THE EQUATORIAL CLOUD-RING. 217 



stration as is the rotation of the earth on its axis. Indeed, Na- 

 ture herself has hung a thermometer under this cloud-belt that is 

 more perfect than any that man can construct, and its indications 

 are not to be mistaken. 



602. Where do the vapors which form this cloud-ring, and 

 which are here condensed and poured down into the sea as rain, 

 come from? They come from the trade-wind regions (§ 162); 

 under the cloud-ring they rise up ; as they rise up, they expand ; 

 and as they expand, they grow cool, form clouds, and then are 

 condensed into rains ; moreover, it requires no mercurial instru- 

 ment of human device to satisfy us that the air Avhich brings the 

 vapor for these clouds can not take it up and let it down at the 

 same temperature. Precipitation and evaporation are the converse 

 of each other, and the same air can not precipitate and evaporate, 

 take up and let down water, at one and the same temperature. 

 As the temperature of the air is raised, its capacity for receiving 

 and retaining water in the state of vapor is increased ; as the tem- 

 perature of the air is lessened, its capacity for retaining that moist- 

 ure is diminished. These are physical laws, and therefore, when 

 we see water dripping from the atmosphere, we need no instru- 

 ment to tell us that the elasticity of the vapor so condensed, and 

 falling in drops, is less than was its elasticity when it was taken 

 up from the surface of the ocean as water, and went up into the 

 clouds as vapor. 



603. Hence we infer that, when the vapors of sea water are 

 condensed, the heat which was necessary to sustain them in the 

 vapor state, and which was borrowed from the ocean, is parted 

 with, and that therefore they were subjected, in the act of con- 

 densation, to a lower temperature than they were in the act of 

 evaporation. Ceaseless precipitation goes on under this cloud- 

 ring. Evaporation under it is suspended almost entirely. We 

 know that the trade- winds encircle the earth ; that they blow per- 

 petually ; that they come from the north and the south, and meet 

 each other near the equator ; therefore we infer that this line of 

 meeting extends around the world. By the rainy seasons of the 

 torrid zone, except where it may be broken by the continents, we 

 can trace the declination of this cloud-ring, stretched like a girdle 



