EXPERIMENT STATION BULLETINS. 



509 



seemingly tend to exaggerate the thermal relations of these series of 

 soils very greatly. 



DIRECTION OF SOLAR HEAT IN FIELD SOILS. 



It will be interesting to consider at this time the direction in which 

 the solar radiation travels thru the soil and the part that the moisture 

 plays in this progression. At first thought it would seem that the sur- 

 face heat will travel thru the soil equally in all directions, vertically 

 as well as horizontally. Such, however, may not be the case if the sur- 

 face varies in color or in other respects, as the following experiments 

 seem to indicate. This point was studied in the following manner: On 

 a very level and smoothed surface ground was spread quartz sand 

 dyed black and very white quartz sand. On the line where the two 

 sands came together, as well as on each side of this line, were placed 

 at various distances, thermometers at the same and at different depths. 

 The temperatures of this ground so treated were read on the thermo- 

 meters on clear and sunny days. The results obtained are shown here- 

 with: 



TABLE 12.— MOVEMENT OF SOLAR HEAT IN SOILS UNDER FIELD CONDITIONS. 



Central line sharp, ie., black and white 

 1 " from central line towards black . . . 

 1 " from central line towards white . . . . 

 2" from central line towards black. . . 

 2" from central line towards white . . . . 

 4" from central line towards black. . . 

 4" from central line towards white . . . . 

 8" from central line towards black. . . 

 8" from central line towards white . . . . 

 8" from central line towards black. . . 

 8" from central line towards white . . . . 

 2" from central line towards black. . . 

 2" from central line towards white . . . . 

 8" from central line towards black. . . 

 8" from central line towards white . . . . 

 1 " from central line towards black . . . 

 1 " from central line towards white . . . . 



84.4°F 



84.4 



83.6 



85.0 



83.4 



85. 



82.8 



86.2 



82.4 



84.0 



80.5 



83.8 



82.4 



82.7 



80.0 



81.5 



81.0 



3" deep. 

 3" deep. 

 3" deep. 

 3" deep. 

 3" deep. 

 3" deep. 

 3" deep. 

 3" deep. 

 3" deep. 

 4" deep. 

 4" deep. 

 4" deep. 

 4" deep. 

 5" deep. 

 5" deep. 

 5" deep. 

 5" deep. 



The data show (l)that the temperature of that side of the line having 

 the white sand is much lower than that of the side with the black sand, 

 but that the temperature of the former is much higher near the neutral 

 line than farther away; (2) that a marked difference in temperature 

 may exist within a distance of an inch; and (3) that the temperature 

 of the black sand covered soil is higher at the 4-inch depth than the 

 white sand covered soil at the 3-inch depth when the measurements in 

 both cases are taken at two i-nches from the central line. At 8 inches 

 from the neutral line the black covered soil at the depth of 5 inches 

 has also a slightly higher temperature than the white covered soil at the 

 depth «f only 3 inches. 



These results, therefore, seem to indicate very strongly that while the 

 soil temperature tends to travel vertically as well as horizontally, the 

 amount or the facility with which it goes in the former direction is 

 greater than in the latter path. Theoretically that is what should be 

 expected since heat tends to travel at a greater rate in the presence of 

 larger amounts of moisture and since the moisture content of the soil 

 tends to increase with the depth. 



The important question to consider next is what part heat conduc- 



