324 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



628. TKe calm belts at sea, like mountains on the land, stand 

 Obstructions to the mightily in the way of the voyager, but, like moun- 

 navigator. tains, they have their passes and their gaps. In the 

 regions of light airs, of baffling winds, and deceitful currents, the 

 seaman finds also his marshes and his " mud-holes " on the water. 

 But these, these researches have taught him how best to pass or 

 entirely to avoid. Thus the forks to his road, its turnings, and the 

 crossings by the way, have been so clearly marked by the ^vinds that 

 there is scarcely a chance for liim who studies the lights before him, 

 and pays attention to directions, to miss his way. 



629. The arrows of Plate YIIL are supposed to fly with the 

 Plate vni. wind; the half-bearded and half-feathered arrows 



denote monsoons or periodic winds ; the dotted bands, the regions 

 of calm and baffling winds. Monsoons, properly speaking, are 

 winds which blow one half of the year from one direction, and the 

 other half from an opposite, or nearly an opposite direction. The 

 time of the changing of these winds, and their boundaries at the 

 various seasons of the year, have been discussed in such numbers, 

 and mapped down in such characters, that the navigator who wishes 

 to take advantage of them or to avoid them altogether is no longer 

 in any doubt as to when and where they may be found. 



630. Let us commence the study of the calm belts as they are 

 Deserts. represented on Plates I, and YIII. The monsoons 



and trade-vvinds are also represented on the latter, they often occupy 

 the same region. But, turning to the trade- winds for a moment, 

 we see that the belt or zone of the south-east trade-winds is broader 

 than the belt or zone of north-east trades. This phenomenon is 

 explained by the fact that there is more land in the northern hemi- 

 sphere, and that most of the deserts of the earth — as the great 

 deserts of Asia and Africa — are situated in the rear, or behind the 

 north-east trades ; so that, as these deserts become more or less 

 heated, there is a call — a pulling back, if you please — upon these 

 trades to turn about and restore the equihbrium which the deserts 

 destroy. There being few or no such regions in the rear of the 

 south-east trades, the south-east trade-wind force prevails, and car- 

 ries them over to the northern hemisphere. 



631. We see by the plate that the two opposing cuiTents of 



Diurnal rotation, wiud callcd " thc tradcs," are so unequally balanced 



that the one recedes before the other, and that the current from the 



southern hemisphere is larger in volume ; i. e., it moves a greater 



zone or belt of air. The south-east trade-winds discharo^e them- 



