RAID'S AND EIVERS. 113 



are the prevailing winds in this part of the Atlantic — cair}- into 

 the interior of Europe much more moisture than they brine- with 

 them into the Atlantic. They enter it with a mean annual 

 temperature not far from 60°, and with an average dew-point of 

 about 55°. They leave it at a mean temperature varying from 

 GO^ to 40°, according to tlie latitude in which they reach the 

 shore, and consequently Avith an average dew-point not higher 

 than the mean temperature. Classifying the winds of this part 

 of the ocean according to the halves of the horizon as east and 

 west, the mean of 44,999 observations in the log-books of the 

 Observatory shows that, on the average, the west winds blow 

 annually 230 and the east winds 122 days. 



285. The vapour-strings for all these rivers not in the Atlantic 

 Ocean. — Taking all these facts and circumstances into considera- 

 tion, and without pretending to determine how much of the 

 water which the rivers of America and Europe carry into this 

 part of the ocean comes from it again, we may with confidence 

 assume that the winds do not get vapour enough from this part 

 of the ocean to give rain to Europe, to the Mississippi Valley, to 

 our Atlantic slopes, and the western half of Asiatic Russia. We 

 have authority for this conclusion, just as we have authority to 

 say that the evaporation from the Mediterranean is greater in 

 amount than the volume of water discharged into it again by the 

 rivers and the rains ; only in this case the reverse takes place, 

 for the rivers empty more water into the Atlantic than the winds 

 carry from it. This fact also is confirmed by the hydrometer, 

 for it shows that the water of the North Atlantic is, parallel for 

 parallel, lighter than water in the Southern Ocean. 



286, The places in the sea whence come the rivers of the north, 

 discovered — proves the crossing at tlie calm belts. — The inference, 

 then, from all this is, that the place in the sea (§ 276) whence 

 come the waters of the Mississippi and other great rivers of the 

 northern hemisphere is to be found in these southern oceans, and 

 the channels by which they come are to be searched out aloft, in 

 the upper currents of the air. Thus we bring evidence and 

 facts which seem to call for a crossing of air at the calm belts, as 

 represented by the diagram of the winds, Plate I. It remains 

 for those who den^^ that there is any such crossing — who also 

 deny that extra-tropical rivers of the northern are fed by rains 

 condensed from vapours taken up in the southern hemisphere — 

 to show whence come the hundreds of cubic miles of water which 



I 



