Teacher's Leaflet. 



1 143 



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Materials. — A glass tube about 36 inches long, closed at one end; a 

 little glass funnel about an inch in diameter at the top ; a small cup, a 

 bird's bathtub is a good size, since it allows plenty of room for the 

 fingers; mercury enough to fill the tube and have the mercury an inch 

 or more deep in the cup. Be careful not to spill the mercury in the 

 following process, or you will be as badly off as old Sisyphus with his 

 rolling stone. 



Set the closed end of the tube in the cup so that any spilled mercury 

 will not be lost; with the help of the funnel slowly and carefully fill the 

 tube clear to the top with the mercury ; empty the 

 rest of the mercury into the cup; place the end 

 of one of the fingers of the left hand tightly over 

 the open end of the tube and keep it there; with 

 the right hand invert the tube, keeping the end 

 closed with the finger and place the hand, finger 

 and all, beneath the mercury in the cup; then 

 remove the finger, keeping the open end of the 

 tube all the time below the surface of the mercury. 

 When the mercury has ceased to fall measure the 

 distance from the surface in the cup to the top of 

 the mercury in the tube. 



Obserz'ations: 



(i). How high is the column of mercury in the 

 tube ? 



(2). What keeps the mercury in the tube ? Place 

 the cup and the tube on a table in the corner of 

 the room, place behind the tube a yard-stick and 



note whether the column of mercury is the same height day after day. 

 If it varies, why? 



(3). Would the mercury column be as high in the tube if it were 

 placed on top of a mountain as it would at the foot? Why? 



Facts for teachers. — As the water is kept in the tumbler by the pressure of 

 the atmosphere because it presses only on one side, thus the column of mercury 

 is kept in the tube by the pressure of the air upon the surface of the mercury ih 

 the cup. Since the tube was completely filled with mercury before it was inverted, 

 the space at the top of the tube is a vacuum, that is, has no air in it. Therefore, 

 there is no air pressing upon the mercury in the tube and it, therefore, rises as 

 high as the pressure of the air pressing down on the mercury in the cup is able 

 to push it up. In the tube the mercury will fall until it stands at about 30 inches, 

 and the size of the hole in the tube makes no difference in the height of the 

 column. If the cup and the tube be placed in the corner of the room where it 



A barometer 

 made by pupils. 



