58 THE BOOK OF NATURE STUDY 



by observation before detailed consideration of the earth's move- 

 ments is begun. 



CLOUDS. The general principles of cloud formation have been 

 considered in the previous chapter, but there are many interesting 

 points connected with their shapes which are worth notice. When 

 speaking of the dust particles in air, it will be natural to speak 

 of the probability that clouds consist of minute particles of water, 

 each one condensed round a more minute dust particle. The 

 dust is produced from the earth, and has a limited extension upwards 

 in the air. For this as well as for other reasons we find that clouds 

 have a limited upward extension, 50,000 feet, or under ten miles, 

 being probably the highest level at which they occur. In some 

 such way one might strive to repeat again the lessons of the previous 

 chapter, to show that the cloud mantle is earth-produced, that 

 what the earth gives to the air she gets back again. 



The same thing can be well studied on a summer's day. Choose 

 a clear day and let the class note, before they come to school, and 

 especially if they have been up early, how clear is the morning 

 sky. As the sun sweeps onward, little flecks of cloud gather in 

 the sky. At first quite ragged, they soon acquire a heaped-up form. 

 The under surface of this cloud mass is more or less level, this 

 edge occurring at about the same height in the different cloud 

 groups. The upper edge, on the other hand, is billowy and moun- 

 tainous. The crests of the billows are usually in motion, often 

 rolling over and disappearing. On fine summer days such clouds 

 usually reach their maximum about midday, when the sky may be 

 largely cloud-covered. As afternoon goes on they usually begin 

 to decrease in amount, and as they decrease they flatten out until 

 they form long horizontal sheets of cloud, lying low in the heavens, 

 and often beautifully illuminated by the rays of the sun at sunset. 

 In fine summer weather this is so frequent a series of events, that 

 it may be described as typical. After it has been studied on more 

 than one occasion, it may be found convenient to give the name 

 of cumulus to the billowy midday clouds, and stratus to the sunset 

 layers, 1 but no special emphasis should be laid on the names. 



1 The special name of alto-stratus is sometimes given to these clouds when they are 

 high in the air. 



