MACHINES THAT MAKE USE OF AIR PRESSURE 117 



down on the surface of the mercury in the vessel. The 

 atmosphere is also pressing down on the surface of the mer- 

 cury in the vessel. The one pressure balances the other. 



It makes no difference what the diameter of the column of 

 mercury is, it will stand at just the same height. If then we 

 weigh a column of mercury an inch square at the base and 

 thirty inches tall, we can find what the approximate pressure 

 of the earth's atmosphere is on every square inch of the earth's 

 surface at sea level. Such a column weighs about fifteen 

 pounds. Therefore the pressure of the atmosphere is about 

 fifteen pounds to the square inch at sea level. 



Experiment 49. (Teacher's Experiment.) Take a thick-walled 

 glass tube of about cm. bore and about 80 cm. length and slip 

 tightly over the end of it about 10 cm. of a thick- 

 walled flexible rubber tube 30 cm. in length. 

 Firmly secure the rubber tube to the glass tube 

 by winding tightly around them many turns of 

 string, making it impossible for the rubber tube 

 to slip or admit air. Completely close the rubber 

 tube with a Hoffman's screw just beyond the FIGURE 63 

 place where it leaves the glass tube. Placing this 

 closed end in a large dish so as not to waste any mercury, fill the 

 glass tube with mercury. Place the thumb over the open end of 

 the tube and invert it in a cup of mercury. If the connections 

 were made tight, the mercury will not fall far below the end of the 

 glass tube. The air pressure keeps the mercury up. This is a 

 simple form of barometer. 



While the tube is still standing in the mercury cup take another 

 glass tube similar to the first and attach it to the open end of the 

 rubber tube in the same way as the first was attached. Place 

 the free end of this tube in a dish of colored water and gradually 

 open the Hoffman's screw. (Figure 63.) The water rises in the tube. 

 Why ? What is meant by sucking water up a tube ? 



Machines that Make Use of Air Pressure. Lift Pump. 

 The ordinary lift pump (Figure 64) is a machine which 



