114^ Home Nature-Study Course, 



a flat stone and see how heavy a stone can be Ufted by the sucker. Ask 

 why a sucker clings so to the flat surface. If a little air is allowed 

 to get between the sucker and the stone what happens? Why? 



Facts for teachers. — The pressure of air is greatest at sea level where it equals 

 about 15 pounds to the square inch and where a cubic foot of air weighs about 

 iJ4 ounces. But since air presses with equal force in all directions, it is difficult 

 for us to make the pupils understand that it exerts any pressure whatever. If 

 the palm of the pupil's hand measures 9 square inches and he holds the palm up, 

 then there is pressing down upon it 135 pounds, but he does not feel it because 

 the air below the hand is pressing upward at the same rate. 



Air pressure, that is, the weight of air, decreases with the increase of height 

 above sea level, because the higher we ascend the less air there is above us. 

 However, the air pressure at any given place has a daily oscillation and a non- 

 periodic oscillation, the reasons for which are not perfectly understood. Dry air 

 is composed of a mixture of gases in the following proportion : nitrogen, 79.02 ; 

 oxygen^ 20.95 ; carbon dioxid, .03, and a varying amount of other rare gases, 

 argon about i, and a varying amount of water vapor up to one twentieth part. 

 According to our present knowledge of the laws of gases, the earth's atmosphere 

 should be only about 40 miles high; but our knowledge is imperfect and it is 

 probable that there is no definite height at which we can say there is no atmos- 

 phere. Meteors have been observed at more than 100 miles above the sea level 

 and their light is believed to be due to friction with our atmosphere. However, 

 we know by the experiences of mountain climbers and balloonists that the air 

 becomes rarer as we ascend and it also becomes colder. 



The water is kept in the tumbler in Experiment i by the pressure of the atmos- 

 phere against the paper. If the tumbler is tipped to one side the water still 

 remains in the glass, which shows that the air is pressing against the paper from 

 the side with sufficient force to restrain the water, and if the tumbler is tipped 

 bottomside up it shows the air is pressing upward with sufficient force to keep 

 the water within the glass. 



In the case of Experiment 2, we know that the leather pressing upon the floor 

 or on the stone is not in itself adhesive, but is wet simply that it shall press 

 against the smooth surface more closely. The reason why we cannot pull it off 

 is that the air is pressing down upon it with the force of about fifteen pounds 

 to the square inch. If the experiment is performed at sea level we should be able 

 to lift by the string of the sucker a stone weighing fifteen pounds. The reason 

 why the water falls out of the tumbler after a little air is let beneath the paper 

 is that then the air is pressing on both sides of the paper; and the reason why 

 the sucker will not hold if there is any air between it and the stone is because 

 the air is pressing in both directions upon it. 



Lesson XV. 



THE BAROMETER. 



Purpose. — To teach the children how to make a barometer and to 

 understand the reason why the mercury rises and falls within it. 



