When the valves are set it should also be noticed whether the axle-boxes (whose construction will be explained hereafter) are in the centre of the jaws, and if not they should be moved to the centre by driving wooden wedges between them and the frames, either above or below, as may be required. The position of the boxes has a very material influence on the valve-gear. If it is intended to lay off the notches on the sectors so as to cut off steam at certain definite points of the stroke, these points should be laid off in the guides from the motion of the cross-head. The latter being placed in any of the required positions at which steam is to be cut off, the reverse-lever should then be moved so that the link will just close the admission port. The lever can then be clamped to the sectors, and the wheels turned so as to show whether its position is correct for each end of the stroke. As before stated it is impossible to get the ordinary link-motion to cut off at exactly the same points at both ends of the cylinder, but a very close approximation can be made by proportioning the different parts properly. As has already been stated, it is believed to be a much better plan to put as many notches in the sectors as possible, than to locate them for certain definite points of the stroke. In setting the valves of locomotives, care must be taken to turn the wheels _forward_ for the _forward motion_ and _backward_ for the _backward motion_. After the valves are set the position of the eccentrics on the shaft should be marked, so that in case they become loose on the road they can easily be set again. It is usual, too, to mark the position of the valves with centre-punch marks on the valve-stem and on the stuffing-box of the steam-chest, so that with a gauge made for the purpose the position of the valve can be determined without taking off the steam-chest cover. In some cases the eccentrics are keyed on, which is done after their position is determined by setting the valves. The ends of the set-screws which are used to fasten the eccentrics should be cup-shaped and case-hardened, so as to hold as securely as possible to the axle when they are screwed down. PART XII. THE RUNNING GEAR. QUESTION 246. _What is meant by the running gear of a locomotive?_ _Answer._ It means those parts, such as the wheels, axles and frames, which carry the other parts of the engine. As the Germans express it, it is the “_wagon_” of the locomotive. QUESTION 247. _How may the wheels be classified?_ _Answer._ As _driving_ and _carrying_ or _truck wheels_. QUESTION 248. _What service must the driving wheels perform?_ _Answer._ The driving wheels, as indicated by their name, “drive” or move the locomotive on the track, as was explained in answer to Questions 64, 65 and 66. As their adhesion depends upon the pressure with which they bear upon the rails, they must carry either a part or the whole of the weight of the engine.