THE ATMOSPHERE. 93 



have worn away the hills and polished the rock by their ceaseless 

 abrasion and the scouring effects of the driving sand. Those who 

 have crossed this pass are astonished at the force of the wind and 

 the marks there exhibited of its geological agencies. 



193. Panama is in the region of equatorial calms. This belt 

 of calms travels during the year, back and forth, over about 17° 

 of latitude, coming farther north in the summer, where it tarries 

 for several months, and then returning so as to reach its extreme 

 southern latitude some time in March or April. Where these 

 calms are it is always raining, and the chart* shows that they hang 

 over the latitude of Panama from June to November ; consequent- 

 ly, from June to November is the rainy season at Panama. The 

 rest of the year that place is in the region of the northeast trades, 

 which, before they arrive there, have to cross the mountains of the 

 isthmus, on the cool tops of which they deposit their moisture, 

 and leave Panama rainless and pleasant until the sun returns north 

 with the belt of equatorial calms after him. They then push the 

 belt of northeast trades farther to the north, occupy a part of the 

 winter zone, and refresh that part of the earth with summer rains. 

 This belt of calms moves over more than double of its breadth, 

 and nearly the entire motion from south to north is accomplished 

 generally in two months, May and June. Take the parallel of 4° 

 north as an illustration : during these two months the entire belt 

 of calms crosses this parallel, and then leaves it in the region of 

 the southeast trades. During these two months it was pouring 

 down rain on that parallel. After the calm belt passes it the rains 

 cease, and the people in that latitude have no more wet weather 

 till the fall, when the belt of calms recrosses this parallel on its 

 way to the south. By examining the "Trade-wind Chart," it 

 may be seen what the latitudes are that have two rainy seasons, 

 and that Bogota is within the bi-rainy latitudes. 



194. The Rainless Regions. — The coast of Peru is within the 

 region of perpetual southeast trade-winds. Though the Peruvian 

 shores are on the verge of the great South Sea boiler, yet it never 

 rains there. The reason is plain. 



195. The southeast trade-winds in the Atlantic Ocean first 



* Vide Trade-wind Chart (Maury's Wind and Current). 



