SCIENCE OF THRESHING. 93 



TABLE NO. 3. 



The above table shows the relative heat conducting 

 power of different substances. 



The fact that soot and boiler scale are such poor 

 heat conductors shows how essential it is that the fire- 

 box and flues should be kept free from soot and the 

 boiler from scale, to give the action of the fire free 

 access to heat the water. An ordinary threshing 

 engine will give from two to four horse power more, 

 when clean of soot and scale than where dirty. 



TABLE NO. 4. 



The following table shows the chemical composi- 

 tion of ordinary firewood. 



Carbon 37.5 per cent. 



Hydrogen 4. 5 per cent. 



Oxygen 30.75 per cent. 



Nitrogen 0.75 per cent. 



Ashes 1.5 per cent. 



75.0 per cent. 

 Hygrometric water 25.0 per cent. 



100.0 



