gear with the wooden teeth is called a mortise gear.

Coupling Rods. A rod used to transmit the motion of one crank to another is called a coupling rod. A familiar example of the use of coupling rods will be found in the locomotive. Coupling rods are made of wrought iron or steel, and are generally of rectangular section. The ends are now generally made solid and lined with solid brass bushes, without any adjustment for wear. This form of coupling rod end is found to answer very well in locomotive practice where the workmanship and arrangements for lubrication are excellent. When the brass bush becomes worn it is replaced by a new one.

Fig. 168 shows an example of a locomotive coupling rod end for an outside cylinder engine. In this case it is desirable to have the crank-pin bearings for the coupling rods as short as possible, for a connecting rod and coupling rod in this kind of engine work side by side on the same crankpin, which, being overhung, should be as short as convenient for the sake of strength. The requisite bearing surface is obtained by having a pin of large diameter. The brass bush is prevented from rotating by means of the square key shown. The oil-box is cut out of the solid, and has a wroughtiron cover slightly dovetailed at the edges. This cover fits into a check round the top inner edge of the box, which is originally parallel, but is made

Example 13. Coupling Rod End. Draw first the side elevation and plan, each partly in section as shown in Fig. 168. Then instead of the view to the left, which is an end elevation partly in section, draw a complete end elevation looking to the right, and also a complete vertical cross section through the center of the bearing. Scale 6 inches to 1 foot. Stuffing-boxes. In Fig. 169 is shown a gland and stuffing-box for the piston rod of a vertical engine. A B is the piston rod, CD a portion of the cylinder cover, and E F the stuffing-box. Fitting into the bottom of the stuffing-box is a brass bush H. The space K around the rod A B is filled with packing, of which there is a variety of kinds, the simplest being greased hempen rope. The packing is compressed by screwing down the cast-iron gland L M, which is lined with a brass bush N. In this case the gland is screwed down by means of three stud-bolts P, which are screwed into a flange cast on the stuffing-box. Surrounding the rod on the top of the gland there is a recess R for holding the lubricant.

The object of the gland and stuffing-box is to allow the piston rod to move backwards and forwards freely without any leakage of steam.

Example 14. Gland and Stuffing-box for a Vertical Rod. Draw the views shown in Fig. 169 to the dimensions given. Scale 6 inches to 1 foot.

Water or Steam Cock. Fig. 170 shows a cock of considerable size, which may be used for water