MECHANICAL PARADOXES. 



up the other seven flaps to the same angle 

 with a similar crease. Take a lead pencil 

 with a dull rounded point, not sharp, but not 

 very wide and flat. Hold it perpendicularly 

 with its point on the intersection point of the 

 lines at the centre of the disc, having placed 

 a few folds of fairly soft paper, such as news- 

 paper, underneath, and twirl it in the fingers, 

 while pressing moderately, so as to make a 

 small conical pivot cup in the centre ; but 

 do not pierce a hole through. The disc, taken 

 up, turned over, and viewed sideways, will 

 then present the appearance shown at Q R. 



Take an ordinary pin, and make the point 

 very slightly dull by rubbing it gently on an 

 oil-stone, or some similar surface. Take care 

 that there is no little hook or tag of sharp 

 metal left sticking to the point, which should 

 be rounded and smooth, though fine. 



Hold up the pin in the position shown at 

 V, with its head between the ends of the finger 

 and thumb, and place the disc upon it, with 

 the vanes on the lower side, and the point of 

 the pin in the little pivot cup. 



This simple apparatus is now a very delicate 

 anemometer. Its weight is very little, and the 

 area of contact between its centre and the 

 supporting pivot is extremely small and is 

 well lubricated by the polished surface of 

 blacklead left by the pencil. It will therefore 

 turn with very great ease. The slightest 



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