The Air We Breathe 39 



the lower opening with a cork. Then fill the can with water 

 and close the opening in the top with a cork. After doing this, 

 remove the cork in the bottom. Does the water run out of the 

 can? Remove the cork in the top. What happens? 



To show that air has weight, weigh on a delicate balance a 

 large corked bottle. Exhaust from the bottle all the air possible 

 by means of an air pump and weigh the bottle again. The differ- 

 ence shows the weight of the air that has been taken out. 



The Barometer. The pressure of the atmosphere at any 

 place on the surface of the earth is liable to change owing to the 

 movements iof the air due to heating and cooling.- The pressure 

 also varies at different heights. Normally at sea level air presses 

 with a force of nearly 15 pounds per square inch. It grows less 

 as the height above sea level increases. 



Evangelista Torricelli (1608-1647) believed that the atmos- 

 phere exerted a pressure on everything. To prove this, he used 

 a glass tube a little more than thirty inches long, closed at one 

 end. This he filled with mercury, put his finger over the open end 

 that he might invert it without the mercury getting out, and placed 

 this open end below the surface of some more mercury in an open 

 dish. When he removed his finger, leaving the long tube in a 

 vertical position with the lower end open and under the mercury 

 in the dish, he found that the mercury lowered in the tube to a 

 certain level and there it remained. He found, also, that it varied 

 some from day to day. This, he thought, was due to a variation 

 in the atmospheric pressure. He reasoned that if this pressure 

 were due to the weight of the air, as he believed it was, the higher 

 he took his apparatus the lower the mercury would settle in the 

 tube. This he found to be true by carrying the apparatus up a 

 mountain. As we know, the reason for the settling of the mercury 

 in the tube as he went up the mountain was because the higher he 

 went the less was the weight of the air on the mercury in the 

 dish. At sea level, the weight of the atmosphere will support a 

 column of mercury about 30 inches in height, or a column of 

 water about 34 feet in height. 



Torricelli's instrument and other instruments for measuring 

 air pressure are called barometers. A barometer may be used to 



