MEASURING INFLUENCES. 15 



local yield for each field concerned and to express other local quanti- 

 ties as a percentage of this yield, rather than to compare different 

 fields by absolute yield. In the case of corn crops the normal yield 

 of each field has been determined at a point far enough from the 

 windbreak to be beyond its influence. For two fields on opposite 

 sides of the same windbreak two normals must be determined. 



In measuring the effect of windbreaks upon orchard crops compari- 

 sons have been made only between protected and unprotected orchards 

 and no attempt has been made to determine a normal yield for each 

 orchard. The orchards studied were all in the same general region and 

 were affected by the same soil and climatic conditions, so that their 

 chances of producing crops in the season of 1908 were equal except 

 as the yield was affected by the presence or absence of windbreaks. 



Other things being equal, the influence of a windbreak is propor- 

 tional to its height ; and as height increases so also the width of branch- 

 ing increases, the shadows extend, and the roots grow and extend in 

 one direction or another. In order to make the measurements under 

 all conditions directly comparable, the height of the trees in any 

 windbreak has been taken as the unit of measurement and horizontal 

 distances have been expressed in terms of this unit. Thus, the terms 

 " 1-ht.," "5-ht.," " 10-ht." indicate distances in terms of tree height, 

 as equal to the height of the trees, five times their height, or ten times, 

 as the case may be. The extent of branches or roots actually meas- 

 ured in feet is everywhere expressed in the form of percentages of 

 the height of the trees. Thus, the average height of a row of cotton- 

 woods may be 50 feet and the average horizontal distance from the 

 trunks of the trees to the ends of their branches on a given side may 

 be 25 feet. The branching will therefore be expressed as 0.50-ht., 

 meaning 50 per cent of the tree height. Similarly, it may be found 

 that the roots of the trees extend 30 feet on a given side and this will 

 be expressed as 0.60-ht. Again it may be found that the windbreak 

 has an appreciable effect upon wind velocity out to 500 feet from the 

 base of the trees, this distance being expressed as 10-ht., and the 

 greatest yield in the field adjacent to this windbreak may be found 

 at a distance of 200 feet, this being expressed as 4-ht. 



In considering areas which may be influenced by a windbreak it is 

 assumed that the quantities determined at any point along the axis 

 of the windbreak represent an average for the entire length of the 

 windbreak. It is, therefore, unnecessary to consider the length of the 

 windbreak or the length of the field adjacent to it, and instead of 

 calculating areas on the basis of acreage or square feet, they are con- 

 sidered to be of indefinite length and of a width which bears a definite 

 relation to the height of the trees. The acreage involved in any case 

 may be directly calculated if the length of the windbreak is known. 



