170 SENSIBILITY 



motions as it is yielding to those of every thing else. 

 The least alteration of temperature, or pressure, in- 

 stantly puts the air into motion. If any thing 

 advances, the air moves off before it to make 

 room ; and if any thing recede, the air follows at 

 the same time to support it. If any thing is 

 heated above the average, the air ascends with 

 the excess of heat ; and if any thing is cooled, the 

 air condenses and closes in upon it, not only as a 

 protection against greater cold, but to impart 

 positive heat. No matter how great or how small 

 the object is, or how long .or how short the 

 distance, the air is sensible to the very smallest 

 cause that can act upon it ; and it is just as ca- 

 pable 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 uncon- 

 fined 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 followed by the corresponding 

 change in the others ; and its freedom of motion 

 enables it to make an instant adjustment by 

 motion. 



Thus 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 toward the earth; and 



