PRESSURE OF THE ATMOSPHERE. 79 



the surface of the earth, as a consequence of the elastic 

 force of the lower portions of air, rather than the direct 

 effect of the weight of the whole air. The weight of 

 the whole atmosphere produces the elastic force of the 

 lower portions by compressing them, and the elastic 

 force of the lower portions exerts the pressure. 



Why are we 186. WHY THE PRESSURE OF THE AIR 

 notcrushedby DOES NOT CRUSH US. If a thin glaSS V6S- 



the pressure of '- . -, -, -, . , 



the atmo- sel were turned upside down, and air-tight, 

 sphere? upon a table, it would collapse but for the 



fact that it is filled with air, which, according to the last 

 paragraph, has elastic force equal to that of the air 

 without. So our bodies would collapse, but for the fact 

 that our lungs, and all of the cavities of the body, are 

 filled with air, possessing the same elastic force as the 

 external air ; a force which it had acquired by compres- 

 sion, before it was taken into our bodies. 



187. QUANTITY OF WATER THE PRES- 



What sustains 



the water in SURE OF THE AIR WILL SUSTAIN. If a 



^uM^rfre- tumD ^ er be filled under water, and then 



sentcd in the lifted OUt bottom Up- 



figure ? 1 



ward, as shown in the 

 figure, it is well known that the wa- 

 ter will not run out. The pressure 

 of the atmosphere on the surface of 

 the water outside, keeps the water forced up on the in- 

 side. 



188. The effect would be the same if 



What quanti- , 



tu of water the tumbler were twice as tall, or if we 



* i *^ . i 



suppose it lengthened into a tube thirty- 

 three feet long. If a still longer tube 



