372 PHYSICAL GEOGRAPHY OP THE SEA, AND ITS METEOROLOGY. 



condensation of the vapour for all this rain expands the air, 

 causing it to boil over, flow off, and leave a low barometer — a 

 diminished atmospheric pressure throughout all the region south 

 of the Himalaya. 



693. Dove and the monsoons. — As long ago as 1831, Dove 

 maintained that the south-west monsoon was the south-east 

 trade-wind rushing forward to fill the vacant places over the 

 northern deserts. Dove admits the proofs of this to be indirect, 

 and acknowledges the difficulty of finding out and demonstrating 

 the problem. 



694. TJie south-east trades passing into south-west monsoons. — 

 But any navigator who, during the summer months, has occasion 

 to traverse the Indian Ocean from north to south, may find that 

 it is so. The outward-bound Indiaman, who, when on his way 

 to Calcutta, crosses the equator in August, fur example, will 

 find the south-east trades, as he approaches the line, to haul 

 more and more to the south. As he advances still farther north 

 they get to the west of south. Finally, he discovers that he has 

 got the regular south-west monsoons, and that he has passed 

 from the south-east trades into them without any intervening 

 calm. This in summer is the rule; it has its exceptions, but 

 they are rare. Examining the logs of a number of vessels taken 

 at random for the passage in August, we find, by 421 obser- 

 vations therein recorded, they had the wind thus : 



Wind from S.E. between Lat. 10^ and 5° S. . with . calms. 



S. „ 50 S. and Equator „ . 3 „ 



„ S.W. „ Equator and 5^ N. , „ . 3 „ 



S.W. „ Lat. 50 and 10° N. . „ . „ 



695. Lieutenant Jansen. — In like manner, and wdth like force, 

 Jansen maintains that the north-west monsoon of Australia is the 

 north-east trade-wind turned aside. 



696. Monsoons in the Pacific. — The influence exerted upon 

 rainless winds by the deserts of Africa and the overheated plains 

 of Asia is felt at sea for a thousand miles or more. Thus, though 

 the desert of Cobi and the sun-burned plains of Asia are, for the 

 most pai-t, north of latitude SO"", their influence in assisting to 

 cause monsoons (§ 692) is felt south of the equator (Plate VIII.). 

 So, too, with the great desert of Sahara and the African monsoons 

 of the Atlantic ; also with the Salt Lake country and the Mexican 

 monsoons on one side, and those of Central America in the 

 Pacific on the otlier. The influence (§ 298) of the deserts of 



