121 



Y. The crowns of the trees diminish the force of the rain falling upon the soil, 

 so that the latter is not as much compacted. 



Prom this knowledge of- the climatic conditions of the forest interior we may 

 judge of the change of conditions on the same area after the removal of the forest. 



The deforested area naturally would show — 



1. Greater extremes of temperature of air as well as of soil. 



2. Average moisture of the air lowered. 



3. On a shallow soil with impenetrable subsoil formation of morasses is possible, 

 while on deep soils more rapid desiccation of the soil is to be anticipated. 



4. Possibly, if the deforested area was extensive, a decrease of atmospheric 

 precipitation might occur, and at least its distribution throughout the year would be 

 changed. 



5. On light and sandy soil, especially near the coast, where winds are strongest, 

 the shifting of sands and formation of dunes is facilitated. 



6. In the mountains deforestation causes torrents, carrying away debris into 

 the valleys ; land slides and snow slides are induced, because the mechanical retard- 

 ing influence of the soil-cover and of the trunks^ upon the superficial flow of water 

 ceases. 



"What the removal of the forest will do in changing the climate of an adjoining 

 country of open ground is much more diflcult to state. In fact, positively, we do 

 not know much about it. The influence of an existing forest may be direct or in- 

 direct — direct by communicating the characteristics of its own climate to the 

 neighbourhood ; indirect by interposing its protective cover against and breaking the 

 force of the winds, which would otherwise have free sweep over the fields. 



Evaporation is the great dissipator of moisture, and the dry north-western and 

 southern winds are its powerful aids. 



A conception of the amount of evaporation may be had from the measurements 

 of the United States Signal Service. In the western country the evaporation ranges 

 from 25 to over 101 inches during the year. To choose a place near the Canadian 

 frontier, take Fort Assiniboine, in Montana. The evaporation during the year was 

 39'5 inches, while during the same year the precipitation was only 17 inches (some- 

 what above the average) ; therefore, a deficit of 22-5 inches to satisfy the demands of 

 evaporation alone. How much of this must be placed to the account of the evapo- 

 rative power of winds we cannot tell, but this amount must be large, since according 

 to experiments of the Signal Office with the temperature of the air 84 degrees and 

 the relative humidity at 50 per cent., the evaporation with a wind at only five miles 

 an hour was 2-2 times that in a calm, and with increased velocity the rate of evapo- 

 ration was six times as large as that observed in a sheltered, calm position. But the 

 wind at Fort Assiniboine during every month in the year reaches a maximum of 

 over 36 miles, and as much as 58 miles, while its average is certainly not l^ss than 12 

 miles an hour, to which corresponds a rate of evaporation of overfour times that in 

 the calm atmosphere. 



"When it comes to such warm, dry winds as the Foeha in Switzerland, and the 

 Chinook on our Eocky Mountain slopes, a snow cover of 2^ feet and more in depth 

 will vanish absolutely in twelve hours' time before its insatiable thirst. 



The mere mechanical action of the forest alone in breaking the force and 

 velocity of winds, which the windbreak, of course, exhibits in a less degree, is most 

 obviously a potent influence in preserving humidity. • , 



Even without taking special account of this influence, direct measurements have 

 established that the evaporation from soil kept saturated with water during the 

 seven months, from April to October, was in the open 40*8, in the pine woods 15-9, 

 and under the shelter of deciduous trees 6-2 — that is to say, the water was evaporated 

 in the field 6 J times as fast as under the forest cover. Here, again, the influence in 

 retarding evaporation is most potent, and explains the beneficial influence of aforest 

 cover on the flow of springs. 



Popular writers have been pleased to compare the forest floor with a sponge 

 which takes up large quantities of water and then gradually gives it up to the soil 



