SCIENCE AND POETRY. 



year, must necessarily be within the tropics : a latitude to which 

 the succeeding lines about the " fond delay " of the sun, and the 

 night which only "draws a thin veil o'er the day," which pro- 

 duces, in other words^ only a few hours of twilight, are utterly 

 inapplicable. 



9. In tropical latitudes the variation of the length of the day is 

 very inconsiderable. It is a little more or a little less than twelve 

 hours, and that is all. The night is, consequently, subject to a 

 variation similarly limited. Instead therefore of the very long 

 days and the very short nights which the poet ascribes to his 

 "isle" in the blue summer ocean, there would necessarily be 

 nights, the duration of which could never be much less than 

 twelve hours in any part of the year. 



But this is not all. Instead of enjoying a constant nocturnal 

 twilight, so beautifully described by the poet as a veil drawn over 

 the day, the inhabitants of the tropics enjoy scarcely any twilight 

 at all, being plunged in nocturnal darkness almost immediately 

 after sunset. This arises from astronomical causes, which will be 

 very easily understood. 



Twilight is produced by the reflection of the sun's light from 

 that part of the visible atmosphere upon which the sun continues 

 to shine after sunset until its depression below the horizon 

 amounts to about 18. Now it is apparent, that the more nearly 

 perpendicular to the horizon the diurnal motion of the sun is, the 

 sooner will its orb attain this depression of 18. In the higher 

 latitudes, where the celestial pole is not very far removed from 

 the zenith, the sun is carried round in a diurnal circle, making 

 a very oblique angle with the horizon; consequently, after it 

 sets, its depression below the horizon increases very slowly, and 

 a long interval elapses before the depression amounts to 18. In 

 some latitudes at the season of Midsummer it is not so much 

 as 18 even at midnight ; and in such places the poet might very 

 truly say, 



" The niglit only draws 

 A thin veil o'er the day." 



But the latitudes in which this can take place are very differ- 

 ent indeed from those in which 



" a leaf never dies in the still blooming bowers, 

 And the bee banquets on through a whole year of flowers." 



The distance of the celestial pole from the northern point of 

 the horizon being always equal to the latitude of the place, as may 

 be seen by reference to our Tract on Latitudes and Longitudes, 

 the depression of the sun below the horizon at midnight will be 

 found by subtracting the latitude of the place from the sun's 

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