Page:Physical Geography of the Sea and its Meteorology.djvu/198

172 great Pacific Ocean, should we look for the place of evaporation? Wondering where, I addressed a circular letter to farmers and planters of the Mississippi Valley, requesting to be informed as to the direction of their rain winds. I received replies from Virginia, Mississippi, Tennessee, Missouri, Indiana, and Ohio; and subsequently, from Colonel W. A. Bird, Buffalo, New York, who says, "The south-west winds are our fair-weather winds; we seldom have rain from the south-west." Buffalo may get much of its rain from the Gulf Stream with easterly winds. But I speak of the Mississippi Valley; all the respondents there, with the exception of one in Missouri, said, "The south-west winds bring us our rains." These winds certainly cannot get their vapours from the Rocky Mountains, nor from the Salt Lake, for they rain quite as much upon that basin as they evaporate from it again; if they did not, they would in the process of time have evaporated all the water there, and the lake would now be dry. These winds, that feed the sources of the Mississippi with rain, like those between the same parallels upon the ocean, are going from a higher to a lower temperature; and the winds in the Mississippi Valley, not being in contact with the ocean, or with any other evaporating surface to supply them with moisture, must bring with them from some sea or another that which they deposit. Therefore, though it may be urged, inasmuch as the winds which brought the rains to Patagonia (§ 355) came direct from the sea, that they therefore took up their vapours as they came along, yet it cannot be so urged in this case; and if these winds could pass with their vapours from the equatorial calms through the upper regions of the atmosphere to the calms of Cancer, and then as surface winds into the Mississippi Valley, it was not perceived why the Patagonian rain winds should not bring their moisture by a similar route. These last are from the north-west, from warmer to colder latitudes; therefore, being once charged with vapours, they must precipitate as they go, and take up less moisture than they deposit. The circumstance that the rainy season in the Mississippi Valley (§ 355) alternates with the dry season on the coast of California and Oregon, indicates that the two regions derive vapour for their rains from the same fountains.

358. Ehrenherg and his microscope.—During the discussion on this subject, my friend Baron von Gerolt, the Prussian minister, had the kindness to place in my hand Ehrenberg's work, "Passat-Staub und Blut-Pegen." Here I found another clew