WHY THE WIND BLOWS. 
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longer held, but falls as rain. So, also, when the 
air is chilled at night by the ground becoming cool, 
it is robbed of its vapour, and the plants and other 
things near the ground consequently become covered 
with moisture, which we call dew. 
11. Amongst the impurities in the air, in addition 
to various gases, there are always numberless specks 
of solid matter; such as dust, hairs, scales of skin, 
starch grains, fragments of plants, and disease-germs. 
WHY THE WIND BLOWS. 
1. Although we cannot see the air we may feel 
it, if we wave our hand quickly through it. It is 
always in motion, as we can tell by the waving of 
the leaves, or by the force of the breeze that blows 
upon us. 
2. This constant movement of the air is one of 
the reasons why its gases are so thoroughly mixed 
together; indeed, they are found in very nearly the 
same proportion everywhere. 
3 . But what is it that sets the air in motion, and 
thus causes the winds? To understand this you 
must know that air expands, or takes up more 
space when it is heated. 
4. If the air in an open bottle be warmed it will 
expand, so that a little of it must pass out. It is 
plain that the air that then fills the bottle will 
