THE STEAM TRACTOR 49 



pump, which is really a small steam engine working a plunger 

 which is connected directly to the piston rod of the pump. 

 The injector draws steam from the upper part of the boiler 

 and feeds cold water into the lower part by the combined 

 effect of the velocity and condensation of the steam. The 

 injector is simple and satisfactory, provided it is properly 

 chosen with reference to the conditions of its work. 



In order to protect the boiler from sudden changes in tem- 

 perature due to the incoming of cold water, the latter is usually 

 passed through a heater located between the pump and the 

 boiler. The pipes are usually surrounded by the exhaust 

 steam, though occasionally live steam is introduced directly 

 into the water to raise its temperature. 



Safety Devices 



To prevent the water from being carried too high or too 

 low, a glass water gauge and several try-cocks are connected 

 to the boiler near the level of the crown sheet. A steam gauge 

 is also provided for indicating the boiler pressure. The essen- 

 tial feature of the steam gauge is a metal tube bent in semi- 

 circular form. One end is attached to the boiler by a siphon, 

 which keeps air in the tube and protects it from the heat of 

 the live steam by a cylinder of water. The pressure of the 

 steam upon the outside circumference of the tube tends to 

 straighten it as water straightens a hose. The pressure is 

 indicated upon a dial by means of a suitable link and needle. 



The steam engine boiler must be equipped with a safety 

 valve for releasing the pressure when it rises to a certain point. 

 Plowing engines usually have what is known as a spring or lock 

 pop valve in which the valve is kept in position by a powerful 

 spring which may be adjusted for action at different pressures. 



A soft metal plug, usually of Banca tin, which fuses at a 

 lower temperature than iron or brass, is placed in the top of 

 the crown sheet. Should the water become low, the plug will 

 melt and the steam, pouring through, will put out the fire. 



