IN POPULAR PHYSICS. 35 



No ; this would cause more friction, and increase the resist- 

 ance to outflow produced by cross currents. 



5. What ivould be the effect if both openings in one of 

 the arms of Barker's Mill were on the same side ? 



It would cease revolving. The pressure in each direction 

 would then be equal, and the arms would balance. 



PNEUMATICS. 



1 45 1. Why must we make tivo openings in a barrel 

 of cider when we tap it ? 



One to let out the cider, and one to admit the air. 



2. What is the weight of 1O cubic feet of air ? 



100 cu. in. weigh 31 grs. ; hence 10 cu. ft. will weigh 31 grs. 

 x 172.8 = .7652 Ibs. avoirdupois. 



3. What is the pressure of the air on one square rod of 

 land ? 



2721 x 144 x 15 Ibs. = 588,060 Ibs. 



4. What is the pressure on a pair of Magdeburg hemi- 

 spheres 4 in. in diameter, when the air is entirely ex- 

 hausted ? 



On each hemisphere the pressure is equal to the area of a 

 great circle, multiplied by the pressure on each unit of area. 

 Hence on each hemisphere the pressure is 



.7854 x 4 2 x 15 Ibs. = 188.5 Ibs. nearly. 

 The sum of the two opposite pressures is thus not quite 277 Ibs. 



5. Hoiv high a column of water can the air sustain 

 when the barometric column stands at 28 in. ? 



28 in. x 13^ = 31 feet. 



6. Tf we should add a pressure of two atmospheres, 

 what ivould be the volume of 1OO cu. in. of common air ? 



The pressure is trebled, and, according to Mariotte's law, the 

 volume will be reduced in the same proportion ; hence it will 

 be 100 cu. in. H- 3 = 33*- cu. in. 



