_a._ A length-of-stroke-index must be provided for each side of pump.
These must be of simple form for at all times rendering obvious the
exact length of stroke which each piston is making, and thus calling
attention to improper adjustments of cushion valves or stuffing boxes.
_b._ The gauge piece over which the index slides must have deep,
conspicuous marks at ends of nominal stroke, and also light marks at
extreme positions; it need contain no other graduations.
_c._ This stroke index must be rigidly secured to cross-head in such a
way that it cannot get loose or out of adjustment.