MECHANICAL PARADOXES. 



In Fig. 12 let the view always be down- 

 wards from above on to the top of the ball, 

 and let the ball in each case be supposed thrown 

 from the left side of the page towards the 

 right, in the direction of the large arrow at 

 the top of the figure. 



Let the ball A at the moment of throwing 

 receive a horizontal spin about a vertical axis, 

 as shown by the small arrows. Then its front 

 face, as it flies, will move towards the left 

 of the thrower, as shown at B, and will tend 

 to brush the air in front of it, against which 

 it is forcibly pressed, in that direction. But 

 action and reaction are equal and opposite. 

 Standing in a boat, if you push someone who 

 is standing on the pier, you not only make 

 him move backwards from you, but you 

 also move yourself and your boat backwards 

 from him. The same thing happens if you 

 push someone while sitting in a swing : you 

 cause movement in yourself as well as in the 

 person you push. A nearer parallel is that 

 of oars pushing the water. This results in 

 the boat moving in the opposite direction. 

 So, when the wings of birds and insects 

 beat the air downwards and backwards, it 

 results in their own movement upwards and 

 forwards. 



In the same way, when the face of the ball 

 drives the air in the direction of B to the 

 left, the ball itself has to move along the dotted 



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