The Weather 13 



Imagine a glass tube with one end sealed, thirty-eight inches 

 long and with a cross-sectional area of one square inch, filled 

 with mercury, and standing with the open end in a basin con- 

 taining a quantity of mercury. The tube is arranged so that 

 the mercury may move freely in and out of the tube ; but the 

 upper part of the tube is a perfect vacuum, which means that 

 there is no air to press upon the mercury column in the tube. 

 The pressure of the air upon the surface of the mercury in the 

 basin prevents the rest of the mercury from escaping from the 

 tube, just as air pressure holds a sheet of paper under a tumbler 

 filled with water and prevents the escape of the water. But 

 the mercury is free to move in or out of the tube as the pressure 

 upon the surface in the basin varies. That is, the mercury 

 column will change in height as the pressure upon the surface 

 in the basin increases or decreases. Now suppose that you 

 find the height of the mercury column to be just thirty inches. 

 What is the pressure of the air which balances this? Multiply 

 the weight of one cubic inch of mercury by the height of the 

 column. 



Exercise. Will the column of mercury in the tube rise or fall 

 as the pressure of the air increases? Why? If the pressure of the 

 air upon the surface decreases, how will the height be changed? 

 Explain. What condition makes it possible for the mercury to rise 

 in the tube when the air pressure upon the surface of the mercury in 

 the basin increases? If the upper space in the tube were filled with 

 air, could the mercury column rise? Explain your answer. 



The mercury column measures 31 inches. What is the air pres- 

 sure in pounds ? 



If the mercury measures 28.4 inches, what is the air pressure? 



The air pressure upon a square inch of surface is 12.4 pounds. 

 What will be the height of a mercury column which balances it? 



IV. THE BAROMETER 



11. Construction of a barometer. It will be a useful experi- 

 ence to make an instrument like that just described. Such an 

 instrument is called the mercury barometer. What does this 



