DIFFERENCES OF SOIL TEMPERATURE. 



45 



Fig. 2 shows tlie dift'eieiices of temperature through the year at the 

 depth of 4 feet. The curves for all tlie stations for Avhich good monthly 

 means for ii 11 the months could be obtained are put on one sheet for 

 comparison. In order not to crowd them too much they are placed at 

 different levels, the zero level for each station being represented by 

 the line running under the name of the station, and the curves belong- 

 ing to each station either unbroken, broken, or dotted lines nearest to 

 its zero line. Seasonal lines are drawn for the monthly values, winter 

 beginning with December, spring with March, summer with June, and 

 autumn with September. The stations are arranged in the order of 

 the greatest summer differences. 



The distances between the horizontal lines representing degrees 

 Fahrenheit, the absolute amount of W — can be i-ead. Thus, for Ebers- 

 walde, during the month of December, W — is between two and three 

 degrees, the soil in the forest being by that amount warmer than that 

 in the open at 4 feet (1.22m) depth, while in the middle of July it is 

 just five degrees lower. 



Fio. 3.— Difl'erence of soil temperature (TF— O), all stations— German observatories. 



It Avill also readily be seen that the values for Hollerath, Carlsberg, 

 an<l Neunmth are never above their zero line; that is to say, are always 

 negative, the soil in the woods being always cooler. 



This figure shows in more detail the differences between the stations 

 Avhich were so readily apparent in <he annual nutans. The time of 

 maximum is generally .lidy, bnt Sclioo, in a. mean often years, gives a 

 sec()ndary minimum at tliis time, and for Oarlsberg and INIelkeiei the 

 maximnm difference falls in August. Mostof the stations give a, posi- 

 tive sign Ibr l^ — O in winter, some of them showing a^ large excess 

 (Eberswalde+2^.41, Fritzen + 2o.20)5 but there arc several exceptions, 

 and all those which do not cross the zero line come very near to it. 

 The maximum monthly difference is — 8o.l7 for Fritzen in -Inly. The 

 least July value is — lo.93 (for S(dioo), less tiian a (juartcr of the pre- 

 ceding. 



The sign of If— generally changes from negative to positive in 

 Decembei', and the reverse in March. The progress of these curves is 



