82 



TOTAL INSOLATION, DIRECT AND DIFFUSED. 



The value of the intensity of the direct solar rays incident nor- 

 mally to any unit surface, as determined by the absolute actinometers 

 of Pouillet, Violle, and others, is not so applicable to the study of 

 the growth of plants as is the sum of the radiation from the sky and 

 other surroundings of the plant, added to the direct solar radiation. 



Comparative measures made in 1866 by Roscoe, at Manchester; 

 Baker, at Kew ; Wollkoff, on the summit of Koenigstuhl, near Hei- 

 delberg (altitude, 550 meters), and Thorpe, at Para, have given the 

 following values of relative intensity of radiation at certain moments 

 when the sun's altitude above the horizon was sensibly the same at all 

 the stations. (See Marie Davy, 1882.) 



Relative intensity of radiation for equal altitudes of the sun. 



r.atitudp ^^°™ ^^'o™ 

 latitude, g^^j^ gjjy 



Manchester - 



Kew 



Koenigstuhl 



Paris 



Para 



N.53.5 

 N.51.5 

 N.49.4 



N.48.8 

 S.00.5 



0.043 

 .150 



0. 140 

 .162 

 .174 

 .501 

 .136 



S^- + Sun/Sky 



0.183 

 .312 

 .437 



0.31 

 0.93 

 1.51 

 0.44 

 1.00 



At Manchester and at Paris the light that comes from the sky is 

 more than double that which comes directly from the sun. AAHien the 

 sun is hidden by clouds, or even partially veiled, it is the radiation 

 from the sky that is of the most importance to agriculture, and in 

 any case this radiation is far from being negligible. 



The Arago-Davy actinometer (believed to have been invented by 

 Arago before 1844, but improved by Marie-Davy and used at the 



