48 PHYSICAL GEOGRAPHY. 



why there has been less rainfall in this section than in eastern Ne- 

 braska. 



As, however, there are trusty indications of a regular rate of in- 

 crease of rainfall for western Nebraska, similar to that going on in 

 eastern Nebraska the probabilities are that when the eastern two- 

 thirds of the State are once properly cultivated, and its rainfall 

 averages forty inches, that of western Nebraska will approximate 

 twenty-eight or thirty inches, and that in this State is sufficient to 

 produce successfully the cereal grains, cultivated grasses and corn. 



The second source of rainfall for Nebraska is the moisture from, 

 the rivers that flow from the mountains. These rivers are the 

 Platte, the Niobrara to a small extent, and the Missouri and its 

 tributaries. The flood time of these rivers is always a rainy season 

 for Nebraska. This rainy season comes earlier or later as the 

 "big rise" is earlier or later. Then the moisture that is wafted 

 here by the winds from the Gulf, is reinforced by the moisture that 

 is evaporated from these rivers; and the consequent precipitation 

 into cloud and rainfall, constitutes the rainy season for Nebraska. 

 A map of Nebraska shows how two of these rivers run the whole 

 length of the State, and that the mighty Missouri is east and north 

 of it. The Missouri too, it should be remembered, has a course of 

 four hundred miles along eastern Nebraska, for though the State 

 is little more than two hundred miles from north to south, the 

 serpentine windings of the river give it at least double that length. 

 We have, therefore, a length of four hundred miles of the Missouri, 

 and (for the same reason as applied to the Missouri) at least six 

 hundred miles of the Platte, or one thousand miles of river aver- 

 aging one mile broad, or one thousand square miles of rapidly mov- 

 ing river surface, exposed to a warm atmosphere, from which the 

 evaporation is simply enormous. The Niobrara, counting its wind- 

 ings, adds five hundred more miles of evaporating surface. Unlike 

 the floods of eastern rivers, these "big rises" last for a considerable 

 length of time, often indeed from its beginning to its close, over 

 two months. What adds greatly to the rapidity of the evapora- 

 tions is the difierence of temperature between the waters of these 

 rivers and the atmosphere. Lewis and Clarke', during their famous 

 expedition up the Missouri in 1804, spoke of the sameness of the 

 temperature of the water of the Missouri and its tributaries with 

 that of the atmosphere. If no difierence existed then, it does now. 

 For example, the signal service at Omaha for June, 1878, report a 



