172 THYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY 



great Pacific Ocean, should we look for the place of evaporation ? 

 Wondering where, 1 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." Bufialo 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 Avinds 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 joarallels upon the ocean, are 

 going from a higher to a lower temperature ; and the winds in 

 the Mississij^pi 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 Can- 

 cer, 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 tliat 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 m^' hand Ehrenberg's work, 

 *' Passat-Staub imd Blut-Pegen." Here I found another clew 



