292 Dr Searle, Some methods of measuring 
is measured by the water manometer* M; the end D is open 
to the atmosphere. From the junction BC a side-tube leads to a — 
cup 4K with a horizontal circular rim on which a soap film is 
placed. On account of the viscosity of the air there is a fall 
of pressure along each tube, and for a given flow of air, the fall of 
pressure in either tube is proportional to the length of the tube 
and inversely proportional to the fourth power of its internal 
radius, provided the flow is so slow that stream-line motion exists. 
The excess of the pressure in the cup K above that of the 
atmosphere causes the film to rise above the rim and to take 
the form of part of a spherical surface. From the distance, A cm., 
of the highest point of the film above the plane of the rim and 
from the radius, c cm., of the rim, the radius, r cm., of the spherical 
surface is deduced. Thus 
h(Qr—h)=e, 
or olen) Ee (7). 
If the length of the tube DC be J, cm. and its internal radius 
be a, cm. and if /,, a, be corresponding quantities for the tube BA, 
and if the pressure excess at B be p and that at A be P, then 
IES) Geb 
P . I Gis. 
It is sometimes convenient to form the portion AB of two or more 
tubes arranged in series. If the lengths and internal radii of these 
be d,, U3, ... cm. and dy, a3, ... cm., then the equation becomes 
JP Sif? Ore ih ik 
= + wos | shiebee Re Ree eee 9). 
Pp L, ie att ) ( ) 
From (8) or (9) the value of p can be found in terms of P, 
the pressure excess observed on the gauge WM. It is convenient 
to arrange the tubes so that P is about 100 times p. The internal 
radii of the tubes may be found by means of mercury. [or 
accurate work they should be calibrated ft. 
The gasometer G is formed of a cylindrical can 16 cm. in 
diameter and 24cm. in height. Part of its weight is supported by 
a string which passes over a ball-bearing pulley and carries a pan 
and weights. By varying these weights the pressure excess in the 
gasometer can be adjusted. The can is furnished with two gas- 
fitter’s taps, as shown in Fig. 6. The lower rim of the can is 
* A petroleum manometer would be better, but the density of the liquid must 
be found. 
+ For details of this process see G. F. C. Searle, ‘‘A simple method of deter- 
mining the viscosity of air.” Proc. Camb. Phil. Soc. Vol. xvit. p. 183. 
