L Dr Searle, A Simple Method of determining, etc. 185 
A Simple Method of determining the Viscosity of Air. By 
G F.C. Smarts, ScD. F.RS., University Lecturer in Experi- 
mental Physics, Fellow of Peterhouse. 
[Read 11 November 1912.] 
§ 1. Introduction. The determination of the viscosity of water 
by the flow through a capillary tube has, for many years, been one 
of the experiments done in my practical class at the Cavendish 
Laboratory. The determination of the viscosity of air was intro- 
duced in October 1912 for the benefit of those students who had 
already determined the viscosity of water in Mr T. G. Bedford's 
class at the Cavendish Laboratory or in other laboratories. The 
apparatus can be constructed at small cost and without the aid of 
a highly-skilled mechanic—advantages which will appeal to many 
teachers. No attempt has been made to introduce refinements 
into the apparatus. 
Air is pumped into a large vessel and is allowed to escape 
through a capillary tube. The pressure of the air at the beginning 
and end of a measured interval of time is determined, and it is 
assumed that the temperature of the air in the vessel and in 
the capillary tube is always equal to that of the surrounding 
atmosphere. 
In the determination of the viscosity of water the difference 
of pressure forcing the liquid through the tube is kept constant 
and the density of the liquid may be treated as uniform. But in 
the present experiment the density of the air is not uniform along 
the flow tube at any given time, and the difference of pressure 
driving the air through the tube is not constant, but diminishes 
as the time increases. The theory must therefore take account of 
these two facts. For the convenience of those who may wish to 
repeat the experiment, I give the details of the necessary 
calculations. 
Fig. 1. 
§ 2. Calculation of velocity. Consider a tube AB (Fig. 1) of 
radius @ cm. and length J cm., through which air is passing in the 
