ySo Rural School Leaflet 



were red, and because the little violet waves were bent more in passing 

 through the prism than were the larger red waves, each fell in a different 

 place on the screen. 



Dust also sorts out the colors in sunlight, but not in the same way 

 that a prism does. In the first lesson we said that many ocean harbors 

 had breakwaters, or great walls, to stop the waves as they came in from 

 the ocean, so that ships might have a quiet place to anchor. Suppose 

 that a wave higher and bigger than the breakwater should come in from 

 the ocean. Would the breakwater stop it? Of course it would not. It 

 would roll right over the breakwater on into the harbor, would it not? 

 Would the breakwater stop a little wave — one that was not so high as 

 the breakwater itself? Certainly it would. Now, the little particles of 

 dust in the air are something like breakwaters, because they stop the little 

 violet and blue waves of light that are smaller than the dust particles; 

 but the coarser waves of red and orange, which are bigger than the dust 

 particles, pass right on over them. We do not see any blue or violet in 

 the sunset colors because the little waves of light that make these colors 

 have been stopped by the dust in the air. In the same way that a break- 

 water will not stop a wave from the ocean that is higher than itself, so 

 the dust in the air will not stop the waves of light that are larger than the 

 dust particles themselves. These larger waves that make the yellow, 

 orange, and red pass on through the dust, and when they fall on the 

 clouds we see them in the gold and crimson colors of the sunset. 



But is there not just as much dust in the air in the middle of the day? 

 Then why are the clouds not colored at noon, as well as at morning or 

 evening? I think you can answer this question if you will try. When 

 the sun is overhead, which side of the clouds does the light fall on? Which 

 side do we always see? When the sun is nearly down, which side of the 

 clouds does the light then fall on? There is another reason. Most of 

 the dust in the air is near the earth, probably below the clouds. So when 

 the sun is nearly or quite down, the light passes through a greater thick- 

 ness of dusty air than when the sun is overhead, and the more dust there 

 is the more the violets and the blues are sorted out, leaving only the yellow 

 and the red to color the evening sky. 



Note. — Weather maps are now published only at the Ithaca, Buffalo, Binghamton 

 Albany, and New York City Weather Bureau ofl&ces. 



