RAIN-FALL. 305 



which sweep over the Pacific Ocean are chilled by contact with the ele- 

 vated ridges and snow-clad peaks of the Rocky Mountains, and precipitate 

 the moisture which they contain as rain or snow. Passing on, these winds 

 descend on the heated surface of the level plains, where in summer the 

 temperature is often as high as 100° Fahrenheit in the shade. Becoming 

 heated and rarified, they sweep eastward to the Missouri, having their 

 capacity for holding moisture greatly increased, and no ordinary change of 

 temperature will cause these winds to part with the little moisture which 

 they possess. Thus it will be seen that, practically, these plains have to 

 depend for their rain-fall on the supply of moisture derived from easterly 

 winds coming from the Atlantic Ocean and the Gulf of Mexico. These 

 winds, in passing over the Alleghanies and the Mississippi Valley, have 

 been subjected to innumerable changes of temperature, and robbed of the 

 greater portion of the moisture they originally contained, to the serious detri- 

 ment of this immense belt of country. As would be expected from the 

 above, it was noticed the past season in the Black Hills that the rain-storms 

 of long duration were preceded by, or occurred during, the prevalence of 

 easterly winds, which were chilled by contact with the colder and more 

 elevated portion of the Hills, precipitating the moisture which they carried. 



While encamped on Rapid Creek, near the edge of the plains on the 

 eastern slope of the Hills, I had an excellent opportunity to observe the 

 formation and condensation of rain-clouds during the succession of rainy 

 days in July. The wind was blowing gently from the east and southeast. 

 Out on the level plains it scarcely rained at all, though dense banks of 

 clouds hung over them and detached masses of mist and fog were sweeping 

 low over the more elevated foothills. The clouds would form over the 

 plains and, drifting on to the Hills, conceal from view the highest peaks and 

 ridges, chill, settle down to the level of the valleys, and immediately rain 

 until the excess of moisture was precipitated, then rise and drift away ; 

 when a short interval would occur before the clouds would again sweep in 

 and produce a shower. 



On examining the accompanying maps of the Black Hills, it will be 

 seen that nearly all the streams rise in the central or western portion and 

 flow east, the slope of the country being in that direction. I think tbn,t 



20 B H 



