Page:Physical Geography of the Sea and its Meteorology.djvu/139

 Rh are the prevailing winds in this part of the Atlantic—carry into the interior of Europe much more moisture than they bring 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 60° to 40°, according to the latitude in which they reach the shore, and consequently with 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 National 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 consideration, 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 the 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 deny 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