G PROCEEDINGS OF THE AMERICAN ACADEMY. 



indicated above, the corresponding value of p can be obtained from the 

 formula by a simple calculation. The numbers thus obtained for the 

 various values of / are given in the third and fourth columns. 



It would, therefore, seem highly probable that so far as hydrogen is 

 concerned the McLeod gauge can be relied upon for pressures as low 

 as the lowest used, and which are recorded in Table I ; and that, in 

 the case of hydrogen, the measurement of friction can be used as a 

 convenient and accurate method of measuring pressure, provided care 

 is taken to exclude mercury vapor. This matter will be discussed at 

 length later. 



The details of the methods used to overcome the difficulties named 

 above follow: 



Measurement of Decrement due to the Friction in the Fibre. 



Referring to Figure 1 it will be seen that the tube C is inserted in 

 such a position that nothing can pass to the viscosity apparatus from the 

 McLeod gauge, B, or from the pump, which is connected to D, without 

 passing through it. This tube, C, therefore, replaces the tubes of sul- 

 phur and silver whose purpose was explained in the earlier paper. C 

 is filled with granular charcoal, and is so arranged that either a cylin- 

 drical electrical heater or a long Dewar vessel can enclose it. When 

 C had been placed in position and sealed in place, the whole apparatus 

 was exhausted through D by means of the mechanical pump, and then 

 dry air was allowed to pass in through an opening placed near the 

 pump. The exhaustion was again performed and the admission of dry 

 air repeated. This exhaustion and admission of air were carried out 

 alternately many times for the purpose of removing the comparatively 

 large quantities of moisture which had been formed in the vessel dur- 

 ing the process of making the various joints in the construction of the 

 apparatus. When it was certain that the whole apparatus had been 

 made fairly dry, the cylindrical electric heater was placed about the 

 tube C, and while the exhaustion proceeded the tube was raised to a 

 temperature of about 150° C, to hasten the removal of the gas present 

 in large quantities in the pores of the charcoal at atmospheric pres- 

 sure, and which separates from the charcoal rather slowly under re- 

 duced pressure if the temperature is kept low. When the mercury 

 pump had been used to secure a fairly high vacuum the other parts of 

 the apparatus, viz., the McLeod gauge, the viscosity apparatus, and 

 the connecting tubes were heated to about 150° C, for the purpose of 

 removing from the glass the occluded gases. After the pumping had 

 proceeded for some time under these conditions, the heater was re- 



