THE INDICATOR 185
The pipes should always be thoroughly blown out by allowing the steam to blow through the open cock during several revolutions of the engine, before con- necting the indicator. If this is not done there is a moral certainty that grit and dirt will get into the cylinder of the indicator, where the pressure of the least atom of grit will cause the delicate instrument to work badly. 'Selecting a Spring. The proper number of spring to use depends upon the boiler pressure in the case of an automatic cut off engine, but for an engine with a fixed cut off and throttling governor the number of the spring to be selected will depend upon the initial pressure in the cylinder. A convenient rule is to select a spring numbered one-half as high as the pres- sure; for instance, if the boiler pressure is 80 lbs., use a No. 40 spring, which will give a diagram 2 in. in height. Care of the Instrument. The indicator should be cleaned and oiled both before and after using. The best material for wiping it is a clean piece of old soft muslin of fine texture, as there is not so much liability of lint sticking to or getting into the small joints. Use good clock oil for the joints and springs, and before taking diagrams it is a good practice to rub a small portion of cylinder oil on the piston and the inside of the cylinder, but when about to put the instrument away these should be oiled with clock oil also. None but the best cord should be used for connecting the paper drum with the reducing motion, as a cord that is liable to stretch will cause trouble. Suitable cord and also blank diagrams can generally be secured from firms manufacturing and selling indicators. After the indicator has been screwed on to the cock connecting with the pipe, the cord must be adjusted to the proper
