xlvi INTRODUCTION TO ASTRONOMY. 



The sun is nearly three of our clays in rising 1 and set (inn- at the poles. 

 About thirty hours, or rather more, before he reaches the exact period of 

 the autumnal equinox, the upper edge or limb of the sun begins to be 

 visible at the south pole ; and it is there seen constantly travelling round 

 the horizon, and rising gradually higher and higher, till at the end of 

 about sixty hours, after revolving nearly 2J times round the horizon, the 

 whole of its orb is visible. 



At the same moment that the edge of the sun becomes visible at the 

 south pole, the same edge which appears as the lower limb at the north 

 pole begins to dip below the horizon ; but the sun still continues visible, 

 travelling round the horizon, more and more of it. being hid, till, at the 

 end of sixty hours, it totally disappears, just at the same moment when it 

 is fully seen at the south pole. As the earth proceeds towards summer, 

 the days lengthen in the northern hemisphere, and shorten in the southern, 

 till the earth reaches our summer solstice, which brings it again to the 

 spot whence we first accompanied her. 



The mind can find no object of contemplation more sublime than the 

 course of this magnificent globe, impelled by the combined powers of pro- 

 jection and attraction to roll in one invariable course around the source 

 of light and heat ; and what can be more delightful than the beneficent 

 effects of this vivifying power on its attendant planet? It is at once the 

 grand principle which animates and fecundates Nature. 



The sun's rays afford less heat when in an oblique direction than when 

 perpendicular, because fewer of them fall upon an equal portion of the 

 earth. This will be understood better by referring to Jig. 1 1, which repre- 

 sents two equal portions of the sun's rays, shining upon different parts of 



Fig. 11. 



the earth. Here it is evident that the same number of rays fall on the 

 space AB as fall on the space B C ; and as A B is less than B C, the 

 heat and light will be much stronger in the former than in the latter. 

 A B, you see, represents the equatorial regions, where the sun shines per- 

 pendicularly; and B C the temperate and frozen climates, where his rays 

 fall more obliquely. This accounts also for the greater heat 'of summer, 

 as the sun shines less obliquely in summer than in winter. 



In Jig. 12, the earth is represented as it is situated on the 21st of June, 

 when England receives less oblique, and consequently a greater number 

 of rays than at any other season; and Jig. 13 shows the situation of Eng- 

 land on the 21st of December, when the rays of the sun fall most obliquely 

 upon her. But there is also another reason why oblique rays give less 

 heat than those which are perpendicular; the former have a greater por- 

 tion of the atmosphere to traverse ; and though it is true that the atmos- 

 phere is itself a transparent body, it does not admit the passage of the 



