176 



THE PHYSICAL GEOGRAPHY OF THE SEA. 



is thus interrapted, and the excess of heat in the earth is deUvered 

 to the air, and by absorption carried up to the clouds, and there 

 transferred to their vapors to prevent excess of precipitation. 



353. In the mean time, the trade-winds north and south are 

 pouring into this cloud-covered receiver, as the calm and rain-belt 

 of the equator may be called, fresh supplies in the shape of cease- 

 less volumes of heated air, w^hich loaded to saturation with vapor 

 has to rise above and get clear of the clouds before it can com- 

 mence the process of coohng by radiation. In the mean time, 

 also, the vapors which the trade-winds bring from the north and 

 the south, expanding and growing cooler as they ascend, are be- 

 ing condensed on the lower side of the cloud stratum, and their 

 latent heat is set free, to check precipitation and prevent a flood. 



354. While this process and these operations are going on upon 

 the nether side of the cloud-ring, one not less important is going 

 on upon the upper side. There, from sunrise to sunset, the rays of 

 the sun are pouring down without intermission. Every day, and 

 all day long, they operate with ceaseless activity upon the upper 

 surface of the cloud stratum. When they become too powerful, 

 and convey more heat to the cloud vapors than the cloud vapors 

 can reflect and give ofl" to the air above them, then, with a beau- 

 tiful elasticity of character, the clouds absorb the surplus heat. 

 They melt away, become invisible, and retain, m a latent and 

 harmless state, until it is wanted at some other place and on some 

 other occasion, the heat thus imparted. 



355. We thus have an insight into the operations which are go- 

 ing on in the equatorial belt of precipitation, and this insight is 

 sufficient to enable us to perceive that exquisite indeed are the ar- 

 rangements which Nature has provided for supplying this calm 

 belt with heat, and for pushing the snow-line there high up above 

 the clouds, in order that the atmosphere may have room to ex- 

 pand, to rise up, overflow, and course back into its channels of 

 healthful circulation. As the vapor is condensed and formed into 

 drops of rain, a twofold object is accomplished : coming from the 

 cooler regions of the clouds, the rain-drops are cooler than the air 

 and earth below ; they descend, and by absorption take up the heat 

 which has been accumulating in the earth's crust during the dry 

 season, and which can not now escape by radiation. Thus this 

 cloud-ring modifies the climate of all places beneath it ; overshad- 

 owing, at different seasons, all parallels from 5° south to 15° north. 



