EXPERIMENT STATION BULLETINS. 737 



waves into the soil and the degree of temperature gradient at the ad- 

 jacent depths at any particular time, are atforded by tables 37 and 38, 

 which contain the temperature of the different types of soil at four dif- 

 ferent depths taken every 20 minutes from the time that the minimum 

 or maximum temperature occurred at the surface to the time they had 

 reached the lowest depth in all the soils. These records are a few of a 

 great number that were taken at various times througliout the four years. 

 The data presented here were obtaind during the fourth year when the 

 thermometers were placed at 2, 4, and 6 inches from the top. The tem- 

 perature at inch depth or at the surface was taken by a mercury ther- 

 mometer which had a very small bulb. This bulb was inserted into the 

 surface soil until it was completely covered. It registered the tem- 

 perature only of the upper 14-inch depth. 

 93 



