FRICTION. 99 



Q,. How do the Indians produce FIRE, by 

 merely RUBBING TWO PIECES of dry WOOD 



TOGETHER ? 



A. They take a piece of dry wood 

 (sharpened to a point), which they rub 

 quickly up and down a flat piece, till a 

 groove is made ; and the saw-dust 

 (collected in this groove) soon catches 

 fire. 



Q. Why does the saw-dust of the WOOD 



CATCH FIRE by RUBBING ? 



A. The latent heat of the wood is 

 developed by friction ; because the par- 

 ticles of the wood are squeezed closer 

 together, and the heat pours out, as 

 water from a sponge. 



(The best woods for this purpose are box-wood 

 against mulberry, or laurel against poplar or ivy.) 



Q. Do not CARRIAGE WHEELS sometimes 



CATCH FIRE ? 



A. Yes; if the wheels be dry, or Jit 

 too tightly, or revolve very rapidly, 

 they often catch fire. 



Q. Why do wheels catch fire in such cases ? 



A. The friction of the wheels against 

 the axle-tree is so great, that their latent 

 heat is disturbed, and produces ignition. 



K2 



