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 (§ 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 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 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 hemisphere.—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 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 hemisphere, we should be able to identify the track of these Andean winds by the droppings of the clouds; for the path of the winds which depend for their moisture upon such sources of supply as the dry land of Central South America and Africa cannot overshadow 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
