THE GEOLOGICAL AGENCY OF THE WINDS. 281 



water-level of those seas ? There are indications (§ 535) that they 

 all once had a higher water-level than they now have, and that 

 formerly the amount of precipitation was greater than it now is ; 

 then what has become of the sources of vapour ? What has 

 diminished its supply ? Its supply would be diminished (§ 538) 

 either by the substitution of dry land for water-surface in those 

 parts of the ocean wliich used to supply that vapour ; or the quan- 

 tity of vapom' deposited in the hydrograpliical basins of those seas 

 would have been lessened if a snow-capped range of mountains 

 (§ 536) had been elevated across the path of these winds, between 

 the places where they were supplied mth vapour and these basins. 

 A chain of evidence which it would be difficult to set aside is 

 cantained in the chapters lY., YI., and YII., going to show that the 

 vapom- which supplies the extra-tropical regions of the north vdih 

 rains comes, in all probability, from the trade-wind regions of the 

 southern hemisphere. 



541. Now if it be true that the trade- winds from that part of 

 The path of the S.E. the world take up there, water which is to be 

 iiuo'^the northern rained in the extra-tropical north, the path ascribed 

 hemisphere. to the south-cast tradcs of Africa and America, after 



they descend and become the prevailing south-west winds of the 

 northern hemisphere, should pass over a region of less precipitation 

 generally than they would do if, while performing the office of 

 south-east trades, they had blown over water instead of land. 

 The south-east trade-winds, with their load of vapour, whether great 

 or small, take, after ascending in the equatorial calms, a north- 

 easterly direction ; they continue to flow in the upper regions of 

 the air in that direction until they cross the tropic of Cancer. 

 The places of least rain, then, between this tropic and the pole, 

 should be precisely those places which depend for their rains upon 

 the vapour which the winds that blow over south-east trade-wind 

 Africa and America convey. Now, if we could trace the path of 

 the winds through the extra-tropical regions of the northern hemi- 

 sphere, we should be able to identify the track of these Andean 

 winds by the droppings of the clouds ; for the path of the Tivdnds 

 which depend for their moisture upon such sources of supply as 

 the dry land of Central South America and Africa cannot over- 

 shadow a country that is watered well. It is a remarkable fact 

 that the countries in the extra-tropical regions of the north that 

 are situated to the north-east of the south-east trade- winds of South 

 Africa and America — that these countries, over which theory 



