232 



214 



191 



172 



169 



157 



130 



118 



29 



26 



15 



10 



9 



7 



3 



2 



5 



4 



2-6 



1-8 



1-5 



1-0 



•37 



•16 



74 



6-3 



51 



4-2 



4-1 



2-6 



2-5 



21 



7-7 



6-8 



3-8 



2-5 



2-2 



1-7 



•72 



•48 



1-0 



•9 



1-3 



17 



1-9 



1-5 



3-5 



44 



96 Mr. W. Sutherland on two New 



according to the equations 



p(^-l + '03) = 2a\ oPo /I), . . . (35) 



with L = -0499, //,='004, and 2a\ Q p /D = 10-5, 



and deflecting force = c7{A>/(l-^) + B /// + l/p\, . (36) 



with e'=4-16, A"=-0006, a=-0016, B'^-Ol. 



10 4 log. dec. ... 324 304 270 253 



def. for 49 45 37 31 



p by gauge ...14-5 12 8 6'5 



p from log. dec. 163 136 102 8'9 



p from def. for. 15-2 133 103 83 



1-1 1-0 10 1-1 



The numbers in the last row are the ratios of the pressures 

 given by Crookes's radiometer when used as a viscometer 

 gauge and as a transpiration gauge, and they show that 

 they agree with one another down to a pressure of about 

 5/10 6 atmo as given by the viscometer gauge, although at 

 that pressure they have become 50 per cent, larger than the 

 pressure given by M'Leod's gauge. Then, just as in the case 

 of air, we find the value of the ratio rise suddenly to be nearly 

 2 instead of 1, and the same explanation holds for the fact 

 here as for air, namely that the mean free path is getting to 

 be about as large as the radius of the bulb. Now as the mean 

 free path in hydrogen is about double that in air, the pressure 

 at which the change in the ratio occurs with hydrogen ought 

 to be about double that with air, and as the change occurs at 

 pressures about 5/10 6 atmo and 2'5/10 6 as measured by the 

 viscometer gauge, the theoretical condition is realized in the 

 experimental results. 



According to the viscometer gauge the lowest pressure is 

 13 times as great as that given by the M'Leod gauge, a result 

 which shows how useful the control of the different gauges 

 by one another will be in measurements of the highest vacua. 

 The change in ratio which we have found both with hydrogen 

 and air seems at first sight to disqualify the transpiration 

 gauge, but it is to be remembered that the magnitude of the 

 change is the result of the unsuitableness of Crookes's torsion 

 radiometer to serve as a type of the ideal transpiration gauge, 

 which, if designed on the lines explained a few pages back, 

 would be liable to only a trifling change of ratio, which could 

 be reduced to zero if steps were taken to bring the whole 



