_Answer_. It is a wrought or cast iron plate which extends across and rests upon the two frames at the back part of the locomotive and behind the boiler, and on which the locomotive runner and fireman stand. It also unites the two frames very securely, and furnishes an attachment for the draw-bar. The foot-board is often made much heavier than is necessary for strength, in order to increase the weight, and thus the adhesion, on the driving-wheels. It is a fact often not suspected that any weight placed on the back end of an ordinary locomotive will increase the load on the driving-wheels by an amount considerably greater than that of the weight itself. The reason of this is that the locomotive rests on the centre of the truck and the centres of the equalizers, and therefore the weight, if applied to the back end of the engine, gains considerable leverage. This will be clear if we take a beam, _A B_, and rest it on two supports, _m_ and _n_, fig. 199. If now we put a weight _W_ on the end, overhanging the point of support, the weight which will rest on _n_ will be equal to that of _W_ multiplied by its distance _C_ from _m_ and divided by the distance _D_ between _m_ and _n_. Thus if a foot-board weighs 1,000 pounds, and its centre of gravity is 5¹⁄₂ feet behind the centre of the equalizer, and the latter 14 feet from the centre of the truck, then the weight thrown on the driving-wheels will be equal to 1,000 × 19¹⁄₂ ------------- = 1,393 pounds. 14 The same thing is of course true of any other weight placed on the back end of the engine. QUESTION 333. _What are the “wheel-guards” of a locomotive?_ _Answer_. They are sheet iron covers over the upper half of the periphery or tread of the wheels, and are placed there to protect the engine from the dirt and mud which adhere to the wheels and are then thrown off on the machinery by the centrifugal force. QUESTION 334. _What are “check” or “safety chains?”_ _Answer_. There are two kinds of such chains, the one attached to the trucks and frames of the locomotive and the tender. The object of these chains is to prevent the trucks from turning around and getting crosswise of the track if the trucks should leave the rails. The other kind of safety chains connects the engine to the tender, so that, in case the draw-bar or coupling-pins should break, the two will not separate. Great care should be exercised to attach the truck chains so as to be strong enough to resist the strains to which they will be subjected in case the trucks run off the track. The grossest carelessness and ignorance are often shown in the construction of these parts. PART XVI. SCREW-THREADS, BOLTS AND NUTS. QUESTION 335. _How must the screws of bolts and nuts be made, in order to fit each other?_ _Answer_. Each size of screw must be made of exactly the same diameter, and their threads of the same form and proportions and _pitch_. QUESTION 336. _What is meant by the “pitch” of a thread?_