OF AIR AND OTHER GASES. 7 



some months before, and had been set into torsional vibrations with various 

 weights attached to it, to determine its moment of torsion. Its moment of 

 torsion and its viscosity seem to have remained afterwards nearly constant, till 

 steam was employed to heat the lower part of the apparatus. Its viscosity 

 then increased, and its moment of torsion diminished permanently, but when 

 the apparatus was again heated, no further change seems to have taken place. 

 During each course of experiments, care was taken not to set the disks vibrating 

 beyond the limits of the scale, so that the viscosity of the wire may be 

 supposed constant in each set of experiments. 



In order to determine how much of the total retardation of the motion is 

 due to the viscosity of the wire, the moving disks were placed in contact with 

 each other, and fixed disks were placed at a measured distance above and 

 below them. The weight and moment of inertia of the system remained as 

 before, but the part of the retardation of the motion due to the viscosity of 

 the air was less, as there were only two surfaces exposed to the action of the 

 air instead of six. Supposing the effect of the viscosity of the wire to remain 

 as before, the difference of retardation is that due to the action of the four 

 additional strata of air, and is independent of the value of the viscosity of the 

 wire. 



In the experiments which were used in determining the viscosity of air, 

 five different arrangements were adopted. 



Arrangement 1. Three disks in contact, fixed disks at 1 inch above and below. 



2. 0-5 inch. 



3. Three disks, each between two fixed disks at distance 0*683. 



4. 0-425. 



5. 0-18475. 



By comparing the results of these different arrangements, the coefficient 

 of viscosity was obtained, and the theory at the same time subjected to a 

 rigorous test. 



Definition of the Coefficient of Viscosity. 



The final result of each set of experiments was to determine the value of 

 the coefficient of viscosity of the gas in the apparatus. This coefficient may be 

 best defined by considering a stratum of air between two parallel horizontal 



