THE DIVING-BELL. 55 



be lower on a mountain top, because the amount of air at an elevation 

 of ten thousand feet is less than at the level of the sea, and consequently 

 exercises less pressure, and the mercury descends. [The pressure is ori 

 the bulb of mercury at the bottom, not on the top, remember.] 



The pressure of the air at the tops of mountains sometimes decreases 

 very much, and it is not sufficiently dense for perfect respiration, as many 



Fig. 53. Diver under water. 



people find. Some climbers suffer from bleeding at the nose, etc., at great 

 altitudes. This is occasioned by the action of the heart, which pumps with 

 great force, and the outward pressure upon the little veins being so much less 

 than usual, they give way. 



Many important instruments depend upon atmospheric pressure. The 

 most important of these is the pump, which will carry us to the considera- 

 tion of water and FLUIDS generally. The fire-engine is another example, 

 but we will now proceed to explain the diving-bell already referred to. 



