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THE POPULAR SCIENCE MONTHLY. 



each side, the other end being left plain. I have two candles, E E, 

 each twelve inches off the pith-bar, one on each side of it. When I 

 remove the screens, H LT, the candle on one side will give the pith a 



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Fig. 6. 



push in one direction, and the candle on the other side will give the 

 pith a push in the opposite direction, and as they are the same dis- 

 tance off they will neutralize each other, and the spot of light will not 

 move. I now take the two screens away : each candle is pushing the 



