_Answer._ This can usually be learned by examining the marks which should always be made on the eccentrics and on the axles. If no such marks have been made by the builder of the engine, the runner himself should make them, after the valves have been set correctly. The effect upon the valve when an eccentric slips is either to increase or diminish the lead. Therefore, by running the engine slowly with the link first in full forward and then in full back gear, and observing whether steam is admitted at each end of the cylinder just before the crank reaches the dead points, it can be known which eccentric has moved. If it has slipped in one direction the lead will be increased and steam will be admitted to the cylinder some time before the piston reaches the end of the stroke. If it has moved the opposite way, the lead will be diminished and steam will not be admitted until after the piston has reached the end of its stroke. The admission of steam will be indicated by its escape from the cylinder cocks. QUESTION 531. _If by any means the valve stem or either of the eccentric rods should be lengthened or shortened, how can it be known?_ _Answer._ The crank on one side should be placed at one of the dead points and the cylinder-cocks opened; then admit a little steam to the cylinder, by opening the throttle-valve slightly, and throw the reverse lever from full gear forward to full gear backward, and observe whether steam escapes all the time from the end of the cylinder at which the piston stands. Then repeat the operation with the crank at the other dead point. If either of the eccentric rods or the valve stem have been lengthened or shortened, it will cause the valve to cover the steam-port either at the front or back end of the cylinder, so that no steam will escape from the cock at that end. If the length of one of the eccentric rods has been changed, then when the altered rod is in gear the valve will have too little or no lead at one end of the cylinder and too much at the other. If, therefore, this occurs when the forward rod is in gear and _not_ in back gear, it indicates that the length of the forward rod has been altered. If the reverse occurs it shows that it is the back-motion rod whose length has been changed. It must be observed that if the length of an eccentric rod is altered the lead will be changed only at that part of the link which is operated by the altered rod. That is, if the forward eccentric rod is too long or too short, the lead at the front and back ends of the cylinder in forward gear only will be affected. If the back eccentric rod is changed the valve will be affected only in back gear. If, however, the length of the valve stem is changed, the lead will be changed in both forward and back gear. The valves on each side of the engine can, of course, be tested in the same way. QUESTION 532. _When it is discovered which eccentric has slipped, how should it be reset?_