46 FOREST INFLUENCES. 



nearly alike, and on eunstructiiii;' the curve lor the average tliis appears 

 smooth aud symmetrical, as is seen in Fig. 3. The dotted line here 

 represents the progress of the differences at the depth of 4 feet. All 

 the irregularities have here disappeared. The temperature is seen to 

 be warmer in the woods than outside during the entire winter. It 

 then falls rapidly during the spring, reaches its lowest point about 

 July 1, and rises again rapidly during the autumn. The broken 

 lines represent the progress of the differences of temi)erature at the 

 depth of <> inches and the unbroken that at the surface. The first have 

 the smaller maximum values and also indicate .. longer time during 

 which it is warmer for woods. The maximum monthly differences are: 

 For 4 feet, — 5^.01 in July; for inches, — 5^.48 in June; for the surface, 

 — T^.O") in June. 



A glance at the curves shows more strikingly the retardation sug- 

 gested by these values. By the course of the curves it appears that 

 the maxima are successively later as we descend, being a full month 

 later at 4 feet than at the surtace. Probal)ly the same thing occurs 

 with diurnal changes; that is, they are retarded by some hours at the 

 lower deptli. It appears from these curves that, on the average, the 

 forest soil is warmer than that of tlie open tields in winter, but cooler 

 in the other seasons, and the total cooling effect is much greater tJian 

 the warming one. The numerical data show that while the maximum 

 negative value of W — O at the surface is 7'^.(>5, the maximum positive 

 value (January) is 0^.66, or only about one eleventh of the former. At 

 6 inches (152.4 mm) the numbers are 5^.48 and 0^.62, and the ratio one 

 to nine. At 4 feet (1.22 m) they are 5°.! and 0°.36, or a ratio of one to 

 fourteen. The forest, therefore, not only cools the soil, but also moder- 

 ates the extremes of its temperature. The amplitudes of the mean 

 monthly values are decreased by 0.6(3 + T'^.O.'j = To.Tl at the surface, 

 0^.(12 + 50.48 = (P.IO and (>o.;3G + 5o.(H = .jo.aj at (i inches ami 4 feet 

 depth, respectively, and through the soil this moderating influence 

 nuist be appreciable in the air. 



The stations of the German service are so happily distributed that a 

 study can be made of the differences in the influence of deciduous and 

 evergreen trees upon soil temperature, and also those for elevation 

 above the sea. For the first, seven stations under deciduous trees and 

 seven under evergreens were taken, Fritzen and Eberswalde being 

 (unitted to get a symmetrical arrangement, and Lintzel, because the 

 trees were young. The reductions Avere made for the two sets of sta- 

 tions independently, and the results are shown in Fig. 4. The results 

 are here plotted separately for the surface, for G inches, and for 4 feet. 

 The dotted lines refer to the deciduous trees, the broken ones to the 

 evergreens. The differences are not great, but they are quite consistent 

 at the three depths. They may be expressed by the statement that the 

 differences vary a little more under deciduous trees. This is what 

 should be expected from the character of their foliage. The same 



