§ Cr,3, G54. SEA ROUTES, CALM BELTS, ETC. 351 



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

 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, w^e must have as much air as these two currents 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 

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



653. Captain 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 dur- 

 ing those ten days rain fell to the depth of 6.15 inches, or at the 

 rate of eighteen feet and upward during the year. In its motions 

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

 of the torrid zone, alwaj^s arriving at certain parallels at stated 

 periods of the year ; consequently, by attentively considering 

 Plate YIII., 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 localit}^. 



654. Were the northeast and the southeast trades, with the belt 

 The appearance of the of CQuatorial calms, of different colors, and visible 



calm belts from a dis- 

 tant planet, to an astronomer m 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 appear- 

 ing stationary, and then commencing 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 there- 

 fore discover that their extremes of declination 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 these zones of winds and calms have their tropics or 

 stationary nodes, about which they linger near three months at a 

 time ; and that they pass from one of their tropics to the other in a 

 little less than another three months. Thus he would observe the 

 whole system of belts to go north from the latter part of May till 

 some time in August. Then they would stop and remain nearly 

 stationarv till winter, in December; wlieu ngnin they would com- 



