448 THE MOUNTAIN. 



beneath the horizon, pours out into cleared spaces, ravines, 

 and valleys, rivers of cold air. This strikes all travelers 

 through the mountains, especially among the higher ranges, 

 and is the cause of the sudden diurnal change of those re- 

 gions. The extremes, monthly and annual, above alluded 

 to are, however, from all authorities, less in Pennsylvania 

 than any other State of the Union, thus giving that State, 

 as already remarked, a more equable and salubrious climate 

 than is exhibited in any other part of North America. Speak- 

 ing of the great general uniformity of the Eastern United 

 States, Lorin Blodget remarks, in his admirable work, the 

 " Climatology of the United States," p. 128 : 



"The district embraced by this uniform climate is very large. 

 Excepting the points of local influence at the coasts and near the 

 great lakes, it may be said to include all the continent east of the 

 one-hundredth meridian ; at which line the arid and extreme charac- 

 ter of the plains sets in. Of this district nearly the whole surface 

 may be practically regarded as level, and very little elevated. The 

 mountains which occur do not break in upon the climate except by 

 reduction of temperature simply, or by changes caused by altitude 

 alone. They do not shelter or expose either side, nor cause any con- 

 trasts in the character of productions respecting them. Western and 

 Eastern Virginia diifer little, and probably not at all, from the influ- 

 ence of the intervening range of mountains. It is still more de- 

 cisively so in Pennsylvania, and at the southern extremity of the 

 Alleghany ranges, where Tennessee may be contrasted with North 

 and South Carolina and Upper Georgia. This absence of interruptions 

 of the general condition, even where mountains of considerable height 

 occur, is one of the most distinguishing features of the North Ameri- 

 can climate, and that which, more than any other, requires it to be 

 treated as a separate district for the area east of the plains." 



Still, as has been observed, there is a difference between 

 the climate of the Atlantic slope and the Appalachian range 

 of mountains, and the western or great valley slope, which 

 constitutes the basin of drainage of the Alleghany and 

 Monongahela Rivers. This gives three minor shades of 

 climate in Pennsylvania, each with its distinctive charac- 

 teristics. 



The climate of the Alleghany Mountain range through 



