SOIL TEMPERATURES. 41 



temi)erature are always, except where otherwiKse stated, the vahies of 

 W—0. A positive sign ( + ) imlicates always that the woods are 

 warmer thau the fields; a negative sign ( — ), that the woods are cooler 

 than the iields, Tlie degrees are FahrtMiheit and the other units of 

 measure are the usual English ones, unless otherwise stated. 



In Fig. 1 are given the mean annual differences of temperature for 

 fields and woods at the surface and at the depths of G inches and •! feet. 

 These are for the German stations, a brief description of which may 

 be found on page 30. They are arranged in the order of their differ- 

 ences at the surface, and at the right hand are given the average 

 values. It will be noted at once that the differences vary much for 

 the different stations. The mean annual difference for Hadersleben is 

 less than one degree at the surface, while that at Melkerei or Neumath 

 is nearly four degrees. It will also be noticed that the differences at 

 6 inches vary with respect to each other and also with respect to those 

 at the surface. They are generally less than the surface differences at 

 the same stations, but HoUerath furnishes a notable exception. On 

 the whole they run with the surface differences, but are somewhat less. 

 Again the differences at the depth of 4 feet differ among themselves, 

 but not so much as do the otliers. Their relative uniformity is due to the 

 depth at which the temperatures are taken, the smaller surface changes, 

 in temperature and in water contents, being little felt. These dififer- 

 ereuces are usually greater than those at the deptii of 6 inches and 

 less than those at the surface. Bat there are exceptions to this, not- 

 ably in the cases of Hollerath and Bt. Johann. These differences in 

 the action of the forest are due to several possible causes, such as dif 

 ferences in soil as well in the field as in the forest station, in kind of 

 trees, in their density, in the exjiosure, in tlie character of the air 

 drainage. 



iuo-s under two sets of conditions, namely, in most cases tlie values which were found 

 for the stations in the woods (W) diminished b\ the values found for the stations iu 

 the open iield (O), or IF — O. 



The value of this difl'ereuce is positive, if the curve runsabove the zero line — that 

 is to say, the records for the woods ( W) showed higher values than that for the open 

 held ( 0) ; it is negative, /. e., the record for tlu'. woods was lower, if tiie curve line runs 

 below the zero line. The greater, therefore, the vertical distance of any point iu 

 the curve from zero line, the greater is the influence of the woods. In temperature 

 readings, for instauce, the curve above the zero line would dtMiote tiiat the woods 

 were warmer; below the zero Hue, that tlio woods were cooler than the open field 

 l»y as many degrees as the curve runs above or below the zero line, the latter repre- 

 senting that state of conditions when W := O, (. c, when there is no dilVcieuce 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 diftereuce betAveen the woods and 

 opea 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. 



