IN POPULAR PHYSICS. 37 



14. When smoke ascends in a straight line, is it a proof 

 of the rarity or density of the air ? 



Of its density, because it shows that the smoke is much 

 lighter than the air, and so rises immediately. 



15. Explain the action of the common leather-sucker. 



There is nearly a vacuum between the sucker and the slab, 

 which is buoyed up by the pressure of the air beneath. 



16. Did you ever see a bottle really empty ? 



No. No absolute vacuum has ever been produced in a bottle 

 or any other vessel by human agency, so far as is known. 



18. How does the variation in the pressure of the air 

 affect those ^vho ascend lofty mountains ? Who descend in 

 diving bells? 



The outward pressure at a great, elevation is partly removed, 

 and the inner pressure remaining the same, the blood is often 

 forced through the ears, nostrils, etc. When one descends into 

 a deep mine the conditions are reversed : the outer pressure be- 

 comes in excess of the inner ; severe pain is felt in the ear-drum, 

 and ringing noises in the head become almost intolerable. These, 

 however, disappear after a time, when the equilibrium between 

 the internal and external pressure is restored. It is said that 

 Humboldt ascended where the mercurial column fell to 14 inches, 

 and descended in a diving-bell where it rose to 45 inches thus 

 making a variation of 31 inches, or a difference of 31,000 Ibs. 

 pressure on the body. 



19. Explain the theory of t( sucking cider " through a 

 straw. 



By the action of the muscles of the chest the lungs are 

 made to expand. A partial vacuum in them is thus produced, 

 and the pressure of the air hence forces liquid through the 

 straw up into the mouth. By closing the glottis at the right 

 moment this is prevented from going through the windpipe, 

 and it is at once swallowed. 



20. Would it make any difference in the action of the 

 siphon if the limbs were of unequal diameter ? 



