All About Engines

4 All About Engines The opening in the boiler is closed by a cup- shaped or “ conical ” piece of brass, which has been ground to fit the end of the seating upon which it rests, and this is held in place by the short rod pinned to the lever just above it. If the area of the opening covered by the valve is 6 square inches and the highest pressure in the boiler is to be 60 lb. on the square inch, the total force, when steam blows off, is 6 x 6o = 360 lb. By hanging a weight at the end of a long lever a much smaller one is required to balance the force on the valve. Thus if it is at eight times the distance of the valve from the fulcrum, the weight need only be 3t°==45 lb. While the safety valve prevents accidents from too high a pressure, the man in charge has to see that enough steam is supplied to enable the engine to do its work ; and in order to know how to manage the fire without continually blowing off steam at the safety-valve, he must know what the pressure in the boiler really is. For this purpose a pressure gauge is provided, and Fig. 3, Plate 1, shows one of the Bourdon type, with the dial partly removed to show the interior. The gauge is connected to the boiler by a U tube or syphon, in which some of the steam con- denses, so that steam never actually enters the curved tube inside the gauge. When the tap A is turned, pressure inside the curved tube B, which is not round but oval in section, rises to that of the boiler. The effect of the steam pressure inside this tube is to make it more nearly circular, and at the same time to straighten it. The end of the tube then pulls