338 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. ^ 



" hand over fist," with fair winds, and crossing the belt after a 

 delay in it of only a few hours instead of days. 



652. Hence we infer that the position of the equatorial calm. 

 It varies with the belt is determined by the difference of strength be- 

 traSiads. ^ tweon the north-east and south-east trade-winds, 

 which difference, in turn, depends upon difierence of barometric 

 pressure (§ 642), and upon difierence in temperature between them; 

 in corresponding latitudes north and south. In it the au' which they 

 bring ascends. Now if we liken this belt of calms to an immense 

 atmospherical trough, extending, as it does, enthely around the 

 earth, and if we liken the north-east and south-east trade-winds to 

 two streams dischargmg themselves into it, we shall see that we 

 have two cm^rents perpetually running in at the bottom, and that, 

 therefore, we must have as much air as these two cm-rents bring in 

 at the bottom, to flow out at the top. What 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-mnds. 



653. Captaui Wilkes, of the Exploring Expedition, when he 

 Precipitation in it. crosscd this belt 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 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 year ; consequently, by attentively considering Plate 

 YIII., one can teU what places within the range of this zone have, 

 dm-ing the year, two rainy seasons, what one, and what are the 

 rainy months for each locahty. 



654. Were the north-east and the south-east, trades, with the 

 The appearance of belt of cquatorial calms, of different colom'S, and 



the calm belts from • -i i , j • jy n i / i 



a distant planet. A^siblo to au astrouomer m one oi the planets, he 

 might, by the motion of these belts or gkdles alone, teU the seasons 

 T^ith us. He would see them at one season going north, then ap- 

 pearing stationary, and then commencing then- return to the south. 

 But, though he would observe (§ 295) that they follow the sun in 

 his annual com^se, he woidd remark that they do not change their 

 latitude as much as the sun does his dechnation ; he would there- 

 fore discover that their extremes of dechnafion are not so far 

 asunder as the tropics of Cancer and Capricorn, though in certain 

 seasons the changes from day to day are very great. He would 

 observe that the zones of ^vinds and calms have their tropics or 



