PHYSICAL AND METEOROLOGICAL RESEARCHES. 



209 



May and June were hot and dry, and plants did not get the necessary 

 moisture. If the average annual temperatures of the soil at different 

 depths are compared, it will be seen that the temperatures diminish 

 from the surface of the soil as far down as 0.10 meter, where it reaches 

 a minimum, from which it rises as far as 1 meter. The increase in tem- 

 perature is less rapid from 0.10 to 1 meter than the decrease from the 

 surface of the soil to 0.10 meter. This phenomena, which at first might 

 appear abnormal, is easily explained. The sun's heat is transmitted 

 through the soil by conductivity. The heating of the soil is either 

 effected through the contact with the warmer air or by the direct action 

 of the sun's rays. The heat is transmitted to the different depths by 

 warming successively the different layers of the soil. The cooling is 

 effected by contact with the colder air, and also by the radiation. The 

 layers of the soil cool from the surface toward the interior, and there is 

 a certain depth where the temperature reaches a minimum. This depth 

 where the temperature is lowest varies according to the different me- 

 teorological conditions during the day. The annual average is 0.10 

 meter below the surface. Wind, rain, and the condition and the humid- 

 ity of the soil are factors which influence heat and cold. These agents 

 influence specially the superficial layer of the soil and modify its tem- 

 perature. It is not possible therefore to state with any precision 

 whether the soil is warmer or colder at this or that depth. The fluc- 

 tuation of the temperature of the soil at different depths is shown in 

 the following table: 



Variation in temperature of soil at different depths. 



On August 4 the temperature of the soil was the same at 0.05, 0.10, 

 and 0.25 meter below the surface, but it was considerably modified the 

 next day. August 8 the temperature at 0.10 meter was lower than at 

 0.05 and higher at 0.25 than at 0.05. At this time there was a general 

 cooling off', which acted rapidly on the superficial layers as far as 0.10, 

 but it was of little effect at the lower depths. Other modifications fol- 

 lowed, and the rise and fall of temperatures at different depths were 

 according to the rapidity and the duration of the atmospheric changes. 

 The temperature of the soil follows that of the air, but it gives better 



