240 MApHIXEKY IX COXXECTIOX WITII AIR, 



lighter it becomes, from the diminished pressure of the 

 weiglit above. At seven miles high, it is four times light- 

 er than at the surface ; at twenty-one miles, it is sixty- 

 four times lighter ; and at fifty miles, about twenty thou- 

 sand times lighter. At this height it ceases to refract the 

 rays of the sun so as to render it visible at the earth's sur- 

 face ; but if it decreases at the same rate upward, at a 

 hundred miles high it must be nearly a thousand million 

 Ihnes rarer than at the earth. 



If the atmosphere were uniformly of the same density, 

 with its present weight, it would reach only five miles 

 high. Although so much lighter than water, yet, from 

 its great height, it presses upon the surface of the earth 

 as heavily as a depth of thirty-three feet of water. This 

 is nearly equal to fifteen pounds on every square inch, or 

 more than two thousand pounds to the square foot. This 

 enormous weight would instantly crush us, did not air, 

 like liquids, press in every direction, so that the upward 

 exactly counterbalances the downward pressure, and the 

 air within the body counteracts that without. 



The weight of the atmosphere is strikingly shown by 

 Pj 203 means of an air- 



pump, which pumps 

 the air from a glass 

 vessel, placed mouth 

 downward upon the 

 brass plate of the 

 machine (fig. 263). 

 When the air is 

 pumped out, and the 

 upward or counter- 

 balancing air remov- 

 ed, so heavy is the 

 load upon the glass 

 ^'''^'"^' vessel, that a strong 



man could scarcely remove it from the plate, although it 



