NO. n.] THE METEOROLOGICAL PERIODS IN THE ARCTIC SEA. 593 



The thermal amount of radiation from the sun on the unit of surface 

 perpendicular to the sun's rays is ^ 



(J 



where C is the solar constant, referred to the mean distance of the earth 

 from the sun, square centimetre and minute of time, r the distance of the 

 earth from the sun, p the coefficient of transparency of the air, and z the 

 ratio of the mass of the atmosphere traversed by the ray to the mass tra- 

 versed by a ray from the zenith. I have assumed that C equals 3 gram 

 calories, p equals 0"75, and z equals cosec h (h the altitude of the sun) for 

 altitudes higher than 20°. For lower altitudes I have taken Maurer's values ^ 

 for z, viz., for 5°, 8'04, and for 0", 14"96, and taken the adopted values for 

 z from a curve. 



The radiation on a unit (sq. cm.) of a horizontal surface becomes 



C 

 dq ^ —^ p* sin h dt. 



I have taken the radiation of the sky from Clausius' numbers given by 

 J. Hann 3, and extrapolated the numbers graphically, making the radiation 

 equal zero, when the apex of the twilight is below the horizon, « = /?' + /? 

 + fc + £ or = 16° 38'. The table then stands thus: 



h = —16° 38' — 10° — 5° 0° 5° 10° 15° 20° 25° 30° 

 p' 0-004 0-013 0-027 0046 007 009 Oil 013 0-14 



The results of these computations are shown in the following Table. 



' A. Angot. Recherches th^oriques sur la distribution de la chaleur k la surface du 

 globe. Annales du Bureau Central m^tSorologique de Fiance. Ann6e 1883. Paris, I8a5. 

 2 Angot. 1. c, p. 131. 

 ^ Lebrbuch der Meteorologie, p. 42. 



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