798 
MR. H. TOMLINSON ON THE COEFFICIENT OF VISCOSITY OF AIR, 
Number of 
observation. 
Logarithmic decrement. 
1 
•0009162 
2 
•0009015 
3 
•0008743 
4 
•0008871 
5 
•0009019 
6 
•0008993 
These, like the others, are consecutive observations, and the mean of them is 
•0008967. 
Applying the corrections, mentioned in the paper, for small differences in the vibra¬ 
tion-periods, temperature, &c., when the two cylinders were used, we have for the 
logarithmic decrement due to a cylinder (60’80 —12'32) centims. or 48'28 centims. in 
length the value 
•0017029. 
It follows, from Professor Stokes’s formulae, that the logarithmic decrement 
arising from the friction of the air against the inner and outer walls taken together 
will be 
log 10 e (a/2/+ y/2 X0-375/- 1 —v/2 X 0-4922/" 3 +&c.), 
f being ecpial to \J 11 p .a, 
where a is the mean radius of the cylinder, r the vibration-period, /x the coefficient of 
viscosity, p the density of the air, M the mass of air which would be contained in a 
cylinder of the same length, and having an internal radius equal to a, and I the 
moment of inertia. 
The values of I and r were 36966 centimetre-grammes and 3‘6038 seconds respec¬ 
tively. The corrected height of the barometer was 29'354 inches, and the tempera¬ 
ture 12 0- 225 C. The value of p was calculated, as usual, on the supposition that the 
air is half saturated with moisture. 
The terms 0’375 f~ l and 0'4922 f~ 3 are so small that we may calculate them by 
using an approximate value of /x, and the series converges so rapidly that it is quite 
unnecessary to include any more terms in it.* 
The value of /x, determined from the data given above, was found to be 
•00017580. 
* Indeed, tlie third term might have been dispensed with in this case, but not in the next experiment. 
