144 Nature's 



phenomenon ran through a period of several 

 weeks, gradually fading away. It was after- 

 ward determined that these, displays were oc- 

 casioned by small particles of ashes or dust 

 floating high in the air, that were thrown off 

 from the volcanic eruption of Krakatoa in 

 the Island of Java. By the general circula- 

 tion of the air the ashes were carried to all 

 parts of the world, making a circuit of the 

 earth in from twelve to thirteen days which 

 showed a velocity of over eighty miles an hour. 

 This is an instance of the high velocity of the 

 air currents in the upper regions of the atmos- 

 phere. The reason why the illumination ex- 

 tended so late in the night was because of the 

 great height that these particles of dust at- 

 tained. The higher the reflecting surfaces are 

 in the air the longer they may be seen after 

 sunset. Ordinary twilight is caused by a re- 

 flection of sunlight from the upper air; and 

 from its duration as ordinarily observed it is 

 estimated that the reflection does not proceed 

 from a point more than thirty-six miles high. 

 In the higher latitudes the twilight is long, 

 from the fact that the sun does not go directly 

 down, and if we go far enough north the whole 

 night is twilight. In the tropical regions the 

 twilight is shorter than at any other point on 

 the globe for reasons that are obvious. The 

 sun there goes directly down and is soon hid- 

 den behind the earth. 



