BAROMETERS 119 



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 pressure of the 

 day plus the" height that the mercury column in the long 

 arm exceeds that in the short arm, we can show by care- 

 ful measurement that the volume of the air decreases just 

 as the pressure increases. 



As was seen in Experiment 1, the volume of 

 the air can be very much decreased by pressure, 

 but when the pressure is removed, it regains its 

 original volume. It cannot be told from this 



experiment whether the volume of the gas de- f H 



creases as the pressure increases or whether it 

 decreases much more rapidly when first pressed 

 upon than afterward. This can be best shown by the use 

 of the Mariotte's tube as in Experiment 66. But if the 

 bicycle pump is a good one, it will answer the question of 

 the rate of decrease quite accurately. It is found that the 

 volume decreases directly as the pressure increases. 



57, Barometers. As the measurement of the atmos- 

 pheric pressure is of great importance in the study of 

 atmospheric conditions, it is necessary to have an instru- 

 ment by which these measurements can be readily made. 

 An instrument designed for this purpose is 

 called a barometer. There are two kinds of 

 barometers in common use, called the mer- 

 curial and the aneroid. 



Experiment 67. (Teacher's Exp.) Take a 

 thick-walled glass tube of about | cm. bore and 

 about 90 cm. long and slip tightly over the end of 

 it about 10 crn. 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 



