_Answer._ After the wheels, axles, main connecting-rods and valve-gear are connected together, put the rocker-arm in its middle position, and lengthen or shorten the valve-stem so that the valve will be in the centre of the valve-face. Then place the crank on the forward centre and the full part of the forward motion eccentric above and that of the backward motion eccentric below the axle, and fasten them to the axle temporarily by tightening up the set-screws. Then throw the link down until the block comes opposite to the end of the eccentric-rod, and turn the wheels,[66] and at the same time, observe whether the travel of the valve is equal to the throw of the eccentric and also whether it travels equally on each side of the centre of the valve-face. If its travel is greater than the throw of the eccentric, raise the link up; if less, lower it down until the two are just equal, and then mark the position for the notches on the sections or quadrants to receive the latch of the reverse-lever. If the valve does not travel equally on each side of the centre of the valve-face, either lengthen or shorten the eccentric-rod, as may be necessary. Repeat this operation for the backward motion, by raising the link up until the block is opposite the end of the lower eccentric-rod. After having done this, go over the whole process again to see whether it is all correct. Now with the crank on the forward centre, and the link in full gear forward, loosen the set-screws in the forward eccentric, and move it around the axle so that the valve will have the required lead and then fasten it again. Now raise the link up into full back gear, and set the backward eccentric in the same way. Then turn the wheels so as to bring the crank on the back centre, and observe whether the lead is correct for the back end of the cylinder. If it is not, lengthen or shorten the valve-stem or eccentric-rod so as to make the lead alike at both ends, and if it is then too much or too little, it can be increased or diminished by moving the eccentrics on the axle. [66] This can be done by moving the engine on the track, or by raising it off its wheels, so that the latter can be turned without moving the former. In some shops a pair of rollers is put in the track so that by placing the driving wheels on them they can be turned without any difficulty. Great care must be taken in setting valves to be sure that the cranks are exactly on the centres or dead points, and it is impossible to set them in that position with sufficient accuracy from the motion of the piston or cross-head, and therefore the centres of the crank-pins should always be set so as to conform to a line drawn through the centre of the cylinder and the axle. When the cylinders are horizontal, it is of course only necessary to place the cranks on a horizontal line drawn through the centre of the axle.