INTENSITY OF SUN'S HEAT AND LIGHT. 



51 



By means of equations (48), (53), I have computed the annual duration of sun- 

 light 2 (2 H), according to the rising and setting of the sun's centre, without regard 

 to refraction. It is the half of 365. 24 days, or 182.62 days increased by the quan- 

 tities in the following table, for the northern hemisphere, and diminished by the 

 same for the southern hemisphere : — 



i 



Annual Inequality of Sunlight. 



Latitude. 



Inequality. 



Latitude. 



Inequality. ■ 



0° 

 10 

 20 

 30 

 40 



h. 00 m. 



3 25 



7 07 

 11 23 

 16 40 



50° 



60 



70 



80 



90 



24 h. 08 m. 

 36 51 

 66 52 

 86 02 

 92 01 



Having thus discussed the duration of Sunlight, let us next consider its increase 

 by Refraction, and by Twilight. The mean horizontal refraction, according to Mr. 

 Lubbock's result, is 2075", or 34'35"; the barometer standing at 30 inches, and the 

 thermometer at 50° F. But as this is somewhat greater than what has been usually 

 employed, we shall adopt 34' as the mean value for determining the increase of 

 daylight by direct refraction. 



With respect to the duration of Twilight, A. Bravais, who has made extensive 

 observations upon the phenomenon, observes, in the Annuaire 3Ieteorologiqtie de la 

 France for 1850, p. 34: " The length of twilight is an element useful to be known: 

 by prolonging the day, it permits the continuance of labor. Unfortunately, philoso- 

 phers are not agreed upon its duration. It depends on the angular quantity by 

 which the sun is depressed below the horizon ; but it is also modified by several 

 other circumstances, of which the principal is the degree of serenity of the air. 

 Immediately after the setting of the sun, the curve which forms the separation 

 between the atmospheric zone directly illuminated by the sun, and that which is 

 only illuminated secondarily, or by reflection, receives the name of the crepuscular 

 curve, or Twilight Bow} Some time after sunset, this bow, in traversing the heavens 

 from east to west, passes the zenith ; this epoch forms the end of Civil Twilight, and 

 is the moment when planets and stars of the first magnitude begin to be visible. 

 The eastern half of the heavens being then removed beyond solar illumination, 

 night commences to all persons in apartments whose windows open to the east. 

 Still later the Twilight bow itself disappears in the western horizon ; it is then the 

 end of Astronomic Twilight ; it is closed night. We may estimate that civil twilight 

 ends, when the sun has declined 6° below the horizon; and that a decline of 16° is 

 necessary to terminate the astronomic twilight. 



"I depart here from the general opinion, which fixes at 18° the solar depression 

 at the end of twilight, and at 9° that which characterizes the end of civil twilight. 



1 The phenomenon is equally conspicuous in the west, before the rising of the sun, and in certain 

 states of the atmosphere is scarcely less beautiful than the rainbow, for the symmetry and vivid tinting 

 of its colors. 



