EXPERIMENT STATION BULLETINS. 521 



really commenced on December 1. Tlie temporaturea were taken daily 

 except Sundays llirougliout the year by means of electrical resistance 

 thermometers at three ditTerent depths, 6, 12 and 18 inches, at 7 a. m., 

 12 m., and 6 p. m. The electrical resistance thermometers were obtained 

 from Leeds and Northrn]), Philadelphia, Pa., and consisted of the resist- 

 ance bulb and P>5 ft. wire leads. Tiie latter were incased in lead pipes 

 so as to keep tlieni dry and well insulated. The ends of these leads 

 were run into a house especially constructed for the purpose. In this 

 house also was kept on a permanent stand, a balance indicator upon 

 which the temperatures were indicated. 



This electrical temperature measuring apparatus gave satisfactory 

 results. If the indicator was in good working order the readings could 

 be relied upon to be accurate to within 0.2° to 0.5° F. The accuracy and 

 sensitiveness was not as high as it was desired, but this lack was some- 

 what compensated or o\-ercome by taking three readings daily and aver- 

 aging them. Furthermore, when it is considered that the high and low 

 temperatures recorded during the day do not represent the real maximum 

 and minimum temperatures, then the accuracy of the apparatus is suffi- 

 ciently high. 



As already stated the soil temperature obtained under field conditions 

 is the resultant of many factors. These factors may be divided into two 

 general groups, intrinsic and external. The intrinsic factors comprise 

 specific heat, heat conductivity, thermal absorption and radiation, speci- 

 fic gravity, texture and structure, topographic position, concentration 

 of the soil solution, moisture content, etc. The external includes the 

 meteorological elements, chief of which are air temperature, sunshine, 

 wind velocity, barometric pressure, precipitation, dew point, humidity, 

 etc. The intrinsic factors vary with the different soils and control their 

 temperature for any particular day or season. They are acted upon by 

 the external factors which cause the soil temperature to vary from day 

 to day as they themselves vary. Variations in temperature, therefore, 

 in the different types of soil for any particular period will depend upon 

 their different intrinsic factors, but the variations of all these soils for 

 succeeding days will depend upon the external factors. These meteoro- 

 logical elements influence the soil temperature either directly or indi- 

 rectly. Pressur-e, for instance, exerts an indirect effect by either depress- 

 ing or accelerating evaporation. The amount of evaporation is least 

 when the barometric pressure is high and greatest when it is low. 



In connection with the study of the soil temperature of the foregoing 

 soils, these external meteorological factors or elements have also been 

 studied. With the exception of the air temperature which was recorded 

 by an air thermograph located near the soil plots, all the other at- 

 mospheric elements were recorded by the proper instruments at the 

 Weather Bureau" office which is situated only a very short distance from 

 the experiment. 



The following tables show the daily and monthly average, maximum 

 and minimum, also the seasonal and yearly average, maximum and mini- 

 mum, temperature of the five differ-ent types of soil. Each table is ac- 

 companied by a diagram to show in a graphic representation its salient 



(17) The writer wishes to express his gratitude to Mr. D. E. Seeley for permission to consult and 

 use these weather records. 



