A ir Wind A t traction. 5 



5. This we call wind. You feel the air, you 

 breathe it, you see the effects of the wind, yet 

 you have never seen air or wind You admit 

 that there is air and that there is wind, although 

 both are invisible. What does invisible mean ? 

 Are houses and trees visible or invisible ? 



6. Now, as the earth is round (or very nearly 

 so) like a great ball, and people travel or sail 

 around on every part of it, what is it that keeps 

 them from falling off from this great ball called 

 the earth ? It is something that is both useful 

 and powerful. It is also invisible. When you 

 throw your ball high in the air, it is brought 

 back again by something which you cannot see, 

 by this other invisible power ; without this 

 power your ball would never come back to 

 you. When chestnuts are ripe, and when you 

 throw a stone into an apple-tree in the autumn, 

 the chestnuts and apples are brought to the 

 ground by this same invisible power. Do you 

 know what we 'call it ? Attraction. Without 

 this attraction which the earth has, those chest- 

 nuts and apples would be as likely to fly away 

 toward the moon or the sun or some of the 

 stars. Without this power which the earth 

 has of drawing or attracting to itself (always 

 downward), the farmer could not sow his seed, 

 for it would be as likely to fly toward the 

 clouds as to fall on the ground ; the carpenter 



