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SECOND BOOK. 
weigh less, or be lighter, than the cold air that 
filled it. So we must remember that any volume of 
warm air is lighter than the same volume of cold air. 
5 . Now let us find out what happens when any 
portion of the earth’s surface has been heated by 
the sun. The heated part warms the air that lies 
next to it. This air at once becomes lighter than 
J \ 
Diagram illustrating Sea-breeze. 
it was, and therefore rises higher; while the colder 
and heavier air that is near rushes in to take its 
place. In this way a wind is produced. 
6. As an instance we may say a few words about 
the land and sea breezes, which are well known in 
the islands of tropical seas. And, in learning the 
cause of these breezes, we must bear in mind that 
the land always takes in heat or gives it out again 
more quickly than water does. 
7 . Therefore, while the sun is shining, the land 
gets much hotter than the sea. The heated land in 
turn heats the air that is over it, thus causing it 
