. Science of railways . t in which the cord runs, the momentum of the Fig. 2. The Thompson Indicator; (Sectional View.) paper cylinder has been reduced to a very smallamount. All of these improvements have lessenedthe amount of friction, which was heretofore verysmall, but is now reduced to a minimum; andfurthermore, they tend to improve, on the whole,an instrument whose principle has always been ofundoubted correctness. 612 LOCOMOTIVE APPLIANCES. READING INDICATOR DIAGRAMS. While the diagram is solely a graphic representa-tion of the pressure in the cylinder, and only one sideof the cylinder,
. Science of railways . t in which the cord runs, the momentum of the Fig. 2. The Thompson Indicator; (Sectional View.) paper cylinder has been reduced to a very smallamount. All of these improvements have lessenedthe amount of friction, which was heretofore verysmall, but is now reduced to a minimum; andfurthermore, they tend to improve, on the whole,an instrument whose principle has always been ofundoubted correctness. 612 LOCOMOTIVE APPLIANCES. READING INDICATOR DIAGRAMS. While the diagram is solely a graphic representa-tion of the pressure in the cylinder, and only one sideof the cylinder, still by a knowledge of the variousoperations of the locomotive slide valve much valuableinformation is gained. In the diagram, Fig. 1, the line D D is the atmos-pheric line, and is made by setting the indicator drumin motion when there is no connection open between thelocomotive cylinder and the indicator piston. Thepressures indicated by lines above and below this linefrom which measurements are taken are those above. or below atmospheric pressure. The line V V is thezero line fourteen and seven-tenths pounds below lineD D, and drawn by hand after the card is taken. A Cis the admission line. Steam is first admitted to thecylinder at C, hence that is called the point of admis-sion. A B is the steam line. B is the point ofcut off, after which the pressure in the cylinder dropsin expanding; hence, B E is the expansion point E the exhaust port begins to open and is thuscalled the point of initial exhaust, or point ofrelease. The exhaust continues to the end of thestroke at Z. Z Y is the line of back pressure. The LOCOMOTIVE APPLIANCES. 613 exhaust port closes at Y and compression begins,Y C being the compression hne. The shghtest irregularities in the steam distribu-tion of a valve may thus be readily detected by takingan indicator card at a not too great speed. It is customary to take an indicator diagram fromeach end of the cyhnder, upon the same card. Thetaking of the
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