas 
pressure in this reservoir was determined before and after every 
experiment, by reading the manometer g. As can be seen, a small 
portion of this volume is not reduced to 0° C., it remains at about 
the temperature of the room. This portion is only about 1.5 °/, of 
the whole volume. 
The three manometers were read by a kathetometer, and were so 
placed that they could all be viewed by turning the kathetometer. 
The temperature of the viscosity capillary was determined by a 
platinum resistance thermometer placed beside it, which was reduced 
to the hydrogen thermometer of the Laboratory by comparison with 
a standard resistance thermometer. 
For the arrangement of the cryostat with stirrer see Com. N°. 123. 
The measuring apparatus were immersed in a cryostat glass exactly 
like that of the helium eryostat. As we said in $ I, it is our intention 
to determine the viscosity of helium at helium temperature with the 
same apparatus. The cryostat glass was covered by a cap, which is 
like that of the helium cryostat, but simplified in an obvious way. 
In fig. 2 the eryostat glass with stirrer and thermometer are omitted. 
§ 4. Course of the experiments. When the tightness of the apparatus 
had been properly tested and all found to be in order, the experiment 
was made in the following way. The volumenometer and the whole 
apparatus were pumped out and the tap « was closed. Then tap 
ec was opened, and regulated so that the manometers 5 and d were 
at the desired height. When this was attained the experiment was 
begun, and simultaneously with the beginning of the regulation of 
tap a the knob of a chronometer was pressed. During the experiment, 
as already said, the taps @ and c were so regulated that the diffe- 
rential manometers which acted as indicators, kept constant, at the 
same time the manometers Q and V were read, and the small 
irregularities which at the most were 1 °/, were noted as well as 
was possible. By the determination of a mean value we find from 
these readings the pressure difference, which existed between the 
extremities of the capillary tube during the experiment. If we reduce 
; aie <3 
the readings of the manometer v by ent we find the mean 
~ 
pressure p. 
The chronometer was compared with the standard clock of the 
laboratory immediately after pressing the knob. The latter served 
