_Answer._ By means of two rings, _A_, _A_, figs. 96 and 97, called _packing-rings_. These rings are turned of the same size or a little larger in diameter than the cylinder. They are then cut open at one point in their circumference so that they can be pressed apart or expanded by the springs _a_, _a_, called _packing springs_, on the inside of the rings. These springs are pressed out by the nuts and bolts _b_, _b_, called _packing-bolts_ and _packing-nuts_, so that when the rings wear they can be expanded so as to fill the cylinder completely. The place where the one ring is cut is placed opposite that of the opening in the other ring, or they are made to _break joints_, as it is called. This is done to prevent the steam which leaks through the opening where the one ring is cut from passing through to the other side of the piston. These rings are usually made of brass and have grooves, _c_, _c_, fig. 97, turned in them, which are filled with what is called Babbitt’s metal. This metal is used because it is less liable to scratch the cylinders than brass alone. Another ring, _l l_, made of cast iron and as wide as the two brass rings, is placed inside of the latter and is intended to furnish a bearing for the springs, and thus distribute the pressure of the springs equally on the packing rings. This iron ring is also cut open at one point. QUESTION 170. _How is the piston-rod made to work steam-tight through the cylinder-head?_ _Answer._ By what is called a _stuffing-box_. This consists of a cylindrical chamber, _r r_, fig. 92, which is made about 1¹⁄₂ inches larger in diameter than the piston-rod. This leaves a space ³⁄₄ of an inch wide all around the rod. This space is filled with hemp or some other fibrous material, called _packing_, saturated with oil or melted tallow. This packing is compressed by a hollow cylinder, _s s_, called a _gland_, the inside of which fits the piston-rod and the outside the stuffing-box. This gland is forced into the stuffing-box by nuts, _t_, _t_, which are screwed down on a flange, _u_, attached to the gland. The packing is thus compressed in the stuffing-box and forced against the piston-rod, which is made smooth and perfectly round and straight, and against the side of the stuffing-box, so that no steam can escape around the piston-rod. A brass ring or “_bushing_” is often put into the cylinder-head and in the gland where it touches the piston-rod,[45] because brass will bear the friction of the rod better than cast iron, and when it is worn out it can be removed and a new one substituted in its place. [45] Locomotive piston-rods are now usually made of steel. QUESTION 171. _Why is the end of the piston-rod made to work in guides?_