33 
change of conditions under the forest cover is brought about, it is by 
masses of trees that the suy’s power is broken, and it-is by large areas 
distributed over the vast expanse that ultimately the force of: the 
winds will be broken. 
I can not too strongly impress you with this idea, that it is a mass 
effect which we expect from the forest cover. Not only will our plan- | 
tation be more successful if we start it with this idea fully before us, 
that it must create “its own favorable conditions of growth” and soon 
become selfsupporting, but its effects upon the surroundings will be 
more readily felt the denser, the larger it is, and the closer the neighbor- 
ing plantations which add to the general effect. Where the single tree 
perishes the forest may live; here, too, ‘in union is strength.” 
FOREST COVER AND MOISTURE. 
For a large part of this now almost treeless area moisture conditions 
will not necessarily be a check to tree growth. 
We know by experience that a naked soil loses by evaporation more 
than six times the amount of moisture that it would under the shade of 
a forest cover. Hence, if we have once established a proper forest 
cover, if we have succeeded in effectively shading the ground by either 
the foliage of the trees or the litter and mulch of the decayed leaves, 
and a check to the sweep of the winds, the amount of water available 
for the tree growth is increased in proportion. What we must never . 
lose sight of is the fact that evaporation is the great dissipator of 
moisture, and that a dense shady forest growth reduces this evapora- 
tion. : 
I must stop long enough to point out what evaporation means to 
the arid or subarid, or, shall I say in deference to my friends who 
do not want to be regarded as quite dry, subhumid regions. If wecom- 
pare the rainfall during the season of vegetation in eastern and western 
stations, it appears that there is not much deficiency, if any, during 
that season on our western plains, and quite sufficient if evaporation 
were not such a rapacious robber. This enormous amount of evapora- 
tion is not alone due to heat and direct insolation, but mainly to the con- 
stant movement of the air, the incessant winds which take up and dis- 
perse the moisture. 
From the interesting experiments recorded in the annals of the 
Weather Bureau the dependence of the rate of evaporation on the ve- 
locity of the wind has been established. With the air ata temperature 
of 84 degrees and a relative humidity at 50 per cent the evaporation 
under a wind of 5 miles an hour will be 2.2 times as rapid as in the 
calm air, at 10 miles 3.8 times, at 15 miles 4.9 times, at 20 miles 5.7 
times, and with a wind at 25 miles velocity the rate of evaporation 
will be 6.1 times as great as in the calm air. And as the average ve- 
locity of the wind on the plains may be set down as 12 miles an hour, 
there is probably at least four times as much water evaporated and 
28975—Bull, 5——3 
