70 SCIENCE OF COMMON THINGS. 



Height of the atmosphere. Weight of the atmosphere. 



430 How are waves of the ocean produced ? 



By the force of the air in motion, or wind striking 

 upon the surface of the water. 



431 Could a bird fly in a space devoid of air, even if it could exist 

 without respiration ? 



It could not ; as the bird rises simply by the resist- 

 ance of the particles of air to the beating of its wings. 



/ 432 How do we know that air is elastic f 



Because a volume of compressed air, the pressure 

 being removed, immediately restores itself to its origi- 

 nal bulk. 



433 When is air said to be rarefied f 



When a given quantity of air is caused to expand 

 and occupy a greater space, it is said to be rarefied. 



"When a part of the air inclosed in any vessel is withdrawn, that which 

 remains, expanding by its elastic property, always tills the dimensions of 

 the vessel as completely as before. If nine-tenths were withdrawn, the 

 remaining one-teiith would occupy the same space that the whole did 

 formerly. 



434 What is the height of the atmosphere above the surface of the earth f 



It is supposed to be about 45 miles ; the zone or shell 

 of air which surrounds the earth to the height of nearly 

 21 miles from its surface, contains one-half of the 

 atmosphere ; and the remaining half being relieved of 

 this superincumbent pressure, expands into another 

 zone or belt of the thickness of 41 or 42 miles. 



Some authorities suppose this last zone to have a much greater area. 



435 What is the weight of air compared with that of water f 



Water is about 840 times the weight of air, taken 

 l>ulkfor bulk. 



430 What is the estimated weight of the whole atmosphere enveloping 

 tJie globe f 



To the weight of a globe of lead sixty miles in dia- 

 meter. 



437' As air has weight, and as the mass of it extends at least 45 miles 

 above the earths surface, what amount of pressure does it exert ? 



At the level of the ocean the atmosphere exerts a 

 pressure of about 1 5 pounds for every square inch of 

 surface. 



