MECHANICAL PARADOXES. 



metal, sloping after the manner of a house- 

 roof, and the boomerang were thrown up the 

 slope, when it had gone as far as its impetus 

 would carry it, it would then simply turn and 

 slide down again to the spot from which it 

 started, being supported all the way by the 

 metallic plane on which it was sliding. 



Well, the boomerang is supported, though 

 on nothing so rigid as a metallic plane. Owing 

 to the fact that it is started with its flat sides 

 up and down, and that its spinning movement 

 keeps it in that position, it can rest on the air 

 like a paper kite, an aeroplane, or the wings 

 of a soaring bird. Like all these, however, 

 it is not rigidly supported ; the air yields, 

 and it slowly descends, so that, instead of 

 keeping to the direction in which it was thrown, 

 marked by the dotted line A C, it follows that 

 of the continuous line, and at D, still slowly 

 dropping, it turns and slides down backwards 

 on the air almost as if on the metallic slide, 

 except that it is all the time falling through it 

 as well as sliding down it. Thus it reaches the 

 level F, at which it started, considerably in 

 advance of its starting point, but, continuing, 

 strikes the ground at E, almost at the feet of 

 the thrower. In this behaviour the boomerang 

 simply imitates the performance of the cards 

 which a trick showman throws up towards 

 the gallery in such a way that they fall back 

 to the stage. At the moment of throwing he 



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