[elworthy-murray] BALLOOX FABRICS 43 



These readings with the necessary temperature and pressure 

 readings yield the following results: 



V = the rate of flow of air expressed in litres per hour of dry gas at 

 15 -SX. and 760 mm. 



r = the percentage of hydrogen or helium in the outflowing air. 



If the area of the fabric exposed = A sq. cms. the permeability, 

 P, is given by the equation 



VXrX 10,000X24 



P = 



lOOA 



In the permeameter employed A = 500 sq. cms. and, on substituting, 

 the formula becomes P = 4-8XVr. 



(b) In the second method, a Katharometer together with a 

 mirror galvanometer was used. The Katharometer, manufactured 

 by the Cambridge Scientific Instrument Co., Ltd., was designed by 

 Dr. G. A. Shakespear and has been described by him elsewhere^ 



The explorer and galvanometer employed have been referred to 

 in another paper^. 



In making Katharometer readings about 1 minute was allowed 

 after adjustment. The permeameter air-chamber was then closed 

 and readings taken at one minute intervals for about 20 minutes. 

 When the readings were completed the exit-trap from the air-chamber 

 of the permeameter was opened, and the air stream re-established. If 

 subsequent readings were made, sufilcient time was given for equili- 

 brium conditions being reached. 



With the Katharometer, the permeability (P) is given by the 

 expression : P = galvanometer-scale divisions X a constant. 



The constant for hydrogen-air was calculated from data supplied 

 by the Cambridge Scientific Instrument Co., Ltd., while that for 

 helium-air was deduced making use of the calibration results obtained 

 by one of the writers,^ The actual values of the constants found were 

 for hydrogen 0-4377, and for helium, 0-6870. 



Results 



The results obtained by the two methods are collected in tables 

 II and III. Column 1 gives the fabric number as described in Table I ; 

 column 2, the sample mark; columns 3 and 4 the permeabilities to 

 the nearest decimal place at 15-5°C. and 760 mm.; column 4, the 



' Shakespear. Advisory Committee for Aeronautics. No. 317. A new Permea- 

 bility Tester for Balloon Fabrics. 1917. 



2 Murray. Proc. Roy. Soc. of Canada, 1919. 

 • Murray loc. cit. 



