292 PHYSICAL GKOGllAPHY OF THE SEA, AND ITS METEOROLOGY. 



seas ? There are indications (§ 535) that tliey 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 (§ 638) either by the 

 substitution of dry land for water-surface in those parts of the 

 ocean which used to supply that vapour ; or the quantity of 

 vapour deposited in the hydrographical basins of those seas 

 Avould 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 with vapour and these 

 basins. A chain of evidence which it would be difficult to set 

 aside is contained in the chapters IV., VI., and VII., going to 

 show that the vapour which supplies the extra-tropical regions of 

 the north with rains comes, in all probability, from the trade-wind 

 regions of the southern hemisphere. 



541. The path of the S.E. trade-winds over into the northern hemi- 

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

 the world take up there water which is to be rained in the 

 extra-tropical north, the path ascribed to the south-east trades cf 

 Africa and America, after they descend and become the pre- 

 vailing 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 couA^ey. Xow, if we could trace the path of 

 the winds through the extra-tropical regions of the northern 

 hemisphere, we should be able to identify the track of these 

 Andean winds b}^ the droppings of the clouds ; for the path of 

 the winds which depend for their moisture upon such sources of 

 supply as the dr}' land of Central South America and Africa 

 cannot overshadow a countr}^ that is watered well. It is a re- 

 markable fact that the countries in the extra-tropical regions of 

 the north that arc situated to the north-east of the south-east 



