THE ATMOSPHERE. 61 



ELASTICITY OF THE AIR. 



We say that body or material is elastic, whose parti- 

 cles admit of condensation or compression, and an im- 

 mediate return to their primitive condition, when the 

 compressing power is removed. All aeriform bodies 

 are perfectly elastic, and admit of compression and 

 condensation in the fullest degree. There is no quali- 

 ty of the air so important to the arts as its elasticity. 

 To it belongs the principle of the airpump, the fire- 

 engine, the airgun, the match syringe and the diving 

 bell. 



There is no spring worthy of comparison with the 

 elastic power of the air. Load it as heavily as we may, 

 it will sink beneath the weight, but it cannot be broken. 

 Confine it for centuries to the same labor, it is there 

 still, faithful and unworn, ever ready to perform its 

 task. And though the being who confines it for his 

 operations, realizes the value of its power, and sinks 

 down to the grave, it still remains the willing slave of 

 his descendants, until the barriers that confine it, conr 

 sumed by the accumulated rust of years, refuse to re- 

 tain the servant longer. 



The instrument by which most of the experiments 

 illustrating the pressure of the atmosphere are shown, 

 is called an airpump. Its object is to remove the air 

 from one side or surface of a vessel, that the natural 

 pressure on the opposite side may not be resisted by 

 the pressure from below. We are apt to associate ideas 

 of great complexity with the name of an airpump, when 

 in fact it is among the most simple instruments of phi- 

 losophy. Any one who has noticed the structure of a 

 common water pump, has seen the whole of the materi- 

 als used in the other; therefore we will first speak 

 of the water pump, which properly belongs to the pres- 

 sure of the air, but is introduced here for facility of de- 

 scription, and then show how nearly they resemble each 

 other. 



