THE YEAR. 



the relative heat above 32 at those points, greatest in 

 the centre of the dome, and diminishing toward the 

 sides, at which it will become nothing. If the effect 

 of the sun were momentary, the successive portions of 

 this dome, which would follow the motion of the central 

 point, would be a perfect index to the temperatures at 

 all places. 



But time is necessary both for acquiring and for losing 

 the full effect, even though there were no local differ- 

 ence of surface, and therefore the portions that are en- 

 tering under the dome are always colder, and those that 

 are passing from under it warmer, than if it were sta- 

 tionary, or than if the heating and cooling were instan- 

 taneous. Now it will be recollected that the central 

 point has two motions, one westward from the rotation 

 which is nearly uniform, and another and smaller one 

 from the declination, which varies northward or south- 

 ward, and fast or slow according to that. 



The changes of the seasons, and some of the pecu- 

 liarities of those changes are the immediate arid neces- 

 sary results of those motions. The distance of the 

 place from the equator, and the declination of the 

 point where the sun is vertical, determine the length of 

 the day ; that and the latitude, determine the degree of 

 temperature; and the rate at which the declination 

 changes, determines the rapidity of change in the 

 season. 



At the natural beginning of the year, the declination 

 is 23 28' south, therefore the whole circumference of 

 the parallel of 66 32' south (the Antarctic polar circle) 

 and all within it, is constantly within the illuminated 

 part; and the illumination just touches, but does not 



