STUDIES IN NATURE 



can sometimes see the nucleus or centre round 

 which they have formed. 



It is very interesting to watch these banks 

 or layers of water-vapour as they float about 

 in the air, disappearing again when they come 

 to a warmer current of air. There are, as 

 we all know, many different sorts of clouds, 

 some of which are connected in our mind with 

 certain sorts of weather, and we can learn a 

 great deal about the state of the air above us if 

 we watch the movements of the clouds. When 

 the drops of water have grown to a certain size 

 they fall to the ground as rain, drawn down by 

 that force called gravity, which holds together 

 the sun and the earth, and holds us on to the 

 surface of the earth. When they are small, 

 they fall so slowly that they seem able to float 

 without falling, as we notice in a fog, which is 

 more often dispelled or destroyed by the sun- 

 shine turning the tiny drops into vapour than by 

 the drops themselves reaching the ground. In 

 towns, where the smoke and dust in the air pro- 

 vide plenty of nuclei, fogs are often very thick 

 and persistent, and prevent the sun's rays coming 

 through. It is probable that in the highest 

 clouds the vapour has been changed into ice, 

 and the effects we see are due to the minute 

 crystals of ice, and not to tiny drops of water. 



Ice is lighter than water, and floats on the 

 top of it. Perhaps this will seem strange, since 

 cold water is heavier than warm water, and 



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