282 THE PHYSICAL GEOGRAPHY OF THE SEA. 



the southeast trade-winds to push themselves over into the north- 

 ern hemisphere. 



815. Hence, also, we infer that, between certain parallels of lat- 

 itude in the northern hemisphere, the sun's rays, by reason of the 

 great extent of land surface, operate with much more intensity 

 than they do between corresponding parallels in the southern ; and 

 that, consequently, the mean summer temperature on shore, north 

 of the equator, is higher than it is south : a beautiful physical fact 

 which the winds have revealed, in corroboration of what observa- 

 tions with the thermometer had already induced meteorologists to 

 suspect. 



816. It appears, from what has been said, 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 ex- 

 amine 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 Em-ope, 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, on the polar side of 

 the southeast trade-winds, no great plains, except in Australia, 

 upon which the rays of the sun, in the summer of the other hem- 

 isphere, 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 (§ 

 810) to draw these trade-winds back, there are numerous other 

 districts in the extra-tropical regions of our hemisphere the sum- 

 mer heat of which, though it be not sufficient to turn the north- 

 east trade-winds back, and make a monsoon of them, yet may be 

 sufficient to weaken them in their force, and by retarding them (§ 

 815), draw the southeast trade- winds over into the northern hem- 

 isphere. 



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

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



