OR INTEENAL FRICTION OF AIR AND OTHER GASES, 267 



Table V. — Results, 



Coefficient of viscosity in dry air. Units — the inch, grain, and second, and Fahrenheit 

 temperature, |x= -00001492(461 + ^)=-006876 + -0000149^. 



At 60° F, the mean temperature of the experiments, ;U/ = •007763. Taking the foot 

 as unit instead of the inch, |M,= '000179(461+^). In metrical units (metre, gramme, 

 second, and Centigrade temperature), 



/t=-01878(l+-00365^). 



The coefficient of viscosity of other gases is to be found from that of air by multiplying 

 [/, by the ratio of the transpiration time of the gas to that of air as determined by Graham*. 



Postscript. — Received December 7, 1865, 



Since the above paper was communicated to the Royal Society, Professor Stokes has 

 directed my attention to a more recent memoir of M. O. E. Meyer, " Ueber die innere 

 Reibung der Gase," in Poggendorff' s Annalen, cxxv. (1S65). M. Meyer has compared 

 the values of the coefficient of viscosity deduced from the experiments of Baily by Stokes, 

 with those deduced from the experiments of Bessel and of Girault. These values are 

 •000104, -000276, and -000384 respectively, the units being the centimetre, the gramme, 

 and the second, M. Meyer's own experiments were made by swinging three disks on a 

 vertical axis in an air-tight vessel. The disks were sometimes placed in contact, and 

 sometimes separate, so as to expose either two or six surfaces to the action of the air. 

 The difference of the logarithmic decrement of oscillation in these two arrangements was 

 employed to determine the viscosity of the air. 



The effects of the resistance of the air on the axis, mirror, &c., and of the viscosity of 

 the suspending wires are thus eliminated. 



The calculations are made on the supposition that the moving disks are so far from 

 each other and from the surface of the receiver which contains them, that the effect of 

 the air upon each is the same as if it were in an infinite space. 



At the distance of 30 miUims., and with a period of oscillation of fourteen seconds, 

 the mutual effect of the disks would be very small in air at the ordinary pressure. In 

 November 1863 I made a series of experiments with an arrangement of three brass disks 

 placed on a vertical axis exactly as in M. Meyer's experiments, except that I had then 

 no aii--tight apparatus, and the disks were protected from currents of air by a wooden box 

 only. 



I attempted to determine the viscosity of air by means of the observed mutual action 

 between the disks at various distances. I obtained the values of this mutual action for 

 distances under 2 inches, but I found that the results were so much involved with the 

 unknown motion of the air near the edge of the disks, that I could place no dependence 

 on the results unless I had a complete mathematical theory of the motion near the edge. 



* Philosophical Transactions, 1849. 



