350 METEOROLOGY. 



be one or more under-currents of salt and heavy water flowing into the 

 Arctic hasin. A considerable body of water at the temperature of 

 40° rising to the surface there — as come to the surface it must, in order 

 to supply the out-going upper currents — would tend mightily to mit- 

 igate the severe cold of these hyperborean regions." 



SECTION IX. 



ON THE DIURNAL AND ANNUAL DURATION OP SUNLIGHT AND TWILIGHT. 



Having thus far considered the intensity of solar radiation upon any 

 part of the earth, we shall lastly pass to examine its duration. 



In several publications it has been stated that " the sun is, in the 

 course of the year, the same length of time above the horizon at all 

 places." On applying an accurate analysis^ however, it appears, as 

 will presently be shown, that the annual duration of sunlight is sub- 

 ject to a very considerable inequality. This annual inequality in- 

 creases with the distance from the eqnator, and is proportional to the 

 sine of the longitude of the sun's perigee. 



The longitude of the perigee on January 1, 1850, was 280° 21' 25", 

 and increasing at the rate of 61". 47 annually ; the sine of the longi- 

 tude of the perigee is therefore decreasing in value every year, and 

 with it, the inequality of sunlight. At the present time it amounts, 

 in the latitude of 60°, to 86 hours — being additive in the northern, 

 and subtractive in the southern hemisphere. That is, in the latitude 

 of 60° north, the total duration of sunlight in a year is 36 hours more, 

 and in the latitude of 60° south, 36 hours less than on the equator. 

 At either pole the inequality amounts to 92 hours, or more than seven 

 and a half average days of twelve hours each. 



Were the earth's orbit a perfect circle, the inequality could not 

 exist ; its physical cause lies in the unequal motion of the earth in its 

 elliptical orbit. During summer of the northern hemisphere, the 

 earth is in and near aphelion, its longitude, and consequently the de- 

 clination on which the length of day depends, changes most slowly 

 from one day to another ; whereas, during summer of the southern 

 hemisphere, it changes the most rapidly, and the longest days are^ 

 fewer in number. 



The ei)och when the annual inequality was at its last maximum, is 

 found by dividing the present excess of the longitude of the perigee 

 above three right angles, by the yeaily change. The excess, in 1850, 

 was 10° 21' 25", which divided by 6 1''. 47 gives a quotient of 606.5 

 years ; which refers back to the period of the middle ages, A. D. 1243. 



At a still earlier epoch, this inequality must have entirely vanished. 

 At that epoch, the line of the apsides evidently coincided witli the 

 line of the equinoxes, which is cotnputed to have been about 4,000- 

 years before the birth of Christ, at which time chronologists have fixed 

 the first residence of man upon the earth. The luminous year was 

 then of the same length, at all latitudes, from pole to pole. 



Though the annual duration of sunlight thus varies from age to^ 

 age, and in the northern hemisphere differs from the southern ; yet,, 

 such is the law of the planet's elliptio motion, that the sun's annual 



