-1859] 



WRITINGS OF JOSEPH HENRY. 



277 



diminution of temperature essential to the formation of rain. 

 They therefore carry their moisture to the belt of confluence, 

 where in the ascent of the air it is precipitated, evolves its 

 latent heat, and develops its ascensional power. To render 

 the ascent of these currents more plain Fig. 7, may be con- 

 sidered a transverse section across the equator at the belt 

 of calms. 





^ ^' 



Fio. 7. 



The air enters below on either side D C, rises upward in 

 the middle space, and spreads out north and south above 

 D' (7. As the air ascends it comes under less pressure, ex- 

 pands, becomes colder, and on this account condenses a por- 

 tion of its vapor, which renders the air warmer and lighter 

 than it would be if this evolution of " latent heat " did not 

 take place. Hence the ascension continues, and the eleva- 

 tion to which the column attains is therefore much greater 

 than it would be if the air were void of moisture. The 

 condensation of the vapor takes place in the form of a large 

 amount of rain which falls by its superior weight through 

 the ascending air, A, B, and deluges the surface immediately 

 below, in some places to such an extent that fresh water on 

 the surface of the ocean has been found floating on the top 

 of the salt water. Indeed more rain falls on the surface 

 within this belt than on the whole earth beside. On either 

 side of the rain belt a cloud will be formed by the spreading 

 out of the ascending air mixed with vapor, as shown in the 

 figure. The falling rain coming from a high elevation and 



