878 



Handbook of Nature-Study 



Compound snow crystals. 

 Photo by W. A. Bentley. 



LESSON CCXXIII 



The Barometer 



Leading thought — The weight of our 

 atmosphere balances a column of meroiry 

 about thirty inches high, and is equal to 

 about fifteen pounds to the square inch. 

 This pressure varies from day to day, and 

 becomes less as the height of the place 

 increases. The barometer is an instru- 

 ment for measuring the atmospheric pres- 

 sure. It is used in finding the height of 

 mountains, and, to a certain extent, it 

 indicates changes of the weather. 



Method — 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. 

 Observations — 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 comer of the 

 room, place behind the tube a yardstick, 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 ic were placed 

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



Supplementary reading — -Chap. II in The Wonderbook of the Atmos- 

 phere, Houston. 



^ 



A barometer made by 

 pupils. 



