FOREST INFLUENCES. 317 



The compilation of the records at these stations into the ingenious graphic form here presented 

 was made by Mr. Mark W. Harrington, formerly Chief of the United States Weather Bureau ; 

 they explain themselves without the need of additional text to any one who will learn to read 

 them with the aid of the following explanation and show at a glance the difference of meteoro- 

 logical conditions prevailing in the forest and in the open. 



NOTE ON THE CONSTRUCTION AND READING OF THE DIAGRAMS. 



The liorizont.il lines ('ordi nates) above or below the zero line represent values or .amounts, degrees of tempera- 

 ture, inches of precipitation or evaporation, percentages, etc. The vertical lines (absciss.e) represent time, dividing 

 the field into twelve seasonal divisions corresponding to the twelve months of the year, the outer lines both stand- 

 ing for the mouth of December or commencement of winter. The curve lines are constructed by noting on each 

 monthly line the values found for tbe month, and then connecting these points by either straight or rounded-off 

 lines. 



Unless otherwise noted, the values so plotted are the differences between the readings under two sets of condi- 

 tions, namely, in most cases the values which were found for the stations in the Avoods (W) diminished by the values 

 found for the stations in the open iield (O), or IV — 0. 



The value of this difference is positive, if the curve runs above the zero line-— that is to say, the records for the 

 woods (IV) showed higher values than for the open field (O); it is negative, i. e., the record for the woods was 

 lower, if the curve line runs below the zero line. The greater, therefore, the vertical distance of any point in the 

 curve from zero line, the greater is the influence of the woods. In tempeiature readings, for instance, the curve 

 above the zero lino would denote that the woodh were warmer,* below the zero line, that the woods were cooler than 

 the open field by as many degrees as the curve runs above or below the zero line, the latter representing that state 

 of conditions when W = 0. i. e., when there is no difference in the readings for the two sets of conditions. 



Where values for each set of conditions are plotted separately, the area included within the two curve lines 

 (hatched) exhibits the difference between the woods and open field. 



To exhibit more readily the amount of influence of the forest, the areas included by the zero line and the curve 

 for mean values is also hatched in most cases. 



