_Answer._ Sometimes they are made with brass wearing pieces called _gibs_, shown at _N_, _N_, fig. 93, which are placed between the slides and the guides. These gibs can either be removed and new ones substituted when they become very much worn, or by inserting thin pieces of metal, called liners, between them and the cross-head, they will be spread apart so as to fill the space between the slides. The slides are now, however, oftener made without gibs, and have recesses either cast or drilled in them, which are filled with either Babbitt’s metal or glass bearings, which latter are said to wear very well. The guides are bolted at each end to blocks, _x_, _x_, called _guide-blocks_, which can be planed off so as to bring the guides nearer together when they and the slides are worn. Sometimes liners are placed between the blocks and the guides, which can be removed when it is necessary to bring the guides nearer together. QUESTION 174. _Are the top and bottom guides worn alike?_ _Answer._ No: the top guide in ordinary engines is worn the most, because the pressure of the slides is always on the top guide when the engine is running forward, and on the bottom guide when it is running backward. This will be understood by referring back to the series of figures from 11 to 24. It will be noticed that in the backward stroke of the piston, represented by figs. 11 to 17, the strain on the connecting-rod tends to _push_ the cross-head upward, and in the forward part of the stroke, figs. 18 to 24, the connecting rod _pulls_ the cross-head in the same direction. If the crank turned the opposite way, this action would be reversed and the cross-head would then be alternately pushed and pulled downward, and the bottom guides would then be worn the most. As nearly all locomotives run forward more than backward, the top guides are usually worn the most. QUESTION 175. _How are the slides oiled?_ _Answer._ Oil cups, _n_, _o_, figs. 92, 93 and 94, are placed about the middle of the top guide. These cups usually have a reservoir to hold a supply of oil, and are so constructed that it will be gradually fed on the slides, which are thus constantly and regularly lubricated. QUESTION 176. _How are the pumps worked from the piston-rods?_ _Answer._ The pump-plunger is attached to a projection, _W_, figs. 92 and 93, called the _pump-lug_, cast on one of the slides of the cross-head. The plunger thus receives a reciprocating motion from the piston. QUESTION 177. _How are the connecting-rods made?_