SHORTNESS OF TWILIGHT. 



21 



lead to the deterioration of the climate and the 

 permanent impoverishment of the country.^ 



Short Tivilight of the Equatorial Zone. — One of the 

 phenomena which markedly distinguish the equatorial 

 from the temperate and polar zones, is the shortness 

 of the twilight and consequent rapid transition from day 

 to night and from night to day. As this depends only 

 on the fact of the sun descending vertically instead 

 of obliquely below the horizon, the difference is most 

 marked when we compare our midsummer twilight with, 

 that of the tropics. Even with us the duration of 

 twilight is very much shorter at the time of the 

 equinoxes, and it is probably not much more than a 

 third shorter than this at the equator. Travellers usually 

 exaggerate the shortness of the tropical twilight, it being 

 sometimes said that if we turn a page of the book we 

 are reading when the sun disappears, b}^ the time we turn 

 over the next page it will be too dark to see to read. 

 With an average book and an average reader this is 

 certainly not true, and it will be w^ell to describe as 

 correctly as we can what really happens. 



In fine weather the air appears to be somewhat more 

 transparent near the equator than with us, and the 

 intensity of sunlight is usually very great up to the 

 moment when the solar orb touches the horizon. As 

 soon as it has disappeared the apparent gloom is propor- 

 tionally great, but this hardly increases perceptibly during 

 the first ten minutes. During the next ten minutes 

 however it becomes rapidly darker, and at the end of 



^ For a terrible picture of the irreparable devastation caused by the reckless 

 clearing of forests see the third chapter of Mr. Marsh's work The Earth as 

 Modified by Human Action. 



