72 SCIENCE OF COMMON THINGS. 



How flies walk on the ceiling. How we breathe. 



447* Why is it often painful and difficult to breathe on a mountain-top f 



Because, owing to the extreme rarity of the air on 

 the top of the mountain, a person, although expanding 

 his chest as much as usual, really takes in only half 

 as much air as he does when at the foot of the moun- 

 tain. 



448 If the lips be applied to the back of the hand, and the breath drawn 

 in so as to produce a partial vacuum in the mouth, why will the skin be 

 drawn or sucked in f 



Not from any force resident in the lips or the mouth 

 drawing the skin in, but from the fact that the usual 

 external pressure of air is removed, and that the pres- 

 sure from within the skin is suffered to prevail. 



449 How is a boy enabled to lift a stone by means of the common 

 sucker ? 



The sucker consists of a disk of moistened leather, 

 with a string by which it may be suspended with any 

 weight attached to it. If its smooth moist surface be 

 pressed so closely against the flat side of a stone or 

 other body that the air cannot enter between them, the 

 weight of the atmosphere pressing upon the upper sur- 

 face of the leather makes it adhere so strongly, that a 

 stone of weight proportioned to the extent of the disk 

 of leather may be raised by lifting the string. 



450 How are flies and other small insects enabled to walk on ceilings 

 and surfaces presented downwards, or upon smooth panes of glass in an 

 upright position ? 



Their feet are formed in such a manner that they 

 act as small air-pumps or suckers, excluding the air 

 between them and the surface with which they are in 

 contact ; and the atmospheric pressure keeps the animal 

 in position. 



451 Why in breathing do we first draw in the breath, as it is termed? 



Because by so doing we make an enlarged space in 

 the chest, and the pressure of the external atmosphere 

 forces the air in to fill it. 



The air enters the lungs, not because they draw it in, but by the weight 

 of the atmosphere forcing it into an empty space. 



453 How is the air caused to escape from the lungs f 



Simply by means of its elasticity ; the lungs by 



