78 HEAT. 



over a glass cylinder, open at both ends, placing the 

 open end air-tight on the plate of an air pump, and 

 then exhausting the air. The pressure of the column 

 of air that stands on the bladder is sufficient to break 

 it, and the air settles in, as effectually as if it were a col- 

 umn of iron. The atmosphere exerts such pressure, 

 amounting to about fifteen pounds to every square inch, 

 on all parts of the surface of the earth. 



184. A SIMPLE MEANS OF PROOF. - 



Describe a 



simple means Wind a stick with cotton and press 

 thatair l * has it; to the bottom of a test-tube, con- 

 wight. taining enough water thoroughly to 



moisten it. It will be found difficult to withdraw 

 the piston. The difficulty arises from the fact that 

 the column of air which rests upon it, must be 

 lifted at the same time. Having raised it a little way 

 and released it, the piston flies with force to the bot- 

 tom, owing to the weight of the same column of air. 



185. ELASTIC FORCE OF THE ATMOSPHERE. 

 Every cubic inch of air at the surface of 



its elastic the earth, may be likened to a piece of in- 

 dia-rubber, which has been compressed into 

 the space of a cubic inch, by a heavy weight placed on 

 it. If we suppose a piece of rubber, while thus com- 

 pressed, to be confined in a strong metallic box, it would 

 evidently exert an elastic force in all directions, equal 

 to the force which compressed it. So the lower por- 

 tions of air, which are kept compressed by the air 

 above, exert elastic force. And it is better to regard 

 the pressure of fifteen pounds on every square inch of 



