1886.] The Coefficient of Viscosity of Air, 41 



merits were made to eliminate the effect of trie friction of the air on 

 their ends. The following are the results : — 



Length in 

 centimetres. 



Diameter in 

 centimetres. 



V lDration- 

 period in 

 seconds. 



Temperature 

 of the air in 



degrees 

 centigrade. 



Coefficient of 

 viscosity of 

 the air in 

 C.G-.S. units. 





— 



\S U L <t ft LLC / O m 







60-875 



2-5636 



6-8373 



12-02 



0-00018171 



60-885 



-9636 



7-0590 



14-63 



0-00018122 



60-875 



2 -5636 



3 0198 



11 -69 



0-00018024 



53 175 



2-5636 



2-9994 



10-64 



0-00017845 







Spheres. 









6 364 



2-8801 



9-35 



0-00017820 



Maxwell has proved* that the coefficient of viscosity of air is inde- 

 pendent of the pressure and directly proportional to the absolute 

 temperature. We can, therefore, calculate from the above data what 

 would be the value of the coefficient of viscosity at 0° C. ; and when 

 this is done, in the case of each of the five sets of experiments, we 

 obtain the following values : — 



Coefficient of 



Set of viscosity of 



experiments. air at 0° C. 



1st 0-00017404 



2nd 0-00017201 



3rd 0-00017284 



4th 0-00017859 



5th .. 0-00017230 



The mean of these numbers is 0*00017296 with a probable error of 

 only 0'14 per cent. The formula for finding /n t , the coefficient of 

 viscosity of air at the temperature t° C, is therefore — 



/i,=0-00017296(l + -Q. 



The value of the coefficient of viscosity of air at 0° C. given above, 

 though much nearer to that obtained by Maxwell than any which 

 has been got by other observers, nevertheless differs from it by more 

 than 8 per cent. Maxwell experimented with dry air freed from 

 carbonic acid, but it does not seem possible that the small amount of 

 aqueous vapour and carbonic acid present in ordinary air can be 

 credited with a diminution of 8 per cent, in the viscosity ; nor can 



* " Phil. Trans.," 1866, vol. 156, Part I. 



