_Answer._ Because it must move in a straight line if it and the piston work steam-tight in the cylinder. By referring back to fig. 2, it is obvious that if a pressure be exerted against the piston _B_ and communicated to the crank-pin _N_ by the connecting-rod _E_, the latter, excepting at the dead-points, will exert a pressure either upward or downward, according to the direction the piston is moving. This pressure would bend the piston-rod if no provision were made to prevent it. For this reason, therefore, the end of the piston-rod is attached to what is called a _cross-head_, _L_, figs. 92 and 94, which works in guides, _M_, _M_. The cross-head is made of cast iron and has slides, _N_, _N_, figs. 93 and 94, one on each side, each of which works between a pair of guide-bars or rods, _M_, _M_, shown in section in fig. 93.[46] These guide-rods, or _guides_ as they are called, are planed and finished with great accuracy so as to be straight and smooth, and are attached to the cylinder-head at one end, and to a support, _O_, called the _guide-yoke_, which is fastened to the frame at _F_, fig. 94, and also usually attached to the boiler. The guides are set with great care, so as to be exactly parallel with the axis or centre line of the cylinder, so that the cross-head will slide in exactly the same path that the piston-rod will if it moves in a straight line. If then the piston-rod and the connecting-rod are attached to the cross-head, all the strain produced by the obliquity of the connecting-rod will be borne by the guides, thus relieving the piston-rod, and making it certain that it will move in a straight line. [46] Fig. 93 is a transverse section through the guides at _n_, fig. 92. QUESTION 172. _How are the piston and connecting-rods attached to the cross-head?_ _Answer._ The end of the piston-rod fits into a tapered hole in the cross-head and is held by a key, _w_, figs. 92 and 94. The connecting-rod is attached to a pin, _Q_, called a _wrist-pin_, which is cast with the cross-head. QUESTION 173. _How is the wear of the slides lessened and compensated?_