THE POLAR CLOCK. 199 



may be stated, that by its means the time may 

 be most accurately ascertained, simply by di- 

 recting the instrument to the North Pole of the 

 sky. It is found that it will indicate the time 

 even before sunrise and after sunset ; in fact, as 

 long as the rays of sunlight are reflected from 

 the atmosphere. 



The beautiful and gorgeous colouring of 

 clouds depends upon the decomposing effect of 

 their watery particles upon the rays of the sun. 

 In tropical countries there is a ' peculiarly beau- 

 tiful appearance in the atmosphere which we 

 do not meet with in temperate zones. Baron 

 Humboldt frequently alludes to it as a sort of 

 thin haze or vapour, which, without changing 

 the transparency of the air, renders its tints 

 more harmonious, and softens its effects. This 

 appearance was also noticed by Mr. Darwin. 

 " The atmosphere," he says, speaking of Bahia 

 in Brazil, " seen through a short space of half 

 or three quarters of a mile, was perfectly lucid, 

 but at a greater distance all colours were blended 

 into a most beautiful haze, of a pale French 

 grey mixed with a little blue. The condition 

 of the atmosphere between the morning and 

 about noon, when the effect was most evident, 

 had undergone little change, excepting in dry- 

 ness." 



The atmosphere contains a large amount of 



