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 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. 
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 VIII., 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- 
