46 INTENSITY OF SUN'S HEAT AND LIGHT. 



for the drainage of the Polar regions and the great Siberian Rivers. To prove that 

 water must actually exist, we have only to observe the icebergs. These floating 

 masses cannot be formed without terra firma, and it is a remarkable fact that, out 

 of 360°, in only 30° are icebergs to be found, showing that land cannot exist in any 

 considerable portion of the country. Again, Baffin's Bay was long thought to be 

 a close bay, but it is now known to be connected with the Arctic Sea. Within the 

 bay, and covering an area of ninety thousand square miles, there is an open sea 

 from June to October. "We find here a vacant space with water at 40° temperature — 

 eight degrees higher than freezing point." 1 



SECTION IX. 



ON THE DIURNAL AND ANNUAL DURATION OF 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 subject to a very considerable inequality. This annual ine- 

 quality increases with the distance from the equator, and is proportional to the sine 

 of the longitude of the sun's perigee. 



The longitude of the perigee on Jan. 1, 1850, was 280° 21' 25", and increasing at 

 the rate of 61".47 annually; the sine of the longitude 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 36 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 ine- 

 quality amounts to 92 hours, or more than seven and a half average days of twelve 

 hours each. 



The epoch when the 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 

 yearly change. The excess, in 1850, was 10° 21' 25", which divided by 61".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 



1 A reference to Plate IV will confirm what was before known from observations that the extremes 

 of summer and winter temperature range through wider and wider limits from the equator towards each 

 Pole. The application of this general law favors an. open Polar sea in summer, as actually seen by- 

 explorers, and more recently by Dr. Kane's party in the month of August. But it equally indicates 

 that the sea is frozen over in winter, when there appears no assignable cause, but a calm atmosphere, 

 to mitigate the most intense cold. 



