82 
Relative quantities of direct and diffused sunshine. 
Horizontal surface. Normal surface. | 
Sun’sal- SS ] = 
titude. | sun (8). |Sky (©).| (go) | Sun @.|Sky(c).| Fetal 
; | | 
10 0.03 | 0.07 0.10 0.19 0.04 0.23 | 
15 09 | 09 .18 . 33 05 38 
15 fr 26 43 | 06 | 49 
25 21 13 | BA 51 08 59 | 
30 .28 14 | 42 56 | 10 66 | 
35 35 159) 50 61 agli 7 
40 Al 16 | BT 64 12 76 
50 58 ali 69 69 14 83 
60 62 51851 Saeee60 We a iG 88 
70 .69 18 | 87 4 | si 91 
#0 74 18 92 1 ee See 
90 15 19 | my 15 19 | 94 | 
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. 
| 
\Latitude. ¥rom | soe pias Sun/sky. 
Manchester se: 222s se ees N.53.5 0.048 0. 140 0.188 0.31 
CSW eee ate his welt eae N.5L.5 150] —-. 162 .312| 0.98 
eGciahfes hall Me Ee N.49. 4 .263| 174 437 | 1.51 
Pamigee eMail eos N. 48.8 (222) B01 12 0.44 
Phare c en Glt Tre eke S.00.5) 136). 136 .272| 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. When 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 im 
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 Marié-Davy and used at the 
utente 
