84 ATMOSPHERE. [Lesson XVI. 



things risible, which arc not to be seen at the time 

 of low water. 



But to return from this digression, and proceed 

 \vith our account of the air and atmosphere: lire 

 science Jvhich treats of the weight, pressure, and 

 elasticity of the air is called Pneumatics. And 

 'here it may be first observed, that the air is a fluid 

 body which surrounds and gravitates upon all parts 

 ' of the earth's surface. But it differs from all other 

 fluids in the following particulars : 1. It can be 

 compressed into a much less space than what it 

 naturally " possesses, which no other fluid can. 

 . It cannot be congealed as other fluids can. 3. It 

 is of a different density in every part upward from 

 the earth's surface, decreasing in its weight, bulk 

 > for bulk, the higher it rises : and therefore must 

 also decrease in density; and the law of diminu- 

 tion is such, that when the heights increase in 

 arithmetical progression, the densities decrease in 

 geometrical progression. 4. It is of an elastic or 

 springy nature : and the law it observes in this 

 respect is, that the force of the spring is equal to 

 its weight, or the density is alway proportional to 

 the force by which it is compressed. We may also 

 observe that heat increases and cold diminishes 

 the elasticity of the air : or heat expands and cold 

 condenses it. 



The weight or pressure of the atmosphere upon 

 any base at the surface of the earth, is equal to the 

 weight of a column of quicksilver of the sarn base, 

 and its height between twenty-eight and thirty-one 



inches: 



