Fluid Pressure 107 



and pressure of this mass of air is therefore (1530 x 750) 

 = 1,147,500, when the pressure is 750 mm. This must 

 also be the product of the volume and pressure when 

 the latter is 735 mm., according to our first statement 

 of Boyle's Law. If we denote the volume when the 

 pressure is 735 mm. by F, we have 735 F = 1,147,500, 

 whence F= 156T2 cu. ft. Thus the volume of air that 

 escapes from the room is (1561-2 - 1530) = 31'2 cu. ft. 

 A similar result is obtained by making use of the 

 second form of statement of Boyle's Law. Thus we 

 may find the volume of the room as above, and since 

 the volume of the air contained in it will increase on 

 account of the diminution of pressure, and we know 

 that the increase of volume will be inversely propor- 

 tional to the decreased pressure, we have 



New volume of air= (1530 x 750)^-735; 

 i.e. 1,147,500 -=-735, as before. 



QUESTIONS 



1. Why is it a somewhat difficult matter to keep a fully inflated 

 football entirely under water ? 



2. The standard pressure of the atmosphere is equivalent to 

 that of a column of mercury 760 mm. in height. If the specific 

 gravity of mercury is 13 '6, find the standard pressure of the atmo- 

 sphere in grams per square centimetre, and in Ibs. per square foot- 



3. If a glass tumbler be completely filled with water, the mouth 

 of the tumbler closed with a postcard held firmly against the edge 

 of the mouth with the hand, and the tumbler then inverted, the 

 hand may be removed without the water being spilt. How do you 

 explain this phenomenon ? 



4. An egg weighs 61-24 g. When weighed in water, its apparent 

 weight is 1'46 g. What is the average density of the constituents 

 of the egg ? 



