122 THE ATMOSPHERE AND ITS SERVICE TO MAN 



breathe. Carpet sweepers and oiled dust cloths do much 

 to prevent stirring up the dust and bacteria, but vacuum 

 cleaners are even more effective. 



The vacuum cleaner is a device to utilize air pressure for 

 cleaning. By means of a pump or a rapidly rotating fan, 

 the air in the machine is exhausted. Atmospheric pres- 

 sure forces the air up through the mouth of the machine, 

 driving the dust and dirt particles with it. This dust- 

 laden air passes into a closely woven bag, which sifts out and 

 collects most of the dust. By using this machine no dust is 

 scattered through the air of the room. 



Vacuum cleaners have also been invented for street 

 cleaning, but outdoor conditions make them less satisfactory 

 than vacuum cleaners for the household. Most cities de- 

 pend upon washing or sweeping to keep the streets clean. 

 Where sweepers are used, they should always be preceded 



by sprinklers in order to keep down as much of 



the dust as possible. 



Decrease of Volume Due to Pressure. Experi- 

 ment 52. In a Mariotte's tube (Figure 66) cause 

 about a centimeter of mercury in the short arm to 

 balance the same amount in the long arm. The 

 pressure inside the short tube will then be equal to 

 that outside the long tube and will be that of the air 

 upon the day of the experiment. The short arm will 

 now be sealed with mercury so that no air can get in 

 or out. Pour mercury into the long arm. The air 

 in the short arm will be gradually compressed and will 

 occupy less and less space. If we remember that the 

 pressure upon the air in the short arm is the air pres- 



FlQUBE 66 . ,, , 1,1 f , i 



sure of the day plus the pressure of the mercury 

 column in the long arm that rises above the mercury level in the 

 short arm, we can show by careful measurement that the volume of 

 the air decreases just as the pressure increases. 



