THE CLOUD REGION, ETC. 271 



has proyided for supi^lying tliis calm belt vdth heat, and of pushing 

 the snow-line there high up above the clouds, in order that the 

 atmosphere may have room to expand, to rise up, overflow, and 

 com'se back into its channels of healthful chculation. As the vapour 

 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 dm^ing the dry season, and which cannot now 

 escape by radiation. 



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

 snow-iine mounts otlicr liaud, liavc sct free a vast quantity of latent 

 equSJiiarcaim belt, licat, which has bccu gathered up with the vapour 

 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 

 upper 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, but always so mounting as to overleap 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 rmg from the equator, 

 north or south, is regulated according to the seasons. If we 

 imagine the atmospherical equator to be always where the calm belt 

 is which separates the north-east from the south-east trade-winds, 

 then the loop in the thermal curve, winch should represent the line 

 of perpetual congelation in the air, would be always found to stride 

 tliis equator; and it may be supposed that a thermometer, kept 

 sliding on the surface of the earth so as always to be in the midcQe 

 of this rain-belt, would show very nearly the same temperatm-e all 

 the year roimd ; and so, too, would a barometer the same pressure, 

 though the height of the atmosphere over this calm belt would, m 

 consequence of so much heat and expansion, be very much greater 

 than it is over the trade-winds or tropical cahns. 



521. Keturning and taking up the train of contemplation as to 

 Offices of the cioixd- tlio oflfice v/liicli tliis belt of clouds, as it encircles 

 ^^°°- the earth, performs in the system of oceanic adapta- 

 tions, we may see how the cloud-ring and calm zone which it 

 overshadows perform the office both of ventricle and auricle in the 

 immense atmospherical heart, where the heat and the forces which 



