84 Mr. W. Sutherland on two New 



But it is very desirable to have an independent method and 

 instrument for controlling the indications of the M'Leod 

 gauge, reaching to higher vacua and simpler in its prac- 

 tical working ; and one outcome of the paper on " Thermal 

 Transpiration and Radiometer Motion " * is the furnishing 

 of two instruments capable of independently measuring the 

 pressure in high vacua. These two instruments were in a 

 manner united into a single one in Crookes' s torsion radio- 

 meter briefly described but rather fully discussed in that 

 paper ; and though for some purposes there might be 

 advantage in uniting the two forms, each would be capable 

 of its best performance when separate. The two instruments 

 are a disk viscosity-meter and a torsional radiometer of special 

 design to be discussed in this paper. Crookes used his 

 torsion radiometer as a viscosity-meter by forcing it to 

 vibrate and determining the decrement of the logarithm 

 of the angular amplitude of successive vibrations. At pres- 

 sures near one at mo the vertical plate of the radiometer 

 in oscillating round a vertical axis in the bulb sweeps air 

 bodily in front of it and draws air behind it in a very complex 

 manner, so that its motion is conditioned partly by the mass 

 motion of the gas, as well as by viscosity of gas and viscosity 

 of suspending fibre ; but as Crookes's experiments showed, in 

 a manner brought out by Stokes's theoretical discussion (Phil. 

 Trans, clxxii.), the effect of the mass motion of the gas on the 

 log. dec. disappears rapidly, and the log. dec. is practically 

 constant for a considerable range of pressure, proving that 

 viscosity alone is effective in retarding the motion of the 

 vibrating vertical plate. The following values of 10 4 times 

 the log. dec. in air are taken from Crookes's results (Phil. 

 Trans, clxxii.) : — 



p (mm.) ... 760 550 301 155 47 12 4 3 1 



1124 1073 1022 1006 1001 1000 1000 994 988 



These show that from 47 mm. to 4 mm., the latter being only 

 1/12 of the former, the log. dec. is constant ; so that at this 

 range of pressure the mass motion of the gas is negligible, 

 and the viscosity is independent of pressure, as it ought to be 

 according to Maxwell's discovery, so long as slipping of the 

 gas on the solid surfaces of the edge of the plate and the walls 

 of the bulb is negligible ; but then at 3 mm. and 1 mm. the 

 log. dec. diminishes decidedly in a manner more fully illus- 

 trated by the following further data from Crookes, the unit of 

 pressure being now 1/10 6 atmo :— 



p 1000 495 300 100 53 24 13 8 



988 966 952 876 774 620 500 390 

 * Phil. Mag. for November and December 1896. 



