92 
RELATIVE TOTAL HEAT RECEIVED DURING CERTAIN MONTHS. 
By adding the amount for each day of any month in the following 
table we get the relative numbers for the total amount of heat received 
direct from the sun at various latitudes during certain months by a 
unit of horizontal surface under a clear sky, and after absorption by 
ordinary clear air, plus the amount received from the diffuse sky hight 
or the atmospheric reflection, all expressed in terms of the amount that 
unit surface would receive if the sun were constantly in the zenith 
during twelve hours. The coefficient of transmission through one 
atmosphere for zenithal sun is, as before, 0.75, and the added sky- 
light is 0.125, to accord with the Arago-Davy conjugate thermometers, 
since these are affected by the sum of the heat received by their sur- 
faces from the sun anda from the atmospheric particles in the visible 
celestial vault. ; 
Relative quantities of total heat received monthly at different latitudes in the 
northern hemisphere. 
Month. 0. 10. 30. 50. 70. | 80. 90. 
NUT RC TOO oleae! as ee ane ee a7 37| . 8538) = 2ai adel more 0.2 
(eS adn ee AE AE) Nie Dee LG |} 100) 106] 101] 80| 54] 39 | 34 
Ry rete heels he mes Sntenes 0. QU Snlee LON 7e pe eelleyalleecT Oa 9.0 8.6 | 8.7 
DRS (s) ee oR eed et) ne ORE Qi2:| 1004) OaTS9N aaa 10599) 9 11907 eae 
STi yA 19 ee AR en is Ce eee 9.7 | AONT|)) QE We Tass 1083) )/ Ose eons 
| | | 
PTS be siee e Oen s aren separa Sree ioc | 10.1] 10.7] 10.9 9.2 6.8| 5.9] 5.8 
September l toes: -.2-.- sas se eee eas eee As Oe) 
TG balleee nei Deena 2 ko dap as ae | 60.2 | 64.6] 67.1| 57.8] 460) 41.6] 40.3 
| a 
PHOTO-CHEMICAL INTENSITY OF SUNSHINE. 
Bunsen and Roscoe, in a series of memoirs published in the 
Philosophical Transactions, London, 1857, 1859, and 1863, entitled, 
“ Photo-chemical researches,” discussed the methods of measuring the 
chemical action of light by help of photographic tints, and endeay- 
ored to improve upon the methods of Herschel, Jordan, Claudet, and 
Hankel. They adopted as a standard unit for measurement that 
intensity of the light which in one second of time produces the 
standard tint of blackness upon the standard paper. Their methods 
are too laborious for the ordinary meteorological observer, but have 
furnished some important data as to the chemical activity of diffuse 
sunlight and of total daylight. 
In his memoir of 1864, Roscoe states that he and Bunsen had 
developed a method of determining the chemical intensity of both 
direct sunlight and diffuse sunlight, or the total daylight, that is, 
based upon the law that the intensity of the hght multiphed by the 
duration of exposure of chloride of silver paper of uniform sensi- 
tiveness gives a series of numbers proportional to the shades of tints, 
_— 
a 
