Transpiration and Radiometer Motion. 385 



2R X say) is 



P2— Pi _ ^2—^i 



(19) 



where 



B / = 6a(E 2 + R 1 )r /7 ?0 (^ 2 + Vl ) 2 5 



while for a uniform tube we have seen that the solution (17) is 

 what this becomes whenH 2 = Ri = R. The simplicity of the case 

 of frustra breaks down when R x becomes only a small fraction 

 of R 2 , for then we cannot neglect (R 2 — Ri) a as we did above. 

 To make clear the comparison between (17) and (19) 

 and experiment a brief description of Reynolds's arrange- 

 ments is necessary : imagine a cylinder divided into five 

 compartments by planes perpendicular to its axis, the middle 

 one filled by a plate of porous material, those on each side of it 

 made into small gas-holders connectable with gas supply and 

 manometers and separated from the end chambers by metal 

 plates, the end chambers being intended to act as a sort of 

 jacket to each of the gas-holders, the one having a stream of 

 steam carried through it and the other a stream of cold water. 

 When a stationary state of temperature is established along 

 the cylinder, the two faces of the porous plate come to fixed 

 temperatures 2 an d #i, corresponding to the molecular veloci- 

 ties v 2 and v 1} and the gas transpires from the cooler face of 

 the plate to the hotter, till the pressures become p 2 and p 1 

 as given by the equation. The internal diameter of the 

 cylinder was 38 mm. and the thickness of the porous plates 

 varied from 1*5 to 14'2 mm., the materials being meerschaum 

 and stucco. Reynolds gives the temperatures of the two 

 jacket-chambers, but not those of the faces of the porous 

 plate, which are the ones we require ; we will show afterwards 

 how to obtain these approximately, but for the present it 

 suffices to know that in any one series of experiments v 2 and v { 

 remained constant, while the mean pressure {p 2 +P\)l'2< i n the 

 passages of the porous plate varied from about 760 mm. of 

 mercury down to about 4. From any three sets of values of 

 p 2 —pi and {p 2 +pi)/2 for any gas, it is possible by equation 

 (19) to calculate {v 2 + Vi)/(v 2 — v x ) and A' and B', or from the 

 whole series of measurements mean values of these can be 

 calculated, and then at all mean pressures p 2 —p\ can be 

 calculated for comparison with the experimental values. 



For Reynolds's meerschaum plate II., having a thickness of 

 6'3 mm. and with the temperature of the steam-jacket at 

 100° 0. and that of the water-jacket at 8°, the values of the 



