FORESTRY. 143 



great lakes and down the St. Lawrence river between the Alleghanies to 

 the south and the Canadian hills to the north thus reaching the Atlantic 

 ocean. This broad lower land in the center of the continent, designated 

 by parallel lines, is highest at the head of the great father of waters, de- 

 scending gently across the broad expanse, northward to the Artie ocean 

 as well as southward to the mouth of the great river. To the west of 

 this central lower strip the plains rise gently but considerably toward the 

 Eockies, while the land to the east rises toward the foot of the Alleghan- 

 ies. So far as the moisture bearing winds of this entire valley between 

 the two systems of mountains are concerned, they flow up this central 

 trough from the Gulf of Mexico as into the lower end of a funnel. Che 

 great trade winds of the central Atlantic ocean pour in the Gulf of Mex- 

 ico and are deflected northward by the rib of elevated land along the 

 western part of both Americas. These great winds bring into this valley 

 nearly all the moisture they get from the ocean coming in just where the 

 Mississippi returns the surplus water to the ocean. The winds coming 

 from the west have their surplus moisture all taken out by rising up over 

 the cold Rockies and the small amount of air coming into the valley from 

 that direction is only made warmer by passing over the plains and become 

 absorbers rather than suppliers of moisture. The winds which might 

 come from the east over the Alleghanies likewise cannot bring a surplus 

 of moisture from the Atlantic. The winds coming from the north are 

 warmed as they flow southward over this broad intercontinental region. 



Practically the two sources of the moisture in the atmosphere, in this 

 entire region, seem to be that brought in from the gulf and that evapor- 

 ated back into the air from the soil, from lakes or other water surfaces, or 

 through plants from the soil. Some one has estimated that only enough 

 water runs out of the Mississippi valley, through rivers each year, to 

 equal one-fifth of the rainfall, and we can then assume that only 

 one-fifth of the amount annually precipitated is brought into this region 

 by winds from the ocean, otherwise water would be stored up here, 

 and our soils and air would become moister. This we know is not true. 

 The other four-fifths must be supplied to the atmosphere by evaporation, 

 the same water being re-precipitated and re-evaporated often. There are 

 many factors operating to cause precipitation from the air. To roughly 

 illustrate, we may say that the moisture-laden winds from the gulf flow 

 northward meeting the cold winds, flowing from the north, and that a 

 mixture of these two is cool enough to cause the southern wind to give 

 up moisture. In summer the wind from the south seems to have the 

 mastery, while in winter the great sheets of wind from the northward 

 sweep down carrying the gulf wind back. There are a few things which 

 illustrate how these two winds mix or combat each other. Both are di- 

 rected to the eastward, (see map N. W. — S. W.) and as they meet the two 

 combatting winds form a resultant of their combined force. This 

 resultant, of course, flows eastward and finds its escape across'the lakes 

 and down the St. Lawrence river to the ocean. That there is something 

 of such a resultant wind is shown by the fact brought out by meterolo- 

 gists, that most cyclonic storms of this valley pass from the west toward 

 the northeast, having, as some one has said, their focal center in the 

 vicinity of Iceland. The flowing of this air toward the eastward carries 

 most of the moister air to the region east of the Mississippi, there giving 



