Transpiration and Radiometer Motion. 487 



2700, 3000, and 2700, which verify the equation. Beyond 

 the maximum, according to (27), the repulsion is ultimately 

 to vary directly as the pressure, so that the numbers in the 

 third row divided by those in the first are to tend towards 

 constancy : the last three values are 2*0, 2*4, and 2'5, while at 

 lower pressures the value 3 is reached; but the results at 

 these lower pressures have not been reproduced in the last 

 table, because the M'Leod gauge with air becomes less re- 

 liable towards 1/10 6 atmo, and therefore Crookes's results at 

 the lowest pressures will be discussed in a separate paper on 

 the measurement of pressures in the highest attainable vacua. 

 From Crookes's experiments we can calculate </, A", B'" 

 in (27), for with 1/10 6 atmo as unit of pressure and Crookes's 

 arbitrary unit as the unit of repulsion, we have just seen that 

 <//A" is about 3000 and c f about 3'0, so that A" = '001 ; now 

 the deflecting force is a maximum when p 2 =l/A // , so that 

 the maximum value of the deflecting force, namely, 



cV(2 + B // Vv / A 77 ), 



gives a convenient method of finding B //; when A" and c' are 

 known ; thus for air, B"' = , 0l and we have all the data for 

 calculating the deflecting force at any pressure by (27) for 

 comparison with experiment : the calculated values are given 

 in the fourth row of the last table, and show that we have 

 the correct form of equation to represent the experimental 

 facts. But according to the meanings of A" and B', B /2 /16* 

 should be nearly equal to A", whereas B W2 /16 is only the 

 1/160 part of it. 



Now the term in W /f expresses the effect of slipping, and 

 our results for air show that in Crookes's apparatus the effect 

 of slipping is only 1/1 60* or 1/13 of what it would be under 

 the ideal conditions for (25), indeed (25) with the given 

 values of A' and B' stands for one limiting case, and with 

 B' — it stands for the other where slipping is of no account, 

 and the conditions of Crookes's experiments are nearer to 

 those of the latter limit than of the former; indeed, with 

 slightly different values of c' and A" we could put B /7/ = 

 and get nearly as good a representation of the experimental 

 results for air as that just obtained. For nitrogen the values 

 of the repulsive force are about two-thirds of those for air at 

 the same pressures, except in the case of the small values, 

 which are somewhat unreliable, thus for nitrogen A /f and W 

 have about the same values as for air, while c' is about 2*0 ; 

 now according to equation (27) c', as it depends only on the 

 dimensions of the apparatus and the temperatures of the two 



2 M2 



