226 THE PHYSICAL GEOGRAPHY OF THE SEA. 



trade-winds a region of calms, known as the equatorial calms. It 

 has a mean average breadth of about six degrees of latitude. In 

 this region, the air which is brought to the equator by the north- 

 east and southeast trades ascends. This belt of calms always 

 separates these two trade-wind zones, and travels up and down 

 with them. If we liken this belt of equatorial calms to an im- 

 mense atmospherical trough, extending, as it does, entirely around 

 the earth, and if we liken the northeast and southeast trade-winds 

 to two streams discharging themselves into it, we shall see that 

 we have two currents perpetually running in at the bottom, and 

 that, therefore, we must have as much air as the two currents bring 

 in at the bottom to flow out at the top. What flow^s out at the 

 top is carried back north and south by these upper currents, which 

 are thus proved to exist and to flow counter to the trade-winds. 



Using still farther this mode of illustration : if we liken the calm 

 belt of Cancer and the calm belt of Capricorn each to a great 

 atmospherical trough extending around the earth also, we shall see 

 that in this case the currents are running in at the top and out at 

 the bottom (§ 101). 



486. The belt of equatorial calms is a belt of constant precipi- 

 tation. Captain Wilkes, of the Exploring Expedition, when he 

 crossed it in 1838, found it to extend from 4° north to 12° north. 

 He was ten days in crossing it, and during those ten days rain fell 

 to the depth of 6.15 inches, or at the rate of eighteen feet and up- 

 ward during the year. In the summer months this belt of calms 

 is found between the parallels of 8° and 14° of north latitude, and 

 in the spring between 5° south and 4° north. ( Vide Plate VIII.) 



487. This calm belt carries with it the rainy seasons of the tor- 

 rid zone, always, in its motions from south to north and back, ar- 

 riving at certain parallels at stated periods of the year ; conse- 

 quently, by attentively considering Plate VIIL, one can tell what 

 places within the range of this zone have, during the year, two 

 rainy seasons, what one, and what are the rainy months for each 

 locality. 



Were the northeast and the southeast trades, With the belt of 

 equatorial calms, of different colors, and visible to an astronomer 

 in one of the planets, he might, by the motion of these belts or 

 girdles alone, tell the seasons with us. He would see them at one 

 season going north, then appearing stationary, and then commenc- 



