OF THE AIR. 171 



the pressure of foreign substances mixed with it 

 are considered as mere pressures, not of very 

 much consequence. 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 put into a state of action by heat ; and from 

 what has been said about the connexion between 

 apparent heat and resistance to motion, it will 

 readily be understood that if the air were abso- 

 lutely 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 cooled. But 

 light as the air is, even the smallest portion of it 

 has some weight; and softly though it moves, 

 still it has some friction. Both of these offer some 

 resistance to the cause of heat, and thus the air is 

 always a little warmed before it begins to move in 

 obedience to the heat. The resistance is the 

 greatest where the pressure is greatest, and the 

 air in consequence the most dense ; and that is 

 one of the reasons why the air is hotter in hollows 

 than on heights. If one were to ascend till the 

 air had only half the pressure, and consequently 

 only half the density, and the resistance to fric- 

 tion that it has at the mean level of the sea, then 

 it would yield twice as easily to the heating cause; 

 and the same cause that would render it not only 

 warm, but disagreeably hot at the level of the sea, 

 Q 2 



