PLOW BOTTOM METALS 



133 



The shin and point of this bottom show the effect of sand upon steel 

 bottoms. This sort of ground causes a steel bottom to wear out very 

 quickly- 



metal. This sample of chilled metal showing the crystals 

 turned on edge explains why sand does not wear away chilled metal 

 as it does steel. The dirt, passing across the ends of the crystals, has a 

 tendency to polish chilled metal rather than to wear grooves in it. 



