EXPERIMENT STATION BULLETINS. 497 



The moisture content )).v volunie of all the types of soil in which the 

 specific heat has already "been considered, has been determined in the 

 following manner: The weight of a cubic foot of each soil was de- 

 tern)ined in the regular way. Their moisture content was found under 

 the same field conditions many times during the year: in the early 

 spring before they began to thaw, in the middle of the summer, in the 

 late fall when they began to freeze, after a long drought, and imme- 

 diately after a heavv rain. The percentage of the moisture content 

 found*^ under this variety of conditions multiplied by the weight of a 

 cubic foot of soil gives the amount of water contained in that volume 

 of soil. Knowing the specific heat of water, the influence that the 

 moisture content exerts on soil temperature can readily be calculated. 

 The table below represents the number of degrees Centigrade that 100 

 heat units will raise or lower the temperature of a cubic foot of the 

 different types of soil. 



A careful examination of the table reveals many striking and signifi- 

 cant facts. It shows (1) that on account of the great difl'erence in tJie 

 weight of a cubic foot the influence of the specific heat of equal weight 

 is offset §0 that the peat which possesses the greatest heat capacity 

 but the lowest weight has its temperature raised about twice as much 

 as that of the sand and gravel which have the lowest specific heat but 

 the greatest weight; (2) the differences in the moisture content are so 

 great in the different types of soil that the influence of the weight of the 

 soil on the temperature is again offset or overshadowed so that the 

 peat which in the dry condition is about twice as warm as the sand 

 or gravel, is now from two to three times colder than the latter soils. 

 This latter fact was actually obtained also in field conditions. The 

 peat for instance, remained frozen in the spring about 10 days longer 

 and in the fall it usually froze after the other soils had frozen. All 

 these facts lead to the important conclusion that while the specific heat 

 as well as the weight play some important part on the ultimate soil 

 temperature, and especially on its ascent and descent, their influences 

 arte minimized and indeed overshadowed by the tremendous influence 

 of water with its great heat capacity. Hence it cannot be determined 

 with certainty what the rising or lowering of temperature will be of 

 any particular group of soils merely from the specific heat and weight 

 of a cubic foot, unless the water content also is known. 



HEAT CONDUCTIVITY OF SOILS. 



OBJECT AND METHOD OF EXPERIMENTATION. 



Heat conductivity is defined as the quantity of heat in calories which 

 passes across a cube of unit edge (1 cm), in unit time (1 second), under 

 a uniform temperature gradient of one degree centigrade per centimeter. 



Heat conductivity is one of the three modes of heat propagation, the 

 other two are radiation and convection. These three modes of heat 

 63 



