368 NATURE-STUDY REVIEW [14:9— Dec, 1918 



Because the moon has no atmosphere, the stars seen from it do not 

 twinkle for that is caused by seeing the light through the moving 

 air curtain, like the little spots of dust we see dancing in the 

 room between us and the window, in our own houses, or out of 

 doors between us and the sun. 



Scientists have proved that light consists of three colors, red, 

 yellow and blue, there being three parts of the yellow, five of the 

 red and eight of the blue, and these three colors make black. 

 Our curtain of atmosphere sifts out the other colors, and leaves 

 us a blue sky, but the moon having no such sifter, its skies remain 

 black. People who have gone up very high in the sky, where our 

 curtain was thinner, have noticed the sky became darker as they 

 went higher. So our knowledge of this fact tells us that the moon 

 sky must be black as ink. 



There is always a shadow on the opposite side of any object 

 on which light falls ; but the atmosphere diffuses the light so that 

 on the earth the shadows are not black; but on the moon there 

 are no scattered permeating particles of light filling all the tiny 

 cracks of space to lighten up things, so there the shadows are 

 blacker than our blackest night, and all the moon is covered with 

 them except in those places on which the light falls directly. 



The atmosphere is an invisible blanket wrapped around the 

 earth. As sometimes our blankets are made of a mixture of 

 materials, so sometimes our air is a mixture and carries more 

 water or less, according as to whether it is warm or cold. If the 

 day be warm and dry, as it might be in summer, water from the 

 oceans, lakes, rivers, ground or foliage, absorbed, is changed to 

 vapor, which is a mixture of the hot air and the water; as more 

 air is mixed in, all the water disappears in the air. We call this 

 evaporation. The hotter the day the faster it takes up water. 



But just as our regular blankets of cotton or wool can take up 

 a certain amount of water and not drip, but will drip when they 

 have more than a certain amount of water in them ; so our blan- 

 ket of air can only take up about seven inches of water at any one 

 time, without dripping; although that air blanket is 200 miles 

 thick. 



If the sun is very hot all the moisture that is taken up may be 

 carried up into the sky several miles, and spread out so thin that 

 it disappears as gas leaving no clouds at all. But if a current 

 of cold air comes rushing along from the north pole, or some other 



