18 WINDBREAKS 



10 and 20 inches or more, and at intervals of 10 or 20 feet on both 

 sides of the trees, giving for each windbreak a complete soil survey. 

 The amount of moisture found in any soil sample is expressed as a 

 percentage of the dry weight of that sample. Having determined 

 the normal moisture per cent for a given field at the given depth, 

 it is only a step to locate by graphic methods the point at some dis- 

 tance from the trees at which the activity of the roots ceases. This 

 distance, which may be from one-half to three times as great as the 

 height of the tree, is expressed as a percentage of that height. 



MECHANICAL FORCE OF WIND. 







The windbreak directly reduces the mechanical force of winds and 

 through this reduction has several other influences. The measure- 

 ment of velocity of surface air currents has been carried on chiefly in 

 connection with studies of evaporation and is accomplished by the 

 use of the standard anemometer, which expresses the wind velocity 

 in miles per hour. As in the case of all other atmospheric measure- 

 ments in this study, the anemometer is set up at a height of 4 feet 

 from the ground. One instrument, in a fully-exposed situation gives 

 the normal wind velocity for a stated period, while at the same time 

 other instruments exposed at certain points within the area influ- 

 enced by the windbreak show comparable velocities. The distances 

 from the windbreak are always even multiples of the height of the 

 windbreak. 



EVAPORATION. 



The ability of the windbreak to check evaporation must be its 

 source of greatest benefit, since, in the region where windbreaks find 

 their greatest usefulnessSnoisture is almost always insufficient for the 

 best interests of agriculture. To thoroughly understand the influ- 

 ence of a windbreak upon evaporation it should be stated that the 

 evaporation of water from any wet surface and also the transpiration 

 of moisture from the leaves of plants is accelerated by three condi- 

 tions heat, dryness of the air, and rapid air circulation. Hence, 

 anything which reduces the movement of the air reduces the rate of 

 evaporation and may effect an appreciable saving of the moisture 

 supply. 



Evaporation has been measured in a way which shows about the 

 same effects of windbreak protection as are felt by the plants of a field 

 crop. In other words, the evaporating surface of the instrument 

 responds to the same influences as do the leaves of plants. The 

 evaporometer consists of a circular sheet of filter paper resting on a 

 thin glass plate and continually moistened by a supply of water 

 which is fed onto the center of the paper. The moist paper 

 is exposed to sunlight, and the circulation of the air above it is 



