THE WINDS. 225 



observations with the thermometer heid abeady induced meteorol- 

 ogists to suspect. 



482. It appears, from what has been said (§ 474), that it is the 

 rays of the sun operating upon the land, not upon the water, which 

 causes the monsoons. Now let us turn to Plate VIIL, and exam- 

 ine into this view. The monsoon regions are marked with half- 

 bearded and half-feathered arrows ; and we perceive, looking at 

 the northern hemisphere, that all of Europe, some of Africa, most 

 of Asia, and nearly the whole of North America, are to the north, 

 or on the polar side of the northeast trade-wind zone ; whereas but 

 a small part of Australia, less of South America, and still less of 

 South Africa, are situated on the polar side of the zone of south- 

 east trade-winds. In other words, there are no great plains on 

 the polar side of the southeast trade-winds upon which the rays 

 of the sun, in the summer of the other hemisphere, can play with 

 force enough to rarefy the air sufficiently to materially interrupt 

 these winds in their course. But, besides the vast area of such 

 plains in the northern hemisphere, on the polar side of its trade- 

 wind belt, the heat of which is sufficient (§ 479) to draw these 

 trade-winds back, there are numerous other districts in the extra- 

 tropical regions of our hemisphere the summer heat of which, 

 though it be not sufficient to turn the northeast trade-winds back, 

 and make a monsoon of them, yet may be sufficient to weaken 

 them in their force, and, by retarding them (§ 480), draw the south- 

 east trade- winds over into the northern hemisphere. 



483. Now, as this interference from the land takes place in the 

 summer only, we might infer, without appealing to actual observa- 

 tion, that the position of these trade-wind zones is variable ; that 

 is, that the equatorial edge of the southeast trade-wind zones is 

 farther' to the north in our summer, when the northeast trades are 

 most feeble, than it is in winter, when they are strongest. 



484. We have here, then, at work upon these trade-wind zones, 

 a force now weak, now strong, which, of course, would cause these 

 zones to vibrate up and down the ocean, and within certain limits, 

 according to the season of the year. These limits are given on 

 Plate VIIL for spring and autumn. During the latter season 

 these zones reach their extreme northern declination, and in our 

 spring their utmost limits toward the south. 



485. The Calm Belts. — There is between the two systems of 



P 



