MEASURING ATMOSPHERIC PRESSURE 49 



Men often go down under water in a caisson full of compressed air, which 

 keeps the water out. The caisson is divided into chambers. The men go 

 from chamber to chamber, slowly allowing their bodies to adjust themselves 

 to the increased air pressure. Caisson disease or "the bends" is due to rapid 

 passage from high to low atmospheric pressure such as that experienced by 

 men working in caissons under the water, on bridge-piers or tunnels. When 

 they come out into the open air the sudden release of pressure fills the blood 

 with bubbles of dissolved gas in much the same way as soda water in a bottle 

 fills with bubbles when the pressure is released by removing the stopper. It is 

 necessary that the release of pressure should be sudden; hence an aviator 

 would not suffer from " bends" from any increase in altitude due to climbing 

 in an aeroplane. "The bends" is due to the sudden release of a very high 

 pressure which forces an abnormal amount of nitrogen into the blood. When 

 a man has worked in compressed air in a diving-bell in the depths of the sea 

 or in a caisson in water, the nitrogen which is absorbed is dissolved in great 

 quantities in the body fluids. When this pressure is lowered, as it is when 

 a man returns to the surface, the nitrogen begins to work into the joints, 

 nerves and brain in the form of bubbles so rapidly as to cause intense pain, 

 paralysis and sometimes death. 



FIG. 40. What do you think about the amount of air 25 miles above the earth? 

 What figures on the diagram tell us that the density of the air rapidly de- 

 creases as we ascend? It is thought by some that particles of air may pass 

 off to and beyond the planets. 



A French author, Blaise Pascal, became interested in Torricellrs 

 discovery. It occurred to him that if the atmospheric pressure sup- 

 ported the mercury in the tube, as shown in Torricelli's experiment, 

 the height of the column of mercury in the tube should increase or 

 decrease if the pressure increased or decreased. 



He took up his idea with Perier, his brother-in-law, who lived 

 near the high conical mountain of Puy-de-D6me, and requested 

 the latter to test his theory upon this mountain. 



