FOREST INFLUENCES. 317 



The compilation of tho 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 opeu. 



NOTE ON THE CONSTKUCTIO.V AND KKADING OF Till: DIAIiltAMS. 



The horizontal li:ies I'ordinates) above or bolow the zero line represent val.ies ur amounts, degrees of tempera- 

 ture, inches of precipitation or evaporation, percentages, etc. Tho vertical lines (abscissa*) represent time, dividing 

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

 ing lor the month of lleeembor or Commencement of winter. The curve lines are constructed by noting on each 

 monthly line the values found for the month, and then connecting these points by either straight or rouudeil-oll' 

 lines. 



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

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

 found for the stations in the open lield (()), or II' O. 



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

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

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

 curve from zero line, tho greater is the inlluence of the woods. In temperature readings, for instance, tho curve 

 above the /cro lino would denote that the woods 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 lino, the latter representing that stale 

 of conditions when \V_ -O. 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 tho two curve lines 

 (hatched) exhibits Ihe differeiice between tho woods and open field. 



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

 for mean values is also hatched in most cases. 



