THE USE OF WEATHER INSTRUMENTS 



181 



Weight of bottle full of air . . . 786 . 2 grams 



Weight of bottle after removal of air ... 784 . 8 grams 



Therefore, weight of air removed 1.4 grams 



Volume of water in the bottle 1150 c.c. 



From these facts he computed the weight 



of 1 liter of air to be nearly 1 . 22 grams 



Prove the correctness of his computation. 



Air varies much in weight at different times and places. 

 Cold air is heavier, or denser, than warm air, and moist air is 

 lighter, or less densj?, than dry air. At sea level, dry air 

 weighs about 1.29 grams per liter, or about \Y oz. per cu. ft. 

 About 13 cu. ft. of air weigh 1 Ib. 



209. Air Pressure. Though a cubic foot of air weighs but 

 little, the atmosphere is many miles deep and presses down 

 with great force upon the earth's surface. This fact was dis- 

 covered soon after it was found that air has weight. 



Exercise 46. To Study the Pressure of Air 



(a) Place a palm glass upon the stand of the air pump. Place 

 rubber dam over the mouth of the palm glass and tie its edges down 

 tight with a cord. Connect 

 the stand with the pump. 

 With the first stroke of the 

 pump notice the effect on the 

 rubber dam. If the rubber is 

 fresh and strong, continued 

 pumping may stretch it till it 

 very nearly lines the inside of 

 the receiver. 



(6) Remove the rubber dam 

 and carefully fit the hand 

 over the mouth of the palm 

 glass. Work the pump. De- 

 scribe the "feeling" of the 

 hand. Does your hand seem 

 to be sucked in? Is it really being drawn in, or is it being pushed 

 in? What pushes it in (Fig. 128)? 



(c) When all the air possible has been pumped out try lifting your 

 hand steadily in a vertical direction, without prying it from the 



Air pressure 



14.7 If-" 



To air 



pump. 



FIG. 128. 



