8571. Yarrow, A. F. May 9. Testing boiler tubes.-In a water-tube boiler, each tube is tested for leakage by closing its bottom and filling it with water. If the lower end of the tube is accessible, it may be closed by a valve hooked on to a jointed rod, introduced from the upper end and operated by a lever. When the lower end is inaccessible, it may be closed by introdueing a flexible pipe carrying an elastic bulb, and then expanding the bulb with air, water, or other fluid. When the leakage is small, both ends of the tube are closed, air or water is forced into it, and the leakage is detected by the change of pressure. As shown in Fig. 3, the tube is closed by elastic washers h compressed between metal washers i held in place by an apertured distance-piece g and nuts j on a jointed rod f having a passage at its upper end connected by a union n to a force-pump and pressure gauge. As shown in Fig. 5, the tube is closed by a valve a, hooked to a jointed rod b, and a stopper k held in place by a winged nut l, bearing on packing m, and connected by a union n to a pipe leading to a pump and pressure gauge.