ORIGIN OF THE MISSISSIPPI VALLEY RAINFALL. 103 



surface vapor rising off the gulf ; this is evident because of the 

 unusual distance to which the sea-breeze penetrates into Texas 

 and the adjoining region of Mexico. In addition, the water of the 

 gulf is not as warm as that of the Atlantic equatorial current 

 to be noticed presently by an average of ien or twelve degrees 

 Fahr., and in consequence, in proportion, its evaporation is just so 

 much the smaller. The equatorial current penetrates the gulf 

 about five hundred miles, but does not diffuse itself and thus im- 

 part its heat to the adjoining waters, but in a compact body the 

 current turns toward the east and finds its way out through the 

 Florida Strait, and thus becomes the Gulf Stream. 



It is estimated that if the " gulf was landlocked and evapora- 

 tion checked,^' the volume of water poured into it by the Mis- 

 sissippi alone would " raise the level of this great area one and a 

 quarter feet each year." (Appletons' Physical Geography, p. 130.) 

 The height of the surface of the gulf, however, remains uniformly 

 the same year in and year out. It follows from this that the out- 

 flow of water and its evaporation 'combined amount each year to 

 only one foot and a quarter. This leaves twelve feet and three 

 quarters to be obtained elsewhere, in order to furnish the rainfall 

 for the great valley. The question is. Where can this be obtained ? 



The Atlantic equatorial current may furnish an answer. This 

 vast stream is about four thousand miles long and about three 

 thousand wide. Taking its rise in the Gulf of Guinea, it flows 

 westwardly, but, dividing on Cape St. Roque, the much greater 

 portion moves along the north shore of South America, and just 

 before entering the Caribbean Sea it unites with the northern 

 counter current. (See Appletons' Physical Geography, pp. 50, 51.) 

 These currents are both under a broiling tropical sun, and their 

 water is heated from 80 to 82 ; " the evaporation is rapid in the 

 equatorial regions, and most of all in the warm belts constantly 

 swept by the trade winds." Thus, when the warm, saturated air 

 next the surface rises, it is rapidly carried away by the wind, and 

 cooler air flowing from the north takes its place, to be in turn 

 heated and floated upward. Says Captain Maury, U. S. N. (Geog- 

 raphy of the Sea, p. 102), " Off this ocean belt there is, in the form 

 of vapor, annually floated up into the higher air fifteen feet of 

 water." Says Prof. Arnold Guyot, in Earth and Man, p. 85, when 

 speaking of the same, " The sun causes these invisible vapors to 

 rise, which, being lighter than the air itself, increasingly tend to 

 soar into the upper atmosphere, filling it and constituting within 

 it another aqueous atmosphere." This vapor is carried by the 

 trade winds steadily westward at the rate of about thirty or thirty- 

 five miles a day, and meets its first obstruction in the plateau of 

 Mexico, which is five thousand feet above sea-level. On the west 

 coast of Mexico stand the Sierra Madre Mountains, whose altitude 



