352 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



winds, and crossing the belt after a delay in it of only a fc^v 

 Lours instead of days. 



652. It varies with the strength of the trade-winds. — Hence we 

 infer that the position of Ihe equatorial calm belt is determined 

 by the difference of strength between the north-east and south- 

 east trade-winds, which difference, in turn, depends upon dif- 

 ference of barometric pressure (§ 642), and upon difference in 

 temperature between them in corresponding latitudes north and 

 south. In it the air which they bring ascends. Now if we liken 

 this belt of calms to an immense atmospherical trough, extend- 

 ing, as it does, entirely around the earth, and if we liken the 

 north-east and south-east trade-winds to two streams discharging 

 themselves into it, we shall see that we have two currents per- 

 petually running in at the bottom, and that, therefore, we must 

 have as much air as these two currents bring in at the bottom to 

 flow out at the top. AVhat flows 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. 



653. Precipitation in it. — Captain Wilkes, of the Exploring 

 Expedition, when he crossed this belt in 1838, found it to extend 

 from 4° north to 12° north. ELe 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 upwards during the year. In its 

 motions from south to north and back, it carries with it the rainy 

 seasons of the torrid zone, always arriving at certain parallels at 

 stated periods of the j^ear ; consequently, by attentively consider- 

 ing Plate YIIL, one can tell what places within the range of thi& 

 zone have, during the year, two rainy seasons, what one, and 

 what are the rainy months for each locality. 



654. Tlie appearance of the calm belts from a distant planet. — Were 

 the north-east and the south-east trades, with the belt of equato- 

 rial calms, of different colours, and visible to an astronomer in 

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

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

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

 mencing their return to the south. But, though he would 

 observe (§ 295) that they follow the sun in his annual course, he 

 would remark that they do not change their latitude as much as 

 the sun does his declination ; he would therefore discover that 

 their extremes of declination are not so far asunder as the 

 tropics of Cancer and Capricorn, though in certain seasons the 



