94 SCIENCE OF SUCCESSFUL THRESHING. 



shows that the sheets spring or bulge with the pressure, the 

 only way to be sure is to drill a small hole and determine the 

 thickness. If found to be safe, the hole may be made tight 

 by tapping and screwing in a copper plug. 



Another Method. A boiler may be tested without using 

 a pump. In this case the boiler is filled with water to the very 

 top of the dome before the fire is built, and the expansion of 

 the water, as it increases in temperature, gives the desired 

 pressure for testing. The boiler may be filled by removing 

 the whistle or the pop-valve and pouring the water through 

 its pipe. The throttle and all of the openings from the boiler 

 must be closed before the fire is built. Straw should be 

 used as fuel, as a fire of it may be quickly checked. When 

 other fuel, such as pine kindling wood is used, very little 

 should be allowed in the fire-box, and the fire carefully 

 watched. Enough dirt, sand or ashes should be at hand to 

 check the fire at any instant. The pressure must be closely 

 watched, and if it shows a tendency to rise too rapidly, or 

 go too high, the fire must be covered. The pop-valve will 

 open at the point at which it is set, in the same way as for 

 steam pressure. 



Amount of Pressure. An old boiler should not be tested 

 at a greater pressure than one hundred and fifty pounds, as 

 higher ones are apt to strain and weaken the boiler. When 

 a boiler has been tested at one hundred and fifty pounds 

 cold water pressure, it may be used at a working pressure 

 of one hundred and twenty-five pounds. 



