[ 767 ] 
XIX. The Coefficient of Viscosity of A ir. 
By Herbert Tomlinson, B.A. 
Communicated, with the addition of two Notes, by Professor G. G. Stokes, P.R.S. 
Received January 6,—Read January 14th, 1886. 
[Plate 42.] 
Origin and Purpose of the Investigation. 
Three years ago I entered on a series of researches relating to the internal friction of 
metals, little calculating, when I did so, that the task which I had set myself would 
occupy almost the whole of my spare time from that date to this. So, however, it 
has been, and one of the many causes of delay has been the necessity of making a 
re-determination of the coefficient of viscosity of air; for the resistance of the air 
played far too important a part in my investigations to permit of its being either 
neglected or even roughly estimated. The coefficient of viscosity of air may, 
according to Maxwell, be best defined by considering a stratum of air between two 
parallel horizontal planes of indefinite extent, at a distance r from one another. 
Suppose the upper plane to be set in motion in a horizontal direction with a velocity 
of v centimetres per second, and to continue in motion till the air in the different 
parts of the stratum has taken up its final velocity, then the velocity of the air will 
increase uniformly as we pass from the lower plane to the upper. If the air in 
contact with the planes has the same velocity as the planes themselves, then the 
velocity will increase - centimetres per second for every centimetre we ascend. The 
friction between any two contiguous strata of air will then be equal to that between 
either surface and the air in contact with it. Suppose that this friction is equal to a 
tangential force f on every square centimetre, then 
v 
where g is the coefficient of friction. If L, M, T represent the units of length, mass, 
and time, the dimensions of g are L _ 1 MT _1 . 
Several investigators have attempted to determine the coefficient of viscosity of air, 
and the following table shows how very widely the results obtained differ among 
each other :— 
