44 



FOREST INFLUENCES. 



It is not SO easy to explain why the difference at 4 feet averages 

 a little less than at the surface and a little more than at 6 inches. It 

 may be due to retardation in the penetration of diurnal temperatures 

 into the soil. This may be slower in the cool, moist forest soil than in 

 the warmer and drier soil of open fields. If this is the case observa- 

 tions at the same instant at the two stations would determiue the 

 temperature at two different parts of their curves. The diurnal 

 changes probably extend to a greater dei)th than 4 feet, and if they 



Fig. 2. — Difference of temiieraturo ( TT^O) nt the depth of 4 feet. (Tlio line running nnch>r the name 

 of a .station is ita zero line. The curve for tlie station i.s represented by tlie nean\st lirolvin, un- 

 lirolieii, oi' dotted line like that in the margin.) 



have different times of nmxima and minima (later in the forest) the 

 increase of the difference at this depth could be explained. It may be 

 that a part (undoubtedly a small one) of the differences at the surface 

 and at inches may be due to this. The observations were taken at 8 

 a. m. and 2 ]). m. 



Turning now to the progress of the difference, " Woods" minus " Open 

 field" readings, through the seasons, or, as we shall generally express it, 

 W — 0, as shown by the differences of the monthly means, another series 

 of interesting facts appear. 



