316 



The Coefficient of Viscosity of Air. [Nov. 18, 



main part of the loss of energy resulting from the friction of the air 

 may be characterised as being due to the pushing of the air. A small 

 part, however, of the whole logarithmic decrement was due to 

 the rotation of the spheres or cylinders about their axes, and 

 Professor Stokes has kindly added to the paper a note in which are 

 deduced formulae which serve to correct for this effect of rotation. 



Acting on a suggestion of Professor Stokes, the author proceeded 

 to determine the coefficient of viscosity of air by suspending a hollow 

 paper cylinder about 2 feet in length and J foot in diameter, so that 

 its axis should coincide as to its direction with the axis of rotation. 

 The cylinder was supported by a light hollow horizontal bar, about 

 7 inches in length, to the centre of which the vertically suspended 

 wire was soldered. The wire was set in torsional vibration, and the 

 logarithmic decrement determined with the same precautions as 

 before. 



The mode of eliminating the effect of the internal friction of the 

 metal wire, and also the effect of the air on the ends of the cylinder, 

 is fully described in the paper. 



The following were the results : — 



Vibration-period Coefficient of viscosity Temperature in degrees 

 in seconds. of air, jw. Centigrade. 



3-6038 0-00017708 12-225 



8-8656 0-00017783 13-075 



In these experiments the loss of energy arising from the friction of 

 the air may be characterised as being due to the dragging of the air, 

 and it is very remarkable that there should be such close agreement 

 in the values of fi as determined by this and the previous methods. 

 The mean value of the coefficient of viscosity of air obtained by 

 this method is 0*00017746 at a temperature of 12*650° C, and the 

 mean value deduced from the previous experiments when proper 

 correction has been made for the rotation of the spheres and cylinders 

 about their axes is 0*00017711 at a temperature of H'79 C. The 

 above values of fx are given in C.G.S. units. 



The author expresses his great obligation to Professor Stokes for 

 his valuable suggestions and advice during the progress of the in- 

 vestigation. 



