136 ROUGH-AND-TUMBLE ENGINEER. 



and allows the steam to flow down on top of the fire 

 stroys the heat and prevents the burning of the crown 

 sheet. 



Q. Why don't the lead fuse with water over ii ? 



A. Because the water absorbs the heat and prevents 

 it reaching the fusing point. 



Q. \Yhat is the fusing point of lead? 



A. 618. 



Q. Is there any objection to the soft plug? 



A. There is. in the hands of some engineers. 



Q. Why? 



A. It relieves him of the fear of a dry crown sheet, 

 and gives him an apparent excuse for low water. 



Q. Is this a real or legitimate objection ? 



A. It is not. 



Q. What are the two distinct classes of boilers ? 



A. The externally and internally fired boilers. 



Q. Which is the most economical ? 



A. The internally fired boiler. 



Q. Why? 



A. Because the fuel is all consumed in close contact 

 with the sides of the furnace and the loss from radr 

 is less than in the externally fired. 



Q. To what class does the farm or traction engine be- 

 long? 



A. To the internally fired. 



