Objects which are behind others which are in front of them are often shown with dotted lines, so as to indicate their position. The seeds of the apple are thus indicated at _A_. It is also customary, in drawings of machinery, to take great liberties with the objects represented and to show them with parts removed or broken away, if their construction can thus be made plainer. It should be remembered that the purpose of drawings of this kind is not to give a pictorial representation of the object as it appears to the eye, but to make its construction and mode of operation apparent to the mind. In such drawings therefore all perspective is disregarded. It would lead us too far were we to explain the reasons for this, and therefore readers must accept the assertion without the proof. CATECHISM OF THE LOCOMOTIVE. PART I. THE STEAM ENGINE. QUESTION 1. _What is the motive power employed in ordinary steam engines?_ _Answer._ The expansive force of steam. QUESTION 2. _How is this expansive force of steam applied?_ [Illustration: _Fig. 1._ Scale ³⁄₈ in. = 1 foot. _A._ Cylinder. _B._ Piston. _R._ Piston-Rod. _D._ Front Cylinder-Head. _C._ Back Cylinder-Head.] _Answer._ It is applied by admitting it into a _cylinder_ (_A_, fig. 1) in which a _piston_, _B_, is fitted so as to move air-tight from one end of the cylinder to the other. The steam, if admitted at _C_, will force the piston _B_ to the opposite end[2] of the cylinder. When it has reached that end, if the steam is allowed to escape and a fresh supply is admitted to the cylinder through the opening _d_, it will move the piston back again. In this way, by alternately admitting steam at one end and exhausting it from the other, the piston receives a _reciprocating motion_, which is communicated to the outside of the cylinder by a rod, _R_, which is called the _piston-rod_, which works air-tight through an opening in one of the _cylinder-covers_, or _cylinder-heads_, as they are usually called. [2] In all ordinary locomotives, the cylinders are so placed that the head _C_ through which the piston-rod works is behind, and the other head _D_ in front. The two ends of the cylinder are therefore designated the _front_ and _back ends_, respectively. [Illustration: _Fig. 2._ Scale ³⁄₈ in. = 1 foot. _A._ Cylinder. _B._ Piston. _R._ Piston-Rod. _c._ and _d._ Steam-ways. _g._ Exhaust-Port. _V._ Slide-Valve. _H._ Exhaust-Cavity. _f.f._ Valve-Seat. _I. I._ Steam-Chest. _M._ Valve-Stem. _r.r._ Rocker. _F. F._ Fly-Wheel. _s._ Rocker-Shaft. _G._ Eccentric. _K._ Eccentric-Strap. _L._ Eccentric-Rod. _E._ Connecting-Rod. _P._ Crank. _N._ Crank-Pin.] [Illustration: _Fig. 3._] [Illustration: _Fig. 4._ _c._ and _d._ Steam-Ports. _g._ Exhaust-Port.] [Illustration: _Fig. 5._ Scale ³⁄₈ in. = 1 foot.] QUESTION 3. _How is this reciprocating motion of the piston converted into rotary motion?_