EVAPORATION AND CONDENSATION. 291 



nishes from the equator to the poles, it is ob- 

 vious that the amount of water which is cap- 

 able of existing in the atmosphere in the state 

 of transparent elastic vapour, must vary pro- 

 portionally in different latitudes. There is also 

 more vapour in the atmosphere of the ocean, 

 other things being equal, than over extensive 

 bodies of dry land. 



According to the observations of Captain Sa- 

 bine and Mr. Caldcleugh, as detailed by Pro- 

 fessor Daniell, it would appear, that within the 

 tropical ocean, between the western coast of 

 Africa and the eastern coast of South America, 

 the point of deposition, or dew point, is gene- 

 rally about 4 or 5 below the existing tempera- 

 ture ; sometimes only 2 and rarely more than 

 8 ; while in the interior of large continents it 

 is often 20 or 30 below the prevalent tempera- 

 ture. As the atmosphere is in a state of nearly 

 constant circulation, it is removed from one place 

 to another before arriving at the point of satu- 

 ration, and therefore contains less vapour than 

 it is capable of sustaining at the existing tem- 

 perature. Within the torrid zone, it is perpe- 

 tually expanded by solar heat, where it rises into 

 the upper regions, whence it flows to higher 

 latitudes ; while it is removed by horizontal cur- 

 rents termed sea breezes, from the ocean to the 

 heated land. 



If at the temperature of 80, the atmosphere 



