824 CONTtNUOtJS DAYLIGfiT 



the mountain-top catches the rays of the coming sun before 

 they reach the lowland, and at sunset keeps them after they 

 have faded from the regions below, so the particles of dust 

 and vapour which always float in the atmosphere catch the 

 sunlight and reflect it downwards to the earth's surface, while 

 the sun is actually below the horizon. This diifuse and some- 

 what vague light at the beginning and the end of the day — - 

 the dawn and the twilight — depends for its duration upon 

 the observer's latitude and the season of the year. It is 

 generally said to end at night when stars of the sixth mag- 

 nitude begin to be visible at the zenith. This occurs wlien 

 the sun has sunk about eighteen degrees beneath the horizon ; 

 and any circumstance which causes the sun to go down rapidly 

 will shorten the duration of twilight, while anything which 

 retards the downward motion of the sun will correspondingly 

 prolong it. Chief among influences of this kind is the angle 

 which the sun's course makes with the horizon. If the sun 

 moves downwards almost vertically, as in the tropics, a much 

 shorter time will suflice to carry it to a depression of eighteen 

 degrees than will be needed where the motion is very oblique 

 to the horizon. In the tropics the twilight does not last 

 half an hour, and there is no exaggeration in the lines of 

 Coleridge : — 



"The sun's rim dips; the stars rush out; 

 At one stride comes the dark." 



In Scotland, however, at this season of the year, the sun's 

 path is so oblique that twilight lasts all niglit. At midsummer 

 in the latitude of Edinburgh the sun is only between ten and 

 and eleven degrees beneath the Northern hoiizon at midnight. 

 As one goes farther North the path becomes still more oblique 

 and the midnight depression smaller, until at the latitude of 

 the extrejne North of Norway the sun just skirts the Northern 

 horizon without setting, and we reach the ' land of the mid- 

 night sun.' 



When the depression is over ten degrees it is only the 

 higher portion of the atmosphere which can receive the suit's 

 rays and reflect their light earthwards, and as the dust and 

 moistui-e in those higher regions are much less abundant than 



