_Answer._ The cylinders _A_, plates I, II and III, are placed at the front end of the locomotive, and the main driving-axle, _B_, far enough behind them to permit the connecting rods, _C_, to be attached to pins, _D_, in the cranks, called _crank-pins_. In this country these cranks are now universally placed on the outside of the wheels, and therefore the cylinders must be placed far enough apart (as shown in fig. 40 and Plate III) to permit the connecting-rods to be attached to the crank-pins. The cylinders are therefore placed outside the frames, _H_, _H_, Plate III, (the latter are inside of the wheels,) and are now nearly always horizontal, although in old engines they are often inclined. Plate I is a side view of an ordinary eight-wheeled American locomotive, Plate II a longitudinal section, Plate III a plan, and fig. 40, a transverse section through the cylinder and smoke-box. [Illustration: _Fig. 40._ Scale ¹⁄₄ in. = 1 foot.] QUESTION 72. _What are the smaller wheels, E E, called, and what are they for?_ _Answer._ They are called _truck-wheels_ and carry the weight of the cylinders and other parts of the front end of the locomotive, and serve to guide and steady the machine in a manner which will be more fully explained hereafter. QUESTION 73. _Why are more than one pair of driving wheels necessary for locomotives?_ _Answer._ Because if all the weight which is needed to create the requisite adhesion of the wheels of locomotives to pull heavy loads was placed on one pair of wheels, it would be so excessive as to partly crush and injure the rails. It is therefore distributed, usually on two pairs, but sometimes on three or four or even more pairs. QUESTION 74. _Where is the second pair of driving-wheels usually placed?_ _Answer._ These wheels, _F_--called the _back_ or _trailing driving-wheels_--are, in the ordinary type of locomotives used in this country, situated behind the main driving-wheels, far enough back to give the room necessary for the boiler, _G_, between the two axles, as shown in plates I, II and III. QUESTION 75. _How are the axles, cylinders, etc., held in the right position in relation to each other?_ _Answer._ By longitudinal frames, _H_, _H_, _H_, _H_, which hold the axles in the proper position, and are bolted to the cylinders, and also fastened to the boiler at _I_, _I_, Plate I. QUESTION 76. _How is a locomotive engine made to run either backward or forward?_