THE ATLANTIC STATES. 205 



of density (resulting from the unequal expansion,) 

 soon causes movements, by which the colder por- 

 tions change places with the warmer. These being 

 cooled, resume their previous situation, and are again 

 displaced by being again made warmer. Thus, the 

 temperature reversing the situations, and these re- 

 versing the temperatures, a circulation is kept up 

 tending to restore the equilibrium. Precisely similar 

 would be the case with our atmosphere, were it not 

 an elastic fluid, and dependant for its density on 

 pressure, as well as heat. Its temperature would be 

 far more uniform than at present, and all its varia- 

 tions would be gradual. An interchange of position 

 would incessantly take place, between the colder air 

 of the upper regions, and the warmer, and of course 

 lighter air near the earth's surface, where the most 

 heat is evolved from the solar rays. Currents would 

 incessantly set from the poles to the equator below, 

 and from the equator to the poles above. Such cur- 

 rents would constitute our only winds, unless where 

 mountains might produce some deviations. Violent 

 gales, squalls, or tornadoes, would never ensue. 

 Gentler movements would anticipate them. But the 

 actual character of the air with respect to elasticity, 

 is diametrically the opposite of that which we have 

 supposed. It is perfectly elastic. Its density is de- 

 pendant on pressure, as well as on heat, and it does 

 not follow, that air which may be heated in conse- 

 quence of its proximity to the earth, will give place 

 to colder air from above. The pressure of the atmos- 

 phere varying with the elevation, one stratum of air 



