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 light 

 or the atmospheric reflection, all expressed in terms of the amount that 

 unit surface would receive if the sun were constantly in the zenith 

 (luring 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 and from the atmospheric particles in the visible 

 celestial vault. 



Relative quantities of total heat received monthly at different latitudes in the 

 northern hemisphere. 



PHOTO-CHEMICAL INTENSITY OF SXINSHINE. 



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 endeav- 

 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 light multiplied 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, 



