INTELLIGENT BILLIARD BALLS. 



end moves off at the speed, as has been shown, 

 of the striking balls, it takes away, in moving 

 off at that speed, half of the energy given up 

 by the two striking balls in being stopped at 

 the same speed. Therefore, the ball before 

 the last, in delivering to the last ball the blow 

 which started it off at that speed, delivered 

 only half of the energy which had been delivered 

 to it by the striking balls through the inter- 

 vening ones which remain practically stationary. 



Retaining the other half of that energy, 

 it has retained enough to start itself at the 

 same speed. As it has no obstacle in the form 

 of a stationary ball beyond it, it starts, and 

 follows the last ball, to which it had itself 

 transmitted the necessary starting power. 



But the ball preceding it the third ball 

 from the end in starting the two last balls 

 at the speed of the striking balls, has given 

 up to them the whole of the energy transmitted 

 from the latter, and has none left to start itself. 

 It remains, therefore, at rest. 



A similar explanation applies whether the 

 number of balls in motion be more or less 

 than two. 



2. Throwing Balls round a Corner. 



We are familiar enough with this operation 

 when the corner is a vertical one. Every boy 

 knows that if a snowball is to hit a police- 

 man over the lowest tunic button from a 



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