AGRICULTURAL EXPERIMENT STATION. 149 



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, 1889 and from 

 April to October inclusive, 1890. 



Table Showing Loss of Heat by Terrestrial Radiation. 



1889. 



May June July Aug. Sept. Oct. Mean 



o o o o o o o 



Mean of minimum temperatures, 46.63 53.25 55.08 53.65 49.07 33.91 48.50 



Mean of Tern, from Ter.Rad.Ther., 38.48 49.20 50.59 47.66 44.60 28.48 43.17 



Loss of heat by radiation, 8.15 4.05 4.49 5.39 4.74 5.43 5.33 



1890. 



O O OOO 000 



April May June Julv Aug. Sept. Oct. Mean 



Mean of minimum temperatures, 29.17 42.52 4S.71 53.61 53.52 45.32 36.05 44.13 



Mean of Tern, from Ter.Rad.Ther., 19.95 37.10 42.10 44.55 46.25 38.40 27.14 36.50 



Loss of heat hy radiation, 8.22 5.42 6.61 9.06 7.27 6.92 9.91 7.63 



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

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

 maximum. The greatest range observed was 19°. 5. On the morn- 

 ing of July 2d 7 1889, 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 heat absorbed 

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

 that the mean radiation for the two seasons was 6°. 48. 



Solar Radiation. 

 The temperature of the atmosphere 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 

 or 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 

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

 protected from the loss of heat which would ensue if the bulb were 

 exposed, its indications are from 20° to 30° higher, than when 

 placed side by side with a similar instrument with the bulb 



