AGRICULTURAL EXPERIMENT STATION. 273 



table. Comparing soil temperature with the air temperature, it 

 appears that, at the depth of 1 inch, the temperature of the soil 

 was lower than that of the air by 1°.73 ; at the depth of 3 inches, 

 by 1°.46 ; 6 inches, by 2°. 60 ; 9 inches, by 3°.45 ; 12 inches, by 

 3°.97 ; 24 inches, by 5°.83, and at the depth of 36 inches, by 7°.44. 



It is proposed to renew to observations with the soil thermome- 

 ters on the 1st of April, 1890. 



Terrestrial Radiation. 



It is a well known fact that the heat radiated from the surface 

 of the earth during the night reduces its temperature below that 

 of the surrounding atmosphere. The amount of this radiation or 

 the consequent reduction of temperature is approximately shown 

 by comparing the readings of a terrestrial radiation thermometer 

 with those of a minimum thermometer. In obtaining data for the 

 comparison given below, the minimum thermometer was four feet 

 above the ground and the terrestrial radiation thermometer was 

 within six inches of its surface. The results are based on monthly 

 averages from May to October inclusive. 



Table Showing Loss of Heat by Terrestrial Radiation. 



May 



June 



July 



Aug. Sept. 



Get. Mean 



4&°M 



53°.25 



65°.08 



53°.5 49°.07 



33°.9I 48°.50 



38°.48 



49°.20 



50°.59 



iT.m wm 



28°.48 43M7 



Mean of minimum temperatures, 

 Mean of Tern, irom Ter. Rad. Ther. 



Loss ofheat by radiation, 8M5 4°.05 4°.49 5°.39 4°.47 5°.43 5°.33 



On cloudy nights the difference in the readings of the two ther- 

 mometers is small, and on exceptionally clear (dry) nights it is a 

 maximum. The greatest range observed was 10°. 8. On the 

 morning of July 2d, the radiation thermometer was the higher, 

 showing that the moist air resting upon the surface of the ground 

 served as a warm blanket, and that the amount of heal absorbed 

 was greater than that radiated. From the table above it appears 

 that the mean radiation was 5°. 33. 



Solar Radiation. 

 The temperature of the atmosphei'e does not indicate the inten- 

 sity of the sun's heat, as only a small percentage is absorbed as 

 the rays are transmitted through the air. The maximum ther- 

 'mometer in the shade, therefore, does not give the intensity of 

 solar radiation ; neither does exposure of an ordinary thermometer 

 to the direct rays of the sun in consequence of the cooling effects 

 of draughts of air. In order to avoid the effects of currents of 

 air, the vacuum solar radiation thermometer has been devised. 

 "This consists of a blackened bulb radiation thermometer inclosed 

 n a glass tube and globe, from which all air is exhausted. Thus 



