UNIVERSITY 



OF / 



SCIENCE OF THRESHING. 89 



The burning of the fuel in the fire-box of the 

 boiler causes the molecules of the fuel to start 

 vibrating at a high rate of speed, or to rise to 

 a high temperature. This energy of motion in the 

 molecules is communicated to the surrounding air 

 and to the walls of the fire-box. Obviously it is 

 necessary to see that as much of this energy as pos- 

 sible be saved or rather directed to the point- where it 

 can be used, and so the fire-box of the boiler should 

 be such as will best retain the heat and best transmit 

 it to the water in the boiler. 



We have further seen how the heat affects the 

 water to which it is imparted through the walls of the 

 fire-box, and as a final step in our reasoning, we have 

 seen how the vibration of the molecules of the water 

 finally overcomes that mysterious force which tends 

 to hold them together as water, and causes them to fly 

 apart or expand. This power of expansion we make 

 to do work by putting something in the way of the 

 expanding particles which they must move before 

 they can go as far as they otherwise would, and we" 

 thus turn the energy of the vibrating molecules of the 

 burning fuel into an energy which we can harness and 

 direct as we please. 



It will be remembered that when water was heated 

 to 212 degrees, the temperature remained the same, 

 while the heat which was afterwards applied disap- 

 peared as far as the thermometer was concerned, and 

 this we called hidden or latent heat. It is found by 

 careful experiments that it takes about 144 units of 



