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XXXIII. The Distribution of tlie Actinic Sunlight on the 

 Northern Hemisphere at Summer Solstice. By John 

 Sebeliex *. 



THE distribution of the effect of the solar radiation over 

 the different regions of the globe and at different 

 seasons has often been subject to calculation, and attention 

 has often been directed to the favourable position of places 

 at high latitude during summer-time. 



The laws of the intensity and quantity of solar radiation 

 are said to have been examined for the first time by Halley 

 in 1779, later determined by Plane f. Chr. Wieler % 

 published in 1879 a series of curves, showing the relative 

 intensity of the solar radiation during its yearly periods and 

 at various seasons for every tenth degree of latitude. His 

 results were not corrected for atmospheric absorption, which 

 varies for rays of different refrangibility. The calculations 

 of "Wieler show that on the 21st of June the radiation that 

 reaches unit area of the surface of the globe during 24 hours 

 has its maximum value at the North Pole. The relative 

 values of the said radiation at 0°, 60°, 75°, and 90° N. lat. 

 are as 282 : 350 : 362 : 385. The North Pole keeps its 

 superior position not only on that single day but also during 

 the whole period from 5th of May till the 7th of August 

 (the longitude of the sun increasing from 45° to 135°) . If 

 the total radiation during this period is calculated, its values 

 at 0°, 40°, 60°, and 90° X. lat. are as 710 : 886 : 846 : 896. 

 This shows a secondary maximum at about 40°, while the 

 absolute maximum is at 90° X. lat. 



The researches of Langley on the atmospheric absorption 

 of rays of different refrangibility having cleared this question, 

 Spitaler§ calculated the quantity of sunlight reaching a 

 unit of area for each 10th degree of latitude during an 

 average day for each of the twelve months of the year. For 

 this calculation he assumed a coefficient of absorption 0'6, 

 corresponding to the rays of an average refrangibility in the 

 yellow-green part of the spectrum. The results found by 

 Wieler, neglecting the influence of the atmosphere, were 

 now so far modified that the maximum value of the daily 

 quantity of sunlight at midsummer does not fall on the 

 5'orth Pole. Spitaler finds that for a day of June the 

 maximum radiation is at 30° N. lat. The quantity of light 



* Communicated bv Sir II. E. Roscoe, F.R.S. 



t Comptes Rendu8,'t. Iviii. 1864. 



t ZeiUchrift fur Meteor ologie, 1871), p. 114. 



$ Eder's Jahrbuchfur Fhotographie, etc. 1888, fid. xi. p. 377. 



