RESULTS OF CHANGING AIR PRESSURE 61 



How to " Set " Barometers. If the observer is situated at any point above 

 sea level, the barometer, unless corrected, will read lower than the reading 

 at sea level, on account of the lessened pressure due to altitude. 



After finding the altitude, and determining the difference, the indicating 

 hand of the barometer should be lifted off the pin on which it is fitted and 

 replaced at the corrected reading. 



Never make this change by means of the small screw seen in the back. 



Effect of Different Pressures of the Air. The different pressures 

 of the atmosphere produce movements of the air. Suppose the atmos- 

 phere in our schoolroom (effects of temperature on the barometer) is 

 heated to 80 F. The air would now weigh 873 pounds; but suppose 

 the air outside was 60 F. A volume of outside air equal to the vol- 

 ume of air in the schoolroom would weigh about 933 pounds or 60 

 pounds more than the air in the schoolroom. Every cubic foot 

 of air outside would weigh T V of a pound more than every cubic 

 foot inside. This extra weight of air would press on the windows 

 and walls, trying to get into the room. If you should hold your 

 hand near the window you would feel the air rushing in at the places 

 where it does not fit tightly. If you open the window you feel the 

 breeze blowing in. 



Ventilation. The difference in pressure of the air assists us to 

 ventilate our houses. The heavy outside air is allowed to rush in 

 and push out the light, warm air which has become bad. The 

 proper ventilation of rooms is very important. The diagram on 

 page 179 will show the effects of poor ways and good ways of ven- 

 tilating our schoolrooms, as well as our homes. 



Soil Ventilation. The constantly changing pressure of the atmos- 

 phere ventilates the soil. When the pressure is great, large quanti- 

 ties of air are forced among the soil particles. As the atmospheric 

 pressure becomes less, the air expands and a part of it is forced out 

 of the ground. 



Drafts. If we hold a piece of joss stick near the draft of a stove, 

 we will see the smoke rushing toward the opening. The air in the 

 stove is heated, and is much lighter than the air outside of the 

 stove. There is a great pressure caused by the cooler air outside 

 which presses the hot air and smoke up the chimney. More will be 

 said about the use of the difference in air pressure when we study 

 methods of heating and ventilating our homes. 



