26 ENGINE TESTS. 



first observation, after a quarter minute interval, might show a 

 reduction of pressure covering nearly the whole range down to 

 the atmosphere. On the contrary, if the engine is tight, the 

 reduction of pressure to the atmosphere would require from 

 five to ten minutes time. The author finds that the pressure 

 will not fall as a rule more than fifty per cent at the expiration 

 of one minute from the time of shutting the throttle valve, if 

 the engine is fairly tight. 



If, on leakage tests with the blocked engine, it is found that 

 the piston and the two valves leak, whichever stroke the piston 

 is occupying, the piston leakage can be eliminated by discon- 

 necting the valve rods in such a way as to open both steam 

 valves and close both exhaust valves. When this is done, the 

 resulting leakage which is observed applies to the exhaust 

 valves alone. 



The leakage of a piston can always be inspected by removing 

 the cylinder head and applying a pressure behind the piston. 

 The leakage then appears at the open end of the cylinder. On 

 large engines the operation of taking off a cylinder head is 

 attended with considerable labor. The methods which have 

 been described can be brought into use with great facility and 

 save this labor, to say nothing of saving time. 



The blocking of the engine which these tests require is a 

 thing which should not be undertaken in any careless manner. 

 In most cases the masonry foundation of the engine is so 

 arranged that a piece of timber can be placed between the 

 spokes of the wheel, and the two ends laid upon or against the 

 foundation, the strain of a spoke being brought to bear upon 

 the middle of the timber. This timber should be of ample size, 

 say, a 12 in. or 14 in. stick of hard pine for an engine of 1000 

 horse-power, the points of support at the two ends being not 

 over 8 ft. apart. The position of the arm should be brought as 

 nearly as possible to the proper point before the block is intro- 

 duced, the leeway being filled in not by subsequently moving 

 the engine, but by the introduction of wooden filling-pieces and 

 wedges. In the case of an engine having a shaft with two 

 cranks and a solid bed beneath each one, the engine can be 



