280 Common Science 



when water evaporates its vapor mingles with the air ; 

 and when this vapor is sufficiently cooled it condenses 

 and forms droplets of water. 



The barometer. In predicting the weather a great 

 deal of use is made of an instrument called the barometer. 

 The barometer shows how hard the air around it is 

 pressing. If the air is pressing hard, the mercury in 

 the barometer rises. If the air is not pressing hard 

 the mercury sinks. Just before a storm, the air usually 

 does not press so hard on things as at other times ; so 

 usually, just before a storm, the mercury in the barom- 

 eter is lower than in clear weather. You will under- 

 stand the barometer better after you make one. Here 

 are the directions for making a barometer : 



Experiment 87. " To be done by the class with the aid of the 

 teacher. Use a piece of glass tubing not less than 32 inches 

 long, sealed at one end. Fill this tube to the brim with 

 mercury (quicksilver), by pouring the mercury into it through 

 a paper funnel. Have the sealed end of the tube in a cup, 

 to catch any mercury that spills. 1 When the tube is full, 

 pour mercury into the cup until there is at least half an 

 inch of it at the bottom. Now put your forefinger very 

 tightly over the open end of the tube, take hold of the sealed 

 end with your other hand, and turn the tube over. Lower 

 the open end, with your finger over it, into the cup. When 

 the mercury in the cup completely covers your finger and 

 the end of the tube, remove your finger carefully so that no 

 air can get up into the tube of mercury. Let the open end 

 of the tube rest gently on the bottom of the cup, and hold 



1 If mercury spills on the floor or table during this experiment, gather 

 it all into a piece of paper by brushing even the tiny droplets together 

 with a soft brush ; squeeze it through a towel into a cup to clean it. It 

 is expensive ; so try not to lose any of it. 



