§ 520. THE CLOUD REGION, ETC. 281 



clouds absorb the surplus beat. They melt away, become invisi- 

 ble, and retain, in a latent and harmless state, until it is wanted at 

 some other place and on some other occasion, the heat thus im- 

 parted. We thus have an insight into the operations which are 

 going on in the equatorial belt of precipitation, and this insight 

 is sufficient to enable us to perceive that exquisite indeed are the 

 arrangements which Nature has provided for supplying this calm 

 belt with heat, and of 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 two-fold 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. 



520. In the process of condensation, these rain-drops, on the 

 Snow- line mounts othcr hand, havc set free a vast quantity of latent 



upas it crosses the , i«ii i .i i •l^ .-i 



equatorial calm belt, hcat, whicu has bccn gathered up witn the vapor 

 from the sea by the trade-winds and brought hither. The caloric 

 thus liberated is taken by the air and carried up aloft still farther, 

 to keep, at the proper distance from the earth, the line of perpetual 

 congelation. Were it possible to trace a thermal curve in the up- 

 per regions of the air to represent this line, we should no doubt 

 find it mounting sometimes at the equator, sometimes on this side, 

 and sometimes on that of it, but always so mounting as to over- 

 leap this cloud-ring. This thermal line would not ascend always 

 over the same parallels: it would ascend over those between 

 which this ring happens to be ; and the distance of this ring from 

 the equator, north or south, is regulated according to the seasons. 

 If we imagine the atmospherical equator to be alwaj'S where the 

 calm belt is which separates the northeast from the southeast 

 trade-winds, then the loop in the thermal curve, which should rep- 

 resent the line of perpetual congelation in the air, would be al- 

 ways found to stride this equator ; and it may be supposed that 

 a thermometer, kept sliding, on the surface of the earth so as al- 

 ways to be in the middle of this rain-belt, would show very near- 

 ly the same temperature all the year round ; and so, too, would a 

 barometer the same pressure, though the height of the atmos- 



