170 SENSIBILITY OF THE AIR. 



smallest cause that can act upon it ; and it is just as 

 capable of obeying the most powerful. It surrounds 

 all earthly things, and it regulates them all. If the 

 distance is not the thousandth part of a hair's breadth, 

 or if it is " round about the pendent world," the 

 " viewless wind" is perfectly true to it. 



The principle upon which all that is done is an 

 exceedingly simple one. The temperature of the 

 air, its pressure when in its natural and unconfined 

 state, and its density, or the quantity of it in a given 

 space, are all, by the very constitution of its nature, 

 so nicely balanced and adapted to each other that 

 the least change in any one of them is instantly fol- 

 lowed by the corresponding change in the others ; 

 and its freedom of motion enables it to make an 

 instant adjustment by motion. 



There are few or no causes of disturbance arising 

 from pressure in the air itself; because the only 

 pressure which it has in itself is its own weight, or 

 pressure downwards towards the earth ; and the 

 pressures of foreign substances mixed with it are con- 

 sidered as mere pressures, not of very much conse- 

 quence. Thus the principal causes of disturbance 

 or motion in the air are differences of heat : and, 

 from general or local causes of heat, these are almost 

 incessant. 



When it is said that, of all substances in nature, 

 the air is the most sensible to heat, the meaning 

 must not be misunderstood. Sensibility to heat 

 does not mean being actually heated, but only being 



Eut into a state of action by heat ; and from what 

 as been said about the connexion between apparent 

 heat and resistance to motion, it will readily be un- 

 derstood that if the air were absolutely free to move, 

 it would never show any increase of heat at all ; but 

 would expand and yield to the full extent of the 

 heating cause. Nay, if it were allowed to expand 

 faster than that cause operated, it would thereby be 

 pooled. But light as the air is, even the smallest 



