16 PHYSICAL SCIENCE BK. i 



easily resisted by the surrounding air. The rest of 

 the heavenly bodies are equally weak, and unable 

 by their own force to burst through the atmosphere. 

 So their shape is impressed and retained in the 

 more solid and less yielding medium. For, in order 

 to produce the phenomenon, the atmosphere must 

 neither be so thick as to exclude or dissipate the 

 light that streams in on it, nor yet so thin and rare as 

 to furnish no hold to the rays that fall upon it. This 

 particular consistency is obtained at night : the 

 sluggish air is at that time struck with the faint light 

 from moon or stars without violence or rudeness, 

 and, being thicker than it is wont to be by day, is 

 tinged thereby. 



Ill 



1 ON the contrary, the Rainbow does not occur by 

 night, except on very rare occasions, inasmuch as 

 the moon has not sufficient strength to pierce the 

 clouds and suffuse them with hues such as 

 they receive from the brilliant light of the sun. 

 The shape and varied colours of the rainbow are 

 due to the peculiarities of different kinds of clouds. 

 Some parts of the clouds are swollen, others 

 hollow ; some are too dense to transmit sunlight, 



2 others too rare to exclude it. This difference in 

 consistency causes alternations of light and shade, 

 and produces that marvellous variety presented by 

 the rainbow. Another explanation is offered in 

 instances like the following : When a pipe bursts 

 anywhere, the water is observed to be forced by 

 pressure through the small opening ; the drops 

 seen against a slanting sun reproduce the appear- 

 ance of the rainbow. Again, if you will at any 



